EMBEDDED SYSTEMS :REAL-TIMEINTERFACINGTOARM®

CORTEXTM-MMICROCONTROLLERS

Volume2FourthEdition,July2014

JonathanW.ValvanoFourthedition2ndPrintingJuly2014

ARManduVisionareregisteredtrademarksofARMLimited.

CortexandKeilaretrademarksofARMLimited.

StellarisandTivaareregisteredtrademarksTexasInstruments.

CodeComposerStudioisatrademarkofTexasInstruments.

Allotherproductorservicenamesmentionedhereinarethetrademarksoftheirrespectiveowners.

Inordertoreducecosts,thiscollegetextbookhasbeenself-published.Formoreinformationaboutmyclasses,myresearch,andmybooks,seehttp://users.ece.utexas.edu/~valvano/

Forcorrectionsandcomments,pleasecontactmeat:valvano@mail.utexas.edu.Pleasecitethisbookas:J.W.Valvano,EmbeddedSystems:Real-TimeInterfacingtoARM ® CortexTM-MMicrocontrollers,http://users.ece.utexas.edu/~valvano/,ISBN:978-1463590154,2014.

Copyright©2014JonathanW.Valvano

Allrightsreserved.Nopartofthisworkcoveredbythecopyrighthereinmaybereproduced,transmitted,stored,orusedinanyformorbyanymeansgraphic,electronic,ormechanical,includingbutnotlimitedtophotocopying,recording,scanning,digitizing,taping,webdistribution,informationnetworks,orinformationstorageandretrieval,exceptaspermittedunderSection107or108ofthe1976UnitedStatesCopyrightAct,withoutthepriorwrittenpermissionofthepublisher.

ISBN-13:978-1463590154

ISBN-10:1463590156

TableofContents

PrefacetoThirdEdition

PrefacetoFourthEdition

Preface

Acknowledgements

1.IntroductiontoEmbeddedSystems

1.1.ComputerArchitecture

1.2.EmbeddedSystems

1.3.TheDesignProcess

1.4.DigitalLogicandOpenCollector

1.5.DigitalRepresentationofNumbers

1.6.Ethics

1.7.Exercises

1.8.LabAssignments

2.ARMCortex-MProcessor

2.1.CortexTM-MArchitecture

2.2.TexasInstrumentsLM3SandTM4CI/Opins

2.3.ARM CortexTM-MAssemblyLanguage

2.4.ParallelI/Oports

2.5.Phase-Lock-Loop

2.6.SysTickTimer

2.7.ChoosingaMicrocontroller

2.8.Exercises

2.9.LabAssignments

3.SoftwareDesign

3.1.Attitude

3.2.QualityProgramming

3.3.SoftwareStyleGuidelines

3.4.ModularSoftware

3.5.FiniteStateMachines

3.6.Threads

3.7.FirstInFirstOutQueue

3.8.MemoryManagementandtheHeap

3.9.IntroductiontoDebugging

3.10.Exercises

3.11.LabAssignments

4.Hardware-SoftwareSynchronization

4.1.Introduction

4.2.Timing

4.3.PetriNets

4.4.KahnProcessNetworks

4.5.Edge-triggeredInterfacing

4.6.ConfiguringDigitalOutputPins

4.7.Blind-cycleInterfacing

4.8.Busy-WaitSynchronization

4.9.UARTInterface

4.10.KeyboardInterface

4.11.Exercises

4.12.LabAssignments

5.InterruptSynchronization

5.1.Multithreading

5.2.InterthreadCommunicationandSynchronization

5.3.CriticalSections

5.4.NVIContheARM Cortex-MProcessor

5.5.Edge-triggeredInterrupts

5.6.Interrupt-DrivenUART

5.7.PeriodicInterruptsusingSysTick

5.8.Low-PowerDesign

5.9.DebuggingProfile

5.10.Exercises

5.11.LabAssignments

6.TimeInterfacing

6.1.InputCaptureorInputEdgeTimeMode

6.2.OutputCompareorPeriodicTimer

6.3.PulseWidthModulation

6.4.FrequencyMeasurement

6.5.BinaryActuators

6.6.IntegralControlofaDCMotor

6.7.Exercises

6.8.LabAssignments

7.SerialInterfacing

7.1.IntroductiontoSerialCommunication

7.2.RS232Interfacing

7.3.RS422/USB/RS423/RS485BalancedDifferentialLines

7.4.LogicLevelConversion

7.5.SynchronousTransmissionandReceivingusingtheSSI

7.6.Inter-IntegratedCircuit(I2C)Interface

7.7.IntroductiontoUniversalSerialBus(USB)

7.8.Exercises

7.9.LabAssignments

8.AnalogInterfacing

8.1.ResistorsandCapacitors

8.2.OpAmps

8.3.AnalogFilters

8.4.DigitaltoAnalogConverters

8.5.AnalogtoDigitalConverters

8.6.Exercises

8.7.LabAssignments

9.System-LevelDesign

9.1.DesignforManufacturability

9.2.Power

9.3Tolerance

9.4.DesignforTestability

9.5.PrintedCircuitBoardLayoutandEnclosures

9.6.Exercises

9.7.LabAssignments

10.DataAcquisitionSystems

10.1.Introduction

10.2.Transducers

10.3.DiscreteCalculus

10.4.DataAcquisitionSystemDesign

10.5.AnalysisofNoise

10.6.DataAcquisitionCaseStudies

10.7.Exercises

10.8.LabAssignments

11.IntroductiontoCommunicationSystems

11.1.Fundamentals

11.2.CommunicationSystemsBasedontheUARTs

11.3.WirelessCommunication

11.4.InternetofThings

11.5.Exercises

11.6.LabAssignments

Appendix1.Glossary

Appendix2.SolutionstoCheckpoints

IndexReferenceMaterial

PrefacetoThirdEditionThereareanewfeaturesaddedtothisthirdedition.ThenewdevelopmentplatformbasedontheTM4C123iscalledTivaLaunchPad.MaterialinthisbookontheTM4CalsoappliestotheLM4FbecauseTexasInstrumentsrebrandedtheLM4FseriesasTM4C(samechipsnewname),andrebrandedStellarisWare™asTivaWare™.Thesenewmicrocontrollersrunat80MHz,includesingle-precisionfloatingpoint,havetwo12-bitADCs,andsupportDMAandUSB.Awonderfulfeatureofthesenewboardsistheirlowcost.AsofDecember2013,theboardsareavailableonTI.comaspartnumberEK-TM4C123GXLfor$12.99.Theyarealsoavailablefrom$13to$24atregularelectronicsretailerslikearrow.com,newark.com,mouser.com,anddigikey.com.ThebookcanbeusedwitheitheraLM3SorTM4Cmicrocontroller.AlthoughthiseditionnowfocusesontheM4,theconceptsstillapplytotheM3,andthewebsiteassociatedwiththisbookhasexampleprojectsbasedontheLM3S811,LM3S1968,andLM3S8962.

PrefacetoFourthEditionThisfourtheditionincludesthenewTM4C1294-basedLaunchPad.MostofthecodeinthebookisspecificfortheTM4C123-basedLaunchPad.However,thebookwebsiteincludescorrespondingexampleprojectsfortheLM3S811,LM3S1968,LM4F120,andTM4C1294,whichareARM ® Cortex™-MmicrocontrollersfromTexasInstruments.Therearenowtwolost-costdevelopmentplatformscalledTivaLaunchPad.TheEK-TM4C123GXLLaunchPadretailsfor$12.99,andtheEK-TM4C1294XLConnectedLaunchPadretailsfor$19.99.ThevariousLM3S,LM4FandTM4Cmicrocontrollersarequitesimilar,sothisbookalongwiththeexamplecodeonthewebcanbeusedforanyofthesemicrocontrollers.ComparedtotheTM4C123,thenewTM4C1294microcontrollerrunsfaster,hasmoreRAM,hasmoreROM,includesEthernet,andhasmoreI/Opins.ThisfourtheditionswitchesthesyntaxfromCtotheindustry-standardC99,addsaline-trackingrobot,designsanintegralcontrollerforaDCmotor,andincludesanexpandedsectiononwirelesscommunicationandInternetofThings.

PrefaceEmbeddedsystemsareaubiquitouscomponentofoureverydaylives.Weinteractwithhundredsoftinycomputerseverydaythatareembeddedintoourhouses,ourcars,ourtoys,andourwork.Asourworldhasbecomemorecomplex,sohavethecapabilitiesofthemicrocontrollersembeddedintoourdevices.TheARM ® CortexTM-Mfamilyrepresentsanewclassofmicrocontrollersmuchmorepowerfulthanthedevicesavailabletenyearsago.Thepurposeofthisbookistopresentthedesignmethodologytotrainyoungengineerstounderstandthebasicbuildingblocksthatcomprisedeviceslikeacellphone,anMP3player,apacemaker,antilockbrakes,andanenginecontroller.

ThisbookisthesecondinaseriesofthreebooksthatteachthefundamentalsofembeddedsystemsasappliedtotheARM ® CortexTM-Mfamilyofmicrocontrollers.Thethreebooksareprimarilywrittenforundergraduateelectricalandcomputerengineeringstudents.TheycouldalsobeusedforprofessionalslearningtheARMplatform.ThefirstbookEmbeddedSystems:IntroductiontoARMCortex-MMicrocontrollersisanintroductiontocomputersandinterfacingfocusingonassemblylanguageandCprogramming.Thissecondbookfocusesoninterfacingandthedesignofembeddedsystems.ThethirdbookEmbeddedSystems:Real-TimeOperatingSystemsforARMCortex-MMicrocontrollersisanadvancedbookfocusingonoperatingsystems,high-speedinterfacing,controlsystems,androbotics.

Anembeddedsystemisasystemthatperformsaspecifictaskandhasacomputerembeddedinside.Asystemiscomprisedofcomponentsandinterfacesconnectedtogetherforacommonpurpose.Thisbookpresentscomponents,interfacesandmethodologiesforbuildingsystems.Specifictopicsincludethearchitectureofmicrocontrollers,designmethodology,verification,hardware/softwaresynchronization,interfacingdevicestothecomputer,timingdiagrams,real-timeoperatingsystems,datacollectionandprocessing,motorcontrol,analogfilters,digitalfilters,real-timesignalprocessing,wirelesscommunication,andtheinternetofthings.

Ingeneral,theareaofembeddedsystemsisanimportantandgrowingdisciplinewithinelectricalandcomputerengineering.TheeducationalmarketofembeddedsystemshasbeendominatedbysimplemicrocontrollerslikethePIC,the9S12,andthe8051.Thisisbecauseoftheirmarketshare,lowcost,andhistoricaldominance.However,asproblemsbecomemorecomplex,somustthesystemsthatsolvethem.Anumberofembeddedsystemparadigmsmustshiftinordertoaccommodatethisgrowthincomplexity.First,thenumberofcalculationspersecondwillincreasefrommillions/sectobillions/sec.Similarly,thenumberoflinesofsoftwarecodewillalsoincreasefromthousandstomillions.Thirdly,systemswillinvolvemultiplemicrocontrollerssupportingmanysimultaneousoperations.Lastly,theneedforsystemverificationwillcontinuetogrowasthesesystemsaredeployedintosafetycriticalapplications.Thesechangesaremorethanasimplegrowthinsizeandbandwidth.Thesesystemsmustemployparallelprogramming,high-speedsynchronization,real-timeoperatingsystems,faulttolerantdesign,priorityinterrupthandling,andnetworking.Consequently,itwillbeimportanttoprovideourstudentswiththesetypesofdesignexperiences.TheARMplatformisbothlowcostandprovidesthehighperformancefeaturesrequiredinfutureembeddedsystems.AlthoughtheARMmarketshareiscurrentlynothuge,itssharewillgrow.Furthermore,students

trainedontheARMwillbeequippedtodesignsystemsacrossthecompletespectrumfromsimpletocomplex.Thepurposeofwritingthesethreebooksatthistimeistobringengineeringeducationintothe21stcentury.

Thisbookemploysmanyapproachestolearning.Itwillnotincludeanexhaustiverecapitulationoftheinformationindatasheets.First,itbeginswithbasicfundamentals,whichallowsthereadertosolvenewproblemswithnewtechnology.Second,thebookpresentsmanydetaileddesignexamples.Theseexamplesillustratetheprocessofdesign.Therearemultiplestructuralcomponentsthatassistlearning.Checkpoints,withanswersintheback,areshorteasytoanswerquestionsprovidingimmediatefeedbackwhilereading.Simplehomework,withanswerstotheoddquestionsontheweb,providesmoredetailedlearningopportunities.Thebookincludesanindexandaglossarysothatinformationcanbesearched.Themostimportantlearningexperiencesinaclasslikethisareofcoursethelaboratories.Eachchapterhassuggestedlabassignments.Moredetailedlabdescriptionsareavailableontheweb.Specifically,lookatthelabassignmentsforEE445LandEE445M.

Thereisawebsiteaccompanyingthisbookhttp://users.ece.utexas.edu/~valvano/arm.PostedhereareARMKeilTMuVision®projectsforeachtheexampleprogramsinthebook.CodeComposerStudioTMversionsarealsoavailableformostexamples.YouwillalsofinddatasheetsandExcelspreadsheetsrelevanttothematerialinthisbook.

ThesethreebookswillcoverembeddedsystemsforARM ® CortexTM-MmicrocontrollerswithspecificdetailsontheLM3S811,LM3S1968,LM3S8962,LM4F120,TM4C123,andTM4C1294.Mostofthetopicscanberunonthelow-costTM4C123.EthernetexamplescanberunontheLM3S8962andTM4C1294.InthesebooksthetermsLM3SandLM4FandTM4CwillrefertoanyoftheTexasInstrumentsARM ® CortexTM-Mbasedmicrocontrollers.AlthoughthesolutionsarespecificfortheLM3SLM4FandTM4Cfamilies,itwillbepossibletousethesebooksforotherARMderivatives.

AcknowledgementsIoweawonderfuldebtofgratitudetoDanielValvano.Hewroteandtestedmostofthesoftwareexamplesfoundinthisbook.Secondly,hecreatedandmaintainstheexamplewebsite,http://users.ece.utexas.edu/~valvano/arm.Lastly,hemeticulouslyproofreadthismanuscript.

Manysharedexperiencescontributedtothedevelopmentofthisbook.FirstIwouldliketoacknowledgethemanyexcellentteachingassistantsIhavehadthepleasureofworkingwith.Someofthesehard-working,underpaidwarriorsincludePankajBishnoi,RajeevSethia,AdsondaRocha,BaoHua,RajRanderi,SantoshJodh,NareshBhavaraju,AshutoshKulkarni,BryanStiles,V.Krishnamurthy,PaulJohnson,CraigKochis,SeanAskew,GeorgePanayi,JeehyunKim,VikramGodbole,AndresZambrano,AnnMeyer,HyunjinShin,AnandRajan,AnilKottam,Chia-lingWei,JigneshShah,IcaroSantos,DavidAltman,NachiketKharalkar,RobinTsang,ByungGeunJun,JohnPorterfield,DanielFernandez,DeepakPanwar,JacobEgner,SandyHermawan,UsmanTariq,SterlingWei,SeilOh,AntoniusKeddis,LevShuhatovich,GlenRhodes,GeoffreyLuke,KarthikSankar,TimVanRuitenbeek,RaffaeleCetrulo,HarshadDesai,JustinCapogna,ArindamGoswami,JunghoJo,MehmetBasoglu,KathrynLoeffler,EvgeniKrimer,NachiappanValliappan,RazikAhmed,SundeepKorrapati,SongZhang,ZahidulHaq,MatthewHalpern,CruzMonrrealII,PohanWu,SaugataBhattacharyya,OmarBacaAdityaSaraf,andMaheshSrinivasan.Theseteachingassistantshavecontributedgreatlytothecontentsofthisbookandparticularlytoitslaboratoryassignments.Since1981,IestimateIhavetaughtembeddedsystemstoover5000students.Mystudentshaverechargedmyenergyeachsemesterwiththeirenthusiasm,dedication,andquestforknowledge.Ihavedecidednottoacknowledgethemallindividually.However,theyknowIfeelprivilegedtohavehadthisopportunity.

Next,IappreciatethepatienceandexpertiseofmyfellowfacultymembershereattheUniversityofTexasatAustin.FromapersonalperspectiveDr.JohnPearceprovidedmuchneededencouragementandsupportthroughoutmycareer.Inaddition,Drs.JohnCogdell,JohnPearce,andFrancisBostickhelpedmewithanalogcircuitdesign.ThebookandaccompanyingsoftwareincludemanyfinitestatemachinesderivedfromthedigitallogicexamplesexplainedtomebyDr.CharlesRoth.Overthelastfewyears,IhaveenjoyedteachingembeddedsystemswithDrs.RameshYerraballi,MattanErez,AndreasGerstlauer,VijayJanapaReddi,NinaTelang,andBillBard.Billhascontributedtoboththeexcitementandsubstanceofourlaboratorybasedonthisbook.WithpushingfromBillandTAsRobin,Glen,Lev,andJohn,wehaveaddedlowpower,PCBlayout,systemsleveldesign,surfacemountsoldering,andwirelesscommunicationtoourlabexperience.YoucanseedescriptionsandphotosofourEE445Ldesigncompetitionathttp://users.ece.utexas.edu/~valvano/.ManyofthesuggestionsandcorrectionsfromChrisShoreandDrewBarbierofARMaboutVolume1appliedequallytothisvolume.AustinBlackstonecreatedanddebuggedtheCodeComposerStudioTMversionsoftheexampleprogramspostedontheweb.AustinalsotaughtmehowtoruntheCC3000andCC3100WiFiexamplesontheLaunchPad.

Sincerely,Iappreciatethevaluablelessonsofcharacterandcommitmenttaughttomebymyparentsandgrandparents.Irecallhowhardmyparentsandgrandparentsworkedtomaketheworldabetterplaceforthenextgeneration.Mostsignificantly,Iacknowledgethelove,patienceandsupportofmywife,Barbara,andmychildren,BenDanielandLiz.Inparticular,DandesignedandtestedmostoftheLM3SandLM4F/TM4Csoftwarepresentedinthisbook.

BythegraceofGod,Iamtrulythehappiestmanontheplanet,becauseIamsurroundedbythesefinepeople.Goodluck.

JonathanW.Valvano

Thetrueengineeringexperienceoccursnotwithyoureyesandears,butratherwithyourfingersandelbows.Inotherwords,engineeringeducationdoesnothappenbylisteninginclassorreadingabook;ratherithappensbydesigningunderthewatchfuleyesofapatientmentor.So,gobuildsomethingtoday,thenshowittosomeoneyourespect!

1.IntroductiontoEmbeddedSystemsChapter1objectivesareto:•Reviewcomputerarchitecture

•Introduceembeddedsystems

•Presentaprocessfordesign

•Discusspracticalaspectsofdigitallogic,includingopencollector

•Reviewhownumbersarerepresentedinbinary

•Defineethics

Theoverallobjectiveofthisbookistoteachthedesignofembeddedsystems.Itiseffectivetolearnnewtechniquesbydoingthem.But,thedilemmainteachingalaboratory-basedtopiclikeembeddedsystemsisthatthereisatremendousvolumeofdetailsthatfirstmustbelearnedbeforehardwareandsoftwaresystemscanbedesigned.Theapproachtakeninthisbookistolearnbydoing,startingwithverysimpleproblemsandbuildinguptomorecomplexsystemslaterinthebook.

Inthischapterwebeginbyintroducingsometerminologyandbasiccomponentsofacomputersystem.Inordertounderstandthecontextofourdesigns,wewilloverviewthegeneralcharacteristicsofembeddedsystems.Itisinthesediscussionsthatwedevelopafeelfortherangeofpossibleembeddedapplications.Nextwewillpresentatemplatetoguideusindesign.Webeginaprojectwitharequirementsdocument.Embeddedsystemsinteractwithphysicaldevices.Often,wecandescribethephysicalworldwithmathematicalmodels.Ifamodelisavailable,wecanthenuseittopredicthowtheembeddedsystemwillinterfacewiththerealworld.Whenwewritesoftware,wemistakenlythinkofitasonedimensional,becausethecodelookssequentialonthecomputerscreen.Dataflowgraphs,callgraphs,andflowchartsaremultidimensionalgraphicaltoolstounderstandcomplexbehaviors.Becausecoursestaughtusingthisbooktypicallyhavealabcomponent,wewillreviewsomepracticalaspectsofdigitallogic.

Next,weshowmultiplewaystorepresentnumbersinthecomputer.Choosingthecorrectformatisnecessarytoimplementefficientandcorrectsolutions.Fixed-pointnumbersarethetypicalwayembeddedsystemsrepresentnon-integervalues.Floating-pointnumbers,typicallyusedtorepresentnon-integervaluesonageneralpurposecomputer,willalsobepresented.

Becauseembeddedsystemscanbeemployedinsafetycriticalapplications,itisimportantforengineersbebotheffectiveandethical.Throughoutthebookwewillpresentwaystoverifythesystemisoperatingwithinspecifications.

1.1.ComputerArchitecture

1.1.1.Computers,microprocessors,memory,andmicrocontrollersAcomputercombinesaprocessor,randomaccessmemory(RAM),readonlymemory(ROM),andinput/output(I/O)ports.ThecommonbusinFigure1.1definesthevonNeumannarchitecture,whereinstructionsarefetchedfromROMonthesamebusasdatafetchedfromRAM.Softwareisanorderedsequenceofveryspecificinstructionsthatarestoredinmemory,definingexactlywhatandwhencertaintasksaretobeperformed.Theprocessorexecutesthesoftwarebyretrievingandinterpretingtheseinstructionsoneatatime.Amicroprocessorisasmallprocessor,wheresmallreferstosize(i.e.,itfitsinyourhand)andnotcomputationalability.Forexample,IntelXeon,AMDFXandSunSPARCaremicroprocessors.AnARM ® CortexTM-Mmicrocontrollerincludesaprocessortogetherwiththebusandsomeperipherals.Amicrocomputerisasmallcomputer,whereagainsmallreferstosize(i.e.,youcancarryit)andnotcomputationalability.Forexample,adesktopPCisamicrocomputer.

Figure1.1.Thebasiccomponentsofacomputersystemincludeprocessor,memoryandI/O.Averysmallmicrocomputer,calledamicrocontroller,containsallthecomponentsofacomputer(processor,memory,I/O)onasinglechip.AsshowninFigure1.2,theAtmelATtiny,theTexasInstrumentsMSP430,andtheTexasInstrumentsTM4C123areexamplesofmicrocontrollers.Becauseamicrocomputerisasmallcomputer,thistermcanbeconfusingbecauseitisusedtodescribeawiderangeofsystemsfroma6-pinATtiny4runningat1MHzwith512bytesofprogrammemorytoapersonalcomputerwithstate-of-the-art64-bitmulti-coreprocessorrunningatmulti-GHzspeedshavingterabytesofstorage.

ThecomputercanstoreinformationinRAMbywritingtoit,oritcanretrievepreviouslystoreddatabyreadingfromit.MostRAMsarevolatile;meaningifpowerisinterruptedandrestoredtheinformationintheRAMislost.MostmicrocontrollershavestaticRAM(SRAM)usingsixmetal-oxide-semiconductorfield-effecttransistors(MOSFET)tocreateeachmemorybit.Fourtransistorsareusedtocreatetwocross-coupledinvertersthatstorethebinaryinformation,andtheothertwoareusedtoreadandwritethebit.

Figure1.2.Amicrocontrollerisacompletecomputeronasinglechip.InformationisprogrammedintoROMusingtechniquesmorecomplicatedthanwritingtoRAM.Fromaprogrammingviewpoint,retrievingdatafromaROMisidenticaltoretrievingdatafromRAM.ROMsarenonvolatile;meaningifpowerisinterruptedandrestoredtheinformationintheROMisretained.SomeROMsareprogrammedatthefactoryandcanneverbechanged.AProgrammableROM(PROM)canbeerasedandreprogrammedbytheuser,buttheerase/programsequenceistypically10000timesslowerthanthetimetowritedataintoaRAM.PROMsusedtoneedultravioletlighttoerase,andthenweprogrammedthemwithvoltages.Now,mostPROMsnowareelectricallyerasable(EEPROM),whichmeanstheycanbebotherasedandprogrammedwithvoltages.WecannotprogramonesintotheROM.WefirsterasetheROM,whichputsonesintoitsstoragememory,andthenweprogramthezerosasneeded.FlashROMisapopulartypeofEEPROM.EachflashbitrequiresonlytwoMOSFETtransistors.Theinput(gate)ofonetransistoriselectricallyisolated,soifwetrapchargeonthisinput,itwillremainthereforyears.Theothertransistorisusedtoreadthebitbysensingwhetherornottheothertransistorhastrappedcharge.InregularEEPROM,youcaneraseandprogramindividualbytes.FlashROMmustbeerasedinlargeblocks.OnmanyofLM3S/LM4F/TM4Cmicrocontrollers,wecanerasetheentireROMorjusta1024-byteblock.BecauseflashissmallerthanregularEEPROM,mostmicrocontrollershavealargeflashintowhichwestorethesoftware.Forallthesystemsinthisbook,wewillstoreinstructionsandconstantsinflashROMandplacevariablesandtemporarydatainstaticRAM.

Checkpoint1.1:Whatarethedifferencesbetweenamicrocomputer,amicroprocessorandamicrocontroller?

Checkpoint1.2:Whichhasahigherinformationdensityonthechipinbitspermm2:staticRAMorflashROM?AssumeallMOSFETsareapproximatelythesamesizeinmm2.

Observation:Memoryisanobjectthatcantransportinformationacrosstime.

Theexternaldevicesattachedtothemicrocontrollerprovidefunctionalityforthesystem.Aninputportishardwareonthemicrocontrollerthatallowsinformationabouttheexternalworldtobeenteredintothecomputer.Themicrocontrolleralsohashardwarecalledanoutputporttosendinformationouttotheexternalworld.MostofthepinsshowninFigure1.2areinput/outputports.

AninterfaceisdefinedasthecollectionoftheI/Oport,externalelectronics,physicaldevices,andthesoftware,whichcombinetoallowthecomputertocommunicatewiththeexternalworld.Anexampleofaninputinterfaceisaswitch,wheretheoperatortogglestheswitch,andthesoftwarecanrecognizetheswitchposition.Anexampleofanoutputinterfaceisalight-emittingdiode(LED),wherethesoftwarecanturnthelightonandoff,andtheoperatorcanseewhetherornotthelightisshining.Thereisawiderangeofpossibleinputsandoutputs,whichcanexistineitherdigitaloranalogform.Ingeneral,wecanclassifyI/Ointerfacesintofourcategories

Parallel-binarydataareavailablesimultaneouslyonagroupoflines

Serial-binarydataareavailableonebitatatimeonasingleline

Analog-dataareencodedasanelectricalvoltage,current,orpower

Time-dataareencodedasaperiod,frequency,pulsewidth,orphaseshift

Checkpoint1.3:Whatarethedifferencesbetweenaninputportandaninputinterface?

Checkpoint1.4:Listthreeinputinterfacesavailableonapersonalcomputer.

Checkpoint1.5:Listthreeoutputinterfacesavailableonapersonalcomputer.

Inthisbook,numbersthatstartwith0x(e.g.,0x64)arespecifiedinhexadecimal,whichisbase16(0x64=6*161+4*160=100).Someassemblersstarthexadecimalnumberswith$(e.g.,$64).Otherassemblylanguagesaddan“H”attheendtospecifyhexadecimal(e.g.,64Hor64h).YalePatt’sLC3assemblerusesjustthe“x”(e.g.,x64).

InasystemwithmemorymappedI/O,asshowninFigure1.1,theI/Oportsareconnectedtotheprocessorinamannersimilartomemory.I/Oportsareassignedaddresses,andthesoftwareaccessesI/OusingreadsandwritestothespecificI/Oaddresses.Thesoftwareinputsfromaninputportusingthesameinstructionsasitwouldifitwerereadingfrommemory.Similarly,thesoftwareoutputsfromanoutputportusingthesameinstructionsasitwouldifitwerewritingtomemory.Abusisdefinedasacollectionofsignals,whicharegroupedforacommonpurpose.Thebushasthreetypesofsignals:addresssignals,datasignals,andcontrolsignals.Together,thebusdirectsthedatatransferbetweenthevariousmodulesinthecomputer.TherearefivebusesonARM ® CortexTM-Mprocessor,asillustratedinFigure1.3.Theaddressspecifieswhichmoduleisbeingaccessed,andthedatacontainstheinformationbeingtransferred.Thecontrolsignalsspecifythedirectionoftransfer,thesizeofthedata,andtiminginformation.TheICodebusisusedtofetchinstructionsfromflashROM.AllICodebusfetchescontain32bitsofdata,whichmaybeoneortwoinstructions.TheDCodebuscanfetchdataordebuginformationfromflashROM.Thesystembuscanread/writedatafromRAMorI/Oports.Theprivateperipheralbus(PPB)canaccesssomeofthe

commonperipheralsliketheinterruptcontroller.Themultiple-busarchitectureallowssimultaneousbusactivity,greatlyimprovingperformanceoversingle-busarchitectures.Forexample,theprocessorcansimultaneouslyfetchaninstructionoutofflashROMusingtheICodebuswhileitwritesdataintoRAMusingthesystembus.Fromasoftwaredevelopmentperspective,thefactthattherearemultiplebusesistransparent.Thismeanswewritecodelikewewouldonanycomputer,andtheparalleloperationsoccurautomatically.TheTM4C123has256kibibytes(218bytes)offlashROMand32768bytesofRAM.TheTM4C1294has1024kibibytes(220bytes)offlashROMand256kibibytesofRAM.TheRAMbeginsat0x2000.0000,andtheflashROMbeginsat0x0000.0000.

TM4C123 TM4C1294

0x0000.0000 256k 0x0000.0000 1024k

∙∙∙ Flash ∙∙∙ Flash

0x0003.FFFF ROM 0x000F.FFFF ROM

0x2000.0000 32k 0x2000.0000 256k

∙∙∙ Static ∙∙∙ Static

0x2000.7FFF RAM 0x2003.FFFF RAM

Figure1.3.HarvardarchitectureofanARM®Cortex -M-basedmicrocontroller.TheCortexTM-M4seriesincludesanadditionalbuscalledtheAdvancedHigh-PerformanceBus(AHBorAHPB).Thisbusimprovesperformancewhencommunicatingwithhigh-speedI/OdeviceslikeUSB.Ingeneral,themoreoperationsthatcanbeperformedinparallel,thefastertheprocessorwillexecute.Insummary:

ICodebusFetchopcodesfromROM

DCodebusReadconstantdatafromROM

SystembusRead/writedatafromRAMorI/O,fetchopcodefromRAM

PPBRead/writedatafrominternalperipheralsliketheNVIC

AHBRead/writedatafromhigh-speedI/Oandparallelports(M4only)

InstructionsanddataareaccessedthesamewayonavonNeumannmachine.TheCortexTM-MprocessorisaHarvardarchitecturebecauseinstructionsarefetchedontheICodebusanddataaccessedonthesystembus.TheaddresssignalsontheARM ®CortexTM-Mprocessorinclude32lines,whichtogetherspecifythememoryaddress(0x0000.0000to0xFFFF.FFFF)thatiscurrentlybeingaccessed.Theaddressspecifiesbothwhichmodule(input,output,RAM,orROM)aswellaswhichcellwithinthemodulewillcommunicatewiththeprocessor.Thedatasignalscontaintheinformationthatisbeingtransferredandalsoinclude32bits.However,onthesystembusitcanalsotransfer8-bitor16-bitdata.Thecontrolsignalsspecifythetiming,thesize,andthedirectionofthetransfer.Wecallacompletedatatransferabuscycle.Twotypesoftransfersareallowed,asshowninTable1.1.Inmostsystems,theprocessoralwayscontrolstheaddress(wheretoaccess),thedirection(readorwrite),andthecontrol(whentoaccess.)

Type AddressDrivenby

DataDrivenby Transfer

ReadCycle Processor RAM,ROMorInput

Datacopiedtoprocessor

WriteCycle Processor Processor DatacopiedtooutputorRAM

Table1.1.Simplecomputersgeneratetwotypesofbuscycles.

Areadcycleisusedtotransferdataintotheprocessor.Duringareadcycletheprocessorfirstplacestheaddressontheaddresssignals,andthentheprocessorissuesareadcommandonthecontrolsignals.Theslavemodule(RAM,ROM,orI/O)willrespondbyplacingthecontentsatthataddressonthedatasignals,andlastlytheprocessorwillacceptthedataanddisablethereadcommand.

TheprocessorusesawritecycletostoredataintomemoryorI/O.Duringawritecycletheprocessoralsobeginsbyplacingtheaddressontheaddresssignals.Next,theprocessorplacestheinformationitwishestostoreonthedatasignals,andthentheprocessorissuesawritecommandonthecontrolsignals.ThememoryorI/Owillrespondbystoringtheinformationintotheproperplace,andaftertheprocessorissurethedatahasbeencaptured,itwilldisablethewritecommand.

ThebandwidthofanI/Ointerfaceisthenumberofbytes/secthatcanbetransferred.IfwewishtotransferdatafromaninputdeviceintoRAM,thesoftwaremustfirsttransferthedatafrominputtotheprocessor,thenfromtheprocessorintoRAM.OntheARM,itwilltakemultipleinstructionstoperformthistransfer.ThebandwidthdependsbothonthespeedoftheI/Ohardwareandthesoftwareperformingthetransfer.InsomemicrocontrollersliketheTM4C123andTM4C1294,wewillbeabletotransferdatadirectlyfrominputtoRAMorRAMtooutputusingdirectmemoryaccess(DMA).WhenusingDMAthesoftwaretimeisremoved,sothebandwidthonlydependsonthespeedoftheI/Ohardware.BecauseDMAisfaster,wewillusethismethodtointerfacehighbandwidthdeviceslikedisksandnetworks.DuringaDMAreadcycledataflowsdirectlyfromthememorytotheoutputdevice.GeneralpurposecomputersalsosupportDMAallowingdatatobetransferredfrommemorytomemory.DuringaDMAwritecycledataflowsdirectlyfromtheinputdevicetomemory.

Input/outputdevicesareimportantinallcomputers,buttheyareespeciallysignificantinanembeddedsystem.InacomputersystemwithI/O-mappedI/O,thecontrolbussignalsthatactivatetheI/Oareseparatefromthosethatactivatethememorydevices.ThesesystemshaveaseparateaddressspaceandseparateinstructionstoaccesstheI/Odevices.TheoriginalIntel8086hadfourcontrolbussignalsMEMR,MEMW,IOR,andIOW.MEMRandMEMWwereusedtoreadandwritememory,whileIORandIOWwereusedtoreadandwriteI/O.TheIntelx86referstoanyoftheprocessorsthatIntelhasdevelopedbasedonthisoriginalarchitecture.Eventhoughwedonotconsiderthepersonalcomputer(PC)anembeddedsystem,thereareembeddedsystemsdevelopedonthisarchitecture.OnesuchplatformiscalledthePC/104Embedded-PC.TheIntelx86processorscontinuetoimplementthisseparationbetweenmemoryandI/O.Ratherthanusetheregularmemoryaccessinstructions,theIntelx86processorusesspecial in and out instructionstoaccesstheI/Odevices.TheadvantagesofI/O-mappedI/OarethatsoftwarecannotinadvertentlyaccessI/Owhenitthinksitisaccessingmemory.Inotherwords,itprotectsI/Odevicesfromcommonsoftwarebugs,suchasbadpointers,stackoverflow,andbufferoverflows.Incontrast,systemswithmemory-mappedI/Oareeasiertodesign,andthesoftwareiseasiertowrite.

1.1.2.CortexTM-MprocessorTheARM ® CortexTM-Mprocessorhasfourmajorcomponents,asillustratedinFigure1.4.Therearefourbusinterfaceunits(BIU)thatreaddatafromthebusduringareadcycleandwritedataontothebusduringawritecycle.BoththeTM4C123andTM4C1294microcontrollerssupportDMA.TheBIUalwaysdrivestheaddressbusandthecontrolsignalsofthebus.Theeffectiveaddressregister(EAR)containsthememoryaddressusedtofetchthedataneededforthecurrentinstruction.CortexTM-MmicrocontrollersexecuteThumb ® instructionsextendedwithThumb-2technology.AnoverviewoftheseinstructionswillbepresentedinChapter2.TheCortexTM-M4Fmicrocontrollersincludeafloating-pointprocessor.However,inthisbookwewillfocusonintegerandfixed-pointarithmetic.

Figure1.4.Thefourbasiccomponentsofaprocessor.Thecontrolunit(CU)orchestratesthesequenceofoperationsintheprocessor.TheCUissuescommandstotheotherthreecomponents.Theinstructionregister(IR)containstheoperationcode(oropcode)forthecurrentinstruction.WhenextendedwithThumb-2technology,opcodesareeither16or32bitswide.Inanembeddedsystemthesoftwareisconvertedtomachinecode,whichisalistofinstructions,andstoredinnonvolatileflashROM.Asinstructionsarefetched,theyareplacedinapipeline.Thisallowsinstructionfetchingtorunaheadofexecution.Instructionsarefetchedinorderandexecutedinorder.However,itcanexecuteoneinstructionwhilefetchingthenext.

Theregistersarehigh-speedstoragedeviceslocatedintheprocessor(e.g.,R0toR15).Registersdonothaveaddresseslikeregularmemory,butrathertheyhavespecificfunctionsexplicitlydefinedbytheinstruction.Registerscancontaindataoraddresses.Theprogramcounter(PC)pointstothememorycontainingtheinstructiontoexecutenext.OntheARM ® CortexTM-Mprocessor,thePCisregister15(R15).Inanembeddedsystem,thePCusuallypointsintononvolatilememorylikeflashROM.Theinformationstoredinnonvolatilememory(e.g.,theinstructions)isnotlostwhenpowerisremoved.Thestackpointer(SP)pointstotheRAM,anddefinesthetopofthestack.Thestackimplementslastinfirstout(LIFO)storage.OntheARM ® CortexTM-Mprocessor,theSPisregister13(R13).Thestackisanextremelyimportantcomponentofsoftwaredevelopment,whichcanbeusedtopassparameters,savetemporaryinformation,andimplementlocalvariables.Theprogramstatusregister(PSR)containsthestatusofthepreviousoperation,aswellassomeoperatingmodeflagssuchastheinterruptenablebit.ThisregisteriscalledtheflagregisterontheIntelcomputers.

Thearithmeticlogicunit(ALU)performsarithmeticandlogicoperations.Addition,subtraction,multiplicationanddivisionareexamplesofarithmeticoperations.And,or,exclusiveor,andshiftareexamplesoflogicaloperations.

Checkpoint1.6:ForwhatdotheacronymsCUDMABIUALUstand?

Ingeneral,theexecutionofaninstructiongoesthroughfourphases.First,thecomputerfetchesthemachinecodefortheinstructionbyreadingthevalueinmemorypointedtobytheprogramcounter(PC).Someinstructionsare16bits,whileothersare32bits.Aftereachinstructionisfetched,thePCisincrementedtothenextinstruction.Atthistime,theinstructionisdecoded,andtheeffectiveaddressisdetermined(EAR).Manyinstructionsrequireadditionaldata,andduringphase2thedataisretrievedfrommemoryattheeffectiveaddress.Next,theactualfunctionforthisinstructionisperformed.Duringthelastphase,theresultsarewrittenbacktomemory.Allinstructionshaveaphase1,buttheotherthreephasesmayormaynotoccurforanyspecificinstruction.

OntheARM ® CortexTM-Mprocessor,aninstructionmayreadmemoryorwritememory,butitdoesnotbothreadandwritememoryinthesameinstruction.Eachofthephasesmayrequireoneormorebuscyclestocomplete.Eachbuscyclereadsorwritesonepieceofdata.Becauseofthemultiplebusarchitecture,mostinstructionsexecuteinoneortwocycles.Formoreinformationonthetimetoexecuteinstructions,seeTable3.1intheCortexTM-MTechnicalReferenceManual.ARMisareducedinstructionsetcomputer(RISC),whichachieveshighperformancebyimplementingverysimpleinstructionsthatrunextremelyfast.

Phase Function Bus Address Comment

1 Instructionfetch

Read PC++ PutintoIR

2 Dataread Read EAR DatapassesthroughALU

3 Operation - - ALUoperations,setPSR

4 Datastore Write EAR Resultsstoredinmemory

Table1.2.Fourphasesofexecution.

AninstructiononaRISCprocessordoesnothavebothaphase2datareadcycleandaphase4datawritecycle.Ingeneral,aRISCprocessorhasasmallnumberofinstructions,instructionshavefixedlengths,instructionsexecutein1or2buscycles,thereareonlyafewinstructions(e.g.,loadandstore)thatcanaccessmemory,nooneinstructioncanbothreadandwritememoryinthesameinstruction,therearemanyidenticalgeneralpurposeregisters,andtherearealimitednumberofaddressingmodes.

Conversely,processorsareclassifiedascomplexinstructionsetcomputers(CISC),becauseoneinstructioniscapableofperformingmultiplememoryoperations.Forexample,CISCprocessorshaveinstructionsthatcanbothreadandwritememoryinthesameinstruction.Assume Data isan8-bitmemoryvariable.ThefollowingIntel8080instructionwillincrementthe8-bitvariable,requiringareadmemorycycle,ALUoperation,andthenawritememorycycle.

INRData;Intel8080

OtherCISCprocessorslikethe6800,9S12,8051,andPentiumalsohavememoryincrementinstructionsrequiringbothaphase2datareadcycleandaphase4datawritecycle.Ingeneral,aCISCprocessorhasalargenumberofinstructions,instructionshavevaryinglengths,instructionsexecuteinvaryingtimes,therearemanyinstructionsthatcanaccessmemory,theprocessorcanbothreadandwritememoryinoneinstruction,theprocessorhasfewerandmorespecializedregisters,andtheprocessorhasmanyaddressingmodes.

1.1.3.HistoryIn1968,twounhappyengineersnamedBobNoyceandGordonMoorelefttheFairchildSemiconductorCompanyandcreatedtheirowncompany,whichtheycalledIntegratedElectronics(Intel).WorkingforIntelin1971,FedericoFaggin,TedHoff,andStanMazorinventedthefirstsinglechipmicroprocessor,theIntel4004.Itwasafour-bitprocessordesignedtosolveaveryspecificapplicationforaJapanesecompanycalledBusicon.Busiconbackedoutofthepurchase,soInteldecidedtomarketitasa“generalpurpose”microprocessingsystem.Theproductwasasuccess,whichleadtoaseriesofmorepowerfulmicroprocessors:theIntel8008in1974,theIntel8080alsoin1974.BoththeIntel8008andtheIntel8080were8-bitmicroprocessorsthatoperatedfromasingle+5VpowersupplyusingN-channelmetal-oxidesemiconductor(NMOS)technology.

Seeingthelongtermpotentialforthistechnology,MotorolareleaseditsMC6800in1974,whichwasalsoan8-bitprocessorwithaboutthesamecapabilitiesofthe8080.Althoughsimilarincomputingpower,the8080and6800hadverydifferentarchitectures.The8080usedisolatedI/Oandhandledaddressesinafundamentallydifferentwaythandata.IsolatedI/Odefinesspecialhardwaresignalsandspecialinstructionsforinput/output.Onthe8080,certainregistershadcapabilitiesdesignedforaddressing,whileotherregistershadcapabilitiesforspecificfordatamanipulation.Incontrast,the6800usedmemory-mappedI/Oandhandledaddressesanddatainasimilarway.Aswedefinedearlier,input/outputonasystemwithmemory-mappedI/Oisperformedinamannersimilartoaccessingmemory.

Duringthe1980sand1990s,MotorolaandInteltraveleddownsimilarpaths.Themicroprocessorfamiliesfrombothcompaniesdevelopedbiggerandfasterproducts:Intel8085,8088,80x86,…andtheMotorola6809,68000,680x0…Duringtheearly1980’sanothertechnologyemerged,themicrocontroller.Insharpcontrasttothemicroprocessorfamily,whichoptimizedcomputationalspeedandmemorysizeattheexpenseofpowerandphysicalsize,themicrocontrollerdevicesminimizedpowerconsumptionandphysicalsize,strivingforonlymodestincreasesincomputationalspeedandmemorysize.OutoftheIntelarchitecturecamethe8051family(www.semiconductors.philips.com),andoutoftheMotorolaarchitecturecamethe6805,6811,and6812microcontrollerfamily(www.freescale.com).Manyofthesamefundamentaldifferencesthatexistedbetweentheoriginal8-bitIntel8080andMotorola6800havepersistedoverfortyyearsofmicroprocessorandmicrocontrollerdevelopments.In1999,Motorolashippedits2billionthMC68HC05microcontroller.In2004,MotorolaspunoffitsmicrocontrollerproductsasFreescaleSemiconductor.Microchipisaleadingsupplierof8-bitmicrocontrollers.

ThefirstARMprocessorwasconceivedinthe1983byAcornComputers,whichatthetimewasoneoftheleadersofbusinesscomputersintheUnitedKingdom.Thefirstchipsweredeliveredin1985.AtthattimeARMreferredtoAcornRISCMachine.In1990,anewcompanyARMLtdwasformedwithAcorn,Apple,andVLSITechnologyasfoundingpartners,changingtheARMacronymtoAdvancedRISCMachine.Asacompany,theARMbusinessmodelinvolvesthedesigningandlicensingofintellectualproperty(IP)ratherthanthemanufacturingandsellingofactualsemiconductorchips.ARMhassold600processorlicensestomorethan200companies.VirtuallyeverycompanythatmanufacturersintegratedcircuitsinthecomputerfieldproducesavariantoftheARMprocessor.ARMcurrentlydominatesthehigh-performancelow-powerembeddedsystemmarket.ARMprocessorsaccountforapproximately90%ofallembedded32-bitRISCprocessorsandareusedinconsumerelectronics,includingPDAs,cellphones,musicplayers,hand-heldgameconsoles,andcalculators.TheARMCortex-Aisusedinapplicationsprocessors,suchassmartphones.TheARMCortex-Risappropriateforreal-timeapplications,andARMCortex-Mtargetsmicrocontrollers.ExamplesofmicrocontrollersbuiltusingtheARM ® CortexTM-McoreareLM3S/TM4CbyTexasInstruments,STM32bySTMicroelectronics,LPC17xxbyNXPSemiconductors,TMPM330byToshiba,EM3xxbyEmber,AT91SAM3byAtmel,andEFM32byEnergyMicro.AsofJune2014over50billionARMprocessorshaveshippedfromover950companies.

Whatwillthefutureunfold?Onewaytopredictthefutureistostudythepast.Howembeddedsystemsinteractwithhumanshasbeenandwillcontinuetobecritical.Improvingthehumanexperiencehasbeenthegoalofmanysystems.Manypredictthenumberofmicrocontrollerswillsoonreachintothetrillions.Asthishappens,communication,security,energy,politics,resources,andeconomicswillbebecomeincreasinglyimportant.Whentherearethismanycomputers,itwillbepossibletomakeguessesabouthowtochange,thenletaprocesslikeevolutionselectwhichchangesarebeneficial.Infact,anetworkofembeddedsystemswithtightcouplingtotherealworld,linkedtogetherforacommonobjective,isnowbeingcalledacyber-physicalsystem(CPS).

Oneconstantdescribingthehistoryofcomputersiscontinuouschangecoupledwithperiodicmonumentalchanges.Therefore,engineersmustfocustheireducationonfundamentalprinciplesratherthanthevoluminousdetails.Theymustembracetheconceptoflifelonglearning.Mosthumansarefundamentallygood,butsomearenot.Therefore,engineersactinginanethicalmannercanguaranteefutureprosperityoftheentireplanet.

1.2.EmbeddedSystemsAnembeddedsystemisanelectronicsystemthatincludesaoneormoremicrocontrollersthatisconfiguredtoperformaspecificdedicatedapplication,drawnpreviouslyasFigure1.1.Tobetterunderstandtheexpression“embeddedsystem,”considereachwordseparately.Inthiscontext,thewordembeddedmeans“acomputerishiddeninsidesoonecan’tseeit.”Theword“system”referstothefactthattherearemanycomponentswhichactinconcertachievingthecommongoal.Asmentionedearlier,input/outputdevicescharacterizetheembeddedsystem,allowingittointeractwiththerealworld.

ThesoftwarethatcontrolsthesystemisprogrammedorfixedintoflashROMandisnotaccessibletotheuserofthedevice.Evenso,softwaremaintenanceisstillextremelyimportant.Softwaremaintenanceisverificationofproperoperation,updates,fixingbugs,addingfeatures,andextendingtonewapplicationsandenduserconfigurations.Embeddedsystemshavethesefourcharacteristics.

First,embeddedsystemstypicallyperformasinglefunction.Consequently,theysolvealimitedrangeofproblems.Forexample,theembeddedsysteminamicrowaveovenmaybereconfiguredtocontroldifferentversionsoftheovenwithinasimilarproductline.But,amicrowaveovenwillalwaysbeamicrowaveoven,andyoucan’treprogramittobeadishwasher.Embeddedsystemsareuniquebecauseofthemicrocontroller’sI/Oportstowhichtheexternaldevicesareinterfaced.Thisallowsthesystemtointeractwiththerealworld.

Second,embeddedsystemsaretightlyconstrained.Typically,systemmustoperatewithinveryspecificperformanceparameters.Ifanembeddedsystemcannotoperatewithspecifications,itisconsideredafailureandwillnotbesold.Forexample,acell-phonecarriertypicallygets832radiofrequenciestouseinacity,ahand-heldvideogamemustcostlessthan$50,anautomotivecruisecontrolsystemmustoperatethevehiclewithin3mphoftheset-pointspeed,andaportableMP3playermustoperatefor12hoursononebatterycharge.

Third,manyembeddedsystemsmustoperateinreal-time.Inareal-timesystem,wecanputanupperboundonthetimerequiredtoperformtheinput-calculation-outputsequence.Areal-timesystemcanguaranteeaworstcaseupperboundontheresponsetimebetweenwhenthenewinputinformationbecomesavailableandwhenthatinformationisprocessed.Anotherreal-timerequirementthatexistsinmanyembeddedsystemsistheexecutionofperiodictasks.Aperiodictaskisonethatmustbeperformedatequaltimeintervals.Areal-timesystemcanputasmallandboundedlimitonthetimeerrorbetweenwhenataskshouldberunandwhenitisactuallyrun.Becauseofthereal-timenatureofthesesystems,microcontrollersintheTM4Cfamilyhavearichsetoffeaturestohandleallaspectsoftime.

Thefourthcharacteristicofembeddedsystemsistheirsmallmemoryrequirementsascomparedtogeneralpurposecomputers.Thereareexceptionstothisrule,suchasthosewhichprocessvideooraudio,butmosthavememoryrequirementsmeasuredinthousandsofbytes.Overtheyears,thememoryinembeddedsystemsasincreased,butthegapmemorysizebetweenembeddedsystemsandgeneralpurposecomputersremains.TheoriginalmicrocontrollershadthousandsofbytesofmemoryandthePChadmillions.Now,microcontrollerscanhavemillionsofbytes,butthePChasbillions.

Therehavebeentwotrendsinthemicrocontrollerfield.Thefirsttrendistomakemicrocontrollerssmaller,cheaper,andlowerpower.TheAtmelATtiny,MicrochipPIC,andTexasInstrumentsMSP430familiesaregoodexamplesofthistrend.Size,cost,andpowerarecriticalfactorsforhigh-volumeproducts,wheretheproductsareoftendisposable.OntheotherendofthespectrumisthetrendoflargerRAMandROM,fasterprocessing,andincreasingintegrationofcomplexI/Odevices,suchasEthernet,radio,graphics,andaudio.Itiscommonforonedevicetohavemultiplemicrocontrollers,wheretheoperationaltasksaredistributedandthemicrocontrollersareconnectedinalocalareanetwork(LAN).Thesehigh-endfeaturesarecriticalforconsumerelectronics,medicaldevices,automotivecontrollers,andmilitaryhardware,whereperformanceandreliabilityaremoreimportantthancost.However,smallsizeandlowpowercontinueasimportantfeaturesforallembeddedsystems.

TheRAMisvolatilememory,meaningitsinformationislostwhenpowerisremoved.Onsomeembeddedsystemsabatterypowersthemicrocontroller.Whenintheoffmode,themicrocontrollergoesintolow-powersleepmode,whichmeanstheinformationinRAMismaintained,buttheprocessorisnotexecuting.TheMSP430andATtinyrequirelessthana Aofcurrentinsleepmode.

Checkpoint1.7:Whatisanembeddedsystem?

Checkpoint1.8:WhatgoesintheRAMonasmartphone?

Checkpoint1.9:WhydoesyoursmartphoneneedsomuchflashROM?

Thecomputerengineerhasmanydesignchoicestomakewhenbuildingareal-timeembeddedsystem.Often,definingtheproblem,specifyingtheobjectives,andidentifyingtheconstraintsareharderthanactualimplementations.Inthisbook,wewilldevelopcomputerengineeringdesignprocessesbyintroducingfundamentalmethodologiesforproblemspecification,prototyping,testing,andperformanceevaluation.

Atypicalautomobilenowcontainsanaverageoftenmicrocontrollers.Infact,upscalehomesmaycontainasmanyas150microcontrollersandtheaverageconsumernowinteractswithmicrocontrollersupto300timesaday.Thegeneralareasthatemployembeddedsystemsencompasseveryfieldofengineering:

•ConsumerElectronics•Home

•Communications•Automotive

•Military•Industrial

•Business•Shipping

•Medical•Computercomponents

Ingeneral,embeddedsystemshaveinputs,performcalculations,makedecisions,andthenproduceoutputs.Themicrocontrollersoftenmustcommunicatewitheachother.Howthesysteminteractswithhumansisoftencalledthehuman-computerinterface(HCI)orman-machineinterface(MMI).Togetasenseofwhat“embeddedsystem”meanswewillpresentbriefdescriptionsoffourexamplesystems.

Example1.1:Thegoalofapacemakeristoregulateandimproveheartfunction.Tobesuccessfultheengineermustunderstandhowtheheartworksandhowdiseasestatescausethehearttofail.Itsinputsaresensorsonthehearttodetectelectricalactivity,anditsoutputscandeliverelectricalpulsestostimulatetheheart.Considerasimplepacemakerwithtwosensors,oneintherightatriumandtheotherintherightventricle.Thesensorallowsthepacemakertoknowifthenormalheartcontractionisoccurring.Thispacemakerhasonerightventricularstimulationoutput.Theembeddedsystemanalyzesthestatusoftheheartdecidingwhereandwhentosendsimulationpulses.Ifthepacemakerrecognizesthenormalbehaviorofatrialcontractionfollowedshortlybyventricularcontraction,thenitwillnotstimulate.Ifthepacemakerrecognizesatrialcontractionwithoutafollowingventricularcontraction,theniswillpacetheventricleshortlyaftereachatrialcontraction.Ifthepacemakersensesnocontractionsorifthecontractionsaretooslow,thenitcanpacetheventricleataregularrate.Apacemakercanalsocommunicateviaradiowiththedoctortodownloadpastperformanceandoptimizeparametersforfutureoperation.Somepacemakerscancallthedoctoronthephonewhenitsensesacriticalproblem.Pacemakersarereal-timesystemsbecausethetimedelaybetweenatrialsensingandventriculartriggeringiscritical.Lowpowerandreliabilityareimportant.

Example1.2:Thegoalofasmokedetectoristowarnpeopleintheeventofafire.Ithastwoinputs.Oneisachemicalsensorthatdetectsthepresenceofsmoke,andtheotherisabuttonthattheoperatorcanpushtotestthebattery.Therearealsotwooutputs:anLEDandthealarm.Mostofthetime,thedetectorisinalow-powersleepmode.Ifthetestbuttonispushed,thedetectorperformsaself-diagnosticandissuesashortsoundifthesensorandbatteryareok.Onceevery30seconds,itwakesupandcheckstoseeifitsensessmoke.Ifitsensessmoke,itwillalarm.Otherwiseitgoesbacktosleep.

Advancedsmokedetectorsshouldbeabletocommunicatewithotherdevicesinthehome.Ifonesensordetectssmoke,allalarmsshouldsound.Ifmultipledetectorsinthehousecollectivelyagreethereisreallyafire,theycouldcommunicatewiththefiredepartmentandwiththeneighboringhouses.Todesignanddeployacollectionofdetectors,theengineermustunderstandhowfiresstartandhowtheyspread.Smokedetectorsarenotreal-timesystems.However,reliabilityandlowpowerareimportant.

Example1.3:Thegoalofamotorcontrolleristocauseamotortospininadesiredmanner.Sometimeswecontrolspeed,asinthecruisecontrolonanautomobile.Sometimeswecontrolpositionasinmovingpaperthroughaprinter.Inacomplexroboticssystem,wemayneedtosimultaneouslycontrolmultiplemotorsandmultipleparameterssuchasposition,speed,andtorque.Torquecontrolisimportantforbuildingarobotthatwalks.Theengineermustunderstandthemechanicsofhowthemotorinteractswithitsworldandthebehavioroftheinterfaceelectronics.Themotorcontrollerusessensorstomeasurethecurrentstateofthemotor,suchasposition,speed,andtorque.Thecontrolleracceptsinputcommandsdefiningthedesiredoperation.Thesystemusesactuators,whichareoutputsthataffectthemotor.Atypicalactuatorallowsthesystemtosettheelectricalpowerdeliveredtothemotor.Periodically,themicrocontrollersensestheinputsandcalculatesthepowerneededtominimizethedifferencebetweenmeasuredanddesiredparameters.Thisneededpowerisoutputtotheactuator.Motorcontrollersarereal-timesystems,becauseperformancedependsgreatlyonwhenandhowfastthecontrollersoftwareruns.Accuracy,stability,andtimeareimportant.

Example1.4:Thegoalofatrafficcontrolleristominimizewaitingtimeandtosaveenergy.Theengineermustunderstandthecivilengineeringofhowcitystreetsarelaidoutandthebehaviorofhumandriversastheyinteractwithtrafficlightsandotherdrivers.Thecontrollerusessensorstoknowthenumberofcarstravelingoneachsegmentofroad.Pedestrianscanalsopushwalkbuttons.Thecontrollerwillacceptinputcommandsfromthefireorpolicedepartmenttohandleemergencies.Theoutputsarethetrafficlightsateachintersection.Thecontrollercollectssensorinputsandcalculatesthetrafficpatternneededtominimizewaitingtime,whilemaintainingsafety.Trafficcontrollersarenotreal-timesystems,becausehumansafetyisnotsacrificedifarequestisdelayed.Incontrast,anairtrafficcontrollermustruninrealtime,becausesafetyiscompromisedifaresponsetoarequestisdelayed.Thesystemmustbeabletooperateunderextremeconditionssuchasrain,snow,freezingtemperature,andpoweroutages.Computationalspeedandsensor/lightreliabilityareimportant.

Checkpoint1.10:Thereisamicrocontrollerembeddedinanalarmclock.Listthreeoperationsthesoftwaremustperform.

Whendesigningembeddedsystemsweneedtoknowhowtointerfaceawiderangeofsignalsthatcanexistindigital,analog,ortimeformats.

Table1.3listsexampleproductsandthefunctionsperformedbytheirembeddedsystems.Themicrocontrolleracceptsinputs,performscalculations,andgeneratesoutputs.

Functionsperformedbythemicrocontroller

Consumer/Home:WashingmachineControlsthewaterandspincycles,savingwaterandenergy

ExerciseequipmentMeasuresspeed,distance,calories,heartrate

RemotecontrolsAcceptskeytouches,sendsinfraredpulses,learnshowtointeractwithuser

ClocksandwatchesMaintainsthetime,alarm,anddisplay

GamesandtoysEntertainstheuser,joystickinput,videooutput

Audio/videoInteractswiththeoperator,enhancesperformancewithsoundsandpictures

Set-backthermostatsAdjustsday/nightthresholdssavingenergy

Communication:AnsweringmachinesPlaysoutgoingmessagesandsavesincomingmessages

TelephonesystemSwitchessignalsandretrievesinformation

CellularphonesInteractswithkeypad,microphone,andspeaker

SatellitesSendsandreceivesmessages

Automotive:AutomaticbrakingOptimizesstoppingonslipperysurfaces

NoisecancellationImprovessoundquality,removingnoise

TheftdeterrentdevicesAllowskeylessentry,controlsalarm

ElectronicignitionControlssparksandfuelinjectors

WindowsandseatsRememberspreferredsettingsforeachdriver

InstrumentationCollectsandprovidesnecessaryinformation

Military:SmartweaponsRecognizesfriendlytargets

MissileguidanceDirectsordnanceatthedesiredtarget

GlobalpositioningDetermineswhereyouareontheplanet,suggests

paths,coordinatestroops

SurveillanceCollectsinformationaboutenemyactivities

Industrial/Business/Shipping:Point-of-salesystemsAcceptsinputsandmanagesmoney,keepscreditinformationsecure

TemperaturecontrolAdjustsheatingandcoolingtomaintaintemperature

RobotsystemsInputsfromsensors,controlsthemotorsimprovingproductivity

InventorysystemsReadsandprintslabels,maximizingprofit,minimizingshippingdelay

AutomaticsprinklersControlsthewetnessofthesoilmaximizingplantgrowth

Medical:InfantapneamonitorsDetectsbreathing,alarmsifstopped

CardiacmonitorsMeasuresheartfunction,alarmsifproblem

CancertreatmentsControlsdosesofradiation,drugs,orheat

ProstheticdevicesIncreasesmobilityforthehandicapped

MedicalrecordsCollect,organize,andpresentmedicalinformation

ComputerComponents:MouseTranslateshandmovementsintocommandsforthemaincomputer

USBflashdriveFacilitatesthestorageandretrievalofinformation

KeyboardAcceptskeystrokes,decodesthem,andtransmitstothemaincomputerTable1.3.Productsinvolvingembeddedsystems.

Incontrast,ageneral-purposecomputersystemtypicallyhasakeyboard,disk,andgraphicsdisplayandcanbeprogrammedforawidevarietyofpurposes.Typicalgeneral-purposeapplicationsincludewordprocessing,electronicmail,businessaccounting,scientificcomputing,cloudcomputing,andwebservers.General-purposecomputershavetheoppositeofthefourcharacteristicslistedabove.First,theycanperformawideanddynamicrangeoffunctions.Becausethegeneral-purposecomputerhasaremovablediskornetworkinterface,newprogramscaneasilybeaddedtothesystem.Theuserofageneral-purposecomputerdoeshaveaccesstothesoftwarethatcontrolsthemachine.Inotherwords,theuserdecideswhichoperatingsystemtorunandwhichapplicationstolaunch.Second,theyarelooselyconstrained.Forexample,theJavamachineusedbyawebbrowserwilloperateonanextremelywiderangeofcomputerplatforms.Third,general-purposemachinesdonotruninreal-time.Yes,wewouldlikethetimetoprintapageontheprintertobefast,andwewouldlikeawebpagetoloadquickly,buttherearenoguaranteedresponsetimesforthesetypesofactivities.Infact,thereal-timetasksthatdoexist(suchassoundrecording,burningCD,andgraphics)areactuallyperformedbyembeddedsystemsbuiltintothesystem.Fourth,generalpurposecomputersemploybillions,ifnottrillionsofmemorycells.

Themostcommontypeofgeneral-purposecomputeristhepersonalcomputer,whichisbasedonthex86architecture(below$3,000).Computersmorepowerfulthanthepersonalcomputercanbegroupedintheworkstation($3,000to$50,000range)orthesupercomputercategories(above$50,000).Seethewebsitewww.top500.orgforalistofthefastestcomputersontheplanet.Thesecomputersoftenemploymultipleprocessorsandhavemuchmorememorythanthetypicalpersonalcomputer.Theworkstationsandsupercomputersareusedforhandlinglargeamountsofinformation(businessapplications),runninglargesimulations(weatherforecasting),searching(www.google.com),orperforminglargecalculations(scientificresearch).Thisbookwillnotcoverthegeneral-purposecomputer,althoughmanyofthebasicprinciplesofembeddedsystemsdoapplytoalltypesofsystems.

TheI/Ointerfacesareacrucialpartofanembeddedsystembecausetheyprovidenecessaryfunctionality.MostpersonalcomputershavethesamebasicI/Odevices(e.g.,mouse,keyboard,videodisplay,CD,USB,andharddrive.)Incontrast,thereisnocommonsetofI/Othatallembeddedsystemhave.ThesoftwaretogetherwiththeI/Oportsandassociatedinterfacecircuitsgiveanembeddedcomputersystemitsdistinctivecharacteristics.AdevicedriverisasetofsoftwarefunctionsthatfacilitatetheuseofanI/Oport.Anothernamefordevicedriverisapplicationprogrammerinterface(API).InthisbookwewillstudyawiderangeofI/OportssupportedbytheLM3S/TM4Cmicrocontrollers.Parallelportsprovidefordigitalinputandoutputs.Serialportsemployawiderangeofformatsandsynchronizationprotocols.Theserialportscancommunicatewithdevicessuchas:

•Sensors

•LiquidCrystalDisplay(LCD)andlightemittingdiode(LED)displays

•Analogtodigitalconverters(ADC)anddigitaltoanalogconverters(DAC)

Analogtodigitalconvertersconvertanalogvoltagestodigitalnumbers.Digitaltoanalogconvertersconvertdigitalnumberstoanalogvoltages.Thetimerfeaturesinclude:

•Fixedrateperiodicexecution

•PulseWidthModulatedoutputs(PWM)

•Inputcaptureusedforperiodandpulsewidthmeasurement

•Outputcompareusedforgeneratingsignalsandfrequencymeasurement.

1.3.TheDesignProcess

1.3.1.RequirementsdocumentBeforebeginninganyproject,itisagoodideatohaveaplan.Thefollowingisonepossibleoutlineofarequirementsdocument.Althoughoriginallyproposedforsoftwareprojects,itisappropriatetousewhenplanninganembeddedsystem,whichincludessoftware,electronics,andmechanicalcomponents.IEEEpublishesanumberoftemplatesthatcanbeusedtodefineaproject(IEEESTD830-1998).Arequirementsdocumentstateswhatthesystemwilldo.Itdoesnotstatehowthesystemwilldoit.Themainpurposeofarequirementsdocumentistoserveasanagreementbetweenyouandyourclientsdescribingwhatthesystemwilldo.Thisagreementcanbecomealegallybindingcontract.Writethedocumentsothatitiseasytoreadandunderstandbyothers.Itshouldbeunambiguous,complete,verifiable,andmodifiable.

Therequirementsdocumentshouldnotincludehowthesystemwillbedesigned.Thisallowstheengineertomakechoicesduringthedesigntominimizecostandmaximizeperformance.Ratheritshoulddescribetheproblembeingsolvedandwhatthesystemactuallydoes.Itcanincludesomeconstraintsplacedonthedevelopmentprocess.Ideally,itisco-writtenbyboththeengineersandthenon-technicalclients.However,itisimperativethatboththeengineersandtheclientsunderstandandagreeonthespecificsinthedocument.

1.Overview

1.1.Objectives:Whyarewedoingthisproject?Whatisthepurpose?

1.2.Process:Howwilltheprojectbedeveloped?

1.3.RolesandResponsibilities:Whowilldowhat?Whoaretheclients?

1.4.InteractionswithExistingSystems:Howwillitfitin?

1.5.Terminology:Definetermsusedinthedocument.

1.6.Security:Howwillintellectualpropertybemanaged?

2.FunctionDescription

2.1.Functionality:Whatwillthesystemdoprecisely?

2.2.Scope:Listthephasesandwhatwillbedeliveredineachphase.

2.3.Prototypes:Howwillintermediateprogressbedemonstrated?

2.4.Performance:Definethemeasuresanddescribehowtheywillbedetermined.

2.5.Usability:Describetheinterfaces.Bequantitativeifpossible.

2.6.Safety:Explainanysafetyrequirementsandhowtheywillbemeasured.

3.Deliverables

3.1.Reports:Howwillthesystembedescribed?

3.2.Audits:Howwilltheclientsevaluateprogress?

3.3.Outcomes:Whatarethedeliverables?Howdoweknowwhenitisdone?

Observation:Tobuildasystemwithoutarequirementsdocumentmeansyouareneverwrong,butneverdone.

1.3.2.ModelingOneofthecommonthreadsintheexampleembeddedsystemspresentedinSection1.2istheneedtounderstandthebehaviorofthephysicalsystemwithwhichtheembeddedsysteminteracts.Sometimesthisunderstandingisonlyhumanintuition.However,thedesignprocesswillbemuchmoresuccessfulifthisunderstandingcanberepresentedinmathematicalform.Scientistsstrivetodescribephysicalprocesseswithclosed-formmathematicalequations.Forexample,Newton’ssecondlawfordampedharmonicoscillatorsis

wherexistheone-dimensionalpositionoftheobject(m),tistime(s),Fistheappliedforce(N),misthemassoftheobject(kg),ciscalledtheviscousdampingcoefficient(kg/s),andkisthespringconstant(N/m).AnotherexampleisMaxwell–Faradayequation(orFaraday’slawofinduction)

or

whereEistheelectricfield(V/m),Bisthemagneticfield(Wb/m2),CistheclosedcurvealongtheboundaryofsurfaceS,dlisdifferentialvectorelementofpathlengthtangentialtothepath/curve(m),and B,SmagneticfluxthroughanysurfaceS(Wb).Athirdexampleisheatconduction

or

wherekisthermalconductivity(W/m/oC),Tistemperature(oC),xisone-dimensionaldistance(m),qisinternalheatgenerations(W/m3), isdensity(kg/m3),cisspecificheatatconstantpressure(W-s/kg/oC)andtistime(s).Thesystemiscausalifitsoutputdependsonlyoncurrentandpastinputs.LetS(x)definetheoutputofamodelforaninputx.AsystemislinearifS(ax1+bx2)=aS(x1)+bS(x2).Alineartime-invariantsystem(LTI)isasystemthatisbothlinearandtimeinvariant.

Someofthedifficultiesinsolvingclosedformequationssuchastheseincludemultidimensionalspace,irregularboundaries,andnon-constantproperties.Thesedifficultiescanbeovercomeusingcomputationalmethodssuchasthefiniteelementmethod(FEM).Stillmanyproblemsremain.Inaccuraciesinpropertyvaluescauseerrorsinthecomputationalmethod.Thebiggestproblemhoweverisintheequationsthemselves.Manyimportantreallifeproblemsexhibitnonlinearbehaviornotdescribedbyscientificequations.

Consequentially,engineerstendtouseempiricalmodelsoftheworldwithwhichtheembeddedsysteminteracts.Theparametersofanempiricalmodelaredeterminedbyexperimentalmeasurementunderconditionssimilartohowthesystemwillbedeployed.Typicallythemodelsarediscreteintime,becausethemeasurementsarediscreteintime.Themodelscanbelinearornonlinearasneeded.Thesemodelsoftenhavememory,meaningtheoutputsareafunctionofboththecurrentinputsandpreviousinputs/outputs.Oneofthesimplestmeasuresofstabilityiscalledbounded-inputbounded-output,whichmeansifallinputsignalsareboundedthenalloutputsignalswillalsobebounded.Forexample,performancemapsareusedinenginecontroltooptimizeperformance.Theyareempiricalequationsrelatingcontrolparameters(suchasappliedpower)andmeasuredparameters(suchasshaftrotationalspeed)todesiredoutputparameters(suchasgeneratedtorque).Evenifdifficult,itisappropriatetodevelopanabstractmodeldescribingtheinteractionbetweenembeddedsystemandtherealworld.Wewillpresentsomemodelswhendesigningmorecomplexsystemslaterinthebook.

1.3.3.Top-downdesignInthissection,wewillpresentthetop-downdesignprocess.Theprocessiscalledtop-down,becausewestartwiththehigh-leveldesignsandworkdowntolow-levelimplementations.Thebasicapproachisintroducedhere,andthedetailsoftheseconceptswillbepresentedthroughouttheremainingchaptersofthebook.Aswelearnsoftware/hardwaredevelopmenttoolsandtechniques,wecanplacethemintotheframeworkpresentedinthissection.AsillustratedinFigure1.5,thedevelopmentofaproductfollowsananalysis-design-implementation-testingcycle.Forcomplexsystemswithlonglife-spans,wetraversemultipletimesaroundthedevelopmentcycle.Forsimplesystems,aone-timepassmaysuffice.Evenafterasystemisdeployed,itcanreenterthelifecycletoaddfeaturesorcorrectmistakes.

Figure1.5.Systemdevelopmentcycleorlife-cycle.Afterthesystemisdoneitcanbedeployed.Duringtheanalysisphase,wediscovertherequirementsandconstraintsforourproposedsystem.Wecanhireconsultantsandinterviewpotentialcustomersinordertogatherthiscriticalinformation.Arequirementisaspecificparameterthatthesystemmustsatisfy,describingwhatthesystemshoulddo.Webeginbyrewritingthesystemrequirements,whichareusuallywrittenasarequirementsdocument.Ingeneral,specificationsaredetailedparametersdescribinghowthesystemshouldwork.Forexample,arequirementmaystatethatthesystemshouldfitintoapocket,whereasaspecificationwouldgivetheexactsizeandweightofthedevice.Forexample,supposewewishtobuildamotorcontroller.Duringtheanalysisphase,wewoulddetermineobviousspecificationssuchasrange,stability,accuracy,andresponsetime.Thefollowingmeasuresareoftenconsideredduringtheanalysisphase:

Safety:Therisktohumansortheenvironment.

Accuracy:Thedifferencebetweentheexpectedtruthandtheactualparameter

Precision:Thenumberofdistinguishablemeasurements

Resolution:Thesmallestchangethatcanbereliablydetected

Responsetime:Thetimebetweenatriggeringeventandtheresultingaction

Bandwidth:Theamountofinformationprocessedpertime

Signaltonoiseratio:Thequotientofthesignalamplitudedividedbythenoise

Maintainability:Theflexibilitywithwhichthedevicecanbemodified

Testability:Theeasewithwhichproperoperationofthedevicecanbeverified

Compatibility:Theconformanceofthedevicetoexistingstandards

Meantimebetweenfailure:Thereliabilityofthedevicedefiningthelifeifaproduct

Sizeandweight:Thephysicalspacerequiredbythesystemanditsmass

Power:Theamountofenergyittakestooperatethesystem

Nonrecurringengineeringcost(NREcost):Theone-timecosttodesignandtest

Unitcost:Thecostrequiredtomanufactureoneadditionalproduct

Time-to-prototype:Thetimerequiredtodesignbuildandtestanexamplesystem

Time-to-market:Thetimerequiredtodelivertheproducttothecustomer

Humanfactors:Thedegreetowhichourcustomersenjoy/like/appreciatetheproduct

Therearemanyparameterstoconsiderandtheirrelativeimportancemaybedifficulttoascertain.Forexample,inconsumerelectronicsthehumaninterfacecanbemoreimportantthanbandwidthorsignaltonoiseratio.Often,improvingtheperformanceononeparametercanbeachievedonlybydecreasingtheperformanceofanother.Thisartofcompromisedefinesthetradeoffsanengineermustmakewhendesigningaproduct.Aconstraintisalimitation,withinwhichthesystemmustoperate.Thesystemmaybeconstrainedtosuchfactorsascost,safety,compatibilitywithotherproducts,useofspecificelectronicandmechanicalpartsasotherdevices,interfaceswithotherinstrumentsandtestequipment,anddevelopmentschedule.

Checkpoint1.11:What’sthedifferencebetweenarequirementandaspecification?

Whenyouwriteapaper,youfirstdecideonatheme,andnextyouwriteanoutline.Inthesamemanner,ifyoudesignanembeddedsystem,youdefineitsspecification(whatitdoes),andbeginwithanorganizationalplan.Inthissection,wewillpresentthreegraphicaltoolstodescribetheorganizationofanembeddedsystem:dataflowgraphs,callgraphs,andflowcharts.Youshoulddrawallthreeforeverysystemyoudesign.

Duringthehigh-leveldesignphase,webuildaconceptualmodelofthehardware/softwaresystem.Itisinthismodelthatweexploitasmuchabstractionasappropriate.Theprojectisbrokeninmodulesorsubcomponents.ModulardesignwillbepresentedinChapter3.Duringthisphase,weestimatethecost,schedule,andexpectedperformanceofthesystem.Atthispointwecandecideiftheprojecthasahighenoughpotentialforprofit.Adataflowgraphisablockdiagramofthesystem,showingtheflowofinformation.Arrowspointfromsourcetodestination.Itisgoodpracticetolabelthearrowswiththeinformationtypeandbandwidth.Therectanglesrepresenthardwarecomponentsandtheovalsaresoftwaremodules.Weusedataflowgraphsinthehigh-leveldesign,becausetheydescribetheoveralloperationofthesystemwhilehidingthedetailsofhowitworks.Issuessuchassafety(e.g.,IsaacAsimov’sfirstLawofRobotics“Arobotmaynotharmahumanbeing,or,throughinaction,allowahumanbeingtocometoharm”)andtesting(e.g.,weneedtoverifyoursystemisoperational)shouldbeaddressedduringthehigh-leveldesign.

AnexampledataflowgraphforamotorcontrollerisshowninFigure1.6.Noticethatthearrowsarelabeledwithdatatypeandbandwidth.Therequirementofthesystemistodeliverpowertoamotorsothatthespeedofthemotorequalsthedesiredvaluesetbytheoperatorusingakeypad.Inordertomakethesystemeasiertouseandtoassistintesting,aliquidcrystaldisplay(LCD)isadded.Thesensorconvertsmotorspeedanelectricalvoltage.Theamplifierconvertsthissignalintothe0to+3.3VvoltagerangerequiredbytheADC.TheADCconvertsanalogvoltageintoadigitalsample.TheADCroutines,usingtheADCandtimerhardware,collectsamplesandcalculatevoltages.Next,thissoftwareusesatabledatastructuretoconvertvoltagetomeasuredspeed.Theuserwillbeabletoselectthedesiredspeedusingthekeypadinterface.Thedesiredandmeasuredspeeddataarepassedtothecontrollersoftware,whichwilladjustthepoweroutputinsuchamannerastominimizethedifferencebetweenthemeasuredspeedandthedesiredspeed.Finally,thepowercommandsareoutputtotheactuatormodule.Theactuatorinterfaceconvertsthedigitalcontrolsignalstopowerdeliveredtothemotor.ThemeasuredspeedandspeederrorwillbesenttotheLCDmodule.

Figure1.6.Adataflowgraphshowinghowsignalspassthroughamotorcontroller.

Thenextphaseisengineeringdesign.Webeginbyconstructingapreliminarydesign.Thissystemincludestheoveralltop-downhierarchicalstructure,thebasicI/Osignals,shareddatastructuresandoverallsoftwarescheme.Atthisstagethereshouldbeasimpleanddirectcorrelationbetweenthehardware/softwaresystemsandtheconceptualmodeldevelopedinthehigh-leveldesign.Next,wefinishthetop-downhierarchicalstructure,andbuildmock-upsofthemechanicalparts(connectors,chassis,cablesetc.)andusersoftwareinterface.Sophisticated3-DCADsystemscancreaterealisticimagesofoursystem.Detailedhardwaredesignsmustincludemechanicaldrawings.Itisagoodideatohaveasecondsource,whichisanalternativesupplierthatcansellourpartsifthefirstsourcecan’tdeliverontime.Acallgraphisadirectedgraphshowingthecallingrelationshipsbetweensoftwareandhardwaremodules.IfafunctioninmoduleAcallsafunctioninmoduleB,thenwedrawanarrowfromAtoB.IfafunctioninmoduleAinput/outputsdatafromhardwaremoduleC,thenwedrawanarrowfromAtoC.IfhardwaremoduleCcancauseaninterrupt,resultinginsoftwarerunninginmoduleA,thenwedrawanarrowfromCtoA.Ahierarchicalsystemwillhaveatree-structuredcallgraph.

AcallgraphforthismotorcontrollerisshowninFigure1.7.Again,rectanglesrepresenthardwarecomponentsandovalsshowsoftwaremodules.Anarrowpointsfromthecallingroutinetothemoduleitcalls.TheI/Oportsareorganizedintogroupsandplacedatthebottomofthegraph.Ahigh-levelcallgraph,liketheoneshowninFigure1.7,showsonlythehigh-levelhardware/softwaremodules.AdetailedcallgraphwouldincludeeachsoftwarefunctionandI/Oport.Normally,hardwareispassiveandthesoftwareinitiateshardware/softwarecommunication,butaswewilllearninChapter5,itispossibleforthehardwaretointerruptthesoftwareandcausecertainsoftwaremodulestoberun.Inthissystem,thetimerhardwarewillcausetheADCsoftwaretocollectasampleataregularrate.Thecontrollersoftwarecallsthekeypadroutinestogetthedesiredspeed,callstheADCsoftwaretogetthemotorspeedatthatpoint,determineswhatpowertodelivertothemotorandupdatestheactuatorbysendingthepowervaluetotheactuatorinterface.ThecontrollersoftwarecallstheLCDroutinestodisplaythestatusofthesystem.Acquiringdata,calculatingparameters,outputtingresultsataregularrateisstrategicwhenperformingdigitalsignalprocessinginembeddedsystems.

Figure1.7.Acallgraphforamotorcontroller.

Checkpoint1.12:WhatconfusioncouldariseiftwosoftwaremoduleswereallowedtoaccessthesameI/Oport?ThissituationwouldbeevidentonacallgraphifthetwosoftwaremoduleshadarrowspointingtothesameI/Oport.

Observation:IfmoduleAcallsmoduleB,andBreturnsdata,thenadataflowgraphwillshowanarrowfromBtoA,butacallgraphwillshowanarrowfromAtoB.

Datastructuresincludeboththeorganizationofinformationandmechanismstoaccessthedata.Againsafetyandtestingshouldbeaddressedduringthislow-leveldesign.

Thenextphaseisimplementation.Anadvantageofatop-downdesignisthatimplementationofsubcomponentscanoccurconcurrently.Themostcommonapproachtodevelopingsoftwareforanembeddedsystemistouseacross-assemblerorcross-compilertoconvertsourcecodeintothemachinecodeforthetargetsystem.Themachinecodecanthenbeloadedintothetargetmachine.Debuggingembeddedsystemswiththissimpleapproachisverydifficultfortworeasons.First,theembeddedsystemlackstheusualkeyboardanddisplaythatassistuswhenwedebugregularsoftware.Second,thenatureofembeddedsystemsinvolvesthecomplexandreal-timeinteractionbetweenthehardwareandsoftware.Thesereal-timeinteractionsmakeitimpossibletotestsoftwarewiththeusualsingle-steppingandprintstatements.

Thenexttechnologicaladvancementthathasgreatlyaffectedthemannerinwhichembeddedsystemsaredevelopedissimulation.Becauseofthehighcostandlongtimesrequiredtocreatehardwareprototypes,manypreliminaryfeasibilitydesignsarenowperformedusinghardware/softwaresimulations.Asimulatorisasoftwareapplicationthatmodelsthebehaviorofthehardware/softwaresystem.Ifboththeexternalhardwareandsoftwareprogramaresimulatedtogether,evenalthoughthesimulatedtimeisslowerthantheclockonthewall,thereal-timehardware/softwareinteractionscanbestudied.

Duringtheinitialiterationsofthedevelopmentcycle,itisquiteefficienttoimplementthehardware/softwareusingsimulation.Onemajoradvantageofsimulationisthatitisusuallyquickertoimplementaninitialproductonasimulatorversusconstructingaphysicaldeviceoutofactualcomponents.Rapidprototypingisimportantintheearlystagesofproductdevelopment.Thisallowsformoreloopsaroundtheanalysis-design-implementation-testingcycle,whichinturnleadstoamoresophisticatedproduct.

Duringthetestingphase,weevaluatetheperformanceofoursystem.First,wedebugthesystemandvalidatebasicfunctions.Next,weusecarefulmeasurementstooptimizeperformancesuchasstaticefficiency(memoryrequirements),dynamicefficiency(executionspeed),accuracy(differencebetweenexpectedtruthandmeasured),andstability(consistentoperation.)Debuggingtechniqueswillbepresentedthroughoutthebook.Testingisnotperformedattheendofprojectwhenwethinkwearedone.Rathertestingmustbeintegratedintoallphasesofthedesigncycle.Oncetestedthesystemcanbedeployed.

Maintenanceistheprocessofcorrectingmistakes,addingnewfeatures,optimizingforexecutionspeedorprogramsize,portingtonewcomputersoroperatingsystems,andreconfiguringthesystemtosolveasimilarproblem.Nosystemisstatic.Customersmaychangeoraddrequirementsorconstraints.Tobeprofitable,weprobablywillwishtotailoreachsystemtotheindividualneedsofeachcustomer.Maintenanceisnotreallyaseparatephase,butratherinvolvesadditionalloopsaroundthedevelopmentcycle.

1.3.4.FlowchartsInthissection,weintroducetheflowchartsyntaxthatwillbeusedthroughoutthebook.Programsthemselvesarewritteninalinearorone-dimensionalfashion.Inotherwords,wetypeonelineofsoftwareafteranotherinasequentialfashion.Writingprogramsthiswayisanaturalprocess,becausethecomputeritselfusuallyexecutestheprograminatop-to-bottomsequentialfashion.Thisone-dimensionalformatisfineforsimpleprograms,butconditionalbranchingandfunctioncallsmaycreatecomplexbehaviorsthatarenoteasilyobservedinalinearfashion.Eventhesimplesystemshavemultiplesoftwaretasks.Furthermore,acomplexapplicationwillrequiremultiplemicrocontrollers.Therefore,weneedamulti-dimensionalwaytovisualizesoftwarebehavior.Flowchartsareonewaytodescribesoftwareinatwo-dimensionalformat,specificallyprovidingconvenientmechanismstovisualizemulti-tasking,branching,andfunctioncalls.Flowchartsareveryusefulintheinitialdesignstageofasoftwaresystemtodefinecomplexalgorithms.Furthermore,flowchartscanbeusedinthefinaldocumentationstageofaprojectinordertoassistinitsuseormodification.

Figuresthroughoutthissectionillustratethesyntaxusedtodrawflowcharts.Theovalshapesdefineentryandexitpoints.Themainentrypointisthestartingpointofthesoftware.Eachfunction,orsubroutine,alsohasanentrypoint,whichistheplacethefunctionstarts.Ifthefunctionhasinputparameterstheyarepassedinattheentrypoint.Theexitpointreturnstheflowofcontrolbacktotheplacefromwhichthefunctionwascalled.Ifthefunctionhasreturnparameterstheyarereturnedattheexitpoint.Whenthesoftwarerunscontinuously,asistypicallythecaseinanembeddedsystem,therewillbenomainexitpoint.

Weuserectanglestospecifyprocessblocks.Inahigh-levelflowchart,aprocessblockmightinvolvemanyoperations,butinalow-levelflowchart,theexactoperationisdefinedintherectangle.Theparallelogramwillbeusedtodefineaninput/outputoperation.Someflowchartartistsuserectanglesforbothprocessesandinput/output.Sinceinput/outputoperationsareanimportantpartofembeddedsystems,wewillusetheparallelogramformat,whichwillmakeiteasiertoidentifyinput/outputinourflowcharts.Thediamond-shapedobjectsdefineabranchpointordecisionblock.Therectanglewithdoublelinesonthesidespecifiesacalltoapredefinedfunction.Inthisbook,functions,subroutinesandproceduresaretermsthatallrefertoawell-definedsectionofcodethatperformsaspecificoperation.Functionsusuallyreturnaresultparameter,whileproceduresusuallydonot.Functionsandproceduresaretermsusedwhendescribingahigh-levellanguage,whilesubroutinesoftenusedwhendescribingassemblylanguage.Whenafunction(orsubroutineorprocedure)iscalled,thesoftwareexecutionpathjumpstothefunction,thespecificoperationisperformed,andtheexecutionpathreturnstothepointimmediatelyafterthefunctioncall.Circlesareusedasconnectors.

Commonerror:Ingeneral,itisbadprogrammingstyletodevelopsoftwarethatrequiresalotofconnectorswhendrawingitsflowchart.

Thereareaseeminglyunlimitednumberoftasksonecanperformonacomputer,andthekeytodevelopinggreatproductsistoselectthecorrectones.Justlikehikingthroughthewoods,weneedtodevelopguidelines(likemapsandtrails)tokeepusfromgettinglost.Oneofthefundamentalissueswhendevelopingsoftware,regardlesswhetheritisamicrocontrollerwith1000linesofassemblycodeoralargecomputersystemwithbillionsoflinesistomaintainaconsistentstructure.Onesuchframeworkiscalledstructuredprogramming.Agoodhigh-levellanguagewillforcetheprogrammertowritestructuredprograms.Structuredprogramsarebuiltfromthreebasicbuildingblocks:thesequence,theconditional,andthewhile-loop.Atthelowestlevel,theprocessblockcontainssimpleandwell-definedcommands.I/Ofunctionsarealsolow-levelbuildingblocks.Structuredprogramminginvolvescombiningexistingblocksintomorecomplexstructures,asshowninFigure1.8.

Figure1.8.Flowchartshowingthebasicbuildingblocksofstructuredprogramming.MaintenanceTip:Remembertoupdatetheflowchartsasmodificationsaremadetothesoftware

Next,wewillrevisitthepacemakerexampleinordertoillustratetheflowchartsyntax.Athreadisthesequenceofactionscausedbyexecutingsoftware.TheflowchartinFigure1.9definesasingle-threadedexecutionbecausethereisonesequence.

Example1.1(continued):Useaflowcharttodescribeanalgorithmthatapacemakermightusetoregulateandimproveheartfunction.

Solution:Thisexampleillustratesacommontraitofanembeddedsystem,thatis,theyperformthesamesetoftasksoverandoverforever.Theprogramstartsatmainwhenpowerisapplied,andthesystembehaveslikeapacemakeruntilthebatteryrunsout.Figure1.9showsaflowchartforaverysimplealgorithm.Iftheheartisbeatingnormallywitharategreaterthanorequalto1beat/sec(60BPM),thentheatrialsensorwilldetectactivityandthefirstdecisionwillgoright.Sincethisisnormalbeating,theventricularactivitywilloccurwithinthenext200ms,andtheventricularsensorwillalsodetectactivity.Inthissituation,nooutputpulseswillbeissued.Ifthedelaybetweenatrialcontractionandventricularcontractwerelongerthanthenormal200ms,thenthepacemakerwillactivatetheventricles200msaftereachatrialcontraction.Iftheventricleisbeatingfasterthan60BPMwithoutanyatrialcontractions,thennoventricularstimulationswillbeissued.Ifthereisnoactivityfromeitheratriumortheventricle(orifthatrateisslowerthan60BPM),thentheventriclesarepacedat60BPM.

Figure1.9.Flowchartillustratingasimplepacemakeralgorithm.

Checkpoint1.13:Assumeyouaregivenasimplewatchthatjusttellsyouthetimeinhours,minutes,andseconds.Lettbeaninputparameter.Explainhowyoucouldusethewatchtowaittseconds.

1.3.5.Parallel,distributed,andconcurrentprogrammingManyproblemscannotbeimplementedusingthesingle-threadedexecutionpatterndescribedintheprevioussection.Parallelprogrammingallowsthecomputertoexecutemultiplethreadsatthesametime.State-of-theartmulti-coreprocessorscanexecuteaseparateprogramineachofitscores.Forkandjoinarethefundamentalbuildingblocksofparallelprogramming.Afterafork,twoormoresoftwarethreadswillberuninparallel.I.e.,thethreadswillrunsimultaneouslyonseparateprocessors.

Twoormoresimultaneoussoftwarethreadscanbecombinedintooneusingajoin.TheflowchartsymbolsforforkandjoinareshowninFigure1.10.Softwareexecutionafterthejoinwillwaituntilallthreadsabovethejoinarecomplete.Asananalogy,ifIwanttodigabigholeinmybackyard,Iwillinvitethreefriendsoverandgiveeveryoneashovel.Theforkoperationchangesthesituationfrommeworkingalonetofourofusreadytodig.Thefourdiggingtasksareruninparallel.Whentheoveralltaskiscomplete,thejoinoperationcausesthefriendstogoaway,andIamworkingaloneagain.Acomplexsystemmayemploymultiplemicrocontrollers,eachrunningitsownsoftware.Weclassifythisconfigurationasparallelordistributedprogramming.

Figure1.10.Flowchartsymbolstodescribeparallel,distributed,andconcurrentprogramming.Concurrentprogrammingallowsthecomputertoexecutemultiplethreads,butonlyoneatatime.Interruptsareonemechanismtoimplementconcurrencyonreal-timesystems.Interruptshaveahardwaretriggerandasoftwareaction.Aninterruptisaparameter-lesssubroutinecall,triggeredbyahardwareevent.TheflowchartsymbolsforinterruptsarealsoshowninFigure1.10.Thetriggerisahardwareeventsignalingitistimetodosomething.Examplesofinterrupttriggerswewillseeinthisbookincludenewinputdatahasarrived,outputdeviceisidle,andperiodicevent.Thesecondcomponentofaninterrupt-drivensystemisthesoftwareactioncalledaninterruptserviceroutine(ISR).Theforegroundthreadisdefinedastheexecutionofthemainprogram,andthebackgroundthreadsareexecutionsoftheISRs.

Considertheanalogyofsittinginacomfychairreadingabook.Readingabookislikeexecutingthemainprogramintheforeground.Becausethereisonlyoneofyou,thisscenarioisanalogoustoacomputerwithoneprocessor.Youstartreadingatthebeginningofthebookandbasicallyreadonepageatatimeinasequentialfashion.Youmightjumptothebackandlooksomethingupintheglossary,thenjumpbacktowhereyouwere,whichisanalogoustoafunctioncall.Similarly,ifyoumightreadthesamepageafewtimes,whichisanalogoustoaprogramloop.Eventhoughyouskiparoundalittle,theorderofpagesyoureadfollowsalogicalandwell-definedsequence.Conversely,ifthetelephonerings,youplaceabookmarkinthebook,andanswerthephone.Whenyouarefinishedwiththephoneconversation,youhangupthephoneandcontinuereadinginthebookwhereyouleftoff.TheringingphoneisanalogoustohardwaretriggerandthephoneconversationislikeexecutingtheISR.

Example1.2(continued):Useaflowcharttodescribeanalgorithmthatastand-alonesmokedetectormightusetowarnpeopleintheeventofafire.

Solution:Thisexampleillustratesacommontraitofalow-powerembeddedsystem.Thesystembeginswithapoweronreset,causingittostartatmain.Theinitializationenablesthetimerinterrupts,andthenitshutsoffthealarm.Inalow-powersystemthemicrocontrollergoestosleepwhentherearenotaskstoperform.Every30secondsthetimerinterruptwakensthemicrocontrollerandexecutestheinterruptserviceroutine.Thefirsttaskistoreadthesmokesensor.Ifthereisnofire,itwillflashtheLEDandreturnfrominterrupt.Atthispoint,themainprogramwillputthemicrocontrollerbacktosleep.Theletters(A-K)inFigure1.11specifythesoftwareactivitiesinthismultithreadedexample.InitiallyitexecutesA-B-Candgoestosleep.Every30seconds,assumingthereisnofire,itexecutes<-D-E-F-G-J-K-J-K-···-J-K-H->CThissequencewillexecuteinabout1ms,dominatedbythetimeittakestoflashtheLED.Thisisalow-powersolutionbecausethemicrocontrollerispoweredforabout0.003%ofthetime,or1msevery30seconds.

Figure1.11.Flowchartillustratingasimplesmokedetectoralgorithm.

Toillustratetheconceptofparallelprogramming,assumewehaveamulti-corecomputerwithfourprocessors.Considertheproblemoffindingthemaximumvalueinalargebuffer.First,wedividethebufferintofourequalparts.Next,weexecuteafork,asshownintheleft-mostflowchartinFigure1.10,launchingfourparallelthreads.Thefourprocessorsruninparalleleachfindingthemaximumofitssubset.Whenallfourthreadsarecomplete,theyperformajoinandcombinethefourresultstofindtheoverallmaximum.Itisimportanttodistinguishparallelprogramminglikethisfrommultithreadingimplementingconcurrentprocessingwithinterrupts.Becausemostmicrocontrollershaveasingleprocessor,thisbookwithfocusonconcurrentprocessingwithinterruptsanddistributedprocessingwithanetworkinvolvingmultiplemicrocontrollers.

1.3.6.Creativediscoveryusingbottom-updesignFigure1.5describestop-downdesignasacyclicprocess,beginningwithaproblemstatementandendingupwithasolution.Withabottom-updesignwebeginwithsolutionsandbuilduptoaproblemstatement.Manyinnovationsbeginwithanidea,“whatif…?”Inabottom-updesign,onebeginswithdesigning,building,andtestinglow-levelcomponents.Figure1.12illustratesatwo-levelprocess,combiningthreesubcomponentstocreatetheoverallproduct.Thishierarchicalprocesscouldhavemorelevelsand/ormorecomponentsateachlevel.Thelow-leveldesignscanoccurinparallel.Thedesignofeachcomponentiscyclic,iteratingthroughthedesign-build-testcycleuntiltheperformanceisacceptable.

Figure1.12.Systemdevelopmentprocessillustratingbottom-updesign.Bottom-updesignisinefficientbecausesomesubsystemsaredesigned,built,andtested,butneverused.Furthermore,inatrulycreativeenvironmentmostideascannotbesuccessfullyconvertedtooperationalsubsystems.Creativelaboratoriesarefilledwithfinished,half-finished,andfailedsubcomponents.Asthedesignprogressesthecomponentsarefittogethertomakethesystemmoreandmorecomplex.Onlyafterthesystemiscompletelybuiltandtesteddoesonedefineitsoverallspecifications.

Thebottom-updesignprocessallowscreativeideastodrivetheproductsacompanydevelops.Italsoallowsonetoquicklytestthefeasibilityofanidea.Ifonefullyunderstandsaproblemareaandthescopeofpotentialsolutions,thenatop-downdesignwillarriveataneffectivesolutionmostquickly.Ontheotherhand,ifonedoesn’treallyunderstandtheproblemorthescopeofitssolutions,abottom-upapproachallowsonetostartoffbylearningabouttheproblem.

Observation:Agoodengineerknowsbothbottom-upandtop-downdesignmethods,choosingtheapproachmostappropriateforthesituationathand.

1.4.DigitalLogicandOpenCollectorDigitallogichastwostates,withmanyenumerationssuchashighandlow,1and0,trueandfalse,onandoff.Therearefourcurrentsofinterest,asshowninFigure1.13,whenanalyzingiftheinputsofthenextstageareloadingtheoutput.IIHandIILarethecurrentsrequiredofaninputwhenhighandlowrespectively.Furthermore,IOHandIOLarethemaximumcurrentsavailableattheoutputwhenhighandlow.Inorderfortheoutputtoproperlydrivealltheinputsofthenextstage,themaximumavailableoutputcurrentmustbelargerthanthesumofalltherequiredinputcurrentsforboththehighandlowconditions.

and

Absolutevalueoperatorsareputintheaboverelationsbecausedatasheetsareinconsistentaboutspecifyingpositiveandnegativecurrents.ThearrowsinFigure1.13definethedirectionofcurrentregardlessofwhetherthedatasheetdefinesitasapositiveornegativecurrent.Itisyourresponsibilitytochoosepartssuchthattheaboveinequalitieshold.

Figure1.13.Sometimesoneoutputmustdrivemultipleinputs.Kirchhoff’sCurrentLaw(KCL)statesthesumofallthecurrentsintoonenodemustbezero.TheaboveinequalitiesarenotaviolationofKCL,becausetheoutputcurrentsaretheavailablecurrentsandtheinputcurrentsaretherequiredcurrents.Oncethesystemisbuiltandrunning,theactualoutputcurrentwillofcourseexactlyequalthesumoftheactualinputcurrents.Asamatterofcompleteness,weincludeKirchhoff’sVoltageLaw(KVL),whichstatesthesumofallthevoltagesinaclosedloopmustbezero.Table1.4showstypicalcurrentvaluesforthevariousdigitallogicfamilies.TheLM3S/TM4Cmicrocontrollersgiveyouthreechoicesofoutputcurrentforthedigitaloutputpins.TheTM4C1294addsa12-mAmode.

Family Example IOH IOL IIH IIL

StandardTTL 7404 0.4mA 16mA 40µA 1.6mA

SchottkyTTL 74S04 1mA 20mA 50µA 2mA

LowPowerSchottky

74LS04 0.4mA 4mA 20µA 0.4mA

74HC04 4mA 4mA 1µA 1µA

HighSpeedCMOS

AdvHighSpeedCMOS

74AHC04 4mA 4mA 1µA 1µA

TM4C2mA-drive

TM4C123 2mA 2mA 2µA 2µA

TM4C4mA-drive

TM4C123 4mA 4mA 2µA 2µA

TM4C8mA-drive

TM4C123 8mA 8mA 2µA 2µA

TM4C12mA-drive

TM4C1294 12mA 12mA 2µA 2µA

Table1.4.Theinputandoutputcurrentsofvariousdigitallogicfamiliesandmicrocontrollers.

Observation:ForTTLdevicesthelogiclowcurrentsaremuchlargerthanthelogichighcurrents.

Whenwedesigncircuitsusingdevicesallfromasinglelogicfamily,wecandefinefanoutasthemaximumnumberofinputs,oneoutputcandrive.Fortransistor-transistorlogic(TTL)logicwecancalculatefanoutfromtheinputandoutputcurrents:

Fanout=minimum((IOH/IIH),(IOL/IIL))

Conversely,thefanoutofhigh-speedcomplementarymetal-oxidesemiconductor(CMOS)devices,whichincludesmostmicrocontrollers,isdeterminedbycapacitiveloadingandnotbythecurrents.Figure1.14showsasimplecircuitmodelofaCMOSinterface.TheidealvoltageoftheoutputdeviceislabeledV1.Forinterfacesincloseproximity,theresistanceRresultsfromtheoutputimpedanceoftheoutputdevice,andthecapacitanceCresultsfromtheinputcapacitanceoftheinputdevice.However,iftheinterfacerequiresacabletoconnectthetwodevices,boththeresistanceandcapacitancewillbeincreasedbythecable.ThevoltagelabeledV2istheeffectivevoltageasseenbytheinput.IfV2isbelow1.3V,theLM3S/LM4F/TM4Cmicrocontrollerswillinterpretthesignalaslow.Conversely,thevoltageisabove2.0V,thesemicrocontrollerswillconsiderithigh.Theslewrateofasignalistheslopeofthevoltageversustimeduringthetimewhenthelogiclevelswitchesbetweenlowandhigh.Asimilarparameteristhetransitiontime,whichisthetimeittakesforanoutputtoswitchfromonelogicleveltoanother.InFigure1.14,thetransitiontimeisdefinedasthetimeittakesV2togofrom1.3to2.0V.Thereisacapacitiveloadfortheoutputandeachinput.Asthiscapacitanceincreasestheslewratedecreases,whichwillincreasethetransitiontime.Signalswithahighslewratecanradiatealotofnoise.So,toreducenoiseemissionswesometimeslimittheslewrateofthesignals.

TherearetwowaystodeterminethefanoutofCMOScircuits.First,somecircuitshaveaminimumtimeitsinputcanexistinthetransitionrange.Forexample,itmightspecifythesignalcannotbeabove1.3andbelow2.0Vformorethan20ns.Clockinputsareoftenspecifiedthisway.Asecondwayistocalculatethetimeconstant,whichisR*Cforthiscircuit.LetTbethepulsewidthofthedigitalsignal.IfTislargecomparedto ,thentheCMOSinterfacefunctionsproperly.Forcircuitsthatmixdevicesfromonefamilywithanother,wemustlookindividuallyattheinputandoutputcurrents,voltagesandcapacitiveloads.Thereisnosimpleformula.

Figure1.14.CapacitanceloadingisanimportantfactorwheninterfacingCMOSdevices.

Figure1.15comparestheinputandoutputvoltagesformanyofthedigitallogicfamilies.VIListhevoltagebelowwhichaninputisconsideredalogiclow.Similarly,VIHisthevoltageabovewhichaninputisconsideredalogichigh.Theoutputvoltagedependsstronglyoncurrentrequiredtodrivetheinputsofthenextstage.VOHistheoutputvoltagewhenthesignalishigh.Inparticular,iftheoutputisalogichigh,andthecurrentislessthanIOH,thenthevoltagewillbegreaterthanVOH.Similarly,VOListheoutputvoltagewhenthesignalislow.Inparticular,iftheoutputisalogiclow,andthecurrentislessthanIOL,thenthevoltagewillbelessthanVOL.ThedigitalinputpinsontheLM3S/LM4F/TM4Cmicrocontrollersare5V-tolerant,meaninganinputhighsignalcanbeanyvoltagefrom2.0to5.0V.

Figure1.15.Voltagethresholdsforvariousdigitallogicfamilies.

Fortheoutputofonecircuittoproperlydrivetheinputsofthenextcircuit,theoutputlowvoltageneedstobelowenough,andtheoutputhighvoltageneedstobehighenough.

forallinputsand forallinputs

ThemaximumoutputcurrentspecificationontheLM3S/LM4F/TM4Cfamilyis25mA,whichisthecurrentabovewhichwillcausedamage.However,wecanselectIOHandIOLtobe2,4,or8mA.Normally,wedesignthesystemsotheoutputcurrentsarelessthanIOHandIOL.Vtisthetypicalthresholdvoltage,whichisthevoltageatwhichtheinputusuallyswitchesbetweenlogiclowandhigh.Formallyhowever,aninputisconsideredinthetransitionregion(valueindeterminate)forvoltagesbetweenVILandVIH.Thefiveparametersthataffectourchoiceoflogicfamiliesare

•Powersupplyvoltage(e.g.,+5V,3.3Vetc.)

•Powersupplycurrent(e.g.,willthesystemneedtorunonbatteries?)

•Speed(e.g.,clockfrequencyandpropagationdelays)

•Outputdrive,IOL,IOH(e.g.,doesitneedtodrivemotorsorlights?)

•Noiseimmunity(e.g.,electromagneticfieldinterference)

•Temperature(e.g.,electromagneticfieldinterference)

Checkpoint1.14:HowwilltheTM4C123interpretaninputpinastheinputvoltagechangesfrom0,1,2,3,4,to5V?I.e.,foreachvoltage,willitbeconsideredasalogiclow,asalogichighorasindeterminate?

Checkpoint1.15:Consideringbothvoltageandcurrent,cantheoutputofa74HC04drivetheinputofa74LS04?Assumebotharerunningat5V.

Checkpoint1.16:Consideringbothvoltageandcurrent,cantheoutputofa74LS04drivetheinputofa74HC04?Assumebotharerunningat5V.

Averyimportantconceptusedincomputertechnologyistristatelogic,whichhasthreeoutputstates:high,low,andoff.OthernamesfortheoffstateareHiZ,floating,andtristate.Tristatelogicisdrawnasatriangleshapewithasignalonthetopofthetriangle.InthisFigure1.16,Aisthedatainput,Gisthegateinput,andBisthedataoutput.Whenthereisnocircleonthegate,itoperatesinpositivelogic,meaningifthegateishigh,thentheoutputdataequalstheinputdata.Ifthepositive-logicgateislow,thentheoutputwillfloat.Whenthereisacircleonthegate,itoperatesinnegativelogic,meaningifthegateislow,thentheoutputdataequalstheinputdata.Ifthenegative-logicgateishigh,thentheoutputwillfloat.

Figure1.16.Digitallogicdrawingoftristatedrivers.Thereareawiderangeoftechnologiesavailablefordigitallogicdesign.Tostudythesedifferencesconsiderthe74LVT245,the74ALVC245,the74LVC245,the74ALB245,the74AC245,the74AHC245,andthe74LV245.

Eachofthesechipsisan8-input8-outputbidirectionaltristatedriver.Table1.5listssomeoftheinterfacingparametersforeachtechnology.ICCisthetotalsupplycurrentrequiredtodrivethechip.tpdisthepropagationdelayfrominputtooutput.VCCisthesupplyvoltage.

FamilyTechnology VILVIH VOLVOH IOL IOH ICC tpd

LVT-Low-VoltageBiCMOS LVTTL LVTTL 64 -32 190 3.5

ALVC-AdvancedLow-VoltageCMOS

LVTTL LVTTL 24 -24 40 3.0

LVC-Low-VoltageCMOS LVTTL LVTTL 24 -24 10 4.0

ALB-AdvancedLow-VoltageBiCMOS

LVTTL LVTTL 25 -25 800 2.0

AC-AdvancedCMOS CMOS CMOS 12 -12 20 8.5

AHC-AdvancedHighSpeedCMOS

CMOS CMOS 4 -4 20 11.9

LV-Low-VoltageCMOS LVTTL LVTTL 8 -8 20 14

units mA mA µA ns

Table1.5.Comparisonoftheoutputdrive,supplycurrentandspeedofvarious3.3Vlogic‘245gates.

Observation:ThereisaninverserelationshipbetweensupplycurrentICCandpropagationdelaytpd.

The74LS04isalow-powerSchottkyNOTgate,asshownontheleftinFigure1.17.ItiscalledSchottkylogicbecausethedevicesaremadefromSchottkytransistors.TheoutputishighwhenthetransistorQ4isactive,drivingtheoutputtoVcc.TheoutputislowwhenthetransistorQ5isactive,drivingtheoutputto0.

Figure1.17.Twotransistor-levelimplementationsofaNOTgate.Itisobviouslynecessarytoreadthedatasheetforyourmicrocontroller.However,itisalsogoodpracticetoreviewtheerratapublishedbythemanufactureraboutyourmicrocontroller.Theerratadefinesituationswheretheactualchipdoesnotfollowspecificationsinthedatasheet.Forexample,theregularTM4C123datasheetstatestheI/Opinsare+5Vtolerant.However,readingtheerratafortheLM3S811versionC2announcesthat“PB6,PC5,andPC6arenot5-Vtolerant.”

The74HC04isahigh-speedCMOSNOTgate,shownontherightinFigure1.17.TheoutputishighwhenthetransistorQ1isactive,drivingtheoutputto3.3V.TheoutputislowwhenthetransistorQ2isactive,drivingtheoutputto0.Sincemostmicrocontrollersaremadewithhigh-speedCMOSlogic,itsoutputsbehaveliketheQ1/Q2“push/pull”transistorpair.Outputportsarenotinverting.I.e.,whenyouwritea“1”toanoutputport,thentheoutputvoltagegoeshigh.Similarly,whenyouwritea“0”toanoutputport,thentheoutputvoltagegoeslow.AnalysesofthecircuitinFigure1.17revealsomeofthebasicpropertiesofhigh-speedCMOSlogic.First,becauseofthecomplementarynatureoftheP-channel(theoneonthetop)andN-channel(theoneonthebottom)transistors,whentheinputisconstant(continuouslyhighorcontinuouslylow),thesupplycurrent,Icc,isverylow.Second,thegatewillrequiresupplycurrentonlywhentheoutputswitchesfromlowtohighorfromhightolow.Thisobservationleadstothedesignrulethatthepowerrequiredtorunahigh-speedCMOSsystemislinearlyrelatedtothefrequencyofitsclock,becausethefrequencyoftheclockdeterminesthenumberoftransitionspersecond.Alongthesamelines,weseethatifthevoltageoninputAexistsbetweenVILandVIHforextendedperiodsoftime,thenbothQ1andQ2arepartiallyactive,causingashortfrompowertoground.Thisconditioncancausepermanentdamagetothetransistors.Third,sincetheinputAisconnectedtothegateofthetwoMOStransistors,theinputcurrentswillbeverysmall(≈1 A).Inotherwords,theinputimpedance(inputvoltagedividebyinputcurrent)ofthegateisveryhigh.Normally,ahighinputimpedanceisagoodthing,exceptiftheinputisnotconnected.Iftheinputisnotconnectedthenittakesverylittleinputcurrentstocausethelogicleveltoswitch.

Commonerror:IfunusedinputpinsonaCMOSmicrocontrollerareleftunconnected,thentheinputsignalmayoscillateathighfrequenciesdependingontheEMfieldsintheenvironment,wastingpowerunnecessarily.

Observation:ItisagooddesignpracticetoconnectunusedCMOSinputstogroundorconnectthemto+3.3V.

NowthatweunderstandthatCMOSdigitallogicisbuiltwithPNPandNPNtransistors,wecanrevisittheinterfacerequirementsforconnectingadigitaloutputfromonemoduletoadigitalinputofanothermodule.Figure1.18showsthemodelwhentheoutputishigh.Tomaketheoutputhigh,aPNPtransistorintheoutputmoduleisconducting(Q1)driving+3.3Vtotheoutput.ThehighvoltagewillactivatethegateofNPNtransistorsintheinputmodule(Q4).TheIIHisthecurrentintotheinputmoduleneededtoactivateallgatesconnectedtotheinput.TheactualcurrentIwillbebetween0andIIH.Forahighsignal,currentflowsfrom+3.3V,acrossthesource-drainofQ1,intothegateofQ4,andthentoground.AstheactualcurrentIincreases,theactualoutputvoltageVwilldrop.IOHisthemaximumoutputcurrentthatguaranteestheoutputvoltagewillbeaboveVOH.AssumingtheactualIislessthanIOH,theactualvoltageVwillbebetweenVOHand+3.3V.IftheinputvoltageisbetweenVIHand+3.3V,theinputsignalisconsideredhighbytheinput.Forthehighsignaltobetransferredproperly,VOHmustbelargerthanVIHandIOHmustbelargerthanIIH.

Figure1.19showsthemodelwhentheoutputislow.Tomaketheoutputlow,anNPN

transistorintheoutputmoduleisconducting(Q2)drivingtheoutputto0V.ThelowvoltagewillactivatethegateofPNPtransistorsintheinputmodule(Q3).TheIIListhecurrentoutoftheinputmoduleneededtoactivateallgatesconnectedtotheinput.TheactualcurrentIwillbebetween0andIIL.Foralowsignal,currentflowsfrom+3.3Vintheinputmodule,acrossthesource-gateofQ3,acrossthesource-draingateofQ2,andthentoground.AstheactualcurrentIincreases,theactualoutputvoltageVwillincrease.IOListhemaximumoutputcurrentthatguaranteestheoutputvoltagewillbelessthanVOL.AssumingtheactualIislessthanIOL,theactualvoltageVwillbebetween0andVOL.Iftheinputvoltageisbetween0andVIL,theinputsignalisconsideredlowbytheinput.Forthelowsignaltobetransferredproperly,VOLmustbelessthanVILandIOLmustbelargerthanIIL.

Figure1.18.Modelfortheinput/outputcharacteristicswhentheoutputishigh.

Figure1.19.Modelfortheinput/outputcharacteristicswhentheoutputislow.Opencollectorlogichasoutputswithtwostates:lowandoff.The74LS05isalow-powerSchottkyopencollectorNOTgate,asshowninFigure1.20.Whendrawinglogicdiagrams,weaddthe‘x’ontheoutputtospecifyopencollectorlogic.

The74HC05isahigh-speedCMOSopencollectorNOTgateisalsoshowninFigure1.20.ItiscalledopencollectorbecausethecollectorpinofQ2isnotconnected,orleftopen.Theoutputisoffwhenthereisnoactivetransistordrivingtheoutput.Inotherwords,whentheinputislow,theoutputfloats.This“notdriven”conditioniscalledtheopencollectorstate.Whentheinputishigh,theoutputwillbelow,causedbymakingthetransistorQ2isactivedrivingtheoutputto0.Technically,the74HC05implementsopendrainratherthanopencollector,becauseitisthedrainpinofQ2thatisleftopen.Inthisbook,wewillusethetermsopencollectorandopendraininterchangeablytorefertodigitallogicwithtwooutputstates(lowandoff).Becauseofthemultipleusesofopencollector,manymicrocontrollerscanimplementopencollectorlogic.OnLM3S/TM4Cmicrocontrollers,wecanaffectthismodebydefininganoutputasopendrain.

Figure1.20.TwotransistorimplementationsofanopencollectorNOTgate.Wecanuseabipolarjunctiontransistor(BJT)tosourceorsinkcurrent.Formostofthecircuitsinthisbookthetransistorsareusedinsaturatedmode.Ingeneral,wewilluseNPNtransistorstosinkcurrenttoground.WeturnonanNPNtransistorbyapplyingapositiveVbe.ThismeanswhentheNPNtransistorison,currentflowsfromthecollectortotheemitter.WhentheNPNtransistorisoff,nocurrentflowsfromthecollectortotheemitter.Eachtransistorhasaninputandoutputimpedance,hieandhoerespectively.Thecurrentgainishfeor .Thehybrid-pismallsignalmodelforthebipolarNPNtransistorisshowninFigure1.21.

Figure1.21.NPNtransistormodel.TherearefivebasicdesignruleswhenusingindividualbipolarNPNtransistorsinsaturatedmode:

1)NormallyVc>Ve

2)Currentcanonlyflowinthefollowingdirections

frombasetoemitter(inputcurrent)

fromcollectortoemitter(outputcurrent)

frombasetocollector(doesn’tusuallyhappen,butcouldifVb>Vc)

3)Eachtransistorhasmaximumvaluesforthefollowingtermsthatshouldnotbeexceeded

IbIcVceandIc•Vce4)Thetransistoractslikeacurrentamplifier

Ic=hfe•Ib5)ThetransistorwillactivateifVb>Ve+Vbe(SAT)whereVbe(SAT)istypicallyabove0.6V

Ingeneral,wewillusePNPtransistorstosourcecurrentfromapositivevoltagesupply.WeturnonaPNPtransistorbyapplyingapositiveVeb.ThismeanswhenthePNPtransistorison,currentflowsfromtheemittertothecollector.WhenthePNPtransistorisoff,nocurrentflowsfromtheemittertothecollector.Eachtransistorhasaninputandoutputimpedance,hieandhoerespectively.Thecurrentgainishfeor .Thehybrid-pismallsignalmodelforthebipolarPNPtransistorisshowninFigure1.22.

Figure1.22.PNPtransistormodel.

TherearefivebasicdesignruleswhenusingindividualbipolarPNPtransistorsinsaturatedmode:

1)NormallyVe>Vc2)Currentcanonlyflowinthefollowingdirections

fromemittertobase(inputcurrent)

fromemittertocollector(outputcurrent)

fromcollectortobase(doesn’tusuallyhappen,butcouldifVc>Vb)

3)Eachtransistorhasmaximumvaluesforthefollowingtermsthatshouldnotbeexceeded

IbIcVceandIc•Vce

4)Thetransistoractslikeacurrentamplifier

Ic=hfe•Ib5)ThetransistorwillactivateifVb<Ve-Vbe(SAT)whereVbe(SAT)istypicallyabove0.6V

PerformanceTip:Agoodtransistordesignisonethattheinput/outputresponseisindependentofhfe.WecandesignasaturatedmodecircuitsothatIbis2to5timesaslargeasneededtosupplythenecessaryIc.

TheTable1.6illustratesthewiderangeofbipolartransistorsthatwecanuse.

Type NPN PNP package Vbe(SAT) Vce(SAT) hfemin/max

Ic

generalpurpose

2N3904 2N3906 TO-92 0.85V 0.2V 100 10mA

generalpurpose

PN2222 PN2907 TO-92 1.2V 0.3V 100 150mA

generalpurpose

2N2222 2N2907 TO-18 1.2V 0.3V 100/300 500mA

powertransistor

TIP29A TIP30A TO-220 1.3V 0.7V 15/75 1A

powertransistor

TIP31A TIP32A TO-220 1.8V 1.2V 25/50 3A

powertransistor

TIP41A TIP42A TO-220 2.0V 1.5V 15/75 3A

powerdarlington

TIP120 TIP125 TO-220 2.5V 2.0V 1000min

3A

Table1.6.Parametersoftypicaltransistorsusedtosourceorsinkcurrent.

UndermostconditionsweplacearesistorinserieswiththebaseofaBJTwhenusingitasacurrentswitch.Thevalueofthisresistoristypically100 to10k .Thepurposeofthisbaseresistoristolimitthecurrentintothebase.Thehieistypicallyaround60 .ItyouconnectamicrocontrolleroutputportdirectlytothebaseofaBJT(withouttheresistor),thenwhentheoutputishighitwilltryandgenerateacurrentof3.3V/60 =50mA,potentiallydamagingthemicrocontroller.Ifthereisa1k resistorbetweenthemicrocontrollerandbaseoftheNPN,thentheVbevoltagegoestoVbeSAT(0.7)and(3.3-0.7)/1000 or2.6mA.

Ingeneral,wecanuseanopencollectorNOTgatetocontrolthecurrenttoadevice,suchasarelay,alightemittingdiode(LED),asolenoid,asmallmotor,andasmalllight.The74HC05canhandleupto4mA.The7405and7406canhandleupto16and40mArespectively,buttheymustbepoweredat+5V.Forcurrentsupto150mAwecanuseaPN2222transistor,asshowninFigure1.23,tocreatealow-costbuteffectivesolution.Whenoutputofthemicrocontrollerishigh,thetransistorison,makingitsoutputlow(VOLorVCE).Inthisstate,a10mAcurrentisappliedtothediode,anditlightsup.Butwhenoutputofthemicrocontrollerislow,thetransistorisoff,makingitsoutputfloat,whichisneitherhighnorlow.ThisfloatingoutputstatecausestheLEDcurrenttobezero,andthediodeisdark.TheresistorisselectedtosettheLEDcurrent.AssumetheVCEis0.3V,andthedesiredLEDoperatingpointis1.9V10mA.Inthiscase,thecorrectresistorvalueis(3.3-1.9-0.3V)/10mA=110 .

Figure1.23.Opencollectorusedtointerfacealightemittingdiode.Checkpoint1.17:WhatresistorvaluewouldyouchoosetooperatetheLEDat2V20mA?

Whenneededfordigitallogic,wecanconvertanopencollectoroutputtoadigitalsignalusingapull-upresistorfromtheoutputtoVCC.Inthisway,whentheopencollectoroutputfloats,thesignalwillbeadigitalhigh.Howdoweselectthevalueofthepull-upresistor?Ingeneralthesmallertheresistor,thelargertheIOHitwillbeabletosupplywhentheoutputishigh.Ontheotherhand,alargerresistordoesnotwasteasmuchIOLcurrentwhentheoutputislow.Onewaytocalculatethevalueofthispull-upresistoristofirstdeterminetherequiredoutputhighvoltage,Vout,andoutputhighcurrent,Iout.TosupplyacurrentofatleastIoutatavoltageaboveVout,theresistormustbelessthan:

R≤(VCC-Vout)/Iout

AsanexamplewewillcalculatetheresistorvalueforthesituationwherethecircuitneedstodrivefiveregularTTLloads,withVCCequalto5V.WeseefromFigure1.15thatVoutmustbeaboveVIH(2V)inorderfortheTTLinputstosenseahighlogiclevel.WecanaddasafetyfactorandsetVoutat3V.InorderforthehighoutputtodriveallfiveTTLinputs,IoutmustbemorethanfiveIIH.FromTable1.4,weseethatIIHis40µA,soIoutshouldbelargerthan5times40µAor0.2mA.Forthissituation,theresistormustbelessthan(5-3V)/0.2mA=10kΩ.

Anotherexampleofopencollectorlogicoccurswheninterfacingswitchestothemicrocontroller.ThecircuitontheleftofFigure1.24showsamechanicalswitchwithoneterminalconnectedtoground.Inthiscircuit,whentheswitchispressed,thevoltageriszero.Whentheswitchisnotprocessed,thesignalrfloats.

ThecircuitonthemiddleofFigure1.24showsthemechanicalswitchwitha10k pull-upresistorattachedtheotherside.Whentheswitchispressedthevoltageatsstillgoestozero,becausetheresistanceoftheswitch(lessthan0.1 )ismuchlessthanthepull-upresistor.Butnowwhentheswitchisnotpressedthepull-upresistorcreatesa+3.3Vats.Thiscircuitisshownconnectedtoaninputpinofthemicrocontroller.Thesoftware,byreadingtheinputport,candeterminewhetherornottheswitchispressed.Iftheswitchispressedthesoftwarewillreadzero,andiftheswitchisnotpressedthesoftwarewillreadone.Thismiddlecircuitiscallednegativelogicbecausetheactivestate,switchisbeingpressed,hasalowervoltagethantheinactivestate.

ThecircuitontherightofFigure1.24alsointerfacesamechanicalswitchtothemicrocontroller,butitimplementspositivelogicusingapull-downresistor.Thesignaltwillbehighiftheswitchispressedandlowifitisreleased.Thisrightcircuitiscalledpositivelogicbecausetheactivestate,switchisbeingpressed,hasahighervoltagethantheinactivestate.

Figure1.24.SinglePoleSingleThrow(SPST)Switchinterface.Observation:Wecanactivatepull-uporpull-downresistorsontheportsonmostmicrocontrollers,sotheinterfacesinFigure1.24canbemadewithouttheexternalresistor.

Earlierweusedavoltageandcurrentargumenttodeterminethevalueofthepull-upresistor.Inthissectionwepresentanothermethodonecouldusetoselectthisresistor.TheLM3S/TM4Cmicrocontrollershaveaninputcurrentof2 A.At3.3V,thisistheequivalentofaninputimpedanceofabout1M .Aswitchhasanon-resistanceoflessthan1 .Wewanttheresistortobesmallwhencomparedto1M ,butlargecomparedto1 .The10k pull-upresistoris100timessmallerthantheinputimpedanceand10,000timeslargerthantheswitchresistance.Theinternalpull-upmodeonLM3Smicrocontrollersrangesfromanequivalentof50to110k .Theinternalpull-downmode

rangesfromanequivalentof55to180k .FortheTM4C,theinternalpull-upresistorrangesfrom13to30k ,andtheinternalpull-downresistorrangesfrom13to35k .

1.5.DigitalRepresentationofNumbers

1.5.1.FundamentalsInformationisstoredonthecomputerinbinaryform.Abinarybitcanexistinoneoftwopossiblestates.Inpositivelogic,thepresenceofavoltageiscalledthe‘1’,true,asserted,orhighstate.Theabsenceofavoltageiscalledthe‘0’,false,notasserted,orlowstate.Converselyinnegativelogic,thetruestatehasalowervoltagethanthefalsestate.Figure1.25showstheoutputofatypicalcomplementarymetaloxidesemiconductor(CMOS)circuit.Theleftsideshowstheconditionwithatruebit,andtherightsideshowsafalse.Theoutputofeachdigitalcircuitconsistsofap-typetransistor“ontopof”ann-typetransistor.Indigitalcircuits,eachtransistorisessentiallyonoroff.Ifthetransistorison,itisequivalenttoashortcircuitbetweenitstwooutputpins.Conversely,ifthetransistorisoff,itisequivalenttoanopencircuitbetweenitsoutputspins.OnLM3S/TM4Cmicrocontrollerspoweredwith3.3Vsupply,avoltagebetween2and5Visconsideredhigh,andavoltagebetween0and1.3Visconsideredlow.Separatingthetworegionsby0.7Vallowsdigitallogictooperatereliablyatveryhighspeeds.Thedesignoftransistor-leveldigitalcircuitsisbeyondthescopeofthisbook.However,itisimportanttoknowthatdigitaldataexistasbinarybitsandencodedashighandlowvoltages.

Figure1.25.Abinarybitistrueifavoltageispresentandfalseifthevoltageis0.

Numbersarestoredonthecomputerinbinaryform.Inotherwords,informationisencodedasasequenceof1’sand0’s.Onmostcomputers,thememoryisorganizedinto8-bitbytes.Thismeanseach8-bitbytestoredinmemorywillhaveaseparateaddress.Precisionisthenumberofdistinctordifferentvalues.Weexpressprecisioninalternatives,decimaldigits,bytes,orbinarybits.AlternativesaredefinedasthetotalnumberofpossibilitiesaslistedinTable1.7.Lettheoperation[[x]]bethegreatestintegerofx.E.g.,[[2.1]]isroundedupto3.Forexample,an8-bitnumberschemecanrepresent256differentnumbers,whichmeans256alternatives.An8-bitdigitaltoanalogconverter(DAC)cangenerate256differentanalogoutputs.An8-bitanalogtodigitalconverter(ADC)canmeasure256differentanaloginputs.

Binarybits Bytes Alternatives

8 1 256

10 1024

12 4096

16 2 65,536

20 1,048,576

24 3 16,777,216

30 1,073,741,824

32 4 4,294,967,296

n [[n/8]] 2n

Table1.7.Relationshipbetweenbits,bytesandalternativesasunitsofprecision.

Observation:Agoodruleofthumbtorememberis210•nisapproximately103•n.

Decimaldigitsareusedtospecifyprecisionofmeasurementsystemsthatdisplayresultsasnumericalvalues,asdefinedinTable1.8.Afulldecimaldigitcanbeanyvalue0,1,2,3,4,5,6,7,8,or9.Adigitthatcanbeeither0or1isdefinedasa½decimaldigit.Theterminologyofa½decimaldigitdidnotarisefromamathematicalperspectiveofprecision,butratheritarosefromthephysicalwidthoftheLED/LCDmoduleusedtodisplayablankor’1’ascomparedtothewidthofafulldigit.Similarly,wedefineadigitthatcanbe+or-alsoasahalfdecimaldigit,becauseithastwochoices.Adigitthatcanbe0,1,2,3isdefinedasa¾decimaldigit,becauseitiswiderthana½digitbutnarrowerthanafulldigit.Wealsodefineadigitthatcanbe-1,-0,+0,or+1asa¾decimaldigit,becauseitalsohasfourchoices.Weusetheexpression4½decimaldigitstomean20,000alternativesandtheexpression4¾decimaldigitstomean40,000alternatives.Theuseofa½decimaldigittomeantwicethenumberofalternativesoroneadditionalbinarybitiswidelyaccepted.Ontheotherhand,theuseof¾decimaldigittomeanfourtimesthenumberofalternativesortwoadditionalbinarybitsisnotascommonlyaccepted.Forexample,considerthetwoohmmeters.Assumebotharesettothe0to200k range.A3½digitohmmeterhasaresolutionof0.1k withmeasurementsrangingfrom0.0to199.9k .Ontheotherhand,a4½digitohmmeterhasaresolutionof0.01k withmeasurementsrangingfrom0.00to199.99k .Table1.8illustratesdecimal-digitrepresentationofprecision.

Decimaldigits Alternatives

3 1000

3½ 2000

3¾ 4000

4 10,000

4½ 20,000

4¾ 40,000

5 100,000

n 10n

Table1.8.Definitionofdecimaldigitsasaunitofprecision.

Checkpoint1.18:Howmanybinarybitscorrespondto2½decimaldigits?

Checkpoint1.19:Howmanydecimaldigitscorrespondto10binarybits?

Checkpoint1.20:Howmanybinarybitscorrespondto6½decimaldigits?

Checkpoint1.21:Abouthowmanydecimaldigitscanbepresentedina64-bit8-bytenumber?Youcananswerthiswithoutacalculator,justusingthe“ruleofthumb”.

Thehexadecimalnumbersystemusesbase16asopposedtoourregulardecimalnumbersystemthatusesbase10.Hexadecimalisaconvenientmechanismforhumanstorepresentbinaryinformation,becauseitisextremelysimpleforustoconvertbackandforthbetweenbinaryandhexadecimal.Hexadecimalnumbersystemisoftenabbreviatedas“hex”.Anibbleisdefinedasfourbinarybits,whichwillbeonehexadecimaldigit.Inmathematics,asubscriptof2meansbinary,butinthisbookwewilldefinebinarynumbersbeginningwith%.Inassemblylanguagehowever,wewillusehexadecimalformatwhenweneedtodefinebinarynumbers.Thehexadecimaldigitsare0,1,2,3,4,5,6,7,8,9,A,B,C,D,E,andF.Someassemblylanguagesusetheprefix$tosignifyhexadecimal,andinCweusetheprefix0x.Toconvertfrombinarytohexadecimal,yousimplyseparatethebinarynumberintogroupsoffourbinarybits(startingontheright),thenconverteachgroupoffourbitsintoonehexadecimaldigit.Forexample,ifyouwishedtoconvert101001112,firstyouwouldgroupitintonibbles10100111,thenyouwouldconverteachgroup1010=Aand0111=7,yieldingtheresultof0xA7.Toconverthexadecimaltobinary,yousimplysubstitutethe4-bitbinaryforeachhexadecimaldigit.Forexample,ifyouwishedtoconvert0xB5D1,yousubstituteB=1011,5=0101,D=1101,and1=0001,yieldingtheresultof10110101110100012.

Checkpoint1.22:Convertthebinarynumber1110111010112tohexadecimal.

Checkpoint1.23:Convertthehexnumber0x3800tobinary.

Checkpoint1.24:Howmanybinarybitsdoesittaketorepresent0x12345?

Agreatdealofconfusionexistsovertheabbreviationsweuseforlargenumbers.In1998theInternationalElectrotechnicalCommission(IEC)definedanewsetofabbreviationsforthepowersof2,asshowninTable1.9.ThesenewtermsareendorsedbytheInstituteofElectricalandElectronicsEngineers(IEEE)andInternationalCommitteeforWeightsandMeasures(CIPM)insituationswheretheuseofabinaryprefixisappropriate.Theconfusionarisesoverthefactthatthemainstreamcomputerindustry,suchasMicrosoft,Apple,andDell,continuestousetheoldterminology.Accordingtothecompaniesthatmarkettoconsumers,a1GHzis1,000,000,000Hzbut1Gbyteofmemoryis1,073,741,824bytes.ThecorrectterminologyistousetheSI-decimalabbreviationstorepresentpowersof10,andtheIEC-binaryabbreviationstorepresentpowersof2.Thescientificmeaningof2kilovoltsis2000volts,but2kibibytesistheproperwaytospecify2048bytes.Thetermkibibyteisacontractionofkilobinarybyteandisaunitofinformationorcomputerstorage,abbreviatedKiB.

1KiB=210bytes=1024bytes

1MiB=220bytes=1,048,576bytes

1GiB=230bytes=1,073,741,824bytes

Theseabbreviationscanalsobeusedtospecifythenumberofbinarybits.Thetermkibibitisacontractionofkilobinarybit,andisaunitofinformationorcomputerstorage,abbreviatedKibit.

1Kibit=210bits=1024bits

1Mibit=220bits=1,048,576bits

1Gibit=230bits=1,073,741,824bits

Amebibyte(1MiBis1,048,576bytes)isapproximatelyequaltoamegabyte(1MBis1,000,000bytes),butmistakingthetwohasnonethelessledtoconfusionandevenlegaldisputes.Intheengineeringcommunity,itisappropriatetousetermsthathaveaclearandunambiguousmeaning.

Value SIDecimal

SIDecimal

Value IECBinary

IECBinary

10001 k kilo- 10241 Ki kibi-

10002 M mega- 10242 Mi mebi-

10003 G giga- 10243 Gi gibi-

10004 T tera- 10244 Ti tebi-

10005 P peta- 10245 Pi pebi-

10006 E exa- 10246 Ei exbi-

10007 Z zetta- 10247 Zi zebi-

10008 Y yotta- 10248 Yi yobi-

Table1.9.Commonabbreviationsforlargenumbers.

1.5.2.8-bitnumbersAbytecontains8bitsasshowninFigure1.26,whereeachbitb7,…,b0isbinaryandhasthevalue1or0.Wespecifyb7asthemostsignificantbitorMSB,andb0astheleastsignificantbitorLSB.InC,theunsignedcharoruint8_tdatatypecreatesanunsigned8-bitnumber.

Figure1.26.8-bitbinaryformat,createdusingeithercharorunsignedchar(inC99int8_toruint8_t).Ifabyteisusedtorepresentanunsignednumber,thenthevalueofthenumberis

N=128•b7+64•b6+32•b5+16•b4+8•b3+4•b2+2•b1+b0Noticethatthesignificanceofbitnis2n.Thereare256differentunsigned8-bitnumbers.Thesmallestunsigned8-bitnumberis0andthelargestis255.Forexample,100001002is128+4or132.

Checkpoint1.25:Convertthebinarynumber011010012tounsigneddecimal.

Checkpoint1.26:Convertthehexnumber0x23tounsigneddecimal.

Thebasisofanumbersystemisasubsetfromwhichlinearcombinationsofthebasiselementscanbeusedtoconstructtheentireset.Thebasisrepresentsthe“places”ina“place-value”system.Forpositiveintegers,thebasisistheinfiniteset1,10,100…andthe“values”canrangefrom0to9.Eachpositiveintegerhasauniquesetofvaluessuchthatthedot-productofthevalue-vectortimesthebasis-vectoryieldsthatnumber.Forexample,2345is(…,2,3,4,5)•(…,1000,100,10,1),whichis2*1000+3*100+4*10+5.Fortheunsigned8-bitnumbersystem,thebasisis

1,2,4,8,16,32,64,128

Thevaluesofabinarynumbersystemcanonlybe0or1.Evenso,each8-bitunsignedintegerhasauniquesetofvaluessuchthatthedot-productofthevaluestimesthebasisyieldsthatnumber.Forexample,69is(0,1,0,0,0,1,0,1)•(128,64,32,16,8,4,2,1),whichequals0*128+1*64+0*32+0*16+0*8+1*4+0*2+1*1.

Checkpoint1.27:Givetherepresentationsofdecimal37in8-bitbinaryandhexadecimal.

Checkpoint1.28:Givetherepresentationsofdecimal202in8-bitbinaryandhexadecimal.

Oneofthefirstschemestorepresentsignednumberswascalledone’scomplement.Itwascalledone’scomplementbecausetonegateanumber,youcomplement(logicalnot)eachbit.Forexample,if25equals00011001inbinary,then–25is11100110.An8-bitone’scomplementnumbercanvaryfrom127to+127.Themostsignificantbitisasignbit,whichis1ifandonlyifthenumberisnegative.Thedifficultywiththisformatisthattherearetwozeros+0is00000000,and–0is11111111.Anotherproblemisthatonescomplementnumbersdonothavebasiselements.Theselimitationsledtotheuseoftwo’scomplement.

InC,thecharorint8_tdatatypecreatesasigned8-bitnumber.Thetwo’scomplementnumbersystemisthemostcommonapproachusedtodefinesignednumbers.Itwascalledtwo’scomplementbecausetonegateanumber,youcomplementeachbit(likeone’scomplement),andthenadd1.Forexample,if25equals00011001inbinary,then‑25is11100111.Ifabyteisusedtorepresentasignedtwo’scomplementnumber,thenthevalueis

N=-128•b7+64•b6+32•b5+16•b4+8•b3+4•b2+2•b1+b0Thereare256differentsigned8-bitnumbers.Thesmallestsigned8-bitnumberis-128andthelargestis127.Forexample,100000102equals-128+2or-126.

Checkpoint1.29:Arethesignedandunsigneddecimalrepresentationsofthe8-bithexnumber0x35thesameordifferent?

Forthesigned8-bitnumbersystemthebasisis

1,2,4,8,16,32,64,-128

Themostsignificantbitinatwo’scomplementsignednumberwillspecifythesign.Anerrorwilloccurifyouusesignedoperationsonunsignednumbers,oruseunsignedoperationsonsignednumbers.Toimprovetheclarityofoursoftware,alwaysspecifytheformatofyourdata(signedversusunsigned)whendefiningoraccessingthedata.

Checkpoint1.30:Givetherepresentationsof-31in8-bitbinaryandhexadecimal.

Observation:Totakethenegativeofatwo’scomplementsignednumber,wefirstcomplement(flip)allthebits,thenadd1.

Manybeginningstudentsconfuseasignednumberwithanegativenumber.Asignednumberisonethatcanbeeitherpositiveornegative.Anegativenumberisonelessthanzero.Noticethatthesamebinarypatternof111111112couldrepresenteither255or-1.Itisveryimportantforthesoftwaredevelopertokeeptrackofthenumberformat.Thecomputercannotdeterminewhetherthe8-bitnumberissignedorunsigned.You,astheprogrammer,willdeterminewhetherthenumberissignedorunsignedbythespecificassemblyinstructionsyouselecttooperateonthenumber.Someoperationslikeaddition,subtraction,andshiftleft(multiplyby2)usethesamehardware(instructions)forbothunsignedandsignedoperations.Ontheotherhand,multiply,divide,andshiftright(divideby2)requireseparatehardware(instruction)forunsignedandsignedoperations.

1.5.3.CharacterinformationWecanusebytestorepresentcharacterswiththeAmericanStandardCodeforInformationInterchange(ASCII)code.StandardASCIIisactuallyonly7bits,butisstoredusing8-bitbyteswiththemostsignificantbyteequalto0.Somecomputersystemsusethe8thbitoftheASCIIcodetodefineadditionalcharacterssuchasgraphicsandlettersinotheralphabets.The7-bitASCIIcodedefinitionsaregivenintheTable1.10.Forexample,theletter‘V’isinthe0x50rowandthe6column.Puttingthetwotogetheryieldshexadecimal0x56.TheNULcharacterhasthevalue0andisusedtoterminatestrings.The‘0’characterhasvalue0x30andrepresentsthezerodigit.InC,weusethechardatatypeforcharacters.

BITS4to6

0 1 2 3 4 5 6 7

0 NUL DLE SP 0 @ P ` p

B 1 SOH XON ! 1 A Q a q

I 2 STX DC2 “ 2 B R b r

T 3 ETX XOFF # 3 C S c s

S 4 EOT DC4 $ 4 D T d t

5 ENQNAK % 5 E U e u

0 6 ACKSYN & 6 F V f v

7 BEL ETB ‘ 7 G W g w

T 8 BS CAN ( 8 H X h x

O 9 HT EM ) 9 I Y i y

A LF SUB * : J Z j z

3 B VT ESC + ; K [ k

C FF FS , < L \ l |

D CR GS - = M ] m

E SO RS . > N ^ n ~

F SI US / ? O _ o DEL

Table1.10.Standard7-bitASCII.

Checkpoint1.31:Howisthecharacter‘0’representedinASCII?

Onewaytoencodeacharacterstringistousenull-termination.Inthisway,thecharactersofthestringarestoredonerightaftertheother,andtheendofthestringissignifiedbytheNULcharacter(0).Forexample,thestring“Valvano”isencodedasthefollowingeightbytes0x56,0x61,0x6C,0x76,0x61,0x6E,0x6F,and0x00.

Checkpoint1.32:Howis“HelloWorld”encodedasanull-terminatedASCIIstring?

1.5.4.16-bitnumbersAhalfwordorshortcontains16bits,whereeachbitb15,…,b0isbinaryandhasthevalue1or0,asshowninFigure1.27.Whenwestore16-bitdataintomemoryitrequirestwobytes.Thememorysystemsonmostcomputersarebyteaddressable,whichmeansthereisauniqueaddressforeachbyte.Therefore,therearetwopossiblewaystostoreinmemorythetwobytesthatconstitutethe16-bitdata.Datacouldbestoredineitherlittle-endianorbig-endianformat.Littleendianmeanstheleastsignificantbyteisattheloweraddressandthemostsignificantbyteisatthehigheraddress.Bigendianmeansthemostsignificantbyteisattheloweraddressandtheleastsignificantbyteisatthehigheraddress.Freescalemicrocontrollersimplementthebig-endianformat.Intelcomputersimplementthelittle-endianformat.Someprocessors,liketheARMandthePowerPCarebiendian,becausetheycanbeconfiguredtoefficientlyhandlebothbigandlittleendian.Bit15oftheApplicationInterruptandResetControl(APINT)registerontheARM ®CortexTM-Mspecifieslittle-endian(0)orbig-endian(1)dataaccess.TheStellaris ® andTiva ® microcontrollers(LM3S/TM4C),however,onlyuselittle-endianmode.Whencommunicatingdatabetweencomputersonemustknowtheformatused.

Figure1.27.Ahalfwordisa16-bitbinarynumber.InC99int16_toruint16_t.Ifahalfwordisusedtorepresentanunsignednumber,definedasanunsignedshortoruint16_t,thenthevalueofthenumberis

N=32768•b15+16384•b14+8192•b13+4096•b12+2048•b11+1024•b10+512•b9+256•b8+128•b7+64•b6+32•b5+16•b4+8•b3+4•b2+2•b1+b0

Thereare65536differentunsigned16-bitnumbers.Thesmallestunsigned16-bitnumberis0andthelargestis65535.Forexample,00100001100001002or0x2184is8192+256+128+4or8580.Fortheunsigned16-bitnumbersystemthebasisis

1,2,4,8,16,32,64,128,256,512,1024,2048,4096,8192,16384,32768

Therearealso65536differentsigned16-bitnumbers,definedeitherasshort,signedshort,orint16_t.Thesmallesttwo’scomplementsigned16-bitnumberis-32768andthelargestis32767.Forexample,11010000000001002or0xD004is32768+16384+4096+4or12284.Ifahalfwordisusedtorepresentasignedtwo’scomplementnumber,thenthevalueis

N=-32768•b15+16384•b14+8192•b13+4096•b12+2048•b11+1024•b10+512•b9+256•b8+128•b7+64•b6+32•b5+16•b4+8•b3+4•b2+2•b1+b0

Anerrorwilloccurifyouuse16-bitoperationson8-bitnumbers,oruse8-bitoperationson16-bitnumbers.Toimprovetheclarityofyoursoftware,alwaysspecifytheprecisionofyourdatawhendefiningoraccessingthedata.Forthesigned16-bitnumbersystemthebasisis

1,2,4,8,16,32,64,128,256,512,1024,2048,4096,8192,16384,-32768

1.5.5.32-bitnumbersThenativenumberontheARMisa32-bitword,whereeachbitb31,…,b0isbinaryandhasthevalue1or0,asshowninFigure1.28,whichisstoredinlittle-endianformat.

Figure1.28.Awordis32-bitnumber.InC99int32_toruint32_t.Wedefineunsignedlongoruint32_ttobeanunsigned32-bitnumber.Thevalueis

N=231•b31+230•b30+…+22•b2+21•b1+20b0

Thereare4,294,967,296(232)differentunsigned32-bitnumbers.Thesmallestunsigned32-bitnumberis0andthelargestis4,294,967,295(232).Fortheunsigned32-bitnumbersystemthebasisis

20,21,22,…,230,231

Wedefinelong,signedlong,orint32_ttobeasigned32-bitnumber.Thevalueis

N=-231•b31+230•b30+…+4•b2+2•b1+b0

Thereare4,294,967,296(232)differentsigned32-bitnumbers.Thesmallestunsigned32-bitnumberis-2,147,483,648(-231)andthelargestis+2,147,483,647(231-1).Forthesigned32-bitnumbersystemthebasisis

20,21,22,…,230,-231

Whendealingwith16or32-bitnumberswenormallywouldnotpickoutindividualbytes,butrathercapturetheentiremultiple-bytedataasonenon-divisiblepieceofinformation.Ontheotherhand,ifeachbyteinamultiple-bytedatastructureisindividuallyaddressable,thenboththebig-andlittle-endianschemesstorethedatainfirsttolastsequence.Forexample,assumewewishtostorethefourASCIIcharacters“LM3S”asastring.Thesefivebytes,whichare0x4C4D335300,wouldexistatfivelocationsinmemory.Thefirstletter,theASCII‘L’=0x4Cwouldbestoredinfirstlocation,regardlessofwhichendianformatthecomputeruses.

Theterms“bigandlittleendian”comesfromJonathanSwift’ssatireGulliver’sTravels.InSwift’sbook,thelittlepeopleofBlefuscubelievedthecorrectwaytocrackaneggisonthebigend;hencetheywerecalledBig-Endians.Intherivalkingdom,thelittlepeopleofLilliputwerecalledLittle-Endiansbecausetheyinsistedthattheonlyproperwayistobreakaneggonthelittleend.TheLilliputiansconsideredthepeopleofBlefuscuasinferiors.TheBig-andLittle-Endiansfoughtalongandsenselesswaroverwhichendisbesttocrackanegg.LilliputandBlefuscuweresatiricalreferencesto18thcenturyGreatBritainandFrance.However,onemightarguetheyalsorefertoIntelandMotoroladuringthe1980’s.

1.5.6.Fixed-pointnumbersWewillusefixed-pointnumberswhenwewishtoexpressvaluesinourcomputerthathavenonintegervalues.Afixed-pointnumbercontainstwoparts.Thefirstpartisavariableinteger,calledI.Thesecondpartofafixed-pointnumberisafixedconstant,calledtheresolution.Theintegermaybesignedorunsigned.Anunsignedfixed-pointnumberisonethathasanunsignedvariableinteger.Asignedfixed-pointnumberisonethathasasignedvariableinteger.Theprecisionofanumbersystemisthetotalnumberofdistinguishablevaluesthatcanberepresented.Theprecisionofafixed-pointnumberisdeterminedbythenumberofbitsusedtostorethevariableinteger.OntheARM,wecanuse8,16or32bitsfortheinteger.Extendedprecisionwithmorethe32bitscanbeimplemented,buttheexecutionspeedwillbeslowerbecausethecalculationswillhavetobeperformedusingsoftwarealgorithmsratherthanwithhardwareinstructions.Thisintegerpartissavedinmemoryandismanipulatedbysoftware.Thesemanipulationsincludebutarenotlimitedtoadd,subtract,multiply,divide,andsquareroot.Theresolutionisfixed,andcannotbechangedduringexecutionoftheprogram.Theresolutionisnotstoredinmemory.Usuallywespecifythevalueoftheresolutionusingsoftwarecommentstoexplainourfixed-pointalgorithm.Thevalueofthefixed-pointnumberisdefinedastheproductofthevariableintegerandthefixedconstant:

Fixed-pointvalue≡I•Δ

Observation:Iftherangeofnumbersisknownandsmall,thenthenumberscanberepresentedinafixed-pointformat.

Wespecifytherangeofafixed-pointnumbersystembygivingthesmallestandlargestpossiblevalue.Therangedependsonboththevariableintegerandthefixedconstant.Forexample,ifthesystemuseda16-bitunsignedvariable,thentheintegerpartcanvaryfrom0to65535.Therefore,therangeofanunsigned16-bitfixed-pointsystemis0to65535• .Ingeneral,therangeofthefixed-pointsystemis

Smallestfixed-pointvalue=Imin•,whereIministhesmallestintegervalue

Largestfixed-pointvalue=Imax•,whereImaxisthelargestintegervalue

Checkpoint1.33:Whatistherangeofa16-bitsignedfixed-pointnumberwith =0.001?

Wheninteractingwithahumanoperator,itisusuallyconvenienttousedecimalfixed-point.Withdecimalfixed-pointthefixedconstantisapowerof10.

Decimalfixed-pointvalueI•10mforsomeconstantintegerm

Again,themisfixedandisnotstoredinmemory.Decimalfixed-pointwillbeeasytodisplay,whilebinaryfixed-pointwillbeeasiertousewhenperformingmathematicalcalculations.TheARMprocessorisveryefficientperformingleftandrightshifts.Withbinaryfixed-pointthefixedconstantisapowerof2.AnexampleisshowninFigure1.29.

Binaryfixed-pointvalueI•2nforsomeconstantintegern

Figure1.29.16-bitbinaryfixed-pointformatwith=2-6.Itisgoodpracticetoexpressthefixed-pointresolutionwithunits.Forexample,adecimalfixed-pointnumberwitharesolutionof0.001VisreallythesamethingasanintegerwithunitsofmV.Considerananalogtodigitalconverter(ADC)thatconvertsananalogvoltageintherangeof0to+5Vintoadigitalnumberbetween0and255.ThisADChasaprecisionof8bitsbecauseithas256distinctalternatives.ADCresolutionisdefinedasthesmallestdifferenceininputvoltagethatcanbereliablydistinguished.Becausethe256alternativesarespreadevenlyacrossthe0to+5Vrange,weexpecttheADCresolutiontobeabout5V/256or0.02V.Whenwechooseafixed-pointnumbersystemtorepresentthevoltageswemustsatisfytwoconstraints.First,wewanttheresolutionofthenumberformattobebetterthantheADCresolution( <0.02).Second,wewanttherangeofthenumbersystemtoencompassallofthevoltagesintherangeoftheADC(65535• >5).Itwouldbeappropriatetostorevoltagesas16-bitunsigneddecimalfixed-pointnumberswitharesolutionof0.01V,0.001V,or0.0001V.

Using =0.01V,westore4.23Vbymakingtheintegerpartequalto423.Ifwewishedtousebinaryfixed-point,thenwecouldchoosearesolutionanywhereintherangeof2-6to2-13V.Ingeneral,wewanttochoosethelargestresolutionthatsatisfiesbothconstraints,sotheintegerpartshavesmallervalues.Smallernumbersarelesslikelytocauseoverflowduringcalculations.Morediscussionofoverflowwillbepresentedinthenextchapter.

Checkpoint1.34:Giveanapproximationof usingthedecimalfixed-pointwith =0.001.

Checkpoint1.35:Giveanapproximationof usingthebinaryfixed-pointwith =2-8.

MicrocontrollersintheLM3Sfamilyhavea10-bitADCandarangeof0to+3V.MicrocontrollersintheTM4Cfamilyprovidea12-bitADCandarangeof0to+3.3V.Witha12-bitADC,theresolutionis3.3V/4096orabout0.001V.Itwouldbeappropriatetostorevoltagesas16-bitunsignedfixed-pointnumberswitharesolutionofeither10-3or2-10V.LetVinbetheanalogvoltageinvoltsandNbetheintegerADCoutput,thentheanalogtodigitalconversionisapproximately

N=4096*Vin/3.3

Assumeweuseafixed-pointresolutionof10-3V.Weusethisequationtocalculatetheintegerpartofafixed-pointnumbergiventheADCresultN.Thedefinitionofthefixed-pointis

Vin=I•10-3

Combiningtheabovetwoequationsyields

I=(3300*N)/4096

Itisveryimportanttocarefullyconsidertheorderofoperationswhenperformingmultipleintegercalculations.Therearetwomistakesthatcanhappenwhenwecalculate3300*N/1024.Thefirsterrorisoverflow,anditiseasytodetect.Overflowoccurswhentheresultofacalculationexceedstherangeofthenumbersystem.Inthisexample,ifthemultiplyisimplementedas16-bitoperation,then3000*Ncanoverflowthe0to65535range.Onesolutionoftheoverflowproblemispromotion.Promotionistheactionofincreasingtheinputstoahigherprecision,performingthecalculationatthehigherprecision,checkingforoverflow,thendemotingtheresultbacktothelowerprecision.Inthisexample,the3300,N,and4096areallconvertedto32-bitunsignednumbers.(3300*N)/4096iscalculatedin32-bitprecision.BecauseweknowtherangeofNis0to4095,weknowthecalculationofIwillyieldnumbersbetween0and3300,andthereforeitwillfitbackina16-bitvariableduringdemotion.Theothererroriscalleddrop-out.Drop-outoccursduringarightshiftoradivide,andtheconsequenceisthatanintermediateresultloosesitsabilitytorepresentallofthevalues.Toavoiddrop-out,itisveryimportanttodividelastwhenperformingmultipleintegercalculations.Ifwedividedfirst,e.g.,I=3300*(N/4096),thenthevaluesofIwouldalwaysbe0.WecouldhavecalculatedI=(3300*N+2048)/4096toimplementroundingtotheclosestinteger.Thevalue2048isselectedbecauseitisaboutonehalfofthedenominator.Sometimeswecansimplifythenumbersinanattempttopreventoverflow.InthiscausewecouldhavecalculatedI=(825*N+256)/1024.However,thisformulationcouldstilloverflowandrequirespromotionto32bitstooperatecorrectly.

Whenaddingorsubtractingtwofixed-pointnumberswiththesame ,wesimplyaddorsubtracttheirintegerparts.First,letx,y,andzbethreefixed-pointnumberswiththesame .Letx=I• ,y=J• ,andz=K• .Toperformz=x+y,wesimplycalculateK=I+J.Similarly,toperformz=x-y,wesimplycalculateK=I-J.

Whenaddingorsubtractingfixed-pointnumberswithdifferentfixedparts,thenwemustfirstconvertthetwoinputstotheformatoftheresultbeforeaddingorsubtracting.Thisiswherebinaryfixed-pointismoreefficient,becausetheconversionprocessinvolvesshiftingratherthanmultiplication/division.ManyinstructionsontheARMallowadatashiftoperationtobeperformedatnoaddedexecutiontime.

Formultiplication,wehavez=x•y.Again,wesubstitutethedefinitionsofeachfixed-pointparameter,andsolvefortheintegerpartoftheresult.Ifallthreevariableshavethesameresolution,thenz=x•ybecomesK• =I• •J• yieldingK=I•J• .Ifthethreevariableshavedifferentresolutions,suchasx=I•2n,y=J•2m,andz=K•2p,thenz=x•ybecomesK•2p=I•2n•J•2myieldingK=I•J•2n+m-p.

Fordivision,wehavez=x/y.Again,wesubstitutethedefinitionsofeachfixed-pointparameter,andsolvefortheintegerpartoftheresult.Ifallthreevariableshavethesameresolution,thenz=x/ybecomesK• =(I• )/(J• )yieldingK=I/J/ .Ifthethreevariableshavedifferentresolutions,suchasx=I•2n,y=J•2m,andz=K•2p,thenz=x/ybecomesK•2p=(I•2n)/(J•2m)yieldingK=(I/J)•2n-m-p.Again,itisveryimportanttocarefullyconsidertheorderofoperationswhenperformingmultipleintegercalculations.Wemustworryaboutoverflowanddrop-out.If(n-m-p)ispositivethentheleftshift(I•2n-m-p)shouldbeperformedbeforethedivide(/J).Conversely,if(n-m-p)isnegativethentherightshiftshouldbeperformedafterthedivide(/J).

Wecanapproximateanon-integerconstantasthequotientoftwointegers.Forexample,thedifferencebetween41/29and√2is0.00042.Ifweneedamoreaccuraterepresentation,wecanincreasethesizeoftheintegers;thedifferencebetween239/169and√2isonly1.2E-05.Usingabinaryfixed-pointapproximationwillbefasterontheARMbecauseoftheefficiencyoftheshiftoperation.Forexample,approximating√2as181/128yieldsanerrorof0.0002.Furthermore,approximating√2as11585/8192yieldsanerrorofonly2.9E-05.

Observation:Formostrealnumbersintherangeof0.5to2,wecanfindtwo3-digitintegersIandJsuchthatthedifferencebetweentheapproximationI/Jandtruthislessthan1E-5.

Checkpoint1.36:Whatistheerrorinapproximating√5by161/72?By682/305?

Wecanusefixed-pointnumberstoperformcomplexoperationsusingtheintegerfunctionsofourmicrocontroller.Forexample,considerthefollowingdigitalfiltercalculation.

y=x-0.0532672•x1+x2+0.0506038•y1-0.9025•y2

Inthiscase,thevariablesy,y1,y2,x,x1,andx2areallintegers,buttheconstantswillbeexpressedinbinaryfixed-pointformat.Thevalue-0.0532672willbeapproximatedby-14•2-8.Thevalue0.0506038willbeapproximatedby13•2-8.Lastly,thevalue-0.9025willbeapproximatedby-231•2-8.Thefixed-pointimplementationofthisdigitalfilteris

y=x+x2+(-14•x1+13•y1-231•y2)>>8

CommonError:Lazyorincompetentprogrammersusefloating-pointinmanysituationswherefixed-pointwouldbepreferable.

Observation:Asthefixedconstantismadesmaller,theaccuracyofthefixed-pointrepresentationisimproved,butthevariableintegerpartalsoincreases.Unfortunately,largerintegerswillrequiremorebitsforstorageandcalculations.

Checkpoint1.37:Usingafixedconstantof10-3,rewritethedigitalfiltery=x+0.0532672•x1+x2+0.0506038•y1-0.9025•y2indecimalfixed-pointformat.

Example1.5:Implementafunctiontocalculatethesurfaceareaofacylinderusingfixed-pointcalculations.risradiusofthecylinder,whichcanvaryfrom0to1cm.Theradiusisstoredasafixed-pointnumberwithresolution0.001cm.Thesoftwarevariablecontainingtheintegerpartoftheradiusis n ,whichcanvaryfrom0to1000.Theheightofthecylinderis1cm.Thesurfaceareaisapproximatedby

s=2 *(r2+r)

Solution:Thesurfaceareacanrangefrom0to12.566cm2(2 *(12+1)).Thesurfaceareaisstoredasafixed-pointnumberwithresolution0.001cm2.Thesoftwarevariablecontainingtheintegerpartofthesurfaceareais m ,whichcanvaryfrom0to12566.Inordertobetterunderstandtheproblem,wemakeatableofexpectedresults.

r n s m

0.000 0 0.000 0

0.001 1 0.006 6

0.010 10 0.063 63

0.100 100 0.691 691

1.000 1000 12.566 12566

Tosolvethisproblemweusethedefinitionofafixed-pointnumber.Inthiscase,risequalto n /1000andsisequaltom/1000.Wesubstitutionthesedefinitionsintothedesiredequation.

s=(6.283)*(r2+r)

m /1000=6.283*(( n /1000)2+( n /1000))

m =6.283*( n 2/1000+ n )

m =6283*( n 2+1000* n )/1000000

m =(6283*( n +1000)* n )/1000000

Ifwewishtoroundtheresulttotheclosestintegerwecanadd½thedivisorbeforedividing.

m =(6283*( n +1000)* n +500000)/1000000

Oneoftheproblemswiththisequationistheintermediateresultcanoverflowa32-bitcalculation.Onewaytoremovetheoverflowistoapproximate2 by6.28.However,thisintroduceserror.Abetterwaytoeliminateoverflowistoapproximate2 by289/46.

m =(289*( n +1000)* n +23000)/46000

Ifweset n toitslargestvalue, n =1000,wecalculatethelargestvaluethenumeratorcanbeas(289*(1000+1000)*1000+23000)=578023000,whichfitsina30-bitnumber.

1.5.7.Floating-pointnumbersWecanusefixed-pointwhentherangeofvaluesissmallandknown.Therefore,wewillnotneedfloating-pointoperationsformostembeddedsystemapplicationsbecausefixed-pointissufficient.Furthermore,iftheprocessordoesnothavefloating-pointinstructionsthenafloating-pointimplementationwillrunmuchslowerthanthecorrespondingfixed-pointimplementation.However,itisappropriatetoknowthedefinitionoffloating-point.NASAbelievesthatthereareontheorderof1021starsinourUniverse.Manipulatinglargenumbersliketheseisnotpossibleusingintegerorfixed-pointformats.Otherlimitationwithintegerorfixed-pointnumbersistherearesomesituationswheretherangeofvaluesisnotknownatthetimethesoftwareisbeingdesigned.InaPhysicsresearchproject,youmightbeaskedtocounttherateatwhichparticlesstrikeasensor.Sincetheexperimenthasneverbeenperformedbefore,youdonotknowinadvancewhethertherewillbe1persecondor1trillionpersecond.Theapplicationswithnumbersoflargeorunknownrangecanbesolvedwithfloating-pointnumbers.Floating-pointissimilarinformattobinaryfixed-point,excepttheexponentisallowedtochangeatruntime.Consequently,boththeexponentandthemantissawillbestored.Justlikewithfixed-pointnumberswewillusebinaryexponentsforinternalcalculations,anddecimalexponentswheninterfacingwithhumans.Thisnumbersystemiscalledfloating-pointbecauseastheexponentvariesthebinarypointordecimalpointmoves.

Observation:Iftherangeofnumbersisunknownorlarge,thenthenumbersmustberepresentedinafloating-pointformat.

Observation:Floating-pointimplementationsoncomputersliketheCortexTM-M3thatdonothavehardwaresupportareextremelylongandveryslow.So,ifyoureallyneedfloatingpoint,anLM4F/TM4Cwithfloatingpointhardwaresupportishighlydesirable.

TheIEEEStandardforBinaryFloating-PointArithmeticorANSI/IEEEStd754-1985isthemostwidely-usedformatforfloating-pointnumbers.ThesingleprecisionfloatingpointoperationsontheLM4F/TM4Cmicrocontrollersarecompatiblewiththisstandard.TherearethreecommonIEEEformats:single-precision(32-bit),double-precision(64-bit),anddouble-extendedprecision(80-bits).Onlythe32-bitshortrealformatispresentedhere.Thefloating-pointformat,f,forthesingle-precisiondatatypeisshowninFigure1.30.Computersusebinaryfloating-pointbecauseitisfastertoshiftthanitistomultiply/divideby10.

Bit31Mantissasign,s=0forpositive,s=1fornegative

Bits30:238-bitbiasedbinaryexponent0≤e≤255

Bits22:024-bitmantissa,m,expressedasabinaryfraction,

Abinary1asthemostsignificantbitisimplied.

m=1.m1m2m3…m23

Figure1.30.32-bitsingle-precisionfloating-pointformat.Thevalueofasingle-precisionfloating-pointnumberis

f=(-1)s•2e-127•m

Therangeofvaluesthatcanberepresentedinthesingle-precisionformatisabout±10-38to±10+38.The24-bitmantissayieldsaprecisionofabout7decimaldigits.Thefloating-pointvalueiszeroifbotheandmarezero.Becauseofthesignbit,therearetwozeros,positiveandnegative,whichbehavethesameduringcalculations.Toillustratefloating-point,wewillcalculatethesingle-precisionrepresentationofthenumber10.Tofindthebinaryrepresentationofafloating-pointnumber,firstextractthesign.

10=(-1)0•10

Step2,multiplyordividebytwountilthemantissaisgreaterthanorequalto1,butlessthan2.

10=(-1)0•23•1.25

Step3,theexponenteisequaltothenumberofdividebytwosplus127.

10=(-1)0•2130-127•1.25

Step4,separatethe1fromthemantissa.Recallthatthe1willnotbestored.

10=(-1)0•2130-127•(1+0.25)

Step5,expressthemantissaasabinaryfixed-pointnumberwithafixedconstantof2-23.

10=(-1)0•2130-127•(1+2097152•2-23)

Step6,converttheexponentandmantissacomponentstohexadecimal.

10=(-1)0•2$82-127•(1+$200000•2-23)

Step7,extracts,e,mterms,converthexadecimaltobinary

10=(0,$82,$200000)=(0,10000010,01000000000000000000000)

Sometimesthisconversiondoesnotyieldanexactrepresentation,asinthecaseof0.1.Inparticular,thefixed-pointrepresentationof0.6isonlyanapproximation.

Step10.1=(-1)0•0.1

Step20.1=(-1)0•2-4•1.6

Step30.1=(-1)0•2123-127•1.6

Step40.1=(-1)0•2123-127•(1+0.6)

Step50.1 (-1)0•2123-127•(1+5033165•2-23)

Step60.1 (-1)0•2$7B-127•(1+$4CCCCD•2-23)

Step70.1 (0,$7B,$4CCCCD)=(0,01111011,10011001100110011001101)

Thefollowingexampleshowsthestepsinfindingthefloating-pointapproximationfor .

Step1 =(-1)0•

Step2 (-1)0•21•1.570796327

Step3 (-1)0•2128-127•1.570796327

Step4 (-1)0•2128-127•(1+0.570796327)

Step5 (-1)0•2128-127•(1+4788187•2-23)

Step6 (-1)0•2$80-127•(1+$490FDB•2-23)

Step7 (0,$80,$490FDB)=(0,10000000,10010010000111111011011)

Therearesomespecialcasesforfloating-pointnumbers.Wheneis255,thenumberisconsideredasplusorminusinfinity,whichprobablyresultedfromanoverflowduringcalculation.Wheneis0,thenumberisconsideredasdenormalized.Thevalueofthemantissaofadenormalizednumberislessthan1.Adenormalizedshortresultnumberhasthevalue,

f=(-1)s•2-126•mwherem=0.m1m2m3…m23

Observation:Thefloating-pointzeroisstoredindenormalizedformat.

Whentwofloating-pointnumbersareaddedorsubtracted,thesmalleroneisfirstunnormalized.Themantissaofthesmallernumberisshiftedrightanditsexponentisincrementeduntilthetwonumbershavethesameexponent.Then,themantissasareaddedorsubtracted.Lastly,theresultisnormalized.Toillustratethefloating-pointaddition,considerthecaseof10+0.1.First,weshowtheoriginalnumbersinfloating-pointformat.Themantissaisshowninbinaryformat.

10.0=(-1)0•23•1.01000000000000000000000

+0.1=(-1)0•2-4•1.10011001100110011001101

Everytimetheexponentisincrementedthemantissaisshiftedtotheright.Noticethat7binarydigitsarelost.The0.1numberisunnormalized,butnowthetwonumbershavethesameexponent.Oftentheresultoftheadditionorsubtractionwillneedtobenormalized.Inthiscasethesumdidnotneednormalization.

10.0=(-1)0•23•1.01000000000000000000000

+0.1=(-1)0•23•0.000000110011001100110011001101

10.1=(-1)0•23•1.01000011001100110011001

Whentwofloating-pointnumbersaremultiplied,theirmantissasaremultipliedandtheirexponentsareadded.Whendividingtwofloating-pointnumbers,theirmantissasaredividedandtheirexponentsaresubtracted.Aftermultiplicationanddivision,theresultmustbenormalized.Toillustratethefloating-pointmultiplication,considerthecaseof10*0.1.Letm1,m2bethevaluesofthetwomantissas.Sincetherangeis1≤m1,m2<2,theproductm1*m2willvaryfrom1≤m1*m2<4.

10.0=(-1)0•23•1.01000000000000000000000

*0.1=(-1)0•2-4•1.10011001100110011001101

1.0=(-1)0•2-1•10.00000000000000000000000

Theresultneedstobenormalized.

1.0=(-1)0•20•1.00000000000000000000000

Roundoffistheerrorthatoccursasaresultofanarithmeticoperation.Forexample,themultiplicationoftwo32-bitmantissasyieldsa64-bitproduct.Thefinalresultisnormalizedintoanormalizedfloating-pointnumberwitha32-bitmantissa.Roundoffistheerrorcausedbydiscardingtheleastsignificantbitsoftheproduct.Roundoffduringadditionandsubtractioncanoccurintwoplaces.First,anerrorcanresultwhenthesmallernumberisshiftedright.Second,whentwon-bitnumbersareaddedtheresultisn+1bits,soanerrorcanoccurasthen+1sumissqueezedbackintoann-bitresult.

Truncationistheerrorthatoccurswhenanumberisconvertedfromoneformattoanother.Forexample,whenan80-bitfloating-pointnumberisconvertedto32-bitfloating-pointformat,40bitsarelostasthe64-bitmantissaistruncatedtofitintothe24-bitmantissa.Recall,thenumber0.1couldnotbeexactlyrepresentedasashortrealfloating-pointnumber.Thisisanexampleoftruncationasthetruefractionwastruncatedtofitintothefinitenumberofbitsavailable.

Iftherangeisknownandsmallandafixed-pointsystemcanbeused,thena32-bitfixed-pointnumbersystemwillhavebetterresolutionthana32-bitfloating-pointsystem.Forafixedrangeofvalues(i.e.,onewithaconstantexponent),a32-bitfloating-pointsystemhasonly23bitsofprecision,whilea32-bitfixed-pointsystemhas9morebitsofprecision.

PerformanceTip:Thesingleprecisionfloating-pointprogramswritteninassemblyontheTM4CrunmuchfasterthanequivalentCcodebecauseyoucanwriteassemblytoperformoperationsinthenativefloatingpointassemblyinstructions.

1.6.EthicsBecauseembeddedsystemsareemployedinmanysafety-criticaldevices,injuryordeathmayresultiftherearehardwareand/orsoftwarefaults.Table1.11listsdictionarydefinitionsoftherelatedtermsmoralsandethics.Amoralpersonisonewhoknowsrightfromwrong,butanethicalpersondoestherightthing.

Morals

1.of,pertainingto,orconcernedwiththeprinciplesorrulesofrightconductorthedistinctionbetweenrightandwrong;ethical:moralattitudes.

2.expressingorconveyingtruthsorcounselastorightconduct,asaspeakeroraliterarywork;moralizing:amoralnovel.

3.foundedonthefundamentalprinciplesofrightconductratherthanonlegalities,enactment,orcustom:moralobligations.

4.capableofconformingtotherulesofrightconduct:amoralbeing.

5.conformingtotherulesofrightconduct(opposedtoimmoral):amoralman.

6.virtuousinsexualmatters;chaste.

7.of,pertainingto,oractingonthemind,feelings,will,orcharacter:moralsupport.

8.restinguponconvincinggroundsofprobability;virtual:amoralcertainty.

Ethics

1.(usedwithasingularorpluralverb)asystemofmoralprinciples:theethicsofaculture.

2.therulesofconductrecognizedinrespecttoaparticularclassofhumanactionsoraparticulargroup,culture,etc.:medicalethics;Christianethics.

3.moralprinciples,asofanindividual:Hisethicsforbadebetrayalofaconfidence.

4.(usuallyusedwithasingularverb)thatbranchofphilosophydealingwithvaluesrelatingtohumanconduct,withrespecttotherightnessandwrongnessofcertainactionsandtothegoodnessandbadnessofthemotivesandendsofsuchactions.

Table1.11.Dictionarydefinitionsofmoralsandethicshttp://dictionary.reference.com

Mostcompanieshaveaspecificanddetailedcodeofethics,similartotheIEEECodeofEthicspresentedbelow.Furthermore,patentandcopyrightlawsprovidealegalperspectivetowhatisrightandwrong.Nevertheless,manysituationspresentthemselvesinthegreyarea.Inthesecases,youshouldseekadvicefrompeoplewhoseethicsyoutrust.However,youareultimatelyresponsibleforyourownactions.

IEEECodeofEthics

We,themembersoftheIEEE,inrecognitionoftheimportanceofourtechnologiesinaffectingthequalityoflifethroughouttheworld,andinacceptingapersonalobligationtoourprofession,itsmembersandthecommunitiesweserve,doherebycommitourselvestothehighestethicalandprofessionalconductandagree:

1.toacceptresponsibilityinmakingdecisionsconsistentwiththesafety,health,andwelfareofthepublic,andtodisclosepromptlyfactorsthatmightendangerthepublicortheenvironment;

2.toavoidrealorperceivedconflictsofinterestwheneverpossible,andtodisclosethemtoaffectedpartieswhentheydoexist;

3.tobehonestandrealisticinstatingclaimsorestimatesbasedonavailabledata;

4.torejectbriberyinallitsforms;

5.toimprovetheunderstandingoftechnology;itsappropriateapplication,andpotentialconsequences;

6.tomaintainandimproveourtechnicalcompetenceandtoundertaketechnologicaltasksforothersonlyifqualifiedbytrainingorexperience,orafterfulldisclosureofpertinentlimitations;

7.toseek,accept,andofferhonestcriticismoftechnicalwork,toacknowledgeandcorrecterrors,andtocreditproperlythecontributionsofothers;

8.totreatfairlyallpersonsregardlessofsuchfactorsasrace,religion,gender,disability,age,ornationalorigin;

9.toavoidinjuringothers,theirproperty,reputation,oremploymentbyfalseormaliciousaction;

10.toassistcolleaguesandco-workersintheirprofessionaldevelopmentandtosupporttheminfollowingthiscodeofethics.

AgreatvolumeofsoftwareexistsinbooksandontheInternet.Howyouusethisinformationinyourclassesisuptoyourprofessor.Whenyoubecomeapracticingengineermakingproductsforprofit,youwillwishtousesoftwarewrittenbyothers.Examplesofsoftwareinbooksandontheinternetarecomprisedoftwocomponents.Thefirstcomponentisthesoftwarecodeitself,andthesecondcomponentisthealgorithmusedtosolvetheproblem.Tousethealgorithm,youshouldsearchtoseeifithaspatentprotection.Ifitisprotected,youcouldpurchaseorlicensethetechnology.Ifthealgorithmisnotprotectedandyouwishtousethesoftwarecode,youshouldaskpermissionfromtheauthorandgivecitationtosource.Ifthealgorithmisnotprotectedandtheauthordoesnotgrantpermission,youcanstillimplementthealgorithmbywritingyourownsoftware.Inallcases,youareresponsiblefortesting.

Averydifficultsituationresultswhenyouleaveonecompanyandbeginworkforanother.Technicalexpertise(thingsyouknow)andprocedures(thingsyouknowhowtodo)thatyouhavelearnedwhileworkingforacompanybelongtoyou,notyouremployer.Thisissuchahugeproblemthatmanyemployershaveadetailedandlegalcontractemployeesmustsigntobehired.Anon-competeclause(NCC),alsocalledacovenantnottocompete(CNC),certifiestheemployeeagreesnottopursueasimilarjobwithanycompanyincompetitionwiththeemployer.Companiesusetheseagreementstopreventpresentandformeremployeesfromworkingwiththeircompetitors.Anexampleagreementfollows:

EMPLOYEENON-COMPETEAGREEMENT

Forgoodconsiderationandasaninducementfor________________(Company)toemploy_________________(Employee),theundersignedEmployeeherebyagreesnottodirectlyorindirectlycompetewiththebusinessoftheCompanyanditssuccessorsandassignsduringtheperiodofemploymentandforaperiodof_____yearsfollowingterminationofemploymentandnotwithstandingthecauseorreasonfortermination.Theterm“notcompete”asusedhereinshallmeanthattheEmployeeshallnotown,manage,operate,consultortobeemployeeinabusinesssubstantiallysimilartoorcompetitivewiththepresentbusinessoftheCompanyorsuchotherbusinessactivityinwhichtheCompanymaysubstantiallyengageduringthetermofemployment.TheEmployeeacknowledgesthattheCompanyshallormayinrelianceofthisagreementprovideEmployeeaccesstotradesecrets,customersandotherconfidentialdataandgoodwill.Employeeagreestoretainsaidinformationasconfidentialandnottousesaidinformationonhisorherbehalfordisclosesametoanythirdparty.Thisagreementshallbebindinguponandinuretothebenefitoftheparties,theirsuccessors,assigns,andpersonalrepresentatives.

Signedthis_____dayof_______________________________________________________________Company

_______________________________________Employee

1.7.Exercises1.1IsRAMvolatileornonvolatile?

1.2IsflashROMvolatileornonvolatile?

1.3Foreachtermgiveadefinitionin16wordsorless:microprocessor,microcomputer,andmicrocontroller.

1.4Foreachtermgiveadefinitionin16wordsorless:bandwidth,real-time,latency.

1.5Foreachtermgiveadefinitionin16wordsorless:volatile,nonvolatile.

1.6Listthefourcomponentsofaprocessoranddefineeachin16wordsorless

1.7Foreachparametergiveadefinitionin16wordsorless:precision,range,resolution

1.8WhatarethedifferencesbetweenCISCandRISCprocessors?

1.9Describestructuredprogrammingin16wordsorless.

1.10Whatarethedifferencesbetweenparallelprogrammingandconcurrentprogramming?

1.11Definedistributedprogrammingin16wordsorless.

1.12Whatarethedifferencesbetweentristateandopencollectorlogic?

1.13Defineopendrainlogicin16wordsorless?

1.14Define5-Vtolerantin16wordsorless?

1.15Consideringjustcurrent,howmany74SSchottkyinputscanonemicrocontrolleroutputdriverunningin8mAoutputmode?

1.16Consideringjustcurrent,howmany74LSlow-powerSchottkyinputscanonemicrocontrolleroutputdriverunningin2mAoutputmode?

1.17WhatisthequalitativedifferenceinsupplycurrentbetweentheCMOSdevicesandthenon-CMOSdevices?Whatistheexplanationforthedifference?

1.18UsingthecircuitinFigure1.23,whatresistorvalueoperatesanLEDat1.8Vand15mA?

1.19UsingthecircuitinFigure1.23,whatresistorvalueoperatesanLEDat1.6Vand12mA?

1.20In16wordsorlessdescribethedifferencesbetweenpositivelogicandnegativelogic.

1.21ForeachADCparametergiveadefinitionin20wordsorless:precision,range,resolution

1.22Howmanyalternativesdoesa12-bitADChave?

1.23Ifasystemusesan11-bitADC,abouthowmanydecimaldigitswillithave?

1.24Whatisthedifferencebetweenthetermskilobitandkibibit?

1.25Howmanyalternativesdoesa13-bitADChave?

1.26Ifasystemusesan14-bitADC,abouthowmanydecimaldigitswillithave?

1.27Ifasystemrequires3½decimaldigitsofprecision,whatisthesmallestnumberofbitstheADCneedstohave?

1.28Ifasystemrequires5decimaldigitsofprecision,whatisthesmallestnumberofbitstheADCneedstohave?

1.29Convertthefollowingdecimalnumbersto8-bitunsignedbinary:26,65,124,and202.

1.30Convertthefollowingdecimalnumbersto8-bitsignedbinary:23,61,-122,and-5.

1.31Convertthefollowinghexnumberstounsigneddecimal:0x2A,0x69,0xB3,and0xDE.

1.32Convertthe16-bitbinarynumber00100010011010102tounsigneddecimal.

1.33Convertthe16-bithexnumber0x5678tounsigneddecimal.

1.34Converttheunsigneddecimalnumber12345to16-bithexadecimal.

1.35Converttheunsigneddecimalnumber20000to16-bitbinary.

1.36Convertthe16-bithexnumber0x7654tosigneddecimal.

1.37Convertthe16-bithexnumber0xBCDEtosigneddecimal.

1.38Convertthesigneddecimalnumber23456to16-bithexadecimal.

1.39Convertthesigneddecimalnumber–20000to16-bitbinary.

1.40Giveanapproximationof√7usingthedecimalfixed-point( =0.001)format.

1.41Giveanapproximationof√7usingthebinaryfixed-point( =2-8)format.

1.42Giveanapproximationof√103usingthedecimalfixed-point( =0.01)format.

1.43Giveanapproximationof√93usingthebinaryfixed-point( =2-4)format.

1.44Asigned16-bitbinaryfixed-pointnumbersystemhasa resolutionof1/256.Whatisthecorrespondingvalueofthenumberiftheintegerpartstoredinmemoryis385?

1.45Anunsigned16-bitdecimalfixed-pointnumbersystemhasa resolutionof1/100.Whatisthecorrespondingvalueofthenumberiftheintegerpartstoredinmemoryis385?

1.46Givetheshortrealfloating-pointrepresentationof√ 2.

1.47Givetheshortrealfloating-pointrepresentationof–134.4.

1.48Givetheshortrealfloating-pointrepresentationof–0.0123.

D1.49DrawaflowchartfortheembeddedsystemdescribedinExample1.3.

D1.50Drawaflowchartfortheembeddedsysteminasimplewatchthatjusttellstime.

D1.51SearchtheinternetforadesignofaflashROMcellthatuses2transistors.Labelonthecircuitthevoltagesoccurringwhenthebitiszero,andwhenthebitishigh.

D1.52SearchtheinternetforadesignofaRAMcellthatuses6transistors.Labelonthecircuitthevoltagesoccurringwhenthebitiszero,andthevoltagesoccurringwhenthebitishigh.

D1.53Designthecircuitthatinterfacesa1.5V5mALEDtothemicrocontroller.

D1.53Designthecircuitthatinterfacesa2.5V1mALEDtothemicrocontroller.

D1.55AssumeMandNaretwointegers,eachlessthan1000.FindthebestsetofMandN,suchthatM/Nisapproximately√6.(Like27/11,butmuchmoreaccurate).

D1.56AssumeMandNaretwointegers,eachlessthan1000.FindthebestsetofMandN,suchthatM/Nisapproximately√7.(Like8/3,butmuchmoreaccurate).

D1.57AssumeMandNaretwointegers,eachlessthan1000.FindthebestsetofMandN,suchthatM/Nisapproximatelye.(Like19/7,butmuchmoreaccurate).

D1.58AssumeMandNaretwointegers,eachlessthan1000.FindthebestsetofMandN,suchthatM/Nisapproximatelyln(2).(Like9/13,butmuchmoreaccurate).

D1.59First,rewritethefollowingdigitalfilterusingdecimalfixed-pointmath.Assumetheinputsareunsigned8-bitvalues(0to255).Then,rewriteitsothatitcanbecalculatedwithintegermathusingthefactthat0.22222isabout2/9and0.088889isabout4/45and0.8is4/5.Inbothcases,thecalculationsaretobeperformedin16-bitunsignedintegerformwithoutoverflow.y=0.22222•x+0.08889•x1+0.80000•y1

D1.60Performtheoperation2+ inshortrealfloating-pointformat.Determinethedifferencebetweenwhatyougotandwhatyoushouldhavegotten(2+ ).Thiserrorhastwocomponents:truncationerrorthatresultsintheapproximation itselfandroundofferrorthatoccursduringtheaddition.

D1.61Performtheoperation0.2*0.2inshortrealfloating-pointformat.Determinethedifferencebetweenwhatyougotandwhatyoushouldhavegotten(0.04).Thiserrorhastwocomponents:truncationerrorthatresultsintheapproximationof0.2itselfandroundofferrorthatoccursduringthemultiplication.

D1.62Performtheoperation 1+1E9inshortrealfloating-pointformat.Determinethedifferencebetweenwhatyougotandwhatyoushouldhavegotten.

D1.63Considerthefollowingsituation:Supposethatyouareadevelopmentengineerwithresponsibilityforanembeddedsystememployedinoneofyourcompany’smajorproducts.Youseektoimprovetheefficiencyoftheembeddedsystemand,followingsomeresearch,youdiscoveranalgorithmpostedontheWebthatwouldprovideavastimprovementforyoursystem.Thealgorithmiswritteninthesamelanguageasthatusedbyyoursystem.

a)Woulditeverbeethicaltocopythecodethatimplementsthealgorithmandincorporateitinyourembeddedsystem?

b)Woulditeverbegoodengineeringpracticetoincorporatethecodethatimplementsthealgorithmandinyourembeddedsystem?

D1.64SupposethattheWebarticlecontainingthecodestatesthatitmaybecopiedandusedinanymannerprovidingthatitisnotusedinaproductforsale.Arethereanycircumstancesthatwouldpermittheethicaluseofthealgorithm?

D1.65Youareadevelopmentengineerthathasrecentlyleftapositionwithalargecorporationtoworkforasmallembeddedsystemcompany.Yourteamatthenewcompanyisworkingonaprojectthatwouldbevastlyimprovedthroughtheuseofanewprocedurethatwasdevelopedbyyourpreviouscompany.Whileyoudidnotparticipateintheprocedure’sdevelopment,youareawareofallthetechnicaldetailsnecessarytoeffectivelyemployit.Pleaseanswerandexplainyourresponsetoeachofthefollowingquestions:

a)Woulditeverbeethicaltodisclosetheproceduretoyourteamatthenewcompany?

b)Woulditeverbegoodengineeringpracticetoincorporatetheprocedureinyournewteam’sembeddedsystem?

c)Assumingthatyoufeelitcouldbeethicaltodisclosetheprocedure,whatconsiderationsorcircumstanceswouldinfluenceyourdecision?

d)Supposethattheconsiderations/circumstancesin(c)aboveleadyoutotheconclusionthatitwouldnotbeethicaltodisclosetheprocedure.Whatchangestotheconsiderationsandcircumstanceswouldbenecessarytopermityoutoethicallydisclosetheproceduretoyournewteam?

1.8.LabAssignmentsLab1.1Yourmicrocontrollerdevelopmentboardcomeswithstartercode.FindtheexamplethatflashesanLED.Usethisexampletodiscoverhowtoperformthefollowingtasks.1)Howdoyouopenasoftwareproject?2)Howdoyoucompilethesoftwareproject?3)Canthesoftwareberuninasimulator?Ifso,howdoyourunonthesimulator?4)Howdoyoudownloadobjectcodeontotherealboard?5)Withinthedebuggerhowdoyouperformtheseoperations:seetheregisters,observeRAM,start/stopexecution,andsetbreakpoints?ChangethesoftwaresotheLEDblinkstwiceasslow.

Lab1.2Yourmicrocontrollerdevelopmentboardcomeswithstartercode.FindtheexamplethatoutputstoeitheraserialportoranLCD.Usethisexampletodiscoverhowtoperformthefollowingtasks.1)Howdoyouopenasoftwareproject?2)Howdoyoucompilethesoftwareproject?3)Canthesoftwareberuninasimulator?Ifso,howdoyourunonthesimulator?4)Howdoyoudownloadobjectcodeontotherealboard?5)Withinthedebuggerhowdoyouperformtheseoperations:seetheregisters,observeRAM,start/stopexecution,andsetbreakpoints?Changethesoftwaresothesystemoutputsyourname.

Lab1.3ThesystemhasoneLED,twoswitches,andresistors.Inthislabyouwillnotuseamicrocontroller.Design,implement,andtestacircuitthatturnsonanLEDifbothswitchesarepressed.UsingdifferentresistorsmeasurefivedifferentvoltageandcurrentpointsontheLEDoperatingcurvelikeFigure1.23.ComparemeasureddatatoparametersfromtheLEDdatasheet.

Lab1.4ThesystemhasoneLED,twoswitches,andresistors.Inthislabyouwillnotuseamicrocontroller.Design,implement,andtestacircuitthatturnsonanLEDifeitherswitchispressed.UsingdifferentresistorsmeasurefivedifferentvoltageandcurrentpointsontheLEDoperatingcurvelikeFigure1.23.ComparemeasureddatatoparametersfromtheLEDdatasheet.

2.ARMCortex-MProcessorChapter2objectivesareto:•IntroduceCortexTM-Mprocessorarchitecture

•PresentanoverviewoftheCortexTM-Mcoreassemblylanguage

•Definethememory-mappedI/OstructureoftheLM3S/TM4Cfamily

•DescribetheparallelportsontheLM3S/TM4Cfamily

•PresenttheSysTicktimer

•Describethesystemclocks

•Presentgeneralthoughtsabouthowtochooseamicrocontroller

InthischapterwepresentageneraldescriptionoftheARMCortexTM-Mprocessor.Ratherthanreproducingthevoluminousdetailsthatcanbefoundinthedatasheets,wewillpresentgeneralconceptsandgivespecificexamplesillustratingtheseconcepts.Afterreadingthischapter,youshouldbeabletolookupandunderstanddetailedspecificsintheARM CortexTM-MTechnicalReferenceManual.DatasheetscanbefoundonthewebsitesofeitherARMorthecompaniesthatmakethemicrocontrollers,likeTexasInstruments.Someofthesedatasheetsarealsopostedonthewebsiteaccompanyingthisbook.Thiswebsitecanbefoundathttp://users.ece.utexas.edu/~valvano/arm.

Therearetworeasonswemustlearntheassemblylanguageofthecomputerwhichweareusing.Sometimes,butnotoften,wewishtooptimizeourapplicationformaximumexecutionspeedorminimummemorysize,andwritingpiecesofourcodeinassemblylanguageisoneapproachtosuchoptimizations.Themostimportantreason,however,isthatbyobservingtheassemblycodegeneratedbythecompilerforourCcodewecantrulyunderstandwhatoursoftwareisdoing.Basedonthisunderstanding,wecanevaluate,debug,andoptimizeoursystem.

Ourfirstinput/outputinterfaceswillusetheparallelports,allowingustoexchangedigitalinformationwiththeexternalworld.Specifically,wewilllearnhowtoconnectswitchesandLEDstothemicrocontroller.Thesecondtechniquewewilllearnistocontroltime.Wecanselecttheexecutionspeedofthemicrocontrollerusingthephase-lock-loop,andwecanperformtimedelaysusingtheSysTicktimer.

EventhoughwewilldesignsystemsbasedspecificallyontheLM3S/TM4Cfamily,thesesolutionscan,withlittleeffort,beimplementedonotherversionsoftheCortexTM-Mfamily.Wewilldiscussprototypingmethodstobuildembeddedsystemsandpresentasimpleexamplewithbinaryinputsandoutputs.

2.1.CortexTM-MArchitectureFigure2.1showsasimplifiedblockdiagramofamicrocontrollerbasedontheARM ®CortexTM-Mprocessor.ItisaHarvardarchitecturebecauseithasseparatedataandinstructionbuses.TheCortexTM-Minstructionsetcombinesthehighperformancetypicalofa32-bitprocessorwithhighcodedensitytypicalof8-bitand16-bitmicrocontrollers.InstructionsarefetchedfromflashROMusingtheICodebus.DataareexchangedwithmemoryandI/Oviathesystembusinterface.OntheCortexTM-M4thereisasecondI/Obusforhigh-speeddeviceslikeUSB.TherearemanysophisticateddebuggingfeaturesutilizingtheDCodebus.Thenestedvectoredinterruptcontroller(NVIC)managesinterrupts,whicharehardware-triggeredsoftwarefunctions.Someinternalperipherals,liketheNVICcommunicatedirectlywiththeprocessorviatheprivateperipheralbus(PPB).Thetightintegrationoftheprocessorandinterruptcontrollerprovidesfastexecutionofinterruptserviceroutines(ISRs),dramaticallyreducingtheinterruptlatency.

Figure2.1.HarvardarchitectureofanARM® CortexTM-M-basedmicrocontroller.

2.1.1.RegistersTheregistersaredepictedinFigure2.2.R0toR12aregeneralpurposeregistersandcontaineitherdataoraddresses.RegisterR13(alsocalledthestackpointer,SP)pointstothetopelementofthestack.Actually,therearetwostackpointers:themainstackpointer(MSP)andtheprocessstackpointer(PSP).Onlyonestackpointerisactiveatatime.Inahigh-reliabilityoperatingsystem,wecouldactivatethePSPforusersoftwareandtheMSPforoperatingsystemsoftware.Thiswaytheuserprogramcouldcrashwithoutdisturbingtheoperatingsystem.Becauseofthesimpleanddedicatednatureoftheembeddedsystemsdevelopedinthisbook,wewillexclusivelyusethemainstackpointer.RegisterR14(alsocalledthelinkregister,LR)isusedtostorethereturnlocationforfunctions.TheLRisalsousedinaspecialwayduringexceptions,suchasinterrupts.InterruptsarecoveredinChapter5.RegisterR15(alsocalledtheprogramcounter,PC)pointstothenextinstructiontobefetchedfrommemory.TheprocessorfetchesaninstructionusingthePCandthenincrementsthePCby2or4.

Figure2.2.RegistersontheARM® CortexTM-Mprocessor.TheARMArchitectureProcedureCallStandard,AAPCS,partoftheARMApplicationBinaryInterface(ABI),usesregistersR0,R1,R2,andR3topassinputparametersintoaCfunction.AlsoaccordingtoAAPCSweplacethereturnparameterinRegisterR0.

TherearethreestatusregistersnamedApplicationProgramStatusRegister(APSR),theInterruptProgramStatusRegister(IPSR),andtheExecutionProgramStatusRegister(EPSR)asshowninFigure2.3.TheseregisterscanbeaccessedindividuallyorincombinationastheProgramStatusRegister(PSR).TheN,Z,V,C,andQbitsgiveinformationabouttheresultofapreviousALUoperation.Ingeneral,theNbitissetafteranarithmeticalorlogicaloperationsignifyingwhetherornottheresultisnegative.Similarly,theZbitissetiftheresultiszero.TheCbitmeanscarryandissetonanunsignedoverflow,andtheVbitsignifiessignedoverflow.TheQbitisthestickysaturationflag,indicatingthat“saturation”hasoccurred,andissetbythe SSAT and USAT instructions.

Figure2.3.TheprogramstatusregisteroftheARM® CortexTM-Mprocessor.

TheTbitwillalwaysbe1,indicatingtheARM ® CortexTM-MisexecutingThumbinstructions.TheICI/ITbitsareusedbyinterruptsandbytheIF-THENinstructions.TheISR_NUMBERindicateswhichinterruptifanytheprocessorishandling.Bit0ofthespecialregisterPRIMASKistheinterruptmaskbit.Ifthisbitis1mostinterruptsandexceptionsarenotallowed.Ifthebitis0,theninterruptsareallowed.Bit0ofthespecialregisterFAULTMASKisthefaultmaskbit.Ifthisbitis1allinterruptsandfaultsarenotallowed.Ifthebitis0,theninterruptsandfaultsareallowed.Thenonmaskableinterrupt(NMI)isnotaffectedbythesemaskbits.TheBASEPRIregisterdefinesthepriorityoftheexecutingsoftware.Itpreventsinterruptswithlowerorequalprioritybutallowshigherpriorityinterrupts.ForexampleifBASEPRIequals3,thenrequestswithlevel0,1,and2caninterrupt,whilerequestsatlevels3andhigherwillbepostponed.ThedetailsofinterruptprocessingwillbepresentedinChapter5.

2.1.2.MemoryMicrocontrollerswithinthesamefamilydifferbytheamountofmemoryandbythetypesofI/Omodules.AllLM3SandLM4F/TM4CmicrocontrollershaveaCortexTM-Mprocessor.Therearehundredsofmembersinthisfamily;someofthemarelistedinTable2.1.

Partnumber RAM Flash I/O I/Omodules

LM3S811 8 64 32 PWM

LM3S1968 64 256 52 PWM

LM3S2110 16 64 40 PWM,CAN

LM3S3748 64 128 61 PWM,DMA,USB

LM3S8962 64 256 42 PWM,CAN,Ethernet,IEEE1588

LM4F120H5QR 32 256 43 floatingpoint,CAN,DMA,USB

TM4C123GH6PGE 32 256 105 floatingpoint,CAN,DMA,USB,PWM

TM4C123GH6PM 32 256 43 floatingpoint,CAN,DMA,USB,PWM

TM4C123GH6ZRB 32 256 120 floatingpoint,CAN,DMA,USB,PWM

TM4C1294NCPDT 256 1024 90

floatingpoint,CAN,DMA,USB,PWM,Ethernet

KiB KiB pins

Table2.1.MemoryandI/Omodules(allhaveSysTick,RTC,timers,UART,I2C,SSI,andADC).

ThememorymapofTM4C123isillustratedinFigure2.4.AlthoughspecificfortheTM4C123,allARM ® CortexTM-Mmicrocontrollershavesimilarmemorymaps.Ingeneral,FlashROMbeginsataddress0x0000.0000,RAMbeginsat0x2000.0000,theperipheralI/Ospaceisfrom0x4000.0000to0x5FFFF.FFFF,andI/Omodulesontheprivateperipheralbusexistfrom0xE000.0000to0xE00F.FFFF.Inparticular,theonlydifferencesinthememorymapforthevariousmembersoftheLM3SandLM4F/TM4CfamiliesaretheendingaddressesoftheflashandRAM.Havingmultiplebusesmeanstheprocessorcanperformmultipletasksinparallel.Thefollowingissomeofthetasksthatcanoccurinparallel

ICodebusFetchopcodefromROM

DCodebusReadconstantdatafromROM

SystembusRead/writedatafromRAMorI/O,fetchopcodefromRAM

PPBRead/writedatafrominternalperipheralsliketheNVIC

AHBRead/writedatafromhigh-speedI/Oandparallelports(M4only)

TheARM ® CortexTM-Musesbit-bandingtoallowread/writeaccesstoindividualbitsinRAMandsomebitsintheI/Ospace.Therearetwoparametersthatdefinebit-banding:theaddressandthebityouwishtoaccess.AssumeyouwishtoaccessbitbofRAMaddress0x2000.0000+n,wherebisanumber0to7.Thealiasedaddressforthisbitwillbe

0x2200.0000+32*n+4*b

Readingthisaddresswillreturna0ora1.Writinga0or1tothisaddresswillperformanatomicread-modify-writemodificationtothebit.

Figure2.4.MemorymapoftheTM4C123.TheTM4C1294issimilarbutwith1024kROM,256kRAM.Ifweconsider32-bitword-aligneddatainRAM,thesamebit-bandingformulastillapplies.Letthewordaddressbe0x2000.0000+n.nstartsat0andincrementsby4.Inthiscase,wedefinebasthebitfrom0to31.Inlittle-endianformat,bit1ofthebyteat0x2000.0001isthesameasbit9ofthewordat0x2000.0000.Thealiasedaddressforthisbitwillstillbe

0x2200.0000+32*n+4*b

Examplesofbit-bandedaddressingarelistedinTable2.2.Writinga1tolocation0x2200.0018willsetbit6ofRAMlocation0x2000.0000.Readinglocation0x2200.0024willreturna0or1dependingonthevalueofbit1ofRAMlocation0x2000.0001.

Checkpoint2.1:Whataddressdoyouusetoaccessbit5ofthebyteat0x2000.1003?

Checkpoint2.2:Whataddressdoyouusetoaccessbit20ofthewordat0x2000.1000?

Theotherbit-bandingregionistheI/Ospacefrom0x4000.0000through0x400F.FFFF.Inthisregion,lettheI/Oaddressbe0x4000.0000+n,andletbrepresentthebit0to7.Thealiasedaddressforthisbitwillbe

0x4200.0000+32*n+4*b

RAMaddress

Offsetn

Bitb

Bit-bandedalias

0x2000.0000 0 0 0x2200.0000

0x2000.0000 0 1 0x2200.0004

0x2000.0000 0 2 0x2200.0008

0x2000.0000 0 3 0x2200.000C

0x2000.0000 0 4 0x2200.0010

0x2000.0000 0 5 0x2200.0014

0x2000.0000 0 6 0x2200.0018

0x2000.0000 0 7 0x2200.001C

0x2000.0001 1 0 0x2200.0020

0x2000.0001 1 1 0x2200.0024

Table2.2.Examplesofbit-bandedaddressing.

Checkpoint2.3:Whataddressdoyouusetoaccessbit2ofthebyteat0x4000.0003?

2.1.3.StackThestackisalast-in-first-outtemporarystorage.Tocreateastack,ablockofRAMisallocatedforthistemporarystorage.OntheARM ® CortexTM-M,thestackalwaysoperateson32-bitdata.Thestackpointer(SP)pointstothe32-bitdataonthetopofthestack.Thestackgrowsdownwardsinmemoryaswepushdataontoitso,althoughwerefertothemostrecentitemasthe“topofthestack”itisactuallytheitemstoredatthelowestaddress!Topushdataonthestack,thestackpointerisfirstdecrementedby4,andthenthe32-bitinformationisstoredattheaddressspecifiedbySP.Topopdatafromthestack,the32-bitinformationpointedtobySPisfirstretrieved,andthenthestackpointerisincrementedby4.SPpointstothelastitempushed,whichwillalsobethenextitemtobepopped.Theprocessorallowsfortwostacks,themainstackandtheprocessstack,withtwoindependentcopiesofthestackpointer.TheboxesinFigure2.5represent32-bitstorageelementsinRAM.Thegreyboxesinthefigurerefertoactualdatastoredonthestack,andthewhiteboxesrefertolocationsinmemorythatdonotcontainstackdata.ThisfigureillustrateshowthestackisusedtopushthecontentsofRegistersR0,R1,andR2inthatorder.AssumeRegisterR0initiallycontainsthevalue1,R1contains2andR2contains3.Thedrawingontheleftshowstheinitialstack.Thesoftwareexecutesthesesixinstructions

PUSHR0

PUSHR1

PUSHR2

POPR3

POPR4

POPR5

Theinstruction PUSHR0 savesthevalueofR0onthestack.ItfirstdecrementsSPby4,andthenitstoresthe32-bitcontentsofR0intothememorylocationpointedtobySP.Thefourbytesarestoredlittleendian.Theright-mostdrawingshowsthestackafterthepushoccursthreetimes.Thestackcontainsthenumbers12and3,with3ontop.

Figure2.5.Stackpictureshowingthreenumbersfirstbeingpushed,thenthreenumbersbeingpopped.Theinstruction POPR3 retrievesdatafromthestack.ItfirstmovesthevaluefrommemorypointedtobySPintoR3,andthenitincrementsSPby4.AfterthepopoccursthreetimesthestackrevertstoitsoriginalstateandregistersR3,R4andR5contain321respectively.Wedefinethe32-bitwordpointedtobySPasthetopentryofthestack.Ifitexists,wedefinethe32-bitdataimmediatelybelowthetop,atSP+4,asnexttotop.Properuseofthestackrequiresfollowingtheseimportantrules

1.Functionsshouldhaveanequalnumberofpushesandpops

2.Stackaccesses(pushorpop)shouldnotbeperformedoutsidetheallocatedarea

3.Stackreadsandwritesshouldnotbeperformedwithinthefreearea

4.StackpushshouldfirstdecrementSP,thenstorethedata

5.Stackpopshouldfirstreadthedata,andthenincrementSP

Functionsthatviolaterulenumber1willprobablycrashwhenincorrectdataarepoppedoffatalatertime.Violationsofrulenumber2canbecausedbyastackunderfloworoverflow.Overflowoccurswhenthenumberofelementsbecamelargerthantheallocatedspace.Stackunderflowiscausedwhentherearemorepopsthanpushes,andisalwaystheresultofasoftwarebug.Astackoverflowcanbecausedbytworeasons.Ifthesoftwaremistakenlypushesmorethanitpops,thenthestackpointerwilleventuallyoverflowitsbounds.Evenwhenthereisexactlyonepopforeachpush,astackoverflowcanoccurifthestackisnotallocatedlargeenough.Theprocessorwillgenerateabusfaultwhenthesoftwaretriesreadfromorwritetoanaddressthatdoesn’texist.IfvalidRAMexistsbelowthestackthenpushingtoanoverflowedstackwillcorruptdatainthismemory.

First,wewillconsiderthesituationwheretheallocatedstackareaisplacedatthebeginningofRAM.Forexample,assumeweallocate4096bytesforthestackfrom0x2000.0000to0x2000.0FFF,seetheleftsideofFigure2.6.TheSPisinitializedto0x2000.1000,andthestackisconsideredempty.IftheSPbecomeslessthan0x2000.0000astackoverflowhasoccurred.Thestackoverflowwillcauseabusfaultbecausethereisnothingataddress0x1FFF.FFFC.Ifthesoftwaretriestoreadfromorwritetoanylocationgreaterthanorequalto0x2000.1000thenastackunderflowhasoccurred.Atthispointthestackandglobalvariablesmayexistatoverlappingaddresses.Stackunderflowisaverydifficultbugtorecognize,becausethefirstconsequencewillbeunexplainedchangestodatastoredinglobalvariables.

Figure2.6.DrawingsshowingtwopossiblewaystoallocatethestackareainRAM.Next,wewillconsiderthesituationwheretheallocatedstackareaisplacedattheendofRAM.TheTM4C123has32KiBofRAMfrom0x2000.0000to0x2000.7FFF.Sointhiscaseweallocatethe4096bytesforthestackfrom0x2000.7000to0x2000.7FFF,shownontherightsideofFigure2.6.TheSPisinitializedto0x2000.8000,andthestackisconsideredempty.IftheSPbecomeslessthan0x2000.7000astackoverflowhasoccurred.Thestackoverflowwillnotcauseabusfaultbecausethereismemoryataddress0x2000.6FFC.Stackoverflowinthiscaseisaverydifficultbugtorecognize,becausethefirstconsequencewillbeunexplainedchangestodatastoredbelowthestackregion.Ifthesoftwaretriestoreadfromorwritetoanylocationgreaterthanorequalto0x2000.8000thenastackunderflowhasoccurred.Inthiscase,stackunderflowwillcauseabusfault.

Executinganinterruptserviceroutinewillautomaticallypushinformationonthestack.Sinceinterruptsaretriggeredbyhardwareevents,exactlywhentheyoccurisnotundersoftwarecontrol.Therefore,violationsofrules3,4,and5willcauseerraticbehaviorwhenoperatingwithinterrupts.Rules4and5arefollowedautomaticallybythe PUSH and POP instructions.

2.1.4.Operatingmodes

TheARM ® CortexTM-Mhastwoprivilegelevelscalledprivilegedandunprivileged.Bit0oftheCONTROLregisteristhethreadmodeprivilegelevel(TPL).IfTPLis1theprocessorlevelisprivileged.Ifthebitis0,thenprocessorlevelisunprivileged.Runningattheunprivilegedlevelpreventsaccesstovariousfeatures,includingthesystemtimerandtheinterruptcontroller.Bit1oftheCONTROLregisteristheactivestackpointerselection(ASPSEL).IfASPSELis1,theprocessorusesthePSPforitsstackpointer.IfASPSELis0,theMSPisused.Whendesigningahigh-reliabilityoperatingsystem,wewillruntheusercodeatanunprivilegedlevelusingthePSPandtheOScodeattheprivilegedlevelusingtheMSP.

Theprocessorknowswhetheritisrunningintheforeground(i.e.,themainprogram)orinthebackground(i.e.,aninterruptserviceroutine).ARMdefinestheforegroundasthreadmode,andthebackgroundashandlermode.Switchingfromthreadmodetohandlermodeoccurswhenaninterruptistriggered.Theprocessorbeginsinthreadmode,signifiedbyISR_NUMBER=0.Wheneveritisservicinganinterruptitswitchestohandlermode,signifiedbysettingISR_NUMBERtospecifywhichinterruptisbeingprocessed.AllinterruptserviceroutinesrunusingtheMSP.Attheendoftheinterruptserviceroutinetheprocessorisswitchedbacktothreadmode,andthemainprogramcontinuesfromwhereitleftoff.

2.1.5.ResetAresetoccursimmediatelyafterpowerisappliedandcanalsooccurbypushingtheresetbuttonavailableonmostboards.Afterareset,theprocessorisinthreadmode,runningataprivilegedlevel,andusingtheMSPstackpointer.The32-bitvalueatflashROMlocation0isloadedintotheSP.Allstackaccessesarewordaligned.Thus,theleastsignificanttwobitsofSPmustbe0.Aresetalsoloadsthe32-bitvalueatlocation4intothePC.Thisvalueiscalledtheresetvector.Allinstructionsarehalfwordaligned.Thus,theleastsignificantbitofPCmustbe0.However,theassemblerwillsettheleastsignificantbitintheresetvector,sotheprocessorwillproperlyinitializethethumbbit(T)inthePSR.OntheARM ® CortexTM-M,theTbitshouldalwaysbesetto1.Onreset,theprocessorinitializestheLRto0xFFFFFFFF.

2.2.TexasInstrumentsLM3SandTM4CI/OpinsTable2.1listedthememoryconfigurationforsomeoftheTexasInstrumentsmicrocontrollers.Inthissection,wepresenttheI/OpinconfigurationsfortheLM3S811,LM3S1968,TM4C123andTM4C1294microcontrollers.TheregularfunctionofapinistoperformparallelI/O,describedlaterinSection2.4.Mostpins,however,haveanalternativefunction.Forexample,portpinsPA1andPA0canbeeitherregularparallelportpins,oranasynchronousserialportcalleduniversalasynchronousreceiver/transmitter(UART).

JointTestActionGroup(JTAG),standardizedastheIEEE1149.1,isastandardtestaccessportusedtoprogramanddebugthemicrocontrollerboard.EachmicrocontrollerusesfourorfiveportpinsfortheJTAGinterface.

Commonerror:Eventhoughitispossibletousethefour/fiveJTAGpinsasgeneralI/O,debuggingmostmicrocontrollerboardswillbemorestableifthesepinsareleftdedicatedtotheJTAGdebugger.

I/OpinsonStellaris ® andTiva ® microcontrollershaveawiderangeofalternativefunctions:

•UARTUniversalasynchronousreceiver/transmitter•SSISynchronousserialinterface•I2CInter-integratedcircuit•I2SInter-ICSound,IntegratedInterchipSound•TimerPeriodicinterrupts,inputcapture,andoutputcompare•PWMPulsewidthmodulation•ADCAnalogtodigitalconverter,measurementanalogsignals•AnalogComparatorComparingtwoanalogsignals•QEIQuadratureencoderinterface•USBUniversalserialbus•EthernetHighspeednetwork•CANControllerareanetwork

TheUARTcanbeusedforserialcommunicationbetweencomputers.Itisasynchronousandallowsforsimultaneouscommunicationinbothdirections.TheSSIisalternatelycalledserialperipheralinterface(SPI).Itisusedtointerfacemedium-speedI/Odevices.Inthisbook,wewilluseittointerfaceagraphicsdisplay,asecuredigitalcard(SDC),andadigitaltoanalogconverter(DAC).I2CisasimpleI/Obusthatwewillusetointerfacelowspeedperipheraldevices.TheI2Sprotocolisusedtocommunicatesoundinformation.Inputcaptureandoutputcomparewillbeusedtocreateperiodicinterrupts,andtakemeasurementsperiod,pulsewidth,phaseandfrequency.PWMoutputswillbeusedtoapplyvariablepowertomotorinterfaces.Inatypicalmotorcontroller,inputcapturemeasuresrotationalspeedandPWMcontrolspower.APWMoutputcanalsobeusedtocreateaDAC.TheADCwillbeusedtomeasuretheamplitudeofanalogsignals,andwillbeimportantindataacquisitionsystems.Theanalogcomparatortakestwoanaloginputsandproducesadigitaloutputdependingonwhichanaloginputisgreater.TheQEIcanbeusedtointerfaceabrushlessDCmotor.USBisahigh-speedserialcommunicationchannel.TheEthernetportcanbeusedtobridgethemicrocontrollertotheInternetoralocalareanetwork.TheCANcreatesahigh-speedcommunicationchannelbetweenmicrocontrollersandiscommonlyfoundinautomotiveandotherdistributedcontrolapplications.

2.2.1.TexasInstrumentsLM3S811I/OpinsFigure2.7drawstheI/OportstructurefortheLM3S811microcontroller.Mostpinshavetwonames:theportpin(PA0)andthealternatefunctionname(U0Rx).BecausetheI/Oportsareconnectedtothesystembusinterface,themicrocontrollercanperformI/ObuscyclessimultaneouswithinstructionfetchesfromflashROM.Thereare32digitalI/Olinesand4ADCinputs.TheADChas10bitsofprecisionandcansampleupto500ktimespersecond.Table2.3liststheregularandalternatenamesoftheportpins.PC4hasnoalternatefunction.

Regular AlternatePinName AlternateFunction

PA0–PA1 U0RX,U0TX UniversalAsynchronousReceiver/Transmitter,UART0

PA2–PA5 SCLK,SFSS,SRX,STX

SynchronousSerialInterface,SSI0

PB0,PB1 PWM2,PWM3 PulseWidthModulator1

PB2–PB3 SCL,SDA Inter-IntegratedCircuit,I2C0

PB4,PB6,PD7

C0-,C0+,C0o AnalogComparator0

PB5 CCP5 Timer2BCapture/Compare

PB7,PC0–PC3

TRST,TCLK,TMS,TDI,TDO

JTAGDebugger

PC5 CCP1 Timer0BCapture/Compare

PC6 CCP3 Timer1BCapture/Compare

PC7 CCP4 Timer2ACapture/Compare

PD0,PD1 PWM0,PWM1 PulseWidthModulator0

PD2–PD3 U1RX,U1TX UniversalAsynchronousReceiver/Transmitter,UART1

PD4 CCP0 Timer0ACapture/Compare

PD5 CCP2 Timer1ACapture/Compare

PD6 Fault HoldallPWMoutputsinsafestate

PE0,PE1 PWM4,PWM5 PulseWidthModulator2

Table2.3.LM3S811I/Opinsthathavealternatefunctions.

Figure2.7.I/OportpinsfortheLM3S811microcontroller.SysTickandthetimerscangenerateperiodicinterrupts,whichwillbeusefulforexecutingperiodicsoftwaretaskslikedataacquisitionandcontrol.Eachtimermodulehastwopins,whichcanbeinputcaptureoroutputcompare.Therefore,thethreetimermodulesprovidesixtimerpins.EachUARThastwopins,onetransmitterandonereceiver.TheI2Cporthastwobidirectionalpins.TheSSImoduleusesfourpins.TherearesixpossiblePWMoutputpins.TheonefaultinputcanbeconfiguredtoaffectallsixPWMsignals.PortpinPC4hasnoalternatefunction.

TexasInstrumentssellsanevaluationkitfortheLM3S811.ThereisoneswitchandoneLEDontheboard,seeFigure2.8.ThepartnumbersforthekitareEKK-LM3S811,EKI-LM3S811,EKC-LM3S811,EKT-LM3S811,andEKS-LM3S811.ThedifferentversionsspecifywhichcompilerisincludedontheCDinthekit.Table2.4liststhephysicaldevicesattachedtopinsonthekit.Theseconnectionscanbebrokenbyremovingajumperontheboard.Thiswaythepinisavailableforyourcircuits.

Pin Function ToIsolate,Remove…

ADC0 ADCInputfromThumbwheelPotentiometer

JP5

PA0/U0RX VirtualCOMPortReceive JP3

PA1/U0TX VirtualCOMPortTransmit JP4

PB2/I2CSCL I2CSCL0toOLEDDisplay JP1

PB3/I2CSDA I2CSDA0toOLEDDisplay JP2

PB7/TRST JTAGDebuggerTestReset DoNotUse

PC0/TCK/SWCLK JTAGDebuggerClock DoNotUse

PC1/TMS/SWDIO JTAGDebuggerModeSelect DoNotUse

PC2/TDI JTAGDebuggerDataIn DoNotUse

PC3/TDO/SWO JTAGDebuggerDataOut DoNotUse

PC4 UserMomentaryNegativeLogicPushButton

JP6

PC5/CCP1 UserLED JP7

PD7/C0O OLEDDisplayPowerEnable JP8

Table2.4.PortpinsconnectedtophysicaldevicesontheLM3S811evaluationkit.

Figure2.8.SwitchandLEDinterfacesontheLM3S811evaluationboard.

2.2.2.TexasInstrumentsLM3S1968I/OpinsFigure2.9drawstheI/OportstructurefortheLM3S1968microcontroller.Mostpinshavetwonames:theportpin(PA0)andthealternatefunctionname(U0Rx).However,pinsPF5,PF7,PG3,PG5,andPH2havenoalternatefunction.BecausetheI/Oportsareconnectedtothesystembusinterface,themicrocontrollercanperformI/ObuscyclessimultaneouswithinstructionfetchesfromflashROM.Ithas3UARTports,2SSIports,2I2Cports,a10-bitADC,6PWMoutputs,4timerinputcapture/outputcomparepins,2quadratureencoderinterfaces,andthreeanalogcomparators.TheADCcansampleupto1000kpersecond.Thereare52digitalI/Olinesand8ADCinputs.Table2.5liststheregularandalternatenamesoftheportpins.

Figure2.10showsaTexasInstrumentsevaluationkitfortheLM3S1968.TherearefiveswitchesandoneLEDontheboard,seeFigure2.11.ThepartnumbersforthesekitsareEKK-LM3S1968,EKI-LM3S1968,EKC-LM3S1968,EKT-LM3S1968,andEKS-LM3S1968.ThedifferentversionsspecifywhichcompilerisincludedontheCDinthekit.Table2.6liststhephysicaldevicesattachedtopinsonthekit.Theseconnectionscanbebrokenbyremovingajumperontheboard.Byremovingthejumperthepinisavailableforyourcircuits.Youmustenableinternalpull-upstousetheswitchesontheboard.

Figure2.9.I/OportpinsfortheLM3S1968microcontroller.Observation:TousetheswitchesontheLM3S1968boardyouneedtoactivatetheinternalpull-upresistorsfortheport,setbits3 – 7inGPIO_PORTG_PUR_R.

Observation:TheswitchesontheLM3S1968boardarenegativelogic.

Observation:TheLED(PG2)ontheLM3S1968boardispositivelogic.

Observation:ThedebuggerfunctionalitywillbelostifyouconfigurePB7,PC3–0asregulardigitalI/O.

Regular AlternatePinName AlternateFunction

PA0–PA1 U0RX,U0TX UniversalAsynchronousReceiver/Transmitter,UART0

PA2–PA5 S0CLK,S0FS,S0RX,S0TX

SynchronousSerialInterface,SSI0

PA6–PA7 SCL1,SDA1 Inter-IntegratedCircuit,I2C1

PB0 CCP0 Timer0ACapture/Compare

PB1 CCP2 Timer1ACapture/Compare

PB2–PB3 SCL0,SDA0 Inter-IntegratedCircuit,I2C0

PB4,PB6,PF4

C0-,C0+,C0o AnalogComparator0

PB5,PC5 C1-,C1+ AnalogComparator1

PB7,PC0–PC3

TRST,TCLK,TMS,TDI,TDO

JTAGDebugger

PC4,PF0,PD0

PHA0,PHB0,IDX0 QuadratureEncoderInterface,QEI0

PC6,PC7 C2+,C2- AnalogComparator2

PD2–PD3 U1RX,U1TX UniversalAsynchronousReceiver/Transmitter,UART1

PE0–PE3 S1CLK,S1FS,S1RX,S1TX

SynchronousSerialInterface,SSI1

PF2,PF3 PWM4,PWM5 PulseWidthModulator2

PF6 CCP1 Timer0BCapture/Compare

PG0–PG1 U2RX,U2TX UniversalAsynchronousReceiver/Transmitter,UART2

PG2,PD1 PWM0,PWM1 PulseWidthModulator0

PG4 CCP3 Timer1BCapture/Compare

PG6,PG7,PF1

PHA1,PHB1,IDX1 QuadratureEncoderInterface,QEI1

PH0,PH1 PWM2,PWM3 PulseWidthModulator1

PH3 Fault HoldallPWMoutputsinsafestate

Table2.5.LM3S1968I/Opinsthathavealternatefunctions.

Pin Function

ToIsolate,Remove…

PA0/U0RX VirtualCOMPortReceive JP4

PA1/U0TX VirtualCOMPortTransmit JP13

PA2/S0CLK OLEDDisplayClock JP11

PA3/S0FS OLEDDisplayChipSelect JP5

PA5/S0TX OLEDDisplayDataIn JP10

PB7/TRST JTAGDebuggerTestReset DoNotUse

PC0/TCK/SWCLK JTAGDebuggerClock DoNotUse

PC1/TMS/SWDIO JTAGDebuggerModeSelect DoNotUse

PC2/TDI JTAGDebuggerDataIn DoNotUse

PC3/TDO/SWO JTAGDebuggerDataOut DoNotUse

PG2/PWM0 UserLED JP2

PG3 SW3UpMomentaryNegativeLogicPushButton

JP1

PG4 SW4DownMomentaryNegativeLogicPushButton

JP9

PG5 SW5LeftMomentaryNegativeLogicPushButton

JP8

PG6/PHA1 SW6RightMomentaryNegativeLogicPushButton

JP6

PG7/PHB1 SelectMomentaryNegativeLogicPushButton

JP7

PH0/PWM2 Sound+ JP14

PH1/PWM3 Sound- JP15

PH2 OLEDDisplayData/ControlSelect JP12

PH3/Fault OLEDDisplayPowerEnable JP3

Table2.6.PortpinsconnectedtophysicaldevicesontheLM3S1968evaluationkit.

Figure2.10.EvaluationkitfortheLM3S1968microcontroller.Thereareanumberofpossibilitiesfordesigningprototypesystemsusingevaluationkits.Oneoptionistosolderindividualwirestopinsasneeded.Thisapproachissimpleandreliable.Itisappropriateifthekitisbeingusedforoneapplicationandthechoiceofpinsisunlikelytochange.Thedisadvantageischangingpinsrequiresunsolderingandresoldering.

Figure2.11.SwitchandLEDinterfacesontheLM3S1968evaluationboard.Asecondapproachistosolderafemalesocketontotheevaluationkit.Toconnectapintoyourexternalcircuit,youplaceasolidwireintothesocket.Thismethodisconvenientifyouplantomovewiresasthedesignchanges.Afteralongperiod,thefemalesocketcanwearoutortheendsofwiresmaybreakoffinsidethesocket.Changingthesocketisverydifficult.

AthirdapproachisillustratedinFigure2.10.ThebreadboardinterfacewasbuiltusingSamtecTSW-133-09-L-S-REandTSW-133-08-L-S-RAconnectors.Right-anglemale-maleheadersaresolderedtotheboardinsuchawaythatthemalepinscanbeinsertedintoastandardsolderlessbreadboard.Thisapproachisconvenientifyouareprototypingonasolderlessbreadboard.Thisconfigurationisextremelyrobustandcanwithstandmultipleinsertionsandextractions.Pushstraightdowntoinserttheboardintothebreadboard.Toremovetheboard,usetwosmallscrewdriversandwedgebetweentheboardandthebreadboardoneachsidealittleatatime.Toassemblethisinterface,itmaybehelpfultoseparatelyinserteachunsolderedheaderintothebreadboardtoholditinplacewhileitisbeingsoldered.Ifthespacingbetweentheheadersandthedevelopmentboardisnotcorrect,thenitwillnotfitintothebreadboard.Noticehowthedevelopmentboardfitsintotheslitinthemiddleofthebreadboard.

2.2.3.TexasInstrumentsTM4C123LaunchPadI/OpinsFigure2.12drawstheI/OportstructurefortheLM4F120H5QRandTM4C123GH6PM.ThesemicrocontrollersareusedontheEK-LM4F120XLandEK-TM4C123GXLLaunchPads.PinsontheLM3Sfamilyhavetwopossibilities:digitalI/Ooranalternativefunction.However,pinsontheLM4F/TM4CfamilycanbeassignedtoasmanyaseightdifferentI/Ofunctions.PinscanbeconfiguredfordigitalI/O,analoginput,timerI/O,orserialI/O.ForexamplePA0canbedigitalI/Oorserialinput.TherearetwobusesusedforI/O.ThedigitalI/Oportsareconnectedtoboththeadvancedperipheralbus(liketheLM3Sfamily)andtheadvancedhigh-performancebus(runsfaster).Becauseofthemultiplebuses,themicrocontrollercansimultaneouslyperformI/ObuscycleswithinstructionfetchesfromflashROM.TheLM4F120H5QRhaseightUARTports,fourSSIports,fourI2Cports,two12-bitADCs,twelvetimers,aCANport,andaUSBinterface.TheTM4C123GH6PMaddsupto16PWMoutputs.Thereare43I/Olines.TherearetwelveADCinputs;eachADCcanconvertupto1millionsamplespersecond.Table2.7liststheregularandalternatenamesoftheportpins.

EachpinhasoneconfigurationbitintheAMSELregister.WesetthisbittoconnecttheportpintotheADCoranalogcomparator.Fordigitalfunctions,eachpinalsohasfourbitsinthePCTLregister,whichwesettospecifythealternativefunctionforthatpin(0meansregularI/Oport).Table2.7showsthe4-bitPCTLconfigurationusedtoconnecteachpintoitsalternatefunction.Forexample,column“7”meansset4-bitfieldinPCTLto01112.

PinsPC3–PC0wereleftoffTable2.7becausethesefourpinsarereservedfortheJTAGdebuggerandshouldnotbeusedforregularI/O.Notice,mostalternatefunctionmodules(e.g.,U0Rx)onlyexistononepin(PA0).Whileotherfunctionscouldbemappedtotwoorthreepins(e.g.,CAN0Rxcouldbemappedtooneofthefollowing:PB4,PE4,orPF0.)

Forexample,ifwewishedtouseSSI2onpinsPB7–4,wewouldsetbits7–4intheDENregister(enabledigital),clearbits7–4intheAMSELregister(disableanalog),writea0010,0010,0010,0010tobits31–16inthePCTLregister(enableSSI2functionality),andsetbits7–4intheAFSELregister(enablealternatefunction).IfwewishedtosampleananalogsignalonPD3,wewouldsetbit3inthealternatefunctionselectregisterAFSEL,

clearbit3inthedigitalenableregisterDEN(disabledigital),setbit3intheanalogmodeselectregisterAMSEL(enableanalog),andactivateoneoftheADCstosamplechannel4.Additionalexampleswillbepresentedthroughoutthebook.

TheTexasInstrumentsLaunchPadevaluationboard(Figure2.13)isalow-costdevelopmentboardavailableaspartnumberEK-TM4C123GXLfromwww.ti.comandfromregularelectronicdistributorslikeDigikey,Mouser,Newark,Arrow,andAvnet.ThekitprovidesanintegratedIn-CircuitDebugInterface(ICDI),whichallowsprogramminganddebuggingoftheonboardLM4Fmicrocontroller.OneUSBcableisusedbythedebugger(ICDI),andtheotherUSBallowstheusertodevelopUSBapplications(device).Theusercanselectboardpowertocomefromeitherthedebugger(ICDI)ortheUSBdevice(device)bysettingthePowerselectionswitch.

Figure2.12.I/OportpinsfortheLM4F120H5QR/TM4C123GH6PMmicrocontrollers.TherearenoPWMmodulesontheLM4F120.Also,theUSBontheLM4F120supportsdevicemode,whiletheTM4C123addshostandon-the-go(OTG)modes.PinsPA1–PA0createaserialport,whichislinkedthroughthedebuggercabletothePC.TheseriallinkisaphysicalUARTasseenbytheLM4F/TM4CandmappedtoavirtualCOMportonthePC.TheUSBdeviceinterfaceusesPD4andPD5.TheJTAGdebuggerrequirespinsPC3–PC0.TheLaunchPadconnectsPB6toPD0,andPB7toPD1.IfyouwishtousebothPB6andPD0youwillneedtoremovetheR9resistor.Similarly,tousebothPB7andPD1removetheR10resistor.TheUSBconnectoronthesideoftheLM4F120LaunchPadhasfourwiresbecauseitsupportsonlydevicemode.However,theUSBconnectoronthesideoftheTM4C123LaunchPadhasfivewiresbecauseitsupportsdevice,host,andOTGmodes.

IO Ain 0 1 2 3 4 5 6 7 8 9 14

PA0 Port U0Rx CAN1Rx

PA1 Port U0Tx CAN1Tx

PA2 Port SSI0Clk

PA3 Port SSI0Fss

PA4 Port SSI0Rx

PA5 Port SSI0Tx

PA6 Port I2C1SCL M1PWM2

PA7 Port I2C1SDA M1PWM3

PB0 USB0ID Port U1Rx T2CCP0

PB1 USB0VBUS Port U1Tx T2CCP1

PB2 Port I2C0SCL T3CCP0

PB3 Port I2C0SDA T3CCP1

PB4 Ain10 Port SSI2Clk M0PWM2 T1CCP0 CAN0Rx

PB5 Ain11 Port SSI2Fss M0PWM3 T1CCP1 CAN0Tx

PB6 Port SSI2Rx M0PWM0 T0CCP0

PB7 Port SSI2Tx M0PWM1 T0CCP1

PC4 C1- Port U4Rx U1Rx M0PWM6 IDX1 WT0CCP0 U1RTS

PC5 C1+ Port U4Tx U1Tx M0PWM7 PhA1 WT0CCP1 U1CTS

PC6 C0+ Port U3Rx PhB1 WT1CCP0 USB0epen

PC7 C0- Port U3Tx WT1CCP1 USB0pflt

PD0 Ain7 Port SSI3Clk SSI1Clk I2C3SCL M0PWM6 M1PWM0 WT2CCP0

PD1 Ain6 Port SSI3Fss SSI1Fss I2C3SDAM0PWM7 M1PWM1 WT2CCP1

PD2 Ain5 Port SSI3Rx SSI1Rx M0Fault0 WT3CCP0 USB0epen

PD3 Ain4 Port SSI3Tx SSI1Tx IDX0 WT3CCP1 USB0pflt

PD4 USB0DM Port U6Rx WT4CCP0

PD5 USB0DP Port U6Tx WT4CCP1

PD6 Port U2Rx M0Fault0 PhA0 WT5CCP0

PD7 Port U2Tx PhB0 WT5CCP1 NMI

PE0 Ain3 Port U7Rx

PE1 Ain2 Port U7Tx

PE2 Ain1 Port

PE3 Ain0 Port

PE4 Ain9 Port U5Rx I2C2SCL M0PWM4 M1PWM2 CAN0Rx

PE5 Ain8 Port U5Tx I2C2SDAM0PWM5 M1PWM3 CAN0Tx

PF0 Port U1RTS SSI1Rx CAN0Rx M1PWM4 PhA0 T0CCP0 NMI C0o

PF1 Port U1CTS SSI1Tx M1PWM5 PhB0 T0CCP1 C1o TRD1

PF2 Port SSI1Clk M0Fault0 M1PWM6 T1CCP0 TRD0

PF3 Port SSI1Fss CAN0Tx M1PWM7 T1CCP1 TRCLK

PF4 Port M1Fault0 IDX0 T2CCP0 USB0epen

Table2.7.PMCxbitsintheGPIO_PORTx_PCTL_RregisterontheLM4F/TM4Cspecifyalternatefunctions.PB1,PB0,PD4andPD5arehardwiredtotheUSBdevice.PA0andPA1arehardwiredtotheserialport.PWMisnotavailableonLM4F120.

Each32-bitGPIO_PORTx_PCTL_R registerdefinesthealternatefunctionfortheeightpinsofthatport,4bitsforeachpin.Forexample,ifwewishedtospecifyPA5-2asSSI0,wewouldsetPortAPCTLbits23-16to0x2222likethis:

GPIO_PORTA_PCTL_R=(GPIO_PORTA_PCTL_R&0xFF0000FF)+0x00222200;

Figure2.13.TexasInstrumentsLaunchPadbasedontheLM4F120H5QRorTM4C123GH6PM.TheTexasInstrumentsLaunchPadevaluationboardhastwoswitchesandone3-colorLED,asshowninFigure2.14.Theswitchesarenegativelogicandwillrequireactivationoftheinternalpull-upresistors.Inparticular,youwillsetbits0and4in GPIO_PORTF_PUR_R register.TheLEDinterfacesonPF3–PF1arepositivelogic.TousetheLED,makethePF3–PF1pinsanoutput.Toactivatetheredcolor,outputaonetoPF1.ThebluecolorisonPF2,andthegreencoloriscontrolledbyPF3.The0- resistors(R1,R2,R11,R12,R13,R25,andR29)canberemovedtodisconnectthecorrespondingpinfromtheexternalhardware.

Figure2.14.SwitchandLEDinterfacesontheTexasInstrumentsLaunchPadEvaluationBoard.Thezeroohmresistorscanberemovedsothecorrespondingpincanbeusedforitsregularpurpose.

TheLaunchPadhasfour10-pinconnectors,labeledasJ1J2J3J4inFigures2.13and2.15,towhichyoucanattachyourexternalsignals.Thetopsideoftheseconnectorshasmalepinsandthebottomsidehasfemalesockets.Theintentistostackboardstogethertomakealayeredsystem,seeFigure2.15.TexasInstrumentsalsosuppliesBoosterPacks,whicharepre-madeexternaldevicesthatwillplugintothis40-pinconnector.TheBoosterPacksfortheMSP430LaunchPadarecompatible(onesimplyplugsthese20-pinconnectorsintotheoutertworows)withthisboard.Theinner10-pinheaders(connectorsJ3andJ4)arenotintendedtobecompatiblewithotherTILaunchPads.J3andJ4applyonlytoCortex-M4BoosterPacks.

TherearetwomethodstoconnectexternalcircuitstotheLaunchPad.Onemethodistopurchaseamaletofemalejumpercable(e.g.,itemnumber826atwww.adafruit.com).Youcouldcreatelow-costmaletofemalejumperwiresbysolderingasolidwireintoafemalesockets(e.g.,HiroseDF11-2428SCA).Asecondmethodistousesolid22-gaugeor24-gaugewireandconnectoneendofthesolidwireintothebottomorfemalesideoftheLaunchPad.

Figure2.15.InterfaceconnectorsontheTexasInstrumentsLM4F120/TM4C123LaunchPadEvaluationBoard.

2.2.4.TexasInstrumentsTM4C1294ConnectedLaunchPadI/OpinsFigure2.16showsthe90I/OpinsavailableontheTM4C1294NCPDT,whichisthemicrocontrollerusedontheConnectedLaunchPad.PinsontheTM4CfamilycanbeassignedtoasmanyassevendifferentI/Ofunctions,seeTable2.8.PinscanbeconfiguredfordigitalI/O,analoginput,timerI/O,orserialI/O.ForexamplePA0canbedigitalI/O,serialinput,I2Cclock,TimerI/O,orCANreceiver.TherearetwobusesusedforI/O.UnliketheTM4C123,thedigitalI/Oportsareonlyconnectedtotheadvancedhigh-performancebus.ThemicrocontrollercanperformI/ObuscyclessimultaneouswithinstructionfetchesfromflashROM.TheTM4C1294NCPDThaseightUARTports,fourSSIports,tenI2Cports,two12-bitADCs,eighttimers,twoCANports,aUSBinterface,8PWMoutputs,andanEthernetport.Ofthe90I/Olines,twentypinscanbeusedforanaloginputstotheADC.TheADCcanconvertupto1Msamplespersecond.Table2.8liststheregularandalternatefunctionsoftheportpins.

Each32-bit GPIO_PORTx_PCTL_R registerdefinesthealternatefunctionfortheeightpinsofthatport,4bitsforeachpin.Forexample,ifwewishedtospecifyPD5–PD4asUART2,wewouldsetPortDPCTLbits23-16to0x11likethis:

GPIO_PORTD_PCTL_R=(GPIO_PORTD_PCTL_R&0xFF00FFFF)+0x00110000;

Figure2.16.I/OportpinsfortheTM4C1294NCPDTmicrocontroller.Figure2.17showsthepinlocationsofthetwoBoosterPackconnectors.TherearethreemethodstoconnectexternalcircuitstotheConnectedLaunchPad.Onemethodusesmaletofemalejumpercable(e.g.,itemnumber826atwww.adafruit.com)orsolderasolidwireintoafemalesocket(e.g.,HiroseDF11-2428SCA)creatingamale-to-femalejumperwire.Inthismethod,youconnectthefemalesockettothetopoftheLaunchPadandthemalepinintoasolderlessbreadboard.Thesecondmethodusesmale-to-malewiresinterfacingtothebottomoftheLaunchPad.Thethirdmethodusestwo49-pinright-angleheaderssotheentireLaunchPadcanbepluggedintoabreadboard.YouwillneedoneeachofSamtecpartsTSW-149-09-L-S-REandTSW-149-08-L-S-RA.ThisconfigurationisshowninFigure2.18,anddirectionscanbefoundathttp://users.ece.utexas.edu/~valvano/arm/TM4C1294soldering.pdf

TheConnectedLaunchPadhastwoswitchesandfourLEDs.SwitchSW1isconnectedtopinPJ0,andSW2isconnectedtoPJ1.Thesetwoswitchesarenegativelogicandrequireenablingtheinternalpullup(PUR).Aresetswitchwillresetthemicrocontrollerandyoursoftwarewillstartwhenyoureleasetheswitch.PositivelogicLEDsD1,D2,D3,andD4areconnectedtoPN1,PN0,PF4,andPF0respectively.ApowerLEDindicatesthat3.3voltpowerispresentontheboard.R19isa0ΩresistorconnectingPA3andPQ2.Similarly,R20isa0ΩresistorconnectingPA2andPQ3.YouneedtoremoveR19ifyouplantousebothPA3andPQ2.YouneedtoremoveR20ifyouplantousebothPA2andPQ3.SeeFigures2.18and2.19.

Figure2.17.InterfaceconnectorsontheEK-TM4C1294-XLLaunchPadEvaluationBoard.JumperJP1hassixpinscreatingthreerowsoftwo.ExactlyonejumpershouldbeconnectedintheJP1block,whichselectsthepowersource.ThetoppositionisforBoosterPackpower.ThemiddlepositiondrawspowerfromtheUSBconnector,labeledOTG,ontheleftsideoftheboardneartheEthernetjack.WerecommendplacingtheJP1jumpinthebottompositionsopowerisdrawnfromtheICDI(Debug)USBconnection.Undernormalconditions,youshouldplacejumpersinbothJ2andJ3.JumpersJ2andJ3facilitatemeasuringcurrenttothemicrocontroller.WerecommendyouplaceJP4andJP5inthe“UART”positionsoPA1andPA0areconnectedtothePCasavirtualCOMport.Yourcoderunsonthe128-pinTM4C1294microcontroller.ThereisasecondTM4Cmicrocontrollerontheboard,whichactsastheJTAGdebuggerforyourTM4C1294.YouconnecttheDebugUSBtoaPCinordertodownloadanddebugsoftwareontheboard.TheotherUSBisforuserapplications.

Figure2.18.EK-TM4C1294-XLConnectedLaunchPad.

Pin Analog 1 2 3 5 6 7 11 13 14

PA0 - U0Rx I2C9SCL T0CCP0 - - CAN0Rx - - -

PA1 - U0Tx I2C9SDA T0CCP1 - - CAN0Tx - - -

PA2 - U4Rx I2C8SCL T1CCP0 - - - - - -

PA3 - U4Tx I2C8SDA T1CCP1 - - - - - -

PA4 - U3Rx I2C7SCL T2CCP0 - - - - - -

PA5 - U3Tx I2C7SDA T2CCP1 - - - - - -

PA6 - U2Rx I2C6SCL T3CCP0 USB0EPEN - - - SSI0XDAT2 -

PA7 - U2Tx I2C6SDA T3CCP1 USB0PFLT - - USB0EPEN SSI0XDAT3 -

PB0 USB0ID U1Rx I2C5SCL T4CCP0 - - CAN1Rx - - -

PB1 USB0VBUS U1Tx I2C5SDA T4CCP1 - - CAN1Tx - - -

PB2 - - I2C0SCL T5CCP0 - - - - - USB0STP

PB3 - - I2C0SDA T5CCP1 - - - - - USB0CLK

PB4 AIN10 U0CTS I2C5SCL - - - - - - -

PB5 AIN11 U0RTS I2C5SDA - - - - - - -

PC4 C1- U7Rx - - - - - - - -

PC5 C1+ U7Tx - - - - RTCCLK - - -

PC6 C0+ U5Rx - - - - - - - -

PC7 C0- U5Tx - - - - - - - -

PD0 AIN15 - I2C7SCL T0CCP0 C0o - - - - -

PD1 AIN14 - I2C7SDA T0CCP1 C1o - - - - -

PD2 AIN13 - I2C8SCL T1CCP0 C2o - - - - -

PD3 AIN12 - I2C8SDA T1CCP1 - - - - - -

PD4 AIN7 U2Rx - T3CCP0 - - - - - -

PD5 AIN6 U2Tx - T3CCP1 - - - - - -

PD6 AIN5 U2RTS - T4CCP0 USB0EPEN - - - - -

PD7 AIN4 U2CTS - T4CCP1 USB0PFLT - - - - -

PE0 AIN3 U1RTS - - - - - - - -

PE1 AIN2 U1DSR - - - - - - - -

PE2 AIN1 U1DCD - - - - - - - -

PE3 AIN0 U1DTR - - - - - - - -

PE4 AIN9 U1RI - - - - - - - -

PE5 AIN8 - - - - - - - - -

PF0 - - - - EN0LED0 M0PWM0 - - - SSI3XDAT1

PF1 - - - - EN0LED2 M0PWM1 - - - SSI3XDAT0

PF2 - - - - - M0PWM2 - - - SSI3Fss

PF3 - - - - - M0PWM3 - - - SSI3Clk

PF4 - - - - EN0LED1 M0FAULT0 - - - SSI3XDAT2

PG0 - - I2C1SCL - EN0PPS M0PWM4 - - - -

PG1 - - I2C1SDA - - M0PWM5 - - - -

PH0 - U0RTS - - - - - - - -

PH1 - U0CTS - - - - - - - -

PH2 - U0DCD - - - - - - - -

PH3 - U0DSR - - - - - - - -

PJ0 - U3Rx - - EN0PPS - - - - -

PJ1 - U3Tx - - - - - - - -

PK0 AIN16 U4Rx - - - - - - - -

PK1 AIN17 U4Tx - - - - - - - -

PK2 AIN18 U4RTS - - - - - - - -

PK3 AIN19 U4CTS - - - - - - - -

PK4 - - I2C3SCL - EN0LED0 M0PWM6 - - - -

PK5 - - I2C3SDA - EN0LED2 M0PWM7 - - - -

PK6 - - I2C4SCL - EN0LED1 M0FAULT1 - - - -

PK7 - U0RI I2C4SDA - RTCCLK M0FAULT2 - - - -

PL0 - - I2C2SDA - - M0FAULT3 - - - USB0D0

PL1 - - I2C2SCL - - PhA0 - - - USB0D1

PL2 - - - - C0o PhB0 - - - USB0D2

PL3 - - - - C1o IDX0 - - - USB0D3

PL4 - - - T0CCP0 - - - - - USB0D4

Pin Analog 1 2 3 5 6 7 11 13 14

PL5 - - - T0CCP1 - - - - - USB0D5

PL6 USB0DP - - T1CCP0 - - - - - -

PL7 USB0DM - - T1CCP1 - - - - - -

PM0 - - - T2CCP0 - - - - - -

PM1 - - - T2CCP1 - - - - - -

PM2 - - - T3CCP0 - - - - - -

PM3 - - - T3CCP1 - - - - - -

PM4 TMPR3 U0CTS - T4CCP0 - - - - - -

PM5 TMPR2 U0DCD - T4CCP1 - - - - - -

PM6 TMPR1 U0DSR - T5CCP0 - - - - - -

PM7 TMPR0 U0RI - T5CCP1 - - - - - -

PN0 - U1RTS - - - - - - - -

PN1 - U1CTS - - - - - - - -

PN2 - U1DCDU2RTS - - - - - - -

PN3 - U1DSR U2CTS - - - - - - -

PN4 - U1DTR U3RTS I2C2SDA - - - - - -

PN5 - U1RI U3CTS I2C2SCL - - - - - -

PP0 C2+ U6Rx - - - - - - - -

PP1 C2- U6Tx - - - - - - - -

PP2 - U0DTR - - - - - - - USB0NXT

PP3 - U1CTS U0DCD - - - RTCCLK - - USB0DIR

PP4 - U3RTS U0DSR - - - - - - USB0D7

PP5 - U3CTS I2C2SCL - - - - - - USB0D6

PQ0 - - - - - - - - - SSI3Clk

PQ1 - - - - - - - - - SSI3Fss

PQ2 - - - - - - - - - SSI3XDAT0

PQ3 - - - - - - - - - SSI3XDAT1

PQ4 - U1Rx - - - - DIVSCLK - - -

Table2.8.PMCxbitsintheGPIO_PORTx_PCTL_RregisterontheTM4C1294specifyalternatefunctions.PD7canbeNMIbysettingPCTLbits31-28to8.PL6andPL7arehardwiredtotheUSB.

Figure2.19.SwitchandLEDinterfacesontheConnectedLaunchPadEvaluationBoard.Thezeroohmresistorscanberemovedsoallthepinscanbeused.EachpinhasoneconfigurationbitintheAMSELregister.WesetthisbittoconnecttheportpintotheADCoranalogcomparator.Fordigitalfunctions,eachpinalsohasfourbitsinthePCTLregister,whichwesettospecifythealternativefunctionforthatpin(0meansregularI/Oport).Table2.8showsthe4-bitPCTLconfigurationusedtoconnecteachpintoitsalternatefunction.Forexample,column“3”meansset4-bitfieldinPCTLto0011.

PinsPC3–PC0wereleftoffTable2.8becausethesefourpinsarereservedfortheJTAGdebuggerandshouldnotbeusedforregularI/O.Notice,somealternatefunctionmodules(e.g.,U0Rx)onlyexistononepin(PA0).Whileotherfunctionscouldbemappedtotwoorthreepins.Forexample,T0CCP0couldbemappedtooneofthefollowing:PA0,PD0,orPL4.

ThePCTLbitsinTable2.8canbetrickytounderstand.Forexample,ifwewishedtouseUART6onpinsPP0andPP1,wewouldsetbits1,0intheDENregister(enabledigital),clearbits1,0intheAMSELregister(disableanalog),writea0001,0001tobits7–0inthePCTLregister(enableUART6functionality),andsetbits1,0intheAFSELregister(enablealternatefunction).IfwewishedtosampleananalogsignalonPD0,wewouldsetbit0inthealternatefunctionselectregisterAFSEL,clearbit0inthedigitalenableregisterDEN(disabledigital),setbit0intheanalogmodeselectregisterAMSEL(enableanalog),andactivateoneoftheADCstosamplechannel15.Additionalexampleswillbepresentedthroughoutthebook.

JumpersJP4andJP5selectwhethertheserialportonUART0(PA1–PA0)oronUART2(PD5–4)islinkedthroughthedebuggercabletothePC.TheseriallinkisaphysicalUARTasseenbytheTM4C1294andismappedtoavirtualCOMportonthePC.TheUSBdeviceinterfaceusesPL6andPL7.TheJTAGdebuggerrequirespinsPC3–PC0.

Tousethenegativelogicswitches,makethepinsdigitalinputs,andactivatetheinternalpull-upresistors.Inparticular,youwillactivatethePortJclock,clearbits0and1in GPIO_PORTJ_DIR_R register,setbits0and1in GPIO_PORTJ_DEN_R register,andsetbits0and1in GPIO_PORTJ_PUR_R register.TheLEDinterfacesarepositivelogic.TousetheLEDs,makethePN1,PN0,PF4,andPF0pinsanoutput.YouwillactivatethePortNclock,setbits0and1in GPIO_PORTN_DIR_R register,andsetbits0and1in GPIO_PORTN_DEN_R register.YouwillactivatethePortFclock,setbits0and4in GPIO_PORTF_DIR_R register,andsetbits0and4in GPIO_PORTF_DEN_Rregister.

2.3.ARM CortexTM-MAssemblyLanguageThissectionfocusesontheARM ® CortexTM-Massemblylanguage.TherearemanyARM®processors,andthisbookfocusesonCortexTM-Mmicrocontrollers,whichexecutesThumb ® instructionsextendedwithThumb-2technology.ThissectiondoesnotpresentalltheThumbinstructions.Rather,wepresentafewbasicinstructionsinordertounderstandhowtheprocessorworks.Forfurtherdetails,pleaserefertoVolume1(EmbeddedSystems:IntroductiontoARM ® CortexTM-MMicrocontrollers),andtotheARM ® CortexTM-MTechnicalReferenceManual.

2.3.1.SyntaxAssemblylanguageinstructionshavefourfieldsseparatedbyspacesortabs.Thelabelfieldisoptionalandstartsinthefirstcolumnandisusedtoidentifythepositioninmemoryofthecurrentinstruction.Youmustchooseauniquenameforeachlabel.Theopcodefieldspecifiestheprocessorcommandtoexecute.Theoperandfieldspecifieswheretofindthedatatoexecutetheinstruction.Thumbinstructionshave0,1,2,3,or4operands,separatedbycommas.Thecommentfieldisalsooptionalandisignoredbytheassembler,butallowsyoutodescribethesoftwaremakingiteasiertounderstand.Youcanaddoptionalspacesbetweenoperandsintheoperandfield.However,asemicolonmustseparatetheoperandandcommentfields.Goodprogrammersaddcommentstoexplainthesoftware.

LabelOpcodeOperandsComment

FuncMOVR0,#100;thissetsR0to100

BXLR;thisisafunctionreturn

Whendescribingassemblyinstructionswewillusethefollowinglistofsymbols

Ra Rd Rm Rn Rt andRt2 representregisters

#imm12 representsa12-bitconstant,0to4095

#imm16 representsa16-bitconstant,0to65535

operand2 representstheflexiblesecondoperandasdescribedinSection2.2.2

cond representsanoptionallogicalconditionaslistedinTable2.9

type enclosesanoptionaldatatypeaslistedinTable2.10

S isanoptionalspecificationthatthisinstructionsetstheconditioncodebits

Rm,shift specifiesanoptionalshifton Rm asdescribedinSection2.2.2

Rn,#offset specifiesanoptionaloffsetto Rn asdescribedinSection2.2.2

Suffix Flags Meaning

EQ Z=1 Equal

NE Z=0 Notequal

CS or HS C=1 Higherorsame,unsigned≥

CC or LO C=0 Lower,unsigned<

MI N=1 Negative

PL N=0 Positiveorzero

VS V=1 Overflow

VC V=0 Nooverflow

HI C=1andZ=0 Higher,unsigned>

LS C=0orZ=1 Lowerorsame,unsigned≤

GE N=V Greaterthanorequal,signed≥

LT N≠V Lessthan,signed<

GT Z=0andN=V Greaterthan,signed>

LE Z=1andN≠V Lessthanorequal,signed≤

AL Canhaveanyvalue

Always.Thisisthedefaultwhennosuffixisspecified.

Table2.9.Conditioncodesuffixesusedtooptionallyexecutioninstruction.

Forexample,thegeneraldescriptionoftheadditioninstruction

ADDcondRd,Rn,#imm12

couldrefertoanyofthefollowingexamples

ADDR0,#1;R0=R0+1

ADDR0,R1,#10;R0=R1+10

ADDGER5,#100;ifN==V,thenR5=R5+100

ADDEQR12,R1,#100;ifZ=1,thenR12=R1+100

Allobjectcodeishalfword-aligned.Thismeansinstructionscanbe16or32bitswide,andtheprogramcounterbit0willalwaysbe0.Thestackmustremainwordaligned,meaningthebottomtwobitsoftheSPwillalwaysremain0.

2.3.2.AddressingmodesandoperandsAfundamentalissueinprogramdevelopmentisthedifferentiationbetweendataandaddress.Itisinassemblylanguageprogrammingingeneralandaddressingmodesinspecificthatthisdifferentiationbecomesclear.Whenweputthenumber1000intoregisterR0,whetherthisisdataoraddressdependsonhowthe1000isused.Torunefficientlywetrytokeepfrequentlyaccesseddatainregisters.However,weneedtoaccessmemorytofetchparametersorsaveresults.Theaddressingmodeistheformattheinstructionusestospecifythememorylocationtoreadorwritedata.Allinstructionsbeginbyfetchingthemachineinstruction(opcodeandoperand)pointedtobythePC.Someinstructionsoperatecompletelywithintheprocessorandrequirenomemorydatafetches.Forexample,the ADD R1,R2 instructionperformsR1+R2andstoresthesumbackintoR1.Ifthedataisfoundintheinstructionitself,likeMOVR0,#1 ,theinstructionusesimmediateaddressingmode.Aregisterthatcontainstheaddressorlocationofdataiscalledapointerorindexregister.Indexedaddressingmodeusesaregisterpointertoaccessmemory.TheaddressingmodethatusesthePCasthepointeriscalledPC-relativeaddressingmode.Itisusedforbranching,forcallingfunctions,andaccessingconstantdatastoredinROM.TheaddressingmodeiscalledPC-relativebecausethemachinecodecontainstheaddressdifferencebetweenwheretheprogramisnowandtheaddresstowhichtheprogramwillaccess.Therearemanymoreaddressingmodes,butfornow,thesefewaddressingmodes,asillustratedbelow,areenoughtogetusstarted.The LDRinstructionwillreada32-bitwordfrommemoryandplacethedatainaregister.WithPC-relativeaddressing,theassemblerautomaticallycalculatesthecorrectPCoffset.

FuncPUSHR1,R2,LR;saveregistersandreturnaddress

MOVR2,#100;R2=100,immediateaddressing

LDRR1,=Count;R1pointstovariableCount,usingPC-relative

LDRR0,[R1];R0=valueofvariableCount

LDRR0,[R1,#4];R0=wordpointedtobyR1+4

LDRR0,[R1,#4]!;firstR1=R1+4,thenR0=wordpointedtobyR1

LDRR0,[R1],#4;R0=wordpointedtobyR1,thenR1=R1+4

LDRR0,[R1,R2];R0=wordpointedtobyR1+R2

LDRR0,[R1,R2,LSL#2];R0=wordpointedtobyR1+4*R2

BLSubroutine;callSubroutine,usingPC-relativeaddressing

POPR1,R2,PC;restoreregistersandreturn

Checkpoint2.4:Whatistheaddressingmodeusedfor?

Checkpoint2.5:AssumeR3equals0x2000.0000atthetime LDRR2,[R3,#8]isexecuted.Whataddresswillbeaccessed?IfR3ischanged,towhatvaluewillR3become?

Checkpoint2.6:AssumeR3equals0x2000.0000atthetime LDRR2,[R3],#8 isexecuted.Whataddresswillbeaccessed?IfR3ischanged,towhatvaluewillR3become?

Theoperationscausedbythefirsttwo LDR instructionsareillustratedinFigure2.20.Assumea32-bitvariable Count islocatedindataspaceatRAMaddress0x2000.0000.First,LDRR1,=Count makesR1equalto0x2000.0000.I.e.,R1pointsto Count .Theassemblerplacesaconstant0x2000.0000incodespaceandtranslatesthe =Count intothecorrectPC-relativeaccesstotheconstant(e.g., LDRR1,[PC,#28] ).Second,theLDRR0,[R1] instructionwilldereferencethispointer,bringingthecontentsatlocation0x2000.0000intoR0.Since Count islocatedat0x2000.0000,thisinstructionwillreadthevalueofthevariableintoR0.

Figure2.20.IndexedaddressingusingR1asaregisterpointertoaccessmemory.DataismovedintoR0.Codespaceiswhereweplaceprogramsanddataspaceiswhereweplacevariables.Manygeneraldataprocessinginstructionshaveaflexiblesecondoperand.Thisisshownas Operand2 inthedescriptionsofthesyntaxofeachinstruction. Operand2 canbeaconstantoraregisterwithoptionalshift.Wespecifyan Operand2 constantintheform #constant :

ADDRd,Rn,#constant;Rd=Rn+constant

where constant canbe( X and Y arehexadecimaldigits):

Constantproducedbyshiftingan8-bitvalueleftbyanynumberofbitsConstantoftheform 0x00XY00XYConstantoftheform 0xXY00XY00Constantoftheform 0xXYXYXYXY

Wecanalsospecifyan Operand2 registerintheform Rm,shift .Forexample:

ADDRd,Rn,Rm,shift;Rd=Rn+Rm

where Rm istheregisterholdingthedataforthesecondoperand,and shift isanoptionalshifttobeappliedto Rm . shift canbeoneof:

ASR#n Arithmeticshiftright n bits,1≤ n ≤32.

LSL#n Logicalshiftleft n bits,1≤n ≤31.

LSR#n Logicalshiftright n bits,1≤ n ≤32.

ROR#n Rotateright n bits,1≤ n ≤31.

RRX Rotaterightonebit,withextend.

Ifweomittheshift,orspecify LSL#0 ,theinstructionusesthevaluein Rm .Ifwespecifyashift,theshiftisappliedtothevaluein Rm ,andtheresulting32-bitvalueisusedbytheinstruction.However,thecontentsintheregister Rm remainunchanged.Forexample,

ADDR0,R1,LSL#4;R0=R0+R1*16(R1unchanged)

ADDR0,R1,ASR#4;signedR0=R0+R1/16(R1unchanged)

Analignedaccessisanoperationwhereaword-alignedaddressisusedforaword,dualword,ormultiplewordaccess,orwhereahalfword-alignedaddressisusedforahalfwordaccess.Byteaccessesarealwaysaligned.The T specifiestheinstructionisunprivileged.TheCortexTM-Mprocessorsupportsunalignedaccessonlyforthefollowinginstructions:

LDR , LDRT Load32-bitwordLDRH , LDRHT Load16-bitunsignedhalfwordLDRSH , LDRSHT Load16-bitsignedhalfwordSTR , STRT Store32-bitwordSTRH , STRHT Store16-bithalfword

Allotherreadandwritememoryoperationsgenerateausagefaultexceptioniftheyperformanunalignedaccess,andthereforetheiraccessesmustbeaddressaligned.

CommonError:Sincenoteveryinstructionsupportseveryaddressingmode,itwouldbeamistaketouseanaddressingmodenotavailableforthatinstruction.

2.3.3.MemoryaccessinstructionsThissectionpresentsmechanismstoreadfromandwritetomemory.AsillustratedinFigure2.20,toaccessmemorywefirstestablishapointertotheobject,thenuseindexedaddressing.UsuallycodespaceisinROM,butitispossibletoassigncodespacetoRAM.Dataspaceiswhereweplacevariables.Therearefourtypesofmemoryobjects,andtypicallyweuseaspecificregistertoaccessit.

MemoryobjecttypeRegisterExampleoperand

ConstantsincodespacePC =ConstantLocalvariablesonthestackSP [SP,#0x04]GlobalvariablesinRAMR0 – R12 [R0]I/OportsR0 – R12 [R0]The ADR usesPC-relativeaddressingandcanbeusedtoestablishapointertoaconstantinROMortoanaddresswithintheprogram.Thegeneralformfor ADR is

ADRcondRd,label

where cond isanoptionalcondition, Rd isthedestinationregister,and label isalabelwithinthecodespacewithintherangeof-4095to+4095fromtheaddressinthePC.Inreality,theassemblerwillgeneratean ADD or SUB instructiontocalculatethedesiredaddressusinganoffsettothePC. DCD isanassemblerdirectivethatdefinesa32-bitconstant.Weuseittocreateconstantsincodespace(ROM).Inthefollowingexample,afterexecutingthe ADR instruction,R5pointsto Pi ,andafterexecutingthe LDRinstruction,R6containsthedata.

AccessADRR5,Pi;R5=&Pi

LDRR6,[R5];R6=314159

…

BXLR

PiDCD314159

Weusethe LDR instructiontoloaddatafrommemoryintoaregister.Thereisaspecialformof LDR whichinstructstheassemblertoloadaconstantoraddressintoaregister.Thisisa“pseudo-instruction”andtheassemblerwilloutputsuitableinstructionstogeneratethespecifiedvalueintheregister.Thisformfor LDR is

LDRcondRd,=number

LDRcondRd,=label

where cond isanoptionalcondition(seeTable2.9), Rd isthedestinationregister,and label isalabelanywhereinmemory.Figure2.20illustrateshowtocreateapointertoavariableinRAM.AsimilarapproachcanbeusedtoaccessI/Oports.OntheTM4C123,PortAexistsataddress0x4000.43FC.Afterexecutingthefirst LDR instruction,R5equals0x4000.43FC,whichisapointertoPortA,andafterexecutingthesecond LDRinstruction,R6containsthevalueatPortA.

InputLDRR5,=0x400043FC;R5=0x400043FC,R5=&PortA

LDRR6,[R5];InputfromPortAintoR6

;…

BXLR

Theassemblertranslatedtheaboveassemblyintothisequivalent

InputLDRR5,[PC,#16];PC+16istheaddressoftheDCD

LDRR6,[R5]

;…

BXLR

DCD0x400043FC

Weusethe LDR instructiontoloaddatafromRAMtoaregisterandthe STR instructiontostoredatafromaregistertoRAM.Inreallife,whenwemoveaboxtothebasement,pushabroomacrossthefloor,loadbulletsintoagun,storespoonsinadrawer,popacandyintoyourmouth,ortransferemployeestoanewlocation,thereisaphysicalobjectandtheactionchangesthelocationofthatobject.Assemblylanguageusesthesesameverbs,buttheactionwillbedifferent.Inmostcases,itcreatesacopyofthedataandplacesthecopyatthenewlocation.Inotherwords,sincetheoriginaldatastillexistsinthepreviouslocation,therearenowtwocopiesoftheinformation.Theexceptiontothismemory-access-creates-two-copies-ruleisastackpop.Whenwepopdatafromthestack,itnolongerexistsonthestackleavingusjustonecopy.ForexampleinFigure2.20,theinstruction LDRR0,[R1] loadsthecontentsofthevariable Count intoR0.Atthispoint,therearetwocopiesofthedata,theoriginalinRAMandthecopyinR0.Ifwenextadd1toR0,thetwocopieshavedifferentvalues.WhenwelearnaboutinterruptsinChapter5,wewilltakespecialcaretohandlesharedinformationstoredinglobalRAM,makingsureweaccessthepropercopy.

Whenaccessingmemorydata,thetypeofdatacanbe8,16,32,or64bitswide.For8-bitand16-bitaccessesthetypecanalsobesignedorunsigned.Tospecifythedatatypeweaddanoptionalmodifier,aslistedinTable2.10.Whenweloadan8-bitor16-bitunsignedvalueintoaregister,themostsignificantbitsarefilledwith0,calledzeropad.

Whenweloadan8-bitor16-bitsignedvalueintoaregister,thesignbitofthevalueisfilledintothemostsignificantbits,calledsignextension.Thisway,ifweloadan8-bit-10(0xF6)intoa32-bitregister,wegetthe32-bit-10(0xFFFF.FFF6).Whenwestorean8-bitor16-bitvalue,onlytheleastsignificantbitsareused.

type Datatype Meaning

32-bitword 0to4,294,967,295or-2,147,483,648to+2,147,483,647

B Unsigned8-bitbyte 0to255,Zeropadto32bitsonload

SB Signed8-bitbyte -128to+127,Signextendto32bitsonload

H Unsigned16-bithalfword

0to65535,Zeropadto32bitsonload

SH Signed16-bithalfword

-32768to+32768,Signextendto32bitsonload

D 64-bitdata Usestworegisters

Table2.10.Optionalmodifiertospecifydatatypewhenaccessingmemory.

Mostoftheaddressingmodeslistedintheprevioussectioncanbeusedwithloadandstore.Thefollowingliststhegeneralformforsomeoftheloadandstoreinstructions

LDRtypecondRd,[Rn];loadmemoryat[Rn]toRd

STRtypecondRt,[Rn];storeRttomemoryat[Rn]

LDRtypecondRd,[Rn,#n];loadmemoryat[Rn+n]toRd

STRtypecondRt,[Rn,#n];storeRttomemory[Rn+n]

LDRtypecondRd,[Rn,Rm,LSL#n];loadmemoryat[Rn+Rm*2n]toRd

STRtypecondRt,[Rn,Rm,LSL#n];storeRttomemory[Rn+Rm*2n]

Program2.1setseachelementofanarraytotheindex.The AREADATA directivespecifiesthefollowinglinesareplacedindataspace(typicallyRAM).The DataSPACE40 allocatestenuninitializedwords.The AREACODE directivespecifiesthefollowinglinesareplacedincodespace(typicallyROM).The |.text| connectsthisprogramtotheCcodegeneratedbythecompiler. ALIGN=2 willforcethemachinecodetobehalfword-alignedasrequired.Thelocalvariable i containsthearrayindex.Inassembly,theindexi iskeptinregisterR0.The LDR instructionestablishesR1asapointertothebeginningofthearray,orthebaseaddress.Sinceeacharrayelementis32bits,theaddressoftheithelementofthearrayisbase+4*i.Thelogicalshiftleftby2implementsthemultiplyby4.Inparticular,theaddressingmode[R1,R0,LSL#2] createsaneffectiveaddressofR1+4*R0,withneitherR1norR0beingchangedbytheinstruction.

AREADATA

DataSPACE40;32-bitdata,length=10

AREA|.text|,CODE,READONLY,ALIGN=2

SetMOVSR0,#0x00;indexi=0

LDRR1,=Data;R1=&Data

//Clanguageimplementation

uint32_tData[10];

voidSet(void)

inti;

loopSTRR0,[R1,R0,LSL#2]

ADDSR0,R0,#1;i=i+1

CMPR0,#10

BLTloop;repeatifi<10

BXLR

for(i=0;i<10;i++)

Data[i]=i;

Program2.1.AssemblyandCversionsthatinitializeaglobalarrayoftenelements.Checkpoint2.7:ExplainhowtochangeProgram2.1ifthearraywereten16-bitnumbers?

2.3.4.LogicaloperationsSoftwareuseslogicalandshiftoperationstocombineinformation,toextractinformationandtotestinformation.Aunaryoperationproducesitsresultgivenasingleinputparameter.Examplesofunaryoperationsincludenegate,complement,increment,anddecrement.Indiscretedigitallogic,thecomplementoperationiscalledaNOTgate,previouslyshowninFigure1.17,seealsoTable2.11.CMOScircuitsarebuiltwithp-typeandn-typetransistors.TherearejustafewrulesoneneedstoknowforunderstandinghowCMOStransistor-levelcircuitswork.Eachtransistoractslikeaswitchbetweenitssourceanddrainpins.Ingeneral,currentcanflowfromsourcetodrainacrossanactivep-typetransistor,andnocurrentwillflowiftheswitchisopen.Fromafirstapproximation,wecanassumenocurrentflowsintooroutofthegate.Forap-typetransistor,theswitchwillbeclosed(transistoractive)ifitsgateislow.Ap-typetransistorwillbeoff(itsswitchisopen)ifitsgateishigh.Thegateonthen-typeworksinacomplementaryfashion,hencethenamecomplementarymetaloxidesemiconductor.Forann-typetransistor,theswitchwillbeclosed(transistoractive)ifitsgateishigh.Ann-typetransistorwillbeoff(itsswitchisopen)ifitsgateislow.Therefore,considerthetwopossibilitiesforthecircuitinFigure1.17.IftheinputAishigh(+3.3V),thenthep-typeisoffandthen-typeisactive.Theclosedswitchacrossthesource-drainofthen-typewillmaketheoutputlow(0V).Conversely,ifAislow(0V),thenp-typeisactiveandthen-typeisoff.Theclosedswitchacrossthesource-drainofthep-typewillmaketheoutputhigh(+3.3V).

A ~A

0 1

1 0

Table2.11.Logicalcomplement.

Abinaryoperationproducesasingleresultgiventwoinputs.Thelogicaland(&)

operationyieldsatrueresultifbothinputparametersaretrue.Thelogicalor(|)operationyieldsatrueresultifeitherinputparameteristrue.Theexclusiveor(^)operationyieldsatrueresultifexactlyoneinputparameteristrue.ThelogicaloperatorsaresummarizedinTable2.12andshownasdigitalgatesinFigure2.21.ThelogicalinstructionsontheARM ® CortexTM-Mtaketwoinputs,onefromaregisterandtheotherfromtheflexiblesecondoperand.Theseoperationsareperformedinabit-wisefashionontwo32-bitparametersyieldinga32-bitresult.Theresultisstoredintothedestinationregister.Forexample,thecalculationr=m&nmeanseachbitiscalculatedseparately,r31=m31&n31,r30=m30&n30,…,r0=m0&n0.

InC,whenwewrite r=m&n; r=m|n; r=m^n; thelogicaloperationoccursinabit-wisefashionasdescribedbyTable2.12.However,whenwewrite r=m&&n; r=m||n; ,thelogicaloperationoccursinaword-wisefashion.Forexample, r=m&&n; meansr willbecomezeroifeither m iszeroor n iszero.Conversely, r willbecome1ifboth m isnonzeroand n isnonzero.

A

Rn

B

Operand2

A&B

AND

A|B

ORR

A^B

EOR

A&(~B)

BIC

A|(~B)

ORN

0 0 0 0 0 0 1

0 1 0 1 1 0 0

1 0 0 1 1 1 1

1 1 1 1 0 0 1

Table2.12.LogicaloperationsperformedbytheCortex-M.

WecanunderstandtheoperationoftheANDgatebyobservingthebehaviorofitssixtransistors.IfbothinputsAandBarehigh,bothT3andT4willbeactive.Furthermore,ifAandBarebothhigh,T1andT2willbeoff.Inthiscase,thesignallabeled~(A&B)willbelowbecausetheT3–T4switchcombinationwillshortthissignaltoground.IfAislow,T1willbeactiveandT3off.Similarly,ifBislow,T2willbeactiveandT4off.ThereforeifeitherAisloworifBislow,thesignallabeled~(A&B)willbehighbecauseoneorbothoftheT1,T2switcheswillshortthissignalto+3.3V.TransistorsT5andT6createalogicalcomplement,convertingthesignal~(A&B)intothedesiredresultofA&B.Wecanusetheandoperationtoextract,ormask,individualbitsfromavalue.

Figure2.21.Logicaloperationscanbeimplementedwithdiscretetransistorsordigitalgates.WecanunderstandtheoperationoftheORgatebyobservingthebehaviorofitssixtransistors.IfbothinputsAandBarelow,bothT1andT2willbeactive.Furthermore,ifAandBarebothlow,T3andT4willbeoff.Inthiscase,thesignallabeled~(A|B)willbehighbecausetheT1–T2switchcombinationwillshortthissignalto+3.3V.IfAishigh,T3willbeactiveandT1off.Similarly,ifBishigh,T4willbeactiveandT2off.ThereforeifeitherAishighorifBishigh,thesignallabeled~(A|B)willbelowbecauseoneorbothoftheT3,T4switcheswillshortthissignaltoground.TransistorsT5andT6createalogicalcomplement,convertingthesignal~(A|B)intothedesiredresultofA|B.Weusetheoroperationtosetindividualbits.

Allinstructionsplacetheresultintothedestinationregister Rd .If Rd isomitted,theresultisplacedinto Rn ,whichistheregisterholdingthefirstoperand.IftheoptionalSsuffixisspecified,theNandZconditioncodebitsareupdatedontheresultoftheoperation.Let B bethe32-bitvaluegeneratedbytheflexiblesecondoperand, Operand2 .SomeflexiblesecondoperandsmayaffecttheCbit.TheselogicalinstructionswillleavetheVbitunchanged.

ANDScondRd,Rn,Operand2; Rd=Rn&B

ORRScondRd,Rn,Operand2; Rd=Rn|B

EORScondRd,Rn,Operand2; Rd=Rn^B

BICScondRd,Rn,Operand2; Rd=Rn&(~B)

ORNScondRd,Rn,Operand2; Rd=Rn|(~B)

OtherconvenientlogicaloperatorsaresummarizedinTable2.13andshownasdigitalgatesinFigure2.22.TheNANDoperationisdefinedbyanANDfollowedbyaNOT.Ifyoucomparethetransistor-levelcircuitsinFigures2.21and2.22,itwouldbemoreprecisetosayANDisdefinedasaNANDfollowedbyaNOT.Similarly,theORoperationisaNORfollowedbyaNOT.TheexclusiveNORoperationimplementsthebit-wiseequalsoperation.

A B NAND NOR exclusiveNOR

0 0 1 1 1

0 1 1 0 0

1 0 1 0 0

1 1 0 0 1

Table2.13.Convenientlogicaloperations.

Theoutputofanopencollectorgate,drawnwiththe‘x’,hastwostateslow(0V)andHiZ(floating.)Considertheoperationofthetransistor-levelcircuitforthe74HC05.IfAishigh(+3.3V),thetransistorisactive,andtheoutputislow(0V).IfAislow(0V),thetransistorisoff,andtheoutputisneitherhighnorlow.Ingeneral,wecanuseanopencollectorNOTgatetocontrolthecurrenttoadevice,suchasarelay,anLED,asolenoid,orasmallmotor.The74HC05,the7405,andthe7406areallopencollectorNOTgates.74HC04ishighspeedCMOSandcanonlysinkupto4mAwhenitsoutputislow.Sincethe7405and7406aretransistor-transistor-logic(TTL)theycansinkmorecurrent.Inparticular,the7405hasamaximumoutputlowcurrent(IOL)of16mA,whereasthe7406hasamaximumIOLof40mA.

Checkpoint2.8:Usingjustthe74HCgatesshowninFigures1.17,2.21,and2.22,designone-bitBICandORNcircuitsasdefinedinTable2.12.

Figure2.22.Otherlogicaloperationscanalsobeimplementedwithdiscretelogic.

Digitalstoragedevicesareessentialcomponentsusedtomakeregistersandmemory.Thesimpleststoragedeviceistheset-resetflipflop.OnewaytobuildoneisshownontheleftsideofFigure2.23.IftheinputsareS*=0andR*=1,thentheQoutputwillbe1.Conversely,iftheinputsareS*=1andR*=0,thentheQoutputwillbe0.Normally,weleaveboththeS*andR*inputshigh.WemakethesignalS*golow,thenbackhightosettheflip-flip,makingQ=1.Conversely,wemakethesignalR*golow,thenbackhightoresettheflip-flip,makingQ=0.IfbothS*andR*are1,thevalueonQwillberememberedorstored.ThisflipflopentersanunpredictablemodewithS*andR*aresimultaneouslylow.

ThegatedDflipflopisalsoshowninFigure2.23.Thefront-endcircuitstakeadatainput,D,andacontrolsignal,W,andproducetheS*andR*commandsfortheset-resetflipflop.Forexample,ifW=0,thentheflipflopisinitsquiescentstate,rememberingthevalueonQthatwaspreviouslywritten.However,ifW=1,thenthedatainputisstoredintotheflipflop.Inparticular,ifD=1andW=1,thenS*=0andR*=1,makingQ=1.Furthermore,ifD=0andW=1,thenS*=1andR*=0,makingQ=0.So,tousethegatedflipflop,wefirstputthedataontheDinput,nextwemakeWgohigh,thenwemakeWgolow.ThiscausesthedatavaluetobestoredatQ.AfterWgoeslow,thedatadoesnotneedtoexistattheDinputanymore.IftheDinputchangeswhileWishigh,thentheQoutputwillchangecorrespondingly.However,thelastvalueontheDinputisrememberedorlatchedwhentheWfalls,asshowninTable2.14.

TheDflip-flop,shownontherightofFigure2.23,canalsobeusedtostoreinformation.Dflip-flipsarethebasicbuildingblockofRAMandregistersonthecomputer.Tosaveinformation,wefirstplacethedigitalvaluewewishtorememberontheDinput,andthengivearisingedgetotheclockinput.Aftertherisingedgeoftheclock,thevalueisavailableattheQoutput,andtheDinputisfreetochange.TheoperationoftheclockedDflipflopisdefinedontherightsideofTable2.14.The74HC374isan8-bitDflip-flop,suchthatall8bitsarestoredontherisingedgeofasingleclock.The74HC374issimilarinstructureandoperationtoaregister,whichishighspeedmemoryinsidetheprocessor.Ifthegate(G)inputonthe74HC374ishigh,itsoutputswillbeHiZ(floating),andifthegateislow,theoutputswillbehighorlowdependingonthestoredvaluesontheflipflop.

Figure2.23.Digitalstorageelements.

D W Q D clock Q

0 0 Qold 0 0 Qold

1 0 Qold 0 1 Qold

0 1 0 1 0 Qold

1 1 1 1 1 Qold

0 ↓ 0 0 ↑ 0

1 ↓ 1 1 ↑ 1

Table2.14.Dflip-flopoperation.QoldisthevalueoftheDinputatthetimeoftheactiveedgeofonWorclock.

Second,thetristatedriver,showninFigure2.24,canbeuseddynamicallycontrolsignalswithinthecomputer.Thetristatedriverisanessentialcomponentfromwhichcomputersarebuilt.Toactivethedriver,wemakeitsgate(G*)low.Whenthedriverisactive,itsoutput(Y)equalsitsinput(A).Todeactivatethedriver,wemakeitsG*high.Whenthedriverisnotactive,itsoutputYfloatsindependentofA.Wesawthisfloatingstatewiththeopencollectorlogic,anditisalsocalledHiZorhighimpedance.TheHiZoutputmeanstheoutputisneitherdrivenhighnorlow.TheoperationofatristatedriverisdefinedinTable2.15.The74HC244isan8-bittristatedriver,suchthatall8bitsareactiveornotactivecontrolledbyasinglegate.The74HC3748-bitDflip-flopincludestristatedriversonitsoutputs.Normally,wecan’tconnecttodigitaloutputstogether.Thetristatedriverprovidesawaytoconnectmultipleoutputstothesamesignal,aslongasatmostoneofthegatesisactiveatatime.

Figure2.24.A1-bitandan8-bittristatedriver(G*isinnegativelogic).Table2.15describeshowatristatedriverinFigure2.24works.TransistorsT1andT2createthelogicalcomplementofG*.Similarly,transistorsT3andT4createthecomplementofA.AninputofG*=0causesthedrivertobeactive.Inthiscase,bothT5andT8willbeon.WithT5andT8on,thecircuitbehaveslikeacascadeoftwoNOTgates,sotheoutputYequalstheinputA.However,iftheinputG*=1,bothT5andT8willbeoff.SinceT5isinserieswiththe+3.3V,andT8inserieswiththeground,theoutputYwillbeneitherhighnorlow.I.e.,itwillfloat.

A G* T1 T2 T3 T4 T5 T6 T7 T8 Y

0 0 on off on off on off on on 0

1 0 on off off on on on off on 1

0 1 off on on off off off on off HiZ

1 1 off on off on off on off off HiZ

Table2.15.Tristatedriveroperation.HiZisthefloatingstate,suchthattheoutputisnothighorlow.

2.3.5.ShiftoperationsLikeprogramminginC,theassemblyshiftisabinaryoperation.InC,the<<and>>operatorstaketwoinputsandyieldoneoutput,e.g.,therightshiftisR=M>>N.Thelogicalshiftright(LSR)issimilartoanunsigneddivideby2n,wherenisthenumberofbitsshifted,asshowninFigure2.25.Azeroisshiftedintothemostsignificantposition,andthecarryflagwillholdthebitshiftedout.Therightshiftoperationsdonotround.Ingeneral,theLSRdiscardsbitsshiftedout,andtheUDIVtruncatestowards0.Thus,whenusingUDIVtodivideunsignednumbersbyapowerof2,UDIVandLSRyieldidenticalresults.Thearithmeticshiftright(ASR)issimilartoasigneddivideby2n.Noticethatthesignbitispreserved,andthecarryflagwillholdthebitshiftedout.Thisrightshiftoperationalsodoesnotround.Ingeneral,theASRdiscardsbitsshiftedout,andtheSDIVtruncatestowards0.Thelogicalshiftleft(LSL)operationworksforbothunsignedandsignedmultiplyby2n.Azeroisshiftedintotheleastsignificantposition,andthecarrybitwillcontainthebitthatwasshiftedout.Thetworotateoperationscanbeusedtocreatemultiple-wordshiftfunctions.Thereisnorotateleftinstruction,becausearotateleft10bitsisthesameasrotateright22bits.

Allinstructionsplacetheresultintothedestinationregister Rd . Rm istheregisterholdingthevaluetobeshifted.Thenumberofbitstoshiftiseitherinregister Rs ,orspecifiedasaconstant n .Iftheoptional S suffixisspecified,theNandZconditioncodebitsareupdatedontheresultoftheoperation.TheCbitisthecarryoutaftertheshiftasshowninFigure2.25.TheseshiftinstructionswillleavetheVbitunchanged.

LSRScondRd,Rm,Rs; logicalshiftrightRd=Rm>>Rs(unsigned)

LSRScondRd,Rm,#n; logicalshiftrightRd=Rm>>n(unsigned)

ASRScondRd,Rm,Rs; arithmeticshiftrightRd=Rm>>Rs(signed)

ASRScondRd,Rm,#n; arithmeticshiftrightRd=Rm>>n(signed)

LSLScondRd,Rm,Rs; shiftleftRd=Rm<<Rs(signedandunsigned)

LSLScondRd,Rm,#n; shiftleftRd=Rm<<n(signedandunsigned)

RORScondRd,Rm,Rs; rotateright

RORScondRd,Rm,#n; rotateright

RXXScondRd,Rm; rotaterightwithextension

Observation:Uselogicshiftoperationsonunsignednumbersandusearithmeticshiftoperationsonsignednumbers.

Figure2.25.Shiftoperations.

2.3.6.ArithmeticoperationsWhensoftwareexecutesarithmeticinstructions,theoperationsareperformedbydigitalhardwareinsidetheprocessor.Eventhoughthedesignofsuchlogiciscomplex,wewillpresentabriefintroduction,inordertoprovidealittleinsightastohowthecomputerperformsarithmetic.Itisimportanttorememberthatarithmeticoperations(addition,subtraction,multiplication,anddivision)haveconstraintswhenperformedwithfiniteprecisiononaprocessor.Anoverflowerroroccurswhentheresultofanarithmeticoperationcannotfitintothefiniteprecisionoftheregisterintowhichtheresultistobestored.

Forexample,consideran8-bitunsignednumbersystem,wherethenumberscanrangefrom0to255.Ifweaddtwonumberstogethertheresultcanrangefrom0to510,whichisa9-bitunsignednumber.Thesenumbersaresimilartothenumbers1–12onaclock,asdrawninFigure2.26.Ifitis11o’clockandwewait3hours,itbecomes2o’clock.ShowninthemiddleofFigure2.26,ifweadd64to224,theresultbecomes32.Inmostcases,wewouldconsiderthisanerror.Anunsignedoverflowoccursduringadditionwhenwecrossthe255–0barrier(carrysetonoverflow).Ifwesubtracttwo8-bitunsignednumberstheresultcanrangefrom‑255to+255,whichisa9-bitsignednumber.Subtractionmovesinacounter-clockwisedirectiononthenumberwheel.AsshownontherightsideofFigure2.26,ifwesubtract64from32(32-64),wegettheincorrectresultof224.Anunsigned

overflowoccursduringsubtractionifwecrossthe255–0barrierintheotherdirection(carryclearonoverflow).AfterasubtractionontheCortex-Mthecarryisclearifanerroroccurred,andthecarryissetifnoerroroccurredandtheansweriscorrect.

Figure2.26.Thecarrybitissetonadditionwhencrossingthe255–0boundary.Thecarrybitisclearedonsubtractionwhencrossingthe255–0boundary.Similarly,whentwo32-bitnumbersareaddedorsubtracted,theresultmaynotfitbackintoa32-bitregister.Thesameadditionandsubtractionhardware(instructions)canbeusedtooperateoneitherunsignedorsignednumbers.Althoughweusethesameinstructions,wemustuseseparateoverflowdetectionforsignedandunsignedoperations.

Checkpoint2.9:Howmanybitsdoesittaketostoretheresultoftwounsigned32-bitnumbersaddedtogether?

Checkpoint2.10:Howmanybitsdoesittaketostoretheresultoftwosigned32-bitnumbersaddedtogether?

Checkpoint2.11:Whereisthebarrier(discontinuity)onasigned8-bitnumberwheel?

LetMbethe32-bitvaluespecifiedbythe #imm12 constantorgeneratedbytheflexiblesecondoperand, Operand2 .When Rd isabsent,theresultisplacedbackin Rn .Thecompareinstructions CMP and CMN donotsavetheresultofthesubtraction,butalwayssettheconditioncode.Thecompareinstructionsareusedtocreateconditionalexecution,suchasif-then,forloops,andwhileloops.Thecompilermayuse RSB or CMN tooptimizeexecutionspeed.

ADDScondRd,Rn,Operand2; Rd=Rn+M

ADDScondRd,Rn,#imm12; Rd=Rn+M

SUBScondRd,Rn,Operand2; Rd=Rn-M

SUBScondRd,Rn,#imm12; Rd=Rn-M

RSBScondRd,Rn,Operand2; Rd=M-Rn

RSBScondRd,Rn,#imm12; Rd=M-Rn

CMPcondRn,Operand2; Rn-M

CMNcondRn,Operand2; Rn-(-M)

Iftheoptional S suffixispresent,additionandsubtractionsettheconditioncodebitsasshowninTable2.16.Theadditionandsubtractioninstructionsworkforbothsignedandunsignedvalues.Asdesigners,wemustknowinadvancewhetherwehavesignedorunsignednumbers.Thecomputercannottellfromthebinarywhichtypeitis,soitsetsbothCandV.OurjobasprogrammersistolookattheCbitifthevaluesareunsignedandlookattheVbitifthevaluesaresigned.

Bit Name Meaningafteradditionorsubtraction

N negative Resultisnegative

Z zero Resultiszero

V overflow Signedoverflow

C carry Unsignedoverflow

Table2.16.Conditioncodebitscontainthestatusofthepreviousarithmeticoperation.

Ifthetwoinputstoanadditionoperationareconsideredasunsigned,thentheCbit(carry)willbesetiftheresultdoesnotfit.Inotherwords,afteranunsignedaddition,theCbitissetiftheansweriswrong.Ifthetwoinputstoasubtractionoperationareconsideredasunsigned,thentheCbit(carry)willbecleariftheresultdoesnotfit.Ifthetwoinputstoanadditionorsubtractionoperationareconsideredassigned,thentheVbit(overflow)willbesetiftheresultdoesnotfit.Inotherwords,afterasignedaddition,theVbitissetiftheansweriswrong.Iftheresultisunsigned,theN=1meanstheresultisgreaterthanorequalto231.Conversely,iftheresultissigned,theN=1meanstheresultisnegative.Assumingtheoptional S suffixispresent,conditioncodebitsaresetaftertheadditionR=X+M,whereXisinitialregistervalueandRisthefinalregistervalue.

N:resultisnegativeN=R31

Z:resultiszero

V:signedoverflow

C:unsignedoverflow

Iftheoptional S suffixispresent,conditioncodebitsaresetafterthesubtractionR=X-M,whereXisinitialregistervalueandRisthefinalregistervalue.IftheCbitisclearafteranunsignedsubtraction(R=X-M),thentheresultisincorrectbecauseanunsignedoverflowoccurred.

N:resultisnegativeN=R31

Z:resultiszero

V:signedoverflow

C:unsignedoverflow

Webeginthedesignofanaddercircuitwithasimplesubcircuitcalledabinaryfulladder,asshowninFigure2.27.TherearetwobinarydatainputsA,Bandacarryinput,Cin.ThereisonedataoutputSout,andonecarryoutput,Cout.AsshowninTable2.17,CinA,andBarethreeindependentbinaryinputseachofwhichcouldbe0or1.Thesethreeinputsareaddedtogether(thesumcouldbe0,1,2,or3)andtheresultisencodedinthetwo-bitbinaryresultwithCoutasthemostsignificantbitandSoutastheleastsignificantbit.Coutistrueifthesumis2or3,andSoutistrueifthesumis1or3.

Figure2.27.Abinaryfulladder.

A B Cin A+B+Cin Cout Sout

0 0 0 0 0 0

0 0 1 1 0 1

0 1 0 1 0 1

0 1 1 2 1 0

1 0 0 1 0 1

1 0 1 2 1 0

1 1 0 2 1 0

1 1 1 3 1 1

Table2.17.Input/outputresponseofabinaryfulladder.

Webuild32-bitadderbyconcatenating32binaryfulladderstogether.Thecarryintothe32-bitadderiszero,andthecarryoutwillbesavedinthecarrybit.

Checkpoint2.12:Howmanybitsdoesittaketostoretheresultoftwounsigned32-bitnumbersmultipliedtogether?

Checkpoint2.13:Howmanybitsdoesittaketostoretheresultoftwosigned32-bitnumbersmultipliedtogether?

Multiply(MUL ),multiplywithaccumulate(MLA ),andmultiplywithsubtract(MLS )use32-bitoperands,andproducinga32-bitresult.Thesethreemultiplyinstructionsonlysavethebottom32bitsoftheresult.Theycanbeusedforeithersignedorunsignednumbers,butnooverflowflagsaregenerated.Ifthe Rd registerisomitted,the Rnregisteristhedestination.Ifthe S suffixisaddedtoMUL ,thentheZandNbitsaresetaccordingtotheresult.Thedivisioninstructionsdonotsetconditioncodeflags,andwillroundtowardszeroifthedivisiondoesnotevenlydivideintoanintegerquotient.

MULScondRd,Rn,Rm; Rd=Rn*Rm

MLAcondRd,Rn,Rm,Ra; Rd=Ra+Rn*Rm

MLScondRd,Rn,Rm,Ra; Rd=Ra-Rn*Rm

UDIVcondRd,Rn,Rm; Rd=Rn/Rmunsigned

SDIVcondRd,Rn,Rm; Rd=Rn/Rmsigned

Thefollowingfourmultiplyinstructionsuse32-bitoperandsandproducea64-bitresult.Thetworegisters RdLo and RdHi containtheleastsignificantandmostsignificantpartsrespectivelyofthe64-bitresult,signifiedas Rd .Thesemultiplyinstructionsdonotsetconditioncodeflags

UMULLcondRdLo,RdHi,Rn,Rm; Rd=Rn*Rm

SMULLcondRdLo,RdHi,Rn,Rm; Rd=Rn*Rm

UMLALcondRdLo,RdHi,Rn,Rm; Rd=Rd+Rn*Rm

SMLALcondRdLo,RdHi,Rn,Rm; Rd=Rd+Rn*Rm

Checkpoint2.14:Canthe32by32bitmultiplyinstructionsUMULLorSMULLoverflow?

2.3.7.FunctionsandcontrolflowNormallythecomputerexecutesoneinstructionafteranotherinalinearfashion.Inparticular,thenextinstructiontoexecuteisfoundimmediatelyfollowingthecurrentinstruction.Weusebranchinstructionstodeviatefromthisstraightlinepath.Table2.9liststheconditionalexecutionavailableontheARM ® CortexTM-M.Inthissection,wewillusetheconditionalbranchinstructiontoimplementif-then,while-loopandfor-loopcontrolstructures.

Bcondlabel; branchtolabel

BXcondRm; branchindirecttolocationspecifiedbyRm

BLcondlabel; branchtosubroutineatlabel

BLXcondRm; branchtosubroutineindirectspecifiedbyRm

Subroutines,procedures,andfunctionsareprogramsthatcanbecalledtoperformspecifictasks.Theyareimportantconceptualtoolsbecausetheyallowustodevelopmodularsoftware.TheprogramminglanguagesPascal,Fortran,andAdadistinguishbetweenfunctions,whichreturnvalues,andprocedures,whichdonot.Ontheotherhand,theprogramminglanguagesC,C++,Java,andLispdonotmakethisdistinctionandtreatfunctionsandproceduresassynonymous.Object-orientedprogramminglanguagesusethetermmethodtodescribeprogramsthatarepartofobjects;itisalsousedinconjunctionwithtypeclasses.Inassemblylanguage,weusethetermsubroutineforallsubprogramswhetherornottheyreturnavalue.Modularprogrammingallowsustobuildcomplexsystemsusingsimplecomponents.Inthissectionwepresentashortintroductiononthesyntaxfordefiningsubroutines.Wedefineasubroutinebygivingitanameinthelabelfield,followedbyinstructions,whichwhenexecuted,performthedesiredeffect.Thelastinstructioninasubroutinewillbe BXLR ,whichweusetoreturnfromthesubroutine.InProgram2.2,wedefinethesubroutinenamed Change ,whichadds25tothevariable Num .TheflowchartforthisexampleisdrawninFigure2.28.Inassemblylanguage,wewillusethe BL instructiontocallthissubroutine.Atruntime,the BLinstructionwillsavethereturnaddressintheLRregister.Thereturnaddressisthelocationoftheinstructionimmediatelyafterthe BL instruction.Attheendofthesubroutine,the BXLR instructionwillgetthereturnaddressfromtheLRregister,returningtheprogramtotheplacefromwhichthesubroutinewascalled.Moreprecisely,itreturnstotheinstructionimmediatelyaftertheinstructionthatperformedthesubroutinecall.Thecommentsspecifytheorderofexecution.Thewhile-loopcausesinstructions4–10toberepeatedoverandover.

Figure2.28.Aflowchartofasimplefunctionthatadds25toaglobalvariable.

ChangeLDRR1,=Num;5)R1=&Num

LDRR0,[R1];6)R0=Num

ADDR0,R0,#25;7)R0=Num+25

uint32_tNum;

voidChange(void)

Num=Num+25;

STRR0,[R1];8)Num=Num+25

BXLR;9)return

mainLDRR1,=Num;1)R1=&Num

MOVR0,#0;2)R0=0

STRR0,[R1];3)Num=0

loopBLChange;4)functioncall

Bloop;10)repeat

voidmain(void)

Num=0;

while(1)

Change();

Program2.2.AssemblyandCversionsthatdefineasimplefunction.InC,inputparameters,ifany,arepassedinR0–R3.Theoutputparameter,ifneeded,isreturnedinR0.

Recallthatallobjectcodeishalfwordaligned,meaningbit0ofthePCisalwaysclear.Whenthe BL instructionisexecuted,bits31–1ofregisterLRareloadedwiththeaddressoftheinstructionafterthe BL ,andbit0issettoone.Whenthe BXLR instructionisexecuted,bits31–1ofregisterLRareputbackintothePC,andbit0ofLRgoesintotheTbit.OntheARM ® CortexTM-M,theTbitshouldalwaysbe1,meaningtheprocessorisalwaysintheThumbstate.Normally,thepropervalueofbit0isassignedautomatically.

Decisionmakingisanimportantaspectofsoftwareprogramming.Twovaluesarecomparedandcertainblocksofprogramareexecutedorskippeddependingontheresultsofthecomparison.Inassemblylanguageitisimportanttoknowtheprecision(e.g.,16-bit,32-bit)andtheformatofthetwovalues(e.g.,unsigned,signed).Ittakesthreestepstoperformacomparison.Webeginbyreadingthefirstvalueintoaregister.Thesecondstepistocomparethefirstvaluewiththesecondvalue.Wecanuseeitherasubtractinstruction( subs )oracompareinstruction( cmp ).Theseinstructionssettheconditioncodebits.Thelaststepisaconditionalbranch.TheavailableconditionsarelistedinTable2.9.Thebranchwilloccuriftheconditionistrue.

Program2.3illustratesanif-thenstructureinvolvingtestingforunsignedgreaterthanorequalto.Itwillincrement Num ifitislessthan25600.Sincethevariableisunsigned,weuseanunsignedconditional.Furthermore,wewanttoexecutetheincrementif Num islessthan25600,soweperformtheoppositeconditionalbranch(greaterthanorequalto)toskipover.

ChangeLDRR1,=Num;R1=&Num

LDRR0,[R1];R0=Num

CMPR0,#25600

BHSskip

ADDR0,R0,#1;R0=Num+1

STRR0,[R1];Num=Num+1

skipBXLR;return

uint32_tNum;

voidChange(void)

if(Num<25600)

Num=Num+1;

Program2.3.AssemblyandCsoftwareshowinganif-thencontrolstructure.Program2.4illustratesanif-then-elsestructureinvolvingsignednumbers.Itwillincrement Num ifitislessthan100,otherwiseitwillsetitto-100.Sincethevariableissigned,weuseansignedconditional.Again,wewanttoexecutetheincrementif Num islessthan100,soweperformtheoppositeconditionalbranch(greaterthanorequalto)toskipover.

ChangeLDRR1,=Num;R1=&Num

LDRR0,[R1];R0=Num

CMPR0,#100

BGEelse

ADDR0,R0,#1;R0=Num+1

Bskip

elseMOVR0,#-100;-100

skipSTRR0,[R1];updateNum

BXLR;return

int32_tNum;

voidChange(void)

if(Num<100)

Num=Num+1;

else

Num=-100;

Program2.4.AssemblyandCsoftwareshowinganif-then-elsecontrolstructure.Checkpoint2.15:WhydoesProgram2.3use BHS andProgram2.4use BGE ?

If-then-elsecontrolstructuresarecommonlyfoundincomputersoftware.Ifthe BHS inProgram2.3orthe BGE inProgram2.4weretobranch,theinstructionpipelinewouldhavetobeflushedandrefilled.Inordertooptimizeexecutionspeedforshortif-thenandif-then-elsecontrolstructures,theARM ® CortexTM-Memploysconditionalexecution.Theconditionalexecutionbeginswiththe IT instruction,whichspecifiesthenumberofinstructionsinthecontrolstructure(1to4)andtheconditionalforthefirstinstruction.Thesyntaxis

ITxyzcond

where x y and z specifytheexistenceoftheoptionalsecond,third,orfourthconditionalinstructionrespectively.Wecanspecifyx y and z as T forexecuteiftrueor E forelse.The cond fieldchoicesarelistedinTable2.9.Theconditionalsuffixesforthe1to4followinginstructionmustmatchtheconditionalfieldofthe IT instruction.Inparticular,theconditionalforthetrueinstructionsexactlymatchtheconditionalforthe ITinstruction.Furthermore,theelseinstructionsmusthavethelogicalcomplementconditional.Iftheconditionistruetheinstructionisexecuted.Iftheconditionisfalse,theinstructionisfetched,butnotexecuted.Forexample,Program2.3couldhavebeenwrittenasfollows.ThetwoT’sin ITT meanstherearetwotrueinstructions.

ChangeLDRR1,=Num;R1=&Num

LDRR0,[R1];R0=Num

CMPR0,#25600

ITTLO

ADDLOR0,R0,#1;if(R0<25600)R0=Num+1

STRLOR0,[R1];if(R0<25600)Num=Num+1

BXLR;return

Program2.4couldhavebeenwrittenasfollows.TheoneTandoneEin ITE meansthereisonetrueandoneelseinstruction.

ChangeLDRR1,=Num;R1=&Num

LDRR0,[R1];R0=Num

CMPR0,#100

ITELT

ADDLTR0,R0,#1;if(R0<100)R0=Num+1

MOVGER0,#-100;if(R0>=100)R0=-100

STRR0,[R1];updateNum

BXLR;return

Thefollowingassemblyconvertsonehexdigit(0–15)inR0toASCIIinR1.TheoneTandoneEin ITE meansthereisonetrueandoneelseinstruction.

CMPR0,#9;ConvertR0(0to15)intoASCII

ITEGT;Next2areconditional

ADDGTR1,R0,#55;Convert0xA->‘A’

ADDLER1,R0,#48;Convert0x0->‘0’

Bythemselves,theconditionalbranchinstructionsdonotrequireapreceding ITinstruction.However,aconditionalbranchcanbeusedasthelastinstructionofan ITblock.TherearealotofrestrictionsonIT.Formoredetails,refertotheprogrammingreferencemanual.

2.3.8.Stackusage

Figure2.5showsthe push and pop instructionscanbeusedtostoretemporaryinformationonthestack.Ifasubroutinemodifiesaregister,itisamatterofprogrammerstyleastowhetherornotitshouldsaveandrestoretheregister.AccordingtoAAPCSasubroutinecanfreelychangeR0–R3andR12,butsaveandrestoreanyotherregisteritchanges.Inparticular,ifonesubroutinecallsanothersubroutine,thenitmustsaveandrestoretheLR.AAPCSalsorequirespushingandpoppingmultiplesof8bytes,whichmeansanevennumberofregisters.Inthefollowingexample,assumethefunctionmodifiesregisterR0,R4,R7,R8andcallsanotherfunction.TheprogrammingstyledictatesregistersR4R7R8andLRbesaved.NoticethereturnaddressispushedonthestackasLR,butpoppedoffintoPC.Whenmultipleregistersarepushedorpopped,thedataexistinmemorywiththelowestnumberedregisterusingthelowestmemoryaddress.Inotherwords,theregistersinthecanbespecifiedinanyorder.Ofcourseremembertobalancethestackbyhavingthesamenumberofpopsaspushes.

FuncPUSHR4,R7,R8,LR;saveregistersasneeded

;bodyofthefunction

POPR4,R7,R8,PC;restoreregistersandreturn

TheARMprocessorhasalotofregisters,andweappropriatelyshouldusethemfortemporaryinformationsuchasfunctionparametersandlocalvariables.However,whentherearealotofparametersorlocalvariableswecanplacethemonthestack.Program2.5issimilartoProgram2.1,exceptthe data bufferisnowlocal,andplacedonthestack.The SUB instructionallocates10wordsonthestack.Figure2.29showsthestackbeforeandaftertheallocation.TheSPpointstothefirstlocationof data .Thelocalvariable i isheldinR0.TheflexiblesecondoperandfortheSTRinstructionusesSPasthebasepointer,andR0*4astheoffset.The ADD instructiondeallocatesthelocalvariable,balancingthestack.

SetSUBsp,sp,#0x28;allocate

MOVSr0,#0x00;i=0

Btest

loopSTRr0,[sp,r0,LSL#2]

ADDSr0,r0,#1;i++

testCMPr0,#0x0A

BLTloop

ADDsp,sp,#0x28;deallocate

BXLR

//Clanguageimplementation

voidSet(void)

uint32_tdata[10];

inti;

for(i=0;i<10;i++)

data[i]=i;

Program2.5.AssemblyandCversionsthatinitializealocalarrayoftenelements.

Figure2.29.Astackpictureshowingalocalarrayoftenelements(40bytes).

2.3.9.AssemblerdirectivesWeuseassemblerdirectivestoassistandcontroltheassemblyprocess.Thefollowingdirectiveschangethewaythecodeisassembled.

AREACODE; placescodeincodespace(flashROM)

AREADATA; placesobjectsindataspace(RAM)

THUMB; usesThumbinstructions

ALIGN; skips0to3bytestomakenextwordaligned

END; endoffile

Thefollowingdirectivescanaddvariablesandconstants.

DCBexpr,expr; places8-bitbyte(s)intomemory

DCWexpr,expr; places16-bithalfword(s)intomemory

DCDexpr,expr; places32-bitword(s)intomemory

SPACEsize; reservessizebytes,unitialized

The EQU directivegivesasymbolicnametoanumericconstant,aregister-relativevalueoraprogram-relativevalue.*isasynonymfor EQU .WewilluseittodefineI/Oportaddresses.Forexample,thesefourdefinitionswillbeusedtoinitializeandoperatePortD.

GPIO_PORTD_DATA_Requ0x400073FC

GPIO_PORTD_DIR_Requ0x40007400

GPIO_PORTD_DEN_Requ0x4000751C

SYSCTL_RCGCGPIO_Requ0x400FE608

2.4.ParallelI/Oports

2.4.1.BasicconceptsofinputandoutputportsThesimplestI/Oportonamicrocontrolleristheparallelport.AparallelI/Oportisasimplemechanismthatallowsthesoftwaretointeractwithexternaldevices.Itiscalledparallelbecausemultiplesignalscanbeaccessedallatonce.Aninputport,whichallowsthesoftwaretoreadexternaldigitalsignals,isreadonly.Thatmeansareadcycleaccessfromtheportaddressreturnsthevaluesexistingontheinputsatthattime.Inparticular,thetristatedriver(triangleshapedcircuitinFigure2.30)willdrivetheinputsignalsontothedatabusduringareadcyclefromtheportaddress.Awritecycleaccesstoaninputportusuallyproducesnoeffect.Thedigitalvaluesexistingontheinputpinsarecopiedintothemicrocontrollerwhenthesoftwareexecutesareadfromtheportaddress.Therearenoinput-onlyportsonLM3S/TM4Cmicrocontrollers.LM3S/TM4Cmicrocontrollershave5V-tolerantdigitalinputs,meaninganinputhighsignalcanbeanyvoltagefrom2.0to5.0V.OntheSTMicroelectronicsSTM32F10xxfamily,someinputsare5-Vtolerantandothersarenot.

Figure2.30.Areadonlyinputportallowsthesoftwaretosenseexternaldigitalsignals.Checkpoint2.16:WhathappensifthesoftwarewritesfromaninputportlikeFigure2.30?

CommonError:ManyprogramerrorscanbetracedtoconfusionbetweenI/Oportsandregularmemory.Forexample,youcannotwritetoaninputport.

Whileaninputdeviceusuallyjustinvolvesthesoftwarereadingtheport,anoutputportcanparticipateinboththereadandwritecyclesverymuchlikearegularmemory.Figure2.31describesareadableoutputport.Awritecycletotheportaddresswillaffectthevaluesontheoutputpins.Inparticular,themicrocontrollerplacesinformationonthedatabusandthatinformationisclockedintotheDflipflops.Sinceitisareadableoutput,areadcycleaccessfromtheportaddressreturnsthecurrentvaluesexistingontheportpins.Therearenooutput-onlyportsonLM3S/TM4Cmicrocontrollers.

Figure2.31.Areadableoutputportallowsthesoftwaretogenerateexternaldigitalsignals.Checkpoint2.17:WhathappensifthesoftwarereadsfromanoutputportlikeFigure2.31?

Tomakethemicrocontrollermoremarketable,mostportscanbesoftware-specifiedtobeeitherinputsoroutputs.Microcontrollersusetheconceptofadirectionregistertodeterminewhetherapinisaninput(directionregisterbitis0)oranoutput(directionregisterbitis1),asshowninFigure2.32.Wedefineaninitializationritualasaprogramexecutedduringstartupthatinitializeshardwareandsoftware.Iftheritualsoftwaremakesdirectionbitzero,theportbehaveslikeasimpleinput,andifitmakesthedirectionbitone,itbecomesareadableoutputport.Eachdigitalportpinhasadirectionbit.Thismeanssomepinsonaportmaybeinputswhileothersareoutputs.Thedigitalportpinsonmostmicrocontrollersarebidirectional,operatingsimilartoFigure2.32.

Figure2.32.Abidirectionalportcanbeconfiguredasaread-onlyinputportorareadableoutputport.

2.4.2.I/OProgrammingandthedirectionregister

Onmostembeddedmicrocontrollers,theI/Oportsarememorymapped.Thismeansthesoftwareaccessesaninput/outputportsimplybyreadingfromorwritingtotheappropriateaddress.TomakeoursoftwaremorereadableweincludesymbolicdefinitionsfortheI/Oports.Wesetthedirectionregister(e.g., GPIO_PORTD_DIR_R )tospecifywhichpinsareinputandwhichareoutput.Bydefault,thealternatefunctionregisteriszero,specifyingthecorrespondingbitsareregularportpins(e.g., GPIO_PORTD_AFSEL_R ).WewillsetbitsinthealternativefunctionregisterwhenwewishtoactivatethefunctionslistedinTables2.3,2.52.7,and2.8.Typically,wewritetothedirectionandalternatefunctionregistersonceduringtheinitializationphase.Weusethedataregister(e.g., GPIO_PORTD_DATA_R )toperforminput/outputontheport.Conversely,wereadandwritethedataregistermultipletimestoperforminputandoutputrespectivelyduringtherunningphase.Table2.18showstheparallelportregistersfortheTM4C123.TheotherTexasInstrumentsmicrocontrollersaresimilar.TheCR,AMSEL,PCTL,andLOCKregistersexistonlyonLM4F/TM4C.FortheLM3Ssoftware,simplyremoveaccessestothesefourregisters.TheonlydifferencesamongvariousmembersoftheTexasInstrumentsmicrocontrollerfamilayarethenumberofportsandavailablepinsineachport.

Forexample,theTM4C1294hasfifteendigitalI/OportsA(8bits),B(6bits),C(8bits),D(8bits),E(6bits),F(5bits),G(2bits),H(4bits),J(2bits),K(8bits),L(8bits),M(8bits),N(6bits),P(6bits),andQ(5bits).Furthermore,theTM4C1294hasdifferentaddressesforports.Refertothefiletm4c1294ncpdt.hortothedatasheetformorethespecificaddressesofitsI/Oports

ToinitializeanLM3SI/Oportforgeneraluseweperformfoursteps.First,weactivatetheclockfortheport.Second,wesetitsdirectionregister.Third,weclearbitsinthealternatefunctionregister,andlastly,weenablethedigitalport.Weneedtoaddashortdelaybetweenactivatingtheclockandaccessingtheportregisters.Thedirectionregisterspecifiesbitforbitwhetherthecorrespondingpinsareinput(0)oroutput(1).

WeusethePURregistertoactivateaninternalpull-upresistor,andweusethePDRregistertoactivateaninternalpull-downresistor.

CommonError:Youwillgetabusfaultifyouaccessaportwithoutenablingitsclock.Also,youhavetowaitabout5buscyclesafterenablingtheclock,beforeyouaccesstheregisters.

InthisfirstexamplewewillmakePD7-4input,andwewillmakePD3-0output,asshowninProgram2.6.Touseaportwefirstmustactivateitsclockinthe SYSCTL_RCGCGPIO_R register.Thesecondstepistounlocktheport(LM4F/TM4Conly),bywritingaspecialvaluetotheLOCKregister,followedbysettingbitsintheCRregister.PC3-0arelockedtothedebugger.OnlyPD7andPF0ontheTM4C123needtobeunlocked.OntheTM4C1294onlyPD7needstobeunlocked.Alltheotherbitsonthetwomicrocontrollersarealwaysunlocked.Thethirdstepistodisabletheanalogfunctionality(TM4Conly),byclearingbitsintheAMSELregister.ThefourthstepistoselectGPIOfunctionality(LM4F/TM4Conly),byclearingbitsinthePCTLregister,asdescribedinTables2.7and2.8.ThefifthstepistospecifywhetherthepinisaninputoranoutputbyclearingorsettingbitsintheDIRregister.BecauseweareusingthepinsasregulardigitalI/O,thesixthstepistoclearthecorrespondingbitsintheAFSELregister.

Address 7 6 5 4 3 2 1 0 Name

0x400FE608 - - GPIOF GPIOE GPIOD GPIOC GPIOB GPIOA SYSCTL_RCGCGPIO_R

0x400FEA08 - - GPIOF GPIOE GPIOD GPIOC GPIOB GPIOA SYSCTL_PRGPIO_R

0x400043FC DATA DATA DATA DATA DATA DATA DATA DATA GPIO_PORTA_DATA_R

0x40004400 DIR DIR DIR DIR DIR DIR DIR DIR GPIO_PORTA_DIR_R

0x40004420 SEL SEL SEL SEL SEL SEL SEL SEL GPIO_PORTA_AFSEL_R

0x40004510 PUE PUE PUE PUE PUE PUE PUE PUE GPIO_PORTA_PUR_R

0x4000451C DEN DEN DEN DEN DEN DEN DEN DEN GPIO_PORTA_DEN_R

0x40004524 1 1 1 1 1 1 1 1 GPIO_PORTA_CR_R

0x40004528 0 0 0 0 0 0 0 0 GPIO_PORTA_AMSEL_R

0x400053FC DATA DATA DATA DATA DATA DATA DATA DATA GPIO_PORTB_DATA_R

0x40005400 DIR DIR DIR DIR DIR DIR DIR DIR GPIO_PORTB_DIR_R

0x40005420 SEL SEL SEL SEL SEL SEL SEL SEL GPIO_PORTB_AFSEL_R

0x40005510 PUE PUE PUE PUE PUE PUE PUE PUE GPIO_PORTB_PUR_R

0x4000551C DEN DEN DEN DEN DEN DEN DEN DEN GPIO_PORTB_DEN_R

0x40005524 1 1 1 1 1 1 1 1 GPIO_PORTB_CR_R

0x40005528 0 0 AMSEL AMSEL 0 0 0 0 GPIO_PORTB_AMSEL_R

0x400063FC DATA DATA DATA DATA JTAG JTAG JTAG JTAG GPIO_PORTC_DATA_R

0x40006400 DIR DIR DIR DIR JTAG JTAG JTAG JTAG GPIO_PORTC_DIR_R

0x40006420 SEL SEL SEL SEL JTAG JTAG JTAG JTAG GPIO_PORTC_AFSEL_R

0x40006510 PUE PUE PUE PUE JTAG JTAG JTAG JTAG GPIO_PORTC_PUR_R

0x4000651C DEN DEN DEN DEN JTAG JTAG JTAG JTAG GPIO_PORTC_DEN_R

0x40006524 1 1 1 1 JTAG JTAG JTAG JTAG GPIO_PORTC_CR_R

0x40006528 AMSEL AMSEL AMSEL AMSEL JTAG JTAG JTAG JTAG GPIO_PORTC_AMSEL_R

0x400073FC DATA DATA DATA DATA DATA DATA DATA DATA GPIO_PORTD_DATA_R

0x40007400 DIR DIR DIR DIR DIR DIR DIR DIR GPIO_PORTD_DIR_R

0x40007420 SEL SEL SEL SEL SEL SEL SEL SEL GPIO_PORTD_AFSEL_R

0x40007510 PUE PUE PUE PUE PUE PUE PUE PUE GPIO_PORTD_PUR_R

0x4000751C DEN DEN DEN DEN DEN DEN DEN DEN GPIO_PORTD_DEN_R

0x40007524 CR 1 1 1 1 1 1 1 GPIO_PORTD_CR_R

0x40007528 0 0 AMSEL AMSEL AMSEL AMSEL AMSEL AMSEL GPIO_PORTD_AMSEL_R

0x400243FC - - DATA DATA DATA DATA DATA DATA GPIO_PORTE_DATA_R

0x40024400 - - DIR DIR DIR DIR DIR DIR GPIO_PORTE_DIR_R

0x40024420 - - SEL SEL SEL SEL SEL SEL GPIO_PORTE_AFSEL_R

0x40024510 - - PUE PUE PUE PUE PUE PUE GPIO_PORTE_PUR_R

0x4002451C - - DEN DEN DEN DEN DEN DEN GPIO_PORTE_DEN_R

0x40024524 - - 1 1 1 1 1 1 GPIO_PORTE_CR_R

0x40024528 - - AMSEL AMSEL AMSEL AMSEL AMSEL AMSEL GPIO_PORTE_AMSEL_R

0x400253FC - - - DATA DATA DATA DATA DATA GPIO_PORTF_DATA_R

0x40025400 - - - DIR DIR DIR DIR DIR GPIO_PORTF_DIR_R

0x40025420 - - - SEL SEL SEL SEL SEL GPIO_PORTF_AFSEL_R

0x40025510 - - - PUE PUE PUE PUE PUE GPIO_PORTF_PUR_R

0x4002551C - - - DEN DEN DEN DEN DEN GPIO_PORTF_DEN_R

0x40025524 - - - 1 1 1 1 CR GPIO_PORTF_CR_R

0x40025528 - - - 0 0 0 0 0 GPIO_PORTF_AMSEL_R

31-28 27-24 23-20 19-16 15-12 11-8 7-4 3-0

0x4000452C PMC7 PMC6 PMC5 PMC4 PMC3 PMC2 PMC1 PMC0 GPIO_PORTA_PCTL_R

0x4000552C PMC7 PMC6 PMC5 PMC4 PMC3 PMC2 PMC1 PMC0 GPIO_PORTB_PCTL_R

0x4000652C PMC7 PMC6 PMC5 PMC4 0x1 0x1 0x1 0x1 GPIO_PORTC_PCTL_R

0x4000752C PMC7 PMC6 PMC5 PMC4 PMC3 PMC2 PMC1 PMC0 GPIO_PORTD_PCTL_R

0x4002452C ---- ---- PMC5 PMC4 PMC3 PMC2 PMC1 PMC0 GPIO_PORTE_PCTL_R

0x4002552C ---- ---- ---- PMC4 PMC3 PMC2 PMC1 PMC0 GPIO_PORTF_PCTL_R

0x40006520 LOCK(write0x4C4F434Btounlock,otherlocks)(reads1iflocked,0ifunlocked) GPIO_PORTC_LOCK_R

0x40007520 LOCK(write0x4C4F434Btounlock,otherlocks)(reads1iflocked,0ifunlocked) GPIO_PORTD_LOCK_R

0x40025520 LOCK(write0x4C4F434Btounlock,otherlocks)(reads1iflocked,0ifunlocked) GPIO_PORTF_LOCK_R

Table2.18.SomeTM4C123parallelports.Eachregisteris32bitswide.ForPCTLbits,seeTables2.7and2.8.JTAGmeansdonotusethesepinsanddonotchangeanyofthesebits.

ThelaststepistoenablethecorrespondingI/OpinsbywritingonestotheDENregister.TorunthisexampleontheTexasInstrumentsLaunchPad,wealsosetbitsinthePURregisterforthetwoswitchinputs(Figure2.14)tohaveaninternalpull-upresistor.

Whenthesoftwarereadsfromlocation0x400073FCthebottom8bitsarereturnedwiththecurrentvaluesonPortD.Thetop24bitsarereturnedzero.AsshowninFigure2.32,theinputpinsshowthecurrentdigitalstate,andtheoutputpinsshowthevaluelastwrittentotheport.Thefunction PortD_Input willreadfromthefourinputpinsandreturnavalue,0x00to0x0F,dependingonthecurrentstatusoftheinputs.Thefunction PortD_Output willwritenewvaluestothefouroutputpins.

#defineGPIO_PORTD_DATA_R(*((volatileuint32_t*)0x400073FC))

#defineGPIO_PORTD_DIR_R(*((volatileuint32_t*)0x40007400))

#defineGPIO_PORTD_AFSEL_R(*((volatileuint32_t*)0x40007420))

#defineGPIO_PORTD_DEN_R(*((volatileuint32_t*)0x4000751C))

#defineGPIO_PORTD_LOCK_R(*((volatileuint32_t*)0x40007520))

#defineGPIO_PORTD_AMSEL_R(*((volatileuint32_t*)0x40007528))

#defineGPIO_PORTD_PCTL_R(*((volatileuint32_t*)0x4000752C))

#defineSYSCTL_RCGCGPIO_R(*((volatileuint32_t*)0x400FE608))

voidPortD_Init(void)

SYSCTL_RCGCGPIO_R|=0x08;//1)activateclockforPortD

while((SYSCTL_PRGPIO_R&0x08)==0);//ready?

GPIO_PORTD_LOCK_R=0x4C4F434B;//2)unlockGPIOPortD

GPIO_PORTD_CR_R=0xFF;//allowchangestoPD7-0

GPIO_PORTD_AMSEL_R=0x00;//3)disableanalogonPD

GPIO_PORTD_PCTL_R=0x00000000;//4)PCTLGPIOonPD7-0

GPIO_PORTD_DIR_R=0x0F;//5)PD7-4in,PD3-0out

GPIO_PORTD_AFSEL_R=0x00;//6)disablealtfunctonPD7-0

GPIO_PORTD_DEN_R=0xFF;//7)enabledigitalI/OonPD7-0

uint32_tPortD_Input(void)

return(GPIO_PORTD_DATA_R>>4);//readPD7-PD4inputs

voidPortD_Output(uint32_tdata)

GPIO_PORTD_DATA_R=data;//writePD3-PD0outputs

Program2.6.AsetoffunctionsusingPD7 –PD4asinputsandPD3 –PD0asoutputs.Checkpoint2.18:Doestheentireportneedtobedefinedasinputoroutput,orcansomepinsbeinputwhileothersareoutput?

InProgram2.6theassumptionwasthesoftwaremodulehadaccesstoallofPortD.Inotherwords,thissoftwareownedalleightpinsofPortD.Inmostcases,asoftwaremoduleneedsaccesstoonlysomeoftheportpins.Iftwoormoresoftwaremodulesaccessthesameport,aconflictwilloccurifonemodulechangesmodesoroutputvaluessetbyanothermodule.Itisgoodsoftwaredesigntowritefriendlysoftware,whichonlyaffectstheindividualpinsasneeded.Friendlysoftwaredoesnotchangetheotherbitsinasharedregister.Conversely,unfriendlysoftwaremodifiesmorebitsofaregisterthanitneedsto.Thedifficultyofunfriendlycodeiseachmodulewillrunproperlywhentestedbyitself,butweirdbugsresultwhentwoormoremodulesarecombined.

ConsidertheproblemthatasoftwaremoduleneedtooutputtojustPortDbit1.AfterenablingtheclockforPortD,weuseread-modify-writesoftwaretoinitializejustpin1.RememberonlyPD7andPF0requireunlockingontheTM4C123,andonlyPD7requiresunlockingontheTM4C1294,sothiscodedoesnotneedtounlock.

SYSCTL_RCGCGPIO_R|=0x08;//1)activateclockforPortD

while((SYSCTL_PRGPIO_R&0x08)==0);//ready?

GPIO_PORTD_DIR_R|=0x02;//PD1isanoutput

GPIO_PORTD_AFSEL_R&=~0x02;//regularportfunction

GPIO_PORTD_AMSEL_R&=~0x02;//disableanalogonPD1

GPIO_PORTD_PCTL_R&=~0x000000F0;//PCTLGPIOonPD1

GPIO_PORTD_DEN_R|=0x02;//PD1isenabledasadigitalport

ThereisnoconflictiftwoormoremodulesenabletheclockforPortD.TherearetwowaysontheCortexTM-Mtoaccessindividualportbits.Thefirstmethodistouseread-modify-writesoftwaretochangejustpin1.Aread-or-writesequencecanbeusedtosetoneormorebits.

GPIO_PORTD_DATA_R|=0x02;//makePD1high

Aread-and-writesequencecanbeusedtoclearoneormorebits.

GPIO_PORTD_DATA_R&=~0x02;//makePD1low

Thesecondmethodusesthebit-specificaddressing.TheLM3S/LM4F/TM4Cfamilyimplementsamoreflexiblewaytoaccessportpinsthanthebit-bandingdescribedearlierinthechapter.Thisbit-specificaddressingdoesn’tworkforalltheI/Oregisters,justtheparallelportdataregisters.TheLM3S/LM4F/TM4Cmechanismallowscollectiveaccessto1to8bitsinadataport.WedefineeightaddressoffsetconstantsinTable2.19.Basically,ifweareinterestedinbitb,theconstantis4*2b.Thereare256possiblebitcombinationswemightbeinterestedinaccessing,fromallofthemtononeofthem.Eachpossiblebitcombinationhasaseparateaddressforaccessingthatcombination.Foreachbitweareinterestedin,weaddupthecorrespondingconstantsfromTable2.19andthenaddthatsumtothebaseaddressfortheport.ThebaseaddressesforthedataportscanbefoundinGPIOchapterofthemicrocontrollerdatasheet.Forexample,assumeweareinterestedinPortAbits1,2,and3.ThebaseaddressforPortAis0x4000.4000,andtheconstantsare0x0020,0x0010and0x008.Thesumof0x4000.4000+0x0020+0x0010+0x008istheaddress0x4000.4038.Ifwereadfrom0x4000.4038onlybits1,2,and3willbereturned.Ifwewritetothisaddressonlybits1,2,and3willbemodified.

Ifwewishtoaccessbit Constant

7 0x0200

6 0x0100

5 0x0080

4 0x0040

3 0x0020

2 0x0010

1 0x0008

0 0x0004

Table2.19.Addressoffsetsusedtospecifyindividualdataportbits.

ThebaseaddressforPortDis0x4000.7000.Ifwewanttoreadandwriteall8bitsofthisport,theconstantswilladdupto0x03FC.Noticethatthesumofthebaseaddressandtheconstantsyieldsthe0x4000.73FCaddressusedinProgram2.6.Inotherwords,readandwriteoperationsto GPIO_PORTD_DATA_R willaccessall8bitsofPortD.Ifweareinterestedinjustbit1ofPortD,weadd0x0008to0x4000.7000,andwecandefinethisinCas

#definePD1(*((volatileuint32_t*)0x40007008))

Now,asimplewriteoperationcanbeusedtosetPD1.Thefollowingcodeisfriendlybecauseitdoesnotmodifytheother7bitsofPortD.

PD1=0x02;//makePD1high

AsimplewritesequencewillclearPD1.Thefollowingcodeisalsofriendly.

PD1=0x00;//makePD1low

AreadfromPD1 willreturn0x01or0x00dependingonwhetherthepinishighorlow,respectively.Thefollowingcodeisalsofriendly.

PD1=PD1^0x01;//togglePD1

Checkpoint2.19:AccordingtoTable2.3,whathappenstoPortDbit5ifwesetbit5initsalternativefunctionregister?E.g., GPIO_PORTD_AFSEL_R|=0x20;

Checkpoint2.20:Whathappensifwewritetolocation0x4000.7000?

Checkpoint2.21:Specifya#definethatallowsustoaccessbits7and2ofPortD.Usethis#definetomakebothbits7and2ofPortDhigh.

Checkpoint2.22:Specifya#definethatallowsustoaccessbits6,5,0ofPortB.Usethis#definetomakebits6,5and0ofPortBhigh.

YoucanfindthecodeforthenextthreeexamplesonthebookwebsiteasSSR_xxx.zip,GPIO_xxx.zip,andSwitch_xxx.zip,wherexxxreferstothespecificmicrocontrolleronwhichtheexamplewastested.

Example2.1:ThegoalisdevelopameansforthemicrocontrollertoturnonandturnoffanAC-poweredappliance.Theinterfacewilluseasolidstaterelay(SSR)havingacontrolportionequivalenttoanLEDwithparametersof2Vand10mA.Includeappropriatefunctions.

Solution:SinceweneedtointerfaceanLED,weuseanopencollectorNOTgatejustlikeFigure1.23.Wechooseanelectroniccircuitthathasanoutputcurrentlargerthanthe10mAneededbytheSSR.SincethemaximumICEofthePN2222is150mA,itcansinkthe10mArequiredbytheSSR.A7405or7406couldalsohavebeenused,buttheyrequirea+5Vsupply.Theresistorisselectedtocontrolthecurrenttothediode.UsingtheLEDdesignequation,R=(3.3-Vd-VCE)/Id=(3.3-2-0.3V)/0.01A=100 .Thereisastandardvalue5%resistorat100.ThespecificationVCE=0.3Visamaximum.IfVCEisactuallybetween0.1and0.3V,then10to12mAwillflow,andtherelaywillstillactivateproperly.WhentheinputtothePN2222ishigh(p=3.3V),theoutputislow(q=0.3V),seeFigure2.33.Inthisstate,a10mAcurrentisappliedtothediode,andrelayswitchactivates.Thiscauses120VACpowertobedeliveredtotheappliance.But,whentheinputislow(p=0),theoutputfloats(q=HiZ,whichisneitherhighnorlow).ThisfloatingoutputstatecausestheLEDcurrenttobezero,andtherelayswitchopens.Inthiscase,noACpowerisdeliveredtotheappliance.

Figure2.33.SolidstaterelayinterfaceusingaPN2222NPNtransistor.

Theinitializationwillsetbit1ofthedirectionregistertomakePD1anoutput,seeProgram2.7.Thisfunctionshouldbecalledonceatthestartofthesystem.Afterinitialization,theonandofffunctionscanbecalledtocontroltheappliance.

#definePD1(*((volatileuint32_t*)0x40007008))

voidSSR_Init(void)

SYSCTL_RCGCGPIO_R|=0x08;//1)activateclockforPortD

while((SYSCTL_PRGPIO_R&0x08)==0);//ready?

GPIO_PORTD_DIR_R|=0x02;//PD1isanoutput

GPIO_PORTD_AFSEL_R&=~0x02;//regularportfunction

GPIO_PORTD_AMSEL_R&=~0x02;//disableanalogonPD1

GPIO_PORTD_PCTL_R&=~0x000000F0;//PCTLGPIOonPD1

GPIO_PORTD_DEN_R|=0x02;//PD1isenabledasadigitalport

voidSSR_Off(void)

PD1=0x00;//turnofftheappliance

voidSSR_On(void)

PD1=0x01;//turnontheappliance

Program2.7.AsetoffunctionsusingPD7 –PD4asinputsandPD3 –PD0asoutputs(SSR_xxx.zip).

Someproblemsaresouniquethattheyrequiretheengineertoinventcompletelyoriginalsolutions.Mostofthetime,however,theengineercansolveevencomplexproblemsbybuildingthesystemfromcomponentsthatalreadyexist.Creativitywillstillberequiredinselectingthepropercomponents,makingsmallchangesintheirbehavior(tweaking),arrangingtheminaneffectiveandefficientmanner,andthenverifyingthesystemsatisfiesboththerequirementsandconstraints.Whenyoungengineersbegintheirfirstjob,theyaresometimessurprisedtoseethateducationdoesnotstopwithcollegegraduation,butratherisalife-longactivity.Infact,itistheeducationalgoalofallengineerstocontinuetolearnbothprocesses(rulesabouthowtosolveproblems)andproducts(hardwareandsoftwarecomponents).Astheengineerbecomesmoreexperienced,heorshehasalargertoolboxfromwhichprocessesandcomponentscanbeselected.

Thehardeststepformostnewengineersisthefirstone:wheretobegin?Webeginbyanalyzingtheproblemtocreateasetofspecificationsandconstraintsintheformofarequirementsdocument.Next,welookforcomponents,intheformofpreviouslydebuggedsolutions,whicharesimilartoourneeds.Oftenduringthedesignprocess,additionalquestionsorconcernsarise.Weatthatpointconsultwithourclientstoclarifytheproblem.Nextwerewritetherequirementsdocumentandgetitreapprovedbytheclients.

Itisoftendifficulttodistinguishwhetheraparameterisaspecificationoraconstraint.Inactuality,whendesigningasystemitoftendoesn’tmatterintowhichcategoryaparameterfalls,becausethesystemmustsatisfyallspecificationsandconstraints.Nevertheless,whendocumentingthedeviceitisbettertocategorizeparametersproperly.Specificationsgenerallydefineinaquantitativemannertheoverallsystemobjectivesasgiventousbyourcustomers.

Constraints,ontheotherhand,generallydefinetheboundaryspacewithinwhichwemustsearchforasolutiontotheproblem.Ifwemustuseaparticularcomponent,itisoftenconsideredaconstraint.Inthisbook,weconstrainmostdesignstoincludeanLM3S/TM4Cmicrocontroller.Constraintsalsoareoftendefinedasaninequality,suchasthecostmustbelessthan$50,orthebatterymustlastforatleastoneweek.Specificationsontheotherhandareoftendefinedasaquantitativenumber,andthesystemsatisfiestherequirementifthesystemoperateswithinaspecifiedtoleranceofthatparameter.Tolerancecanbedefinedasapercentageerrororasarangewithminimumandmaximumvalues.

Thehigh-leveldesignusesdataflowgraphs.Wethencombinethepiecesanddebugthesystem.Asthepiecesarecombinedwecandrawacallgraphtoorganizetheparts.Ifnewcomponentsaredesigned,wecanuseflowchartstodevelopnewalgorithms.Themorewecansimulatethesystem,themoredesignpossibilitieswecanevaluate,andthequickerwecanmakechanges.Debugginginvolvesbothmakingsureitworks,togetherwithsatisfyingallrequirementsandconstraints.

Observation:Definingrealistictolerancesonourspecificationswillhaveaprofoundeffectonsystemcost.

Checkpoint2.23:Whataretheeffectsofspecifyingatightertolerance(e.g.,1%whentheproblemaskedfor5%)?

Checkpoint2.24:Whataretheeffectsofspecifyingaloosertolerance(e.g.,10%whentheproblemaskedfor5%)?

Example2.2:DesignanembeddedsystemthatflashesLEDsina0101,0110,1010,1001binaryrepeatingpattern.

Solution:ThissystemwillneedfourLEDs,andthecomputermustbeabletoactivate/deactivatethem.Sincetheproblemdidn’tspecifypowersource,speed,color,orbrightness,wecouldeitherputoffthesedecisionsuntiltheengineeringdesignstageinordertosimplifythedesignorminimizecost,orwecouldgobacktotheclientsandaskforadditionalrequirements.Inthiscase,theclientsdidn’tcareaboutpower,speed,colororbrightness,butdidthinkminimizingcostwasagoodidea.Duetothenatureofthisbook,wewillconstrainalldesignstoincludeaLM3S/TM4Cmicrocontroller.Becausewehave+3.3Vmicrocontrollersystems,wewillspecifythesystemtorunon+3.3Vpower.WehaveinstockHLMP-4740greenLEDsthatoperateat1.9Vand2mA,sowewillusethem.Table2.20summarizesthespecificationsandconstraints.Wewillusestandard5%resistorstominimizecost.

Specifications Constraints

Repeatingpatternof5,6,10,9

TM4C123-based

Four1.9V,2mAgreenLEDs

Minimizecost

+3.3Vpowersupply Standard5%resistors

Table2.20.SpecificationsandconstraintsoftheLEDoutputsystem.

ToleranceforthisLEDoutputsystemsaysitisacceptableifithasfourLEDs,butunacceptableifithasthreeorfiveofthem.Similarly,itwillbeacceptableaslongastheLEDcurrentisbetween1.5and2.5mA.Ifthecurrentdropsbelow1.5mA,wewon’tbeabletoseetheLED,andifitgoesabove2.5mA,itmightdamagetheLED.ThedataflowgraphinFigure2.34showsinformationasitflowsfromthecontrollersoftwaretothefourLEDs.Thedataflowgraphwillbeimportantduringthesubsequentdesignphasesbecausethehardwareblockscanbeconsideredasapreliminaryhardwareblockdiagramofthesystem.Thecallgraph,alsoshowninFigure2.34,illustratesthisismaster/slaveconfigurationwherethecontrollersoftwarewillmanipulatethefourLEDs.

Figure2.34.DataflowgraphandcallgraphoftheLEDoutputsystem.

ThehardwaredesignofthissystemcouldhaveusedfourcopiesoftheLEDinterfacepresentedearlierinFigure1.23.However,fromTable1.4weseeLM3S/TM4Cmicrocontrollerscansourceorsinkupto8mA.Wecansavemoneybyusinglow-currentLEDs,whichcanbeconnecteddirectlytothemicrocontrollerwithoutadriver.Figure2.35showsfoursimplenegativelogicLEDinterfaces.AlowoutputwillturnontheLEDandahighoutputwillturnitoff.NoticethesimilarityofthedataflowgraphinFigure2.34withthehardwarecircuitinFigure2.35.IftheVOLofthemicrocontrolleris0.4V,andthevoltageacrosstheLEDis1.9V,thenthevoltageacrosstheresistorshouldbe3.3-1.9-0.4Vor1V.WecalculatetheresistorvalueusingOhm’sLaw,Ris1V/2mAor500Ω.Usingstandardresistorvalueswitha5%tolerancewillbecheapertobuild(seeSection9.1).Inparticular,470Ωand510Ωaretwostandardresistorvaluesnear500Ω.Ifweweretouse470Ω,thentheLEDcurrentwouldbe(3.3-1.9-0.4V)/470Ωor2.1mA.Similarly,ifweweretouse510Ω,thentheLEDcurrentwouldbe(3.3-1.9-0.4V)/510Ωor1.96mA.Bothwouldhavebeenacceptable,butwewillusethe510Ωresistorbecauseitisacceptableforawiderangeofmicrocontrolleroutputvoltages.Morespecifically,ifVOLrangesfrom0.13to0.63V,thentheLEDcurrentremainswithinthe1.5to2.5mA

specification.Itwouldhavebeenmoreexpensivetouse500Ωresistors.

Pseudo-codeissimilartohigh-levellanguages,butwithoutarigidsyntax.Thismeansweutilizewhateversyntaxwelike.Flowchartsaregoodwhenthesoftwareinvolvescomplexalgorithmswithmanydecisionspointscausingcontrolpaths.Ontheotherhand,pseudo-codemaybebetterwhenthesoftwareismoresequentialandinvolvescomplexmathematicalcalculations.

Figure2.35.HardwarecircuitfortheLEDoutputsystem.

Thesoftwaredesignofthissystemalsoinvolvesusingexamplespresentedearlierwithsomeminortweaking.Theonlydatarequiredinthisproblemisthe5–6–10–9sequence.LaterinChapter3,wewillconsidersolutionstothistypeofproblemusingdatastructures,butinthisfirstexample,wewilltakeasimpleapproach,notusingadatastructure.Figure2.36illustratesasoftwaredesignprocessusingflowcharts.Westartwithgeneralapproachontheleft.Flowchart1showsthesoftwarewillinitializetheoutputport,andperformtheoutputsequence.Aswedesignthesoftwaresystem,wefillinthedetails.Thisdesignprocessiscalledsuccessiverefinement.Itisalsoclassifiedastop-down,becausewebeginwithhigh-levelissues,andendatthelow-level.InFlowchart2,wesetthedirectionregister,andthenoutputthesequence5–6–10–9.Itisatthisstagewefiguredouthowtocreatetherepeatingsequence.Flowchart3fillsintheremainingdetails.Tooutputthenegativelogicpattern1010totheLEDs,wewilloutputa5tothebottom4bitsofPortEonthemicrocontroller.

Manysoftwaredevelopersusepseudo-coderatherthanflowcharts,becausethepseudo-codeitselfcanbeembeddedintothesoftwareascomments.Program2.8showstheCimplementationforthissystem.NoticethesimilarityinstructurebetweenFlowchart3andthiscode.The LEDS definitionimplementsfriendlyaccesstopinsPE3–PE0.

Inordertotestthesystemweneedtobuildaprototype.Oneoptionissimulation.AsecondoptionistouseadevelopmentsystemliketheonesshowninFigures2.10,2.13and2.18.Inthisapproach,youbuildtheexternalcircuitsonaprotoboardandusethedebuggertodownloadandtestthesoftware.Athirdapproachistypicallyusedafterasuccessfulevaluationwithoneofthepreviousmethods.Inthisapproach,wedesignaprintedcircuitboard(PCB)includingboththeexternalcircuitsandthemicrocontrolleritself.

DuringthetestingphaseoftheprojectweobservethatallfouroftheLEDsare

continuouslyon.Weusethesoftwaredebuggertosinglestepourprogram,whichcorrectlyoutputsthe1010,1001,0101,0110binaryrepeatingpattern.DuringthissinglesteppingtheLEDsdocomeonandoffintheproperpattern.Usingavoltmeteronthecircuitweobservea0.25Vsignalontheoutputofthemicrocontrolleranda1.9VvoltagedropacrossthediodewheneverthesoftwarewishestoturntheLEDon.BecausetheLEDsareflashingfasterthanoureyescansee,wetestthesystematfullspeedandobservethefouroutputsonalogicanalyzer,collectingdatapresentedasFigure2.37.IfwewishedtobeabletoseetheLEDsflashwithoureyes,wecouldadda0.1seconddelayaftereachoutput.

Portabilityisameasureofhoweasyitistoconvertsoftwarethatrunsononemachinetorunonanothermachine.IngeneralCcodeismoreportablethanassemblylanguage.

Figure2.36.SoftwaredesignfortheLEDoutputsystemusingflowcharts.

#defineLEDS(*((volatileuint32_t*)0x4002403C))

intmain(void)

SYSCTL_RCGCGPIO_R|=0x10;//1)activateclockforPortE

while((SYSCTL_PRGPIO_R&0x10)==0);//ready?

GPIO_PORTE_DIR_R|=0x0F;//PE3-0isanoutput

GPIO_PORTE_AFSEL_R&=~0x0F;//regularportfunction

GPIO_PORTE_AMSEL_R&=~0x0F;//disableanalogonPE3-0

GPIO_PORTE_PCTL_R&=~0x0000FFFF;//PCTLGPIOonPE3-0

GPIO_PORTE_DEN_R|=0x0F;//PE3-0enabledasadigitalport

while(1)

LEDS=10;//1010,LEDis0101

LEDS=9;//1001,LEDis0110

LEDS=5;//0101,LEDis1010

LEDS=6;//0110,LEDis1001

Program2.8.CsoftwarefortheLEDoutputsystem(GPIO_xxx.zip).

Figure2.37.LogicanalyzerwaveformscollectedduringthetestingtheLEDoutputsystem.

Example2.3:Interfaceapushbuttonswitchtothemicrocontrollerandwritesoftwarefunctionsthatinitializeandreadtheswitch.

Solution:Thefirststepistodrawahardwarecircuitconnectingtheswitchtoaninputportofthemicrocontroller.Wewillusepositivelogicinterfacebecausewewantthedigitalsignaltobehighifandonlyiftheswitchispressed,asshowninFigure2.38.SimilartoFigure1.24, PB1 containsasignalthatishighorlowdependingonthepositionoftheswitch.Iftheswitchisnotpressed,the10k resistorcreatesa0Vsignalontheportpin,andvirtuallynocurrentflowsthroughtheresistor(IILis2 A).Iftheswitchispressed,a3.3Vsignalisontheportpinand0.33mAflowsthroughthe10k resistor.Someswitchesbounce,whichmeanstherewillbemultipleopen/closedcycleswhentheswitchischanged.Thissimplesolutioncanbeusediftheswitchdoesn’tbounceorifthebouncingdoesn’tmatter.Thesoftwaresolutionrequirestwofunctions.Theinitializationfunctioniscalledoncewhenthesystemstarts.Wheneverthesoftwarewishestoknowtheswitchstatus,itcallstheinputfunction.WhenthecomputerreadsPortBitgetsall8bitsoftheinputport.ThefollowingCcodewillsetavariabletotrue(nonzero)ifandonlyiftheswitchispressed.

Pressed=GPIO_PORTB_DATA_R&0x02;//trueiftheswitchispressed

Figure2.38.Positivelogicinterfaceofaswitchtoamicrocontrollerinput.

TheinitializationinProgram2.9activatestheclock,clearsthedirectionregisterbitforPB1,andenablesPortBbit1asadigitalport.Thedefinitionof PB1 inProgram2.9usesbit-specificaddressingsothesoftwarejustseesbit1.

#definePB1(*((volatileuint32_t*)0x40005008))

voidSwitch_Init(void)

SYSCTL_RCGCGPIO_R|=0x02;//1)activateclockforPortB

while((SYSCTL_PRGPIO_R&0x02)==0);//ready?

GPIO_PORTB_DIR_R&=~0x02;//PB1isaninput

GPIO_PORTB_AFSEL_R&=~0x02;//regularportfunction

GPIO_PORTB_AMSEL_R&=~0x02;//disableanalogonPB1

GPIO_PORTB_PCTL_R&=~0x000000F0;//PCTLGPIOonPB1

GPIO_PORTB_DEN_R|=0x02;//PB3-0enabledasadigitalport

uint32_tSwitch_Input(void)

returnPB1;//0x02ifpressed,0x00ifnotpressed

Program2.9.AsetoffunctionsthatinterfaceaninputswitchtoPB1(Switch_xxx.zip).

2.5.Phase-Lock-LoopNormally,theexecutionspeedofamicrocontrollerisdeterminedbyanexternalcrystal.BothLaunchPadboardshavea16MHzcrystal.Mostmicrocontrollershaveaphase-lock-loop(PLL)thatallowsthesoftwaretoadjusttheexecutionspeedofthecomputer.Typically,thechoiceoffrequencyinvolvesthetradeoffbetweensoftwareexecutionspeedandelectricalpower.Inotherwords,slowingdownthebusclockwillrequirelesspowertooperateandgeneratelessheat.Speedingupthebusclockobviouslyallowsformorecalculationspersecondatthecostofrequiringmorepowertooperateandgeneratingmoreheat.

ThedefaultbusspeedofanLM3S811microcontrolleristhatofthecrystalattachedtotheOSC1andOSC0pins,meaningthePLLisinitiallynotactiveThedefaultbusspeedofanLM3S1968andTM4Cmicrocontrollersisthatoftheinternaloscillator,alsomeaningthatthePLLisnotinitiallyactive.ThedefaultbusspeedfortheTM4Cinternaloscillatoris16MHz±1%.Theinternaloscillatorissignificantlylessprecisethanthecrystal,butitrequireslesspoweranddoesnotneedanexternalcrystal.ThismeansformostapplicationswewillactivatethemainoscillatorandthePLLsowecanhaveastablebusclock.

TherearetwowaystoactivatethePLL.Wecouldcallalibraryfunction,orwecouldaccesstheclockregistersdirectly.Ingeneral,usinglibraryfunctionscreatesabetterdesignbecausethesolutionwillbemorestable(lessbugs)andwillbemoreportable(easiertoswitchmicrocontrollers).However,theobjectiveofthebookistopresentmicrocontrollerfundamentals.ShowingthedirectaccessdoesillustratesomeconceptsofthePLL.First,wecanincludetheStellaris/Tivalibraryandcallthe SysCtlClockSetfunctiontochangethespeed.Thisfunctionisdefinedinthesysctl.cfile.ThelibraryfunctionactivatesthePLLbecauseofthe SYSCTL_USE_PLL parameter.Themainoscillatoristheonewiththeexternalcrystalattached.Thelastparameterspecifiesthefrequencyoftheattachedcrystal.AssumewewishtorunanTM4Cwitha16MHzcrystalat80MHz.Thedivideby2.5createsabusfrequencyof80MHz,implementedas400MHzdividedby5.

SysCtlClockSet(SYSCTL_SYSDIV_2_5|SYSCTL_USE_PLL|

SYSCTL_OSC_MAIN|SYSCTL_XTAL_16MHZ);

Tomakeourcodemoreportable,itisagoodideatouselibraryfunctionswheneverpossible.However,wewillpresentanexplicitexampleillustratinghowthePLLworks.AnexternalcrystalisattachedtotheTM4Cmicrocontroller,asshowninFigure2.39.ThePLLsontheotherTexasInstrumentsmicrocontrollersoperateinthesamebasicmanner.Table2.21showstheclockregistersusedtodefinewhatspeedtheprocessoroperates.Theoutputofthemainoscillator(MainOsc)isaclockatthesamefrequencyasthecrystal.BysettingtheOSCSRCbitsto0,themultiplexercontrolwillselectthemainoscillatorastheclocksource.

Forexample,themainoscillatorfortheTM4Contheevaluationboardwillbe16MHz.Thismeansthereferenceclock(RefClk)inputtothephase/frequencydetectorwillbe16

MHz.Fora16MHzcrystal,wesettheXTALbitsto10101(seeTable2.21).Inthisway,a400MHzoutputofthevoltagecontrolledoscillator(VCO)willyielda16MHzclockattheotherinputofthephase/frequencydetector.Ifthe400MHzclockistooslow,theupsignalwilladdcharge,increasingtheinputtotheVCO,leadingtoanincreaseinthe400MHzfrequency.Ifthe400MHzclockistoofast,downsignaltothechargepumpwillsubtractcharge,decreasingtheinputtotheVCO,leadingtoadecreaseinthe400MHzfrequency.ThefeedbackloopinthePLLwilldrivetheoutputtoastable400MHzfrequency.

Figure2.39.BlockdiagramofthemainclocktreeontheTM4CincludingthePLL(othersaresimilar).Program2.10showsaprogramtoactivateamicrocontrollerwitha16MHzmainoscillatortorunat80MHz.0)UseRCC2becauseitprovidesformoreoptions.1)ThefirststepissetBYPASS2(bit11).AtthispointthePLLisbypassedandthereisnosystemclockdivider.2)ThesecondstepistospecifythecrystalfrequencyinthefourXTALbitsusingthecodeinTable2.21.TheOSCSRC2bitsareclearedtoselectthemainoscillatorastheoscillatorclocksource.3)ThethirdstepistoclearPWRDN2(bit13)toactivatethePLL.4)Thefourthstepistoconfigureandenabletheclockdividerusingthe7-bitSYSDIV2field.Ifthe7-bitSYSDIV2isn,thentheclockwillbedividedbyn+1.Togetthedesired80MHzfromthe400MHzPLL,weneedtodivideby5.So,weplacea4intotheSYSDIV2field.5)ThefifthstepistowaitforthePLLtostabilizebywaitingforPLLRIS(bit6)inthe SYSCTL_RIS_R tobecomehigh.6)ThelaststepistoconnectthePLLbyclearingtheBYPASS2bit.Tomodifythisprogramtooperateonothermicrocontrollers,youwillneedtochangeXTALandtheSYSDIV2.

XTAL CrystalFreq(MHz)

XTAL CrystalFreq(MHz)

0x4 3.579545MHz 0x10 10.0MHz

0x5 3.6864MHz 0x11 12.0MHz

0x6 4MHz 0x12 12.288MHz

0x7 4.096MHz 0x13 13.56MHz

0x8 4.9152MHz 0x14 14.31818MHz

0x9 5MHz 0x15 16.0MHz

0xA 5.12MHz 0x16 16.384MHz

0xB 6MHz(resetvalue)

0x17 18.0MHz

0xC 6.144MHz 0x18 20.0MHz

0xD 7.3728MHz 0x19 24.0MHz

0xE 8MHz 0x1A 25.0MHz

0xF 8.192MHz others reserved

Address 26-23 22 13 11 10-6 5-4 Name

$400FE060 SYSDIV USESYSDIV PWRDN BYPASS XTAL OSCSRC SYSCTL_RCC_R

$400FE050 PLLRIS SYSCTL_RIS_R

31 30 28-22 13 11 6-4

$400FE070 USERCC2 DIV400 SYSDIV2 PWRDN2 BYPASS2 OSCSRC2 SYSCTL_RCC2_R

Table2.21.Mainclockregisters(RCC2inLM4F/TM4Conly)(othervaluesofXTALarereserved).

voidPLL_Init(void)

SYSCTL_RCC2_R|=0x80000000;//0)UseRCC2

SYSCTL_RCC2_R|=0x00000800;//1)bypassPLLwhileinitializing

SYSCTL_RCC_R=(SYSCTL_RCC_R&~0x000007C0)+0x00000540;//2)16MHz

SYSCTL_RCC2_R&=~0x00000070;//configureformainoscillatorsource

SYSCTL_RCC2_R&=~0x00002000;//3)activatePLLbyclearingPWRDN

SYSCTL_RCC2_R|=0x40000000;//4)use400MHzPLL

SYSCTL_RCC2_R=(SYSCTL_RCC2_R&~ 0x1FC00000)+(4<<22);//80MHz

while((SYSCTL_RIS_R&0x00000040)==0);//5)waitforthePLLtolock

SYSCTL_RCC2_R&=~0x00000800;//6)enablePLLbyclearingBYPASS

Program2.10.ActivatetheLM4F/TM4Cwitha16MHzcrystaltorunat80MHz(PLL_xxx.zip).Checkpoint2.25:HowwouldyouchangeProgram2.10ifyourLM4F/TM4Cmicrocontrollerhadan8MHzcrystalandyouwishtorunat50MHz?

Wecanmakeafirstorderestimateoftherelationshipbetweenworkdoneinthesoftwareandelectricalpowerrequiredtorunthesystem.Therearetwofactorsinvolvedintheperformanceofsoftware.Wedefinesoftwareworkasthedesiredactionsperformedbyexecutingsoftware:

Softwarework=algorithm*speed(ininstructions/sec)

Inotherwords,ifwewanttoimprovesoftwareperformancewecanwritebettersoftwareorincreasetherateatwhichthecomputerexecutesinstructions.RecallthatthecompilerconvertsourCsoftwareintoCortexMmachinecode,sotheefficiencyofthecompilerwillalsoaffectthisrelationship.Furthermore,mostcompilershaveoptimizationsettingsthatallowyoutomakeyoursoftwarerunfasterattheexpenseofusingmorememory.OntheCortexM,mostinstructionsexecutein1or2buscycles.Seesection3.3inCortexM4_TRM_r0p1.pdfformoredetails.InCMOSlogic,mostoftheelectricalpowerrequiredtorunthesystemoccursinmakingsignalschange,thatis,whenadigitalsignalrisesfrom0to1,orfallsfrom1to0.Thereforeweseealinearrelationshipbetweenbusfrequencyandelectricalpower.Letmbetheslopeofthislinearrelationship

Power=m*fBusSomeofthefactorsthataffecttheslopemareoperatingvoltageandfundamentalbehaviorofhowtheCMOStransistorsaredesigned.IfweapproximatetheCortexMprocessorasbeingabletoexecuteoneinstructioneverytwobuscycles,wecancombinetheabovetwoequationstoseethespeed-powertradeoff.

Softwarework=algorithm*½fBus=algorithm*½Power/m

Observation:Tosavepower,weslowdownthebusfrequencyremovingasmuchofthewastedbuscycleswhilestillperformingalloftherequiredtasks.

2.6.SysTickTimerSysTickisasimplecounterthatwecanusetocreatetimedelaysandgenerateperiodicinterrupts.ItexistsonallCortexTM-Mmicrocontrollers,sousingSysTickmeansthesystemwillbeeasytoporttoothermicrocontrollers.Table2.22showssomeoftheregisterdefinitionsforSysTick.CURRENTisa24-bitdowncounterthatrunsatthebusclockfrequency.

Address 31-24

23-17

16 15-3 2 1 0 Name

$E000E010 0 0 COUNT 0 CLK_SRC INTEN ENABLE NVIC_ST_CTRL_R

$E000E014 0 24-bitRELOADvalue NVIC_ST_RELOAD_R

$E000E018 0 24-bitCURRENTvalueofSysTickcounter NVIC_ST_CURRENT_R

Table2.22.SysTickregisters.

TherearefourstepstoinitializetheSysTicktimer.First,wecleartheENABLEbittoturnoffSysTickduringinitialization.Second,wesettheRELOADregister.Third,wewritetothe NVIC_ST_CURRENT_R valuetoclearthecounter.Lastly,wewritethedesiredmodetothecontrolregister,NVIC_ST_CTRL_R .WesettheCLK_SRCbitspecifyingthecoreclockwillbeused.WemustsetCLK_SRC=1,becauseCLK_SRC=0modeisnotimplementedontheLM3Sfamily.LM4F/TM4CmicrocontrollersdosupportCLK_SRC=0internaloscillatormode.InChapter5,wewillsetINTENtoenableinterrupts,butinthisfirstexampleweclearINTENsointerruptswillnotberequested.WeneedtosettheENABLEbitsothecounterwillrun.WhentheCURRENTvaluecountsdownfrom1to0,theCOUNTflagisset.Onthenextclock,theCURRENTisloadedwiththeRELOADvalue.Inthisway,theSysTickcounter(CURRENT)iscontinuouslydecrementing.IftheRELOADvalueisn,thentheSysTickcounteroperatesatmodulon+1(…n,n-1,n-2…1,0,n,n-1,…).Inotherwords,itrollsovereveryn+1counts.TheCOUNTflagcouldbeconfiguredtotriggeraninterrupt.However,inthisfirstexampleinterruptswillnotbegenerated.WesetRELOADto0x00FFFFFFforageneralcounter.Foradelaytimeroraperiodicinterruptthevalue(RELOAD+1)*busperiodwilldeterminethedelaytimeorinterruptperiod.IfweactivatethePLLtorunthemicrocontrollerat80MHz,thentheSysTickcounterdecrementsevery12.5ns.Ingeneral,iftheperiodofthecorebusclockist,thentheCOUNTflagwillbesetevery(n+1)t.Readingthe NVIC_ST_CTRL_R controlregisterwillreturntheCOUNTflaginbit16,andthencleartheflag.Also,writinganyvaluetothe NVIC_ST_CURRENT_R registerwillresetthecountertozeroandcleartheCOUNTflag.

Program2.11usestheSysTicktimertoimplementatimedelay.Forexample,theusercalls SysTick_Wait10ms(123); andthefunctionreturns1.23secondslater.Inthefunction SysTick_Wait() ,the NVIC_ST_RELOAD_R valueissettospecifythedelay.WritingtoCURRENTclearstheCOUNTflagandreloadsthecounter.Whenthecountergoesfrom1to0,theflagCOUNTisset.

TheaccuracyofSysTickdependsontheaccuracyoftheclock.WeusethePLLtoderiveabusclockbasedonthe16MHzcrystal,thetimemeasuredorgeneratedusingSysTickwillbeveryaccurate.Morespecifically,theaccuracyoftheNX5032GAcrystalontheLaunchPadboardis±50partspermillion(PPM),whichtranslatesto0.005%,whichisabout±5secondsperday.Onecouldspendmoremoneyonthecrystalandimprovetheaccuracybyafactorof10.Notonlyarecrystalsaccurate,theyarestable.TheNX5032GAcrystalwillvaryonly±150PPMastemperaturevariesfrom-40to+150ºC.Crystalsaremorestablethantheyareaccurate,typicallyvaryingbylessthan5PPMperyear.

#defineNVIC_ST_CTRL_R(*((volatileuint32_t*)0xE000E010))

#defineNVIC_ST_RELOAD_R(*((volatileuint32_t*)0xE000E014))

#defineNVIC_ST_CURRENT_R(*((volatileuint32_t*)0xE000E018))

voidSysTick_Init(void)

NVIC_ST_CTRL_R=0;//1)disableSysTickduringsetup

NVIC_ST_RELOAD_R=0x00FFFFFF;//2)maximumreloadvalue

NVIC_ST_CURRENT_R=0;//3)anywritetocurrentclearsit

NVIC_ST_CTRL_R=0x00000005;//4)enableSysTickwithcoreclock

voidSysTick_Wait(uint32_tdelay)//delayisin12.5nsunits

NVIC_ST_RELOAD_R=delay-1;//numberofcountstowait

NVIC_ST_CURRENT_R=0;//anyvaluewrittentoCURRENTclears

while((NVIC_ST_CTRL_R&0x00010000)==0)//waitforCOUNTflag

voidSysTick_Wait10ms(uint32_tdelay)//delayisin10msunits

uint32_ti;

for(i=0;i<delay;i++)

SysTick_Wait(800000);//800000*12.5nsequals10ms

Program2.11.Timerfunctionsthatimplementatimedelay(SysTick_xxx.zip).Checkpoint2.26:Howwouldyouchange SysTick_Wait10ms inProgram2.11ifyourmicrocontrollerwererunningat50MHz?

2.7.ChoosingaMicrocontrollerIchosetofocusthisbookontheLM3S/TM4Cfamilyofmicrocontrollers,becauseithasarichsetoffeaturesneededtoteachthefundamentalsrequiredforbothtoday’sandtomorrow’sembeddedsystems.Sometimes,thecomputerengineerisfacedwiththetaskofselectingthemicrocontrollerfortheproject.Whenfacedwiththisdecisionsomeengineerswillonlyconsiderthosedevicesforwhichtheyhavehardwareandsoftwareexperience.Fortunately,thisblindapproachoftenstillyieldsaneffectiveandefficientproduct,becausemanymicrocontrollersoverlapintheircostandperformance.Inotherwords,ifafamiliarmicrocontrollercanimplementthedesiredfunctionsfortheproject,thenitisoftenefficienttobypassthatmoreperfectpieceofhardwareinfavorofafasterdevelopmenttime.Ontheotherhand,sometimeswewishtoevaluateallpotentialcandidates.Itmaybecost-effectivetohireortraintheengineeringpersonnelsothattheyareproficientinawidespectrumofpotentialmicrocontrollerdevices.Therearemanyfactorstoconsiderwhenselectingamicrocontroller.

Afirstgroupoffactorsdealswithcost,maintenanceandproduction:

•Laborcostsincludestraining,development,andtesting

•Materialcostsincludespartsandsupplies

•Manufacturingcostsdependonthenumberandcomplexityofthecomponents

•Maintenancecostsinvolverevisionstofixbugsandperformupgrades

•Secondsourceavailability

Asecondgroupoffactorsdealswithmemoryandtheprocessor:

•ROMsizemustbebigenoughtoholdinstructionsandfixeddataforthesoftware

•RAMsizemustbebigenoughtoholdlocals,parameters,andglobalvariables

•EEPROMtoholdnonvolatilefixedconstantsthatarefield-configurable

•Processormustbecapableofperformingallcalculationsinrealtime

•8-,16-,or32-bitdatasizeshouldmatchmostofthedatatobeprocessed

•Numericaloperationslikemultiply,divide,saturation,floatingpoint

•Specialfunctionslikemultiply/accumulate,fuzzylogic,complexnumbers

•Availabilityofhigh-levellanguagecross-compilers,simulators,anddebuggers

Athirdgroupoffactorsdealswithinputandoutput:

•I/Obandwidthdeterminestheinput/outputrate

•Parallelportsfortheinput/outputdigitalsignals

•SerialportstointerfacewithothercomputersorI/Odevices

•Timerfunctionstogeneratesignals,measurefrequency,andmeasureperiod

•Pulsewidthmodulationfortheoutputsignalsinmanycontrolapplications

•ADCthatisusedtoconvertanaloginputstodigitalnumbers

•DACthatisusedtoconvertdigitalnumberstoanalogoutputs

•SpecialI/OfunctionssuchasCAN,EthernetandUSB

Afourthgroupoffactorsdealswithsystemleveldesign:

•Packagesizeandenvironmentalissuesaffectmanyembeddedsystems

•Powerrequirementsbecausemanysystemswillbebatteryoperated

Whenconsideringspeeditisbesttocomparetimetoexecuteabenchmarkprogramsimilartoyourspecificapplication,ratherthanjustcomparingbusfrequency.Oneofthedifficultiesisthatthemicrocontrollerselectiondependsonthespeedandsizeofthesoftware,butthesoftwarecannotbewrittenwithoutthecomputer.Giventhisuncertainty,itisbesttoselectafamilyofdeviceswitharangeofexecutionspeedsandmemoryconfigurations.Inthiswayaprototypesystemwithlargeamountsofmemoryandperipheralscanbepurchasedforsoftwareandhardwaredevelopment,andoncethedesignisinitsfinalstages,thespecificversionofthecomputercanbeselectednowknowingthememoryandspeedrequirementsfortheproject.Inconclusion,whilethisbookfocusesontheARM ® CortexTM-Mmicrocontrollers,itisexpectedthatoncethestudyofthisbookiscompleted,thereaderwillbeequippedwiththeknowledgetoselectthepropermicrocontrollerandcompletethesoftwaredesign.

2.8.Exercises2.1WhatisspecialaboutRegister13?Register14?Register15?

2.2In20wordsorlessdescribethedifferencesbetweenvonNeumannandHarvardarchitectures.

2.3WhathappenswhenyouloadavalueintoRegister15withbit0set?

2.4WriteCcodethatsetsbit31ofmemorylocation0x2000.1234usingbit-banding.

2.5WriteCcodethatclearsbit16ofmemorylocation0x2000.8000usingbit-banding.

2.6WriteCcodethatsetsbit1ofmemorylocation0x4000.5400usingbit-banding.Whateffectdoesthisoperationhave?

2.7WriteCcodethatclearsbit2ofmemorylocation0x4000.7400usingbit-banding.Whateffectdoesthisoperationhave?

2.8HowmuchRAMandROMareinTM4C123?Whatarethespecificaddressrangesofthesememorycomponents?

2.9HowmuchRAMandROMareinLM3S1968?Whatarethespecificaddressrangesofthesememorycomponents?

2.10HowmuchRAMandROMareinLM3S8962?Whatarethespecificaddressrangesofthesememorycomponents?

2.11WhatarethebitsintheProgramStatusRegister(PSR)ofARM ® CortexTM-M?

2.12Whathappensifyouexecutethesefourassemblyinstructions?

PUSHR1

PUSHR2

POPR1

POPR2

2.13WriteassemblycodethatpushesregistersR1R3andR5ontothestack.

2.14Howdoyouinitializethestack?

2.15Howdoyouspecifywheretobeginexecutionafterareset?

2.16Whatdoesword-alignedmean?

2.17WhendoestheLRhavetobepushedonthestack?

2.18Doestheassociativeprincipleholdforsignedintegermultiplyanddivide?Assume Out1Out2ABC areallthesameprecision(e.g.,32bits).InparticulardothesetwoCcalculationsalwaysachieveidenticaloutputs?Ifnot,giveanexample.

Out1=(A*B)/C;

Out2=A*(B/C);

2.19Doestheassociativeprincipleholdforsignedintegeradditionandsubtraction?Assume Out3Out4ABC areallthesameprecision(e.g.,32bits).InparticulardothesetwoCcalculationsalwaysachieveidenticaloutputs?Ifnot,giveanexample.

Out3=(A+B)-C;

Out4=A+(B-C);

2.20WhatareparallelportsareavailableontheTM4C123?

2.21WhatareparallelportsareavailableontheLM3S1968?

2.22WhatareparallelportsareavailableontheLM3S8962?

2.23Whatisadirectionregister?Whydoesthemicrocontrollerhavedirectionregisters?

2.24Whatisthealternativefunctionregister?

D2.25WritesoftwarethatinitializesLM3S/LM4F/TM4CPortA,sopins7,5,3,1areoutputandtherestareinput.

D2.26WritesoftwarethatinitializesLM3S/LM4F/TM4CPortA,sopins5,4areoutputandtherestareinput.

D2.27WritesoftwarethatinitializesLM3S/LM4F/TM4CPortA,sopins5,4,and3areoutput.Maketheinitializationfriendly.Designanoutputfunctionthattakesa3-bitparameter(0to7)andwritesthevaluetothesethreepins.Usebit-specificaddressingfortheoutput.

D2.28WritesoftwarethatinitializesLM3S/LM4F/TM4CPortE,sopin1isanoutput.Maketheinitializationfriendly.Designanoutputfunctionthattakesa1-bitparameter(0or1)andwritesthevaluetothispin.Usebit-specificaddressingfortheoutput.

D2.29RedesigntheSSRinterfaceshowninFigure2.33usinga+5Vsource.Inparticular,recalculatetherequiredresistorvalueifweweretochangethe+3.3Vto+5V.

D2.30RedesigntheLEDinterfaceshowninFigure2.35ifthefourLEDsoperatedat1.9Vand1mA.Inparticular,recalculatetherequiredresistorvaluesifweweretochangetotheseLEDs.

D2.31RewritethesoftwareinProgram2.8sotheLEDpatternchangesevery0.1sec.

D2.32Designaswitchinterfacethatitisnegativelogic.I.e.,theinputislowiftheswitchispressedandhighiftheswitchisnotpressed.

2.9.LabAssignmentsThelabsinthisbookinvolvethefollowingsteps:

Parta)Duringtheanalysisphaseoftheprojectdetermineadditionalspecificationsandconstraints.Inparticular,discoverwhichmicrocontrolleryouaretouse,whetheryouaretodevelopinassemblylanguageorinC,andwhethertheprojectistobesimulatedthenbuilt,justbuiltorjustsimulated.Forexample,inputscanbecreatedwithswitchesandoutputscanbegeneratedwithLEDs.TheUARTcanbeinterfacedtoaPC,andacommunicationprogramlikePuTTYcanbeusedtointeractwiththesystem.

Partb)Design,build,andtestthehardwareinterfaces.Useacomputer-aided-drawing(CAD)programtodrawthehardwarecircuits.Labelallpins,chips,andresistorvalues.Inthischapter,therewillbeoneswitchforeachinputandoneLEDforeachoutput.Connecttheswitchinterfacestomicrocontrollerinputpins,andconnecttheLEDinterfacestomicrocontrolleroutputpins.Pressingtheswitchwillsignifyahighinputlogicvalue.YoushouldactivatetheLEDtosignifyahighoutputlogicvalue.

Partc)Design,implementandtestthesoftwarethatinitializestheI/Oportsandperformsthespecifiedfunction.Oftenamainprogramisusedtodemonstratethesystem.

Lab2.1TheoverallobjectiveistocreateaNOTgate.Thesystemhasonedigitalinputandonedigitaloutput,suchthattheoutputisthelogicalcomplementoftheinput.Implementthedesignsuchthatthecomplementfunctionoccursinthesoftwareofthemicrocontroller.

Lab2.2Theoverallobjectiveistocreatea3-inputANDgate.Thesystemhasthreedigitalinputsandonedigitaloutput,suchthattheoutputisthelogicalandofthethreeinputs.ImplementthedesignsuchthattheANDfunctionoccursinthesoftwareofthemicrocontroller.

Lab2.3Theoverallobjectiveistocreatea2-inputEXCLUSIVEORgate.Thesystemhastwodigitalinputsandonedigitaloutput,suchthattheoutputisthelogicalexclusiveorofthetwoinputs.ImplementthedesignsuchthattheEXCLUSIVEORfunctionoccursinthesoftware.

Lab2.4Theoverallobjectiveistocreatea3-inputvotinglogic.Thesystemhasthreedigitalinputsandonedigitaloutput,suchthattheoutputishighifandonlyiftwoormoreinputsarehigh.Thismeanstheoutputwillbelowiftwoformoreinputsarelow.Implementthedesignsuchthatthevotingfunctionoccursinthesoftwareofthemicrocontroller.

Lab2.5Theoverallobjectiveistoavariablefrequencyoscillator.Thesystemhastwodigitalinputsandtwodigitaloutputs.Ifinput1istruethedigitaloutput1oscillatesat262Hz.Iftheinput1isfalsetheoutput1remainslow.Ifinput2istruethedigitaloutput2oscillatesat392Hz.Iftheinput2isfalsetheoutput2remainslow.Ifyouconnecteachoutputtoa10k resistorasshowninthefigure,thenyoucanhearthetonesasmiddleCandmiddleG.

3.SoftwareDesignChapter3objectivesareto:•Presentthesoftwaredesignprocess

•Describeasoftwarecodingstyle

•Definemodules,boardsupportpackage,anddevicedrivers

•Presentadesignmethodusingfinitestatemachines

•Definetheconceptofthreads

•ImplementFIFOqueues

•Presentasimplememorymanagerasanintroductiontotheheap

•Introducetheartofdebugging

Theultimatesuccessofanembeddedsystemprojectdependsbothonitssoftwareandhardware.Computerscientistspridethemselvesintheirabilitytodevelopqualitysoftware.Similarlyelectricalengineersarewell-trainedintheprocessestodesignbothdigitalandanalogelectronics.Manufacturers,inanattempttogetdesignerstousetheirproducts,provideapplicationnotesfortheirhardwaredevices.Themainobjectiveofthisbookistocombineeffectivedesignprocessestogetherwithpracticalsoftwaretechniquesinordertodevelopqualityembeddedsystems.Asthesizeandcomplexityofthesoftwareincrease,softwaredevelopmentchangesfromsimple“coding”to“softwareengineering”.Naturally,asthesystemcomplexityincreasessodotheengineeringskillsrequiredtodesignsuchsystems.Thesesoftwareskillspresentedinthischapterincludemodulardesign,layeredarchitecture,abstraction,andverification.Evenifreal-timeembeddedsystemsareonthesmallendofthesizescale,nevertheless,thesesystemscanbequitecomplex.Therefore,theabovementionedskillsareessentialfordevelopingembeddedsystems.Thischapteronsoftwaredevelopmentisplacedearlyinthebookbecausewritinggoodsoftwareisanartthatmustbedevelopedandcannotbeaddedonattheendofaproject.Goodsoftwarecombinedwithaveragehardwarewillalwaysoutperformaveragesoftwareongoodhardware.Inthischapterwewilloutlinevarioustechniquesfordevelopingqualitysoftwareandthenapplythesetechniquesthroughouttheremainderofthebook.

3.1.AttitudeGoodengineersemploywell-defineddesignprocesseswhendevelopingcomplexsystems.Whenweworkwithinastructuredframework,itiseasiertoproveoursystemworks(verification)andtomodifyoursysteminthefuture(maintenance).Asoursoftwaresystemsbecomemorecomplex,itbecomesincreasinglyimportanttoemploywell-definedsoftwaredesignprocesses.Throughoutthisbook,averydetailedsetofsoftwaredevelopmentruleswillbepresented.Thisbookfocusesonreal-timeembeddedsystemswritteninC.Atfirst,itmayseemradicaltoforcesucharigidstructuretosoftware.Wemightwonderifcreativitywillbesacrificedintheprocess.Truecreativityismoreabouteffectivesolutionstoimportantproblemsandnotaboutbeingsloppyandinconsistent.Becausesoftwaremaintenanceisacriticaltask,thetimespentorganizing,documenting,andtestingduringtheinitialdevelopmentstageswillreaphugedividendsthroughoutthelifeofthesoftwareproject.

Observation:Theeasiestwaytodebugistowritesoftwarewithoutanybugs.

Wedefineclientsaspeoplewhowilluseoursoftware.Sometimes,theclientistheend-userwhousestheembeddedsystem.Othertimes,wedevelophardware/softwarecomponentsthatplugintoalargersystem.Inthiscase,theclientdevelopssoftwarethatwillcallourfunctions.Wedefinecoworkersasengineerswhowillmaintainoursoftware.Wemustmakeiteasyforacoworkertodebug,use,andextendoursoftware.

Writingqualitysoftwarehasalottodowithattitude.Weshouldbeembarrassedtoaskourcoworkerstomakechangestoourpoorlywrittensoftware.Sincesomuchsoftwaredevelopmenteffortinvolvesmaintenance,weshouldcreatesoftwaremodulesthatareeasytochange.Inotherwords,weshouldexpecteachpieceofourcodewillbereadbyanotherengineerinthefuture,whosejobitwillbetomakechangestoourcode.Wemightbetemptedtoquitasoftwareprojectoncethesystemisrunning,butthisshorttimewemightsavebynotorganizing,documenting,andtestingwillbelostmanytimesoverinthefuturewhenitistimetoupdatethecode.

Asprojectmanagers,wemustrewardgoodbehaviorandpunishbadbehavior.Acompany,inanefforttoimprovethequalityoftheirsoftwareproducts,implementedthefollowingpolicies.“Theemployeesinthecustomerrelationsdepartmentreceiveabonusforeverysoftwarebugthattheycanidentify.Thesebugsarereportedtothesoftwaredevelopers,whointurnreceiveabonusforeverybugtheyfix.”

Checkpoint3.1:Whydidtheabovepolicyfailhorribly?

Weshoulddemandofourselvesthatwedeliverbug-freesoftwaretoourclients.Again,weshouldbeembarrassedwhenourclientsreportbugsinourcode.Weshouldbeashamedwhenotherprogrammersfindbugsinourcode.Therearefourstepswecantaketofacilitatethisimportantaspectofsoftwaredesign.

Testitnow.Whenwefindabug,fixitimmediately.Thelongerweputofffixingamistakethemorecomplicatedthesystembecomes,makingithardertofind.Rememberthatbugsdonotgoawayautomatically,butwecanmakethesystemsocomplexthatthebugswillmanifestthemselvesinamysteriousandobscurefashion.Forthesamereason,weshouldcompletelytesteachmoduleindividually,beforecombiningthemintoalargersystem.Weshouldnotaddnewfeaturesbeforeweareconvincedtheexistingfeaturesarebug-free.Inthisway,westartwithaworkingsystem,addfeatures,andthendebugthissystemuntilitisworkingagain.

Thisincrementalapproachmakesiteasiertotrackprogress.Itallowsustoundobaddecisions,becausewecanalwaysrevertbacktoapreviousworkingsystem.Addingnewfeaturesbeforetheoldonesaredebuggedisveryrisky.Withthissloppyapproach,wecouldeasilyreachtheprojectdeadlinewith100%ofthefeaturesimplemented,buthaveasystemthatdoesn’trun.Inaddition,onceabugisintroduced,thelongerwewaittoremoveit,theharderitwillbetocorrect.Thisisparticularlytruewhenthebugsinteractwitheachother.Conversely,withtheincrementalapproach,whentheprojectscheduleslips,wecandeliveraworkingsystematthedeadlinethatsupportssomeofthefeatures.

MaintenanceTip:Gofromworkingsystemtoworkingsystem.

Planfortesting.Howtotestshouldbeconsideredatthebeginning,middle,andendofaproject.Inparticular,testingshouldbeincludedaspartoftheinitialdesign.Ourtestingandtheclient’susagegohandinhand.Inparticular,howwetestthesoftwaremodulewillhelptheclientunderstandthecontextandlimitationsofhowoursoftwareistobeused.Itoftenmakessensetoexplainthetestingprocedurestotheclientasanefforttocommunicatethefeaturesandlimitationsofthemodule.Furthermore,aclearunderstandingofhowtheclientwishestouseoursoftwareiscriticalforboththesoftwaredesignanditstesting.Forexample,afterseeinghowyoutestedthemodule,theclientmayrespond,“That’snice,butwhatIreallywantittodois…”.Ifthishappens,itmakessensetorewritetherequirementsdocumenttoreflectthisnewunderstandingoftheclient’sexpectation.

MaintenanceTip:Itisbettertohavesomepartsofthesystemthatrunwith100%reliabilitythantohavetheentiresystemwithbugs.

Gethelp.Usewhateverfeaturesareavailablefororganizationanddebugging.Payattentiontowarnings,becausetheyoftenpointtomisunderstandingsaboutdataorfunctions.Misunderstandingofassumptionscancausebugswhenthesoftwareisupgraded,orreusedinadifferentcontextthanoriginallyconceived.Rememberthatcomputertimeisalotcheaperthanprogrammertime.Itisamistaketodebuganembeddedsystemsimplybyobservingitsinputsandoutputs.Weneedtousebothsoftwareandhardwaredebuggingtoolstovisualizeinternalparameterswithinthesystem.

MaintenanceTip:Itisbettertohaveasystemthatrunsslowlythantohaveonethatdoesn’trunatall.

Divideandconquer.Intheearlydaysofmicrocomputersystems,softwaresizecouldbemeasuredinhundredsoflinesofsourcecodeorthousandsofbytesofobjectcode.Theseearlysystems,duetotheirsmallsize,wereinherentlysimple.Theexplosionofhardwaretechnology(bothinspeedandsize)hasledtoasimilarincreaseinthesizeofsoftwaresystems.Theonlyhopeforsuccessinalargesoftwaresystemwillbetobreakitintosimplemodules.Inmostcases,thecomplexityoftheproblemitselfcannotbeavoided.E.g.,thereisjustnosimplewaytogettothemoon.Nevertheless,acomplexsystemcanbecreatedoutofsimplecomponents.Arealcreativeeffortisrequiredtoorchestratesimplebuildingblocksintolargermodules,whichthemselvesaregrouped.Weuseourcreativitytobreakacomplexproblemintosimplecomponents,ratherthandevelopingcomplexsolutionstosimpleproblems.

Observation:Therearetwowaysofconstructingasoftwaredesign:onewayistomakeitsosimplethatthereareobviouslynodeficienciesandtheotherwayismakeitsocomplicatedthattherearenoobviousdeficiencies.C.A.R.Hoare,“TheEmperor’sOldClothes,”CACMFeb.1981.

3.2.QualityProgrammingSoftwaredevelopmentissimilartootherengineeringtasks.Wecanchoosetofollowwell-definedproceduresduringthedevelopmentandevaluationphases,orwecanmeanderinahaphazardwayandproducecodethatishardtotestandhardertochange.Theultimategoalofthesystemistosatisfythestatedobjectivessuchasaccuracy,stability,andinput/outputrelationships.Neverthelessitisappropriatetoseparatelyevaluatetheindividualcomponentsofthesystem.Thereforeinthissection,wewillevaluatethequalityofoursoftware.Therearetwocategoriesofperformancecriteriawithwhichweevaluatethe“goodness”ofoursoftware.Quantitativecriteriaincludedynamicefficiency(speedofexecution),staticefficiency(ROMandRAMprogramsize),andaccuracyoftheresults.Qualitativecriteriacenteroneaseofsoftwaremaintenance.Anotherqualitativewaytoevaluatesoftwareiseaseofunderstanding.Ifyoursoftwareiseasytounderstandthenitwillbe:

Easytodebug,includingbothfindingandfixingmistakesEasytoverify,meaningwecanproveitiscorrectEasytomaintain,meaningwecanaddnewfeatures

Commonerror:Programmerswhosacrificeclarityinfavorofexecutionspeedoftendevelopsoftwarethatrunsfastbutiserror-proneanddifficulttochange.

GoldenRuleofSoftwareDevelopment:Writesoftwareforothersasyouwishtheywouldwriteforyou.

3.2.1.QuantitativePerformanceMeasurementsInordertoevaluateoursoftwarequality,weneedperformancemeasures.Thesimplestapproachestothisissuearequantitativemeasurements.Dynamicefficiencyisameasureofhowfasttheprogramexecutes.Itismeasuredinsecondsorprocessorbuscycles.BecauseofthecomplexityoftheCortexTM-M,itwillbehardtoestimateexecutionspeedbyobservingtheassemblylanguagegeneratedbythecompiler.Rather,wewillemploymethodstoexperimentallymeasureexecutionspeed.Staticefficiencyisthenumberofmemorybytesrequired.SincemostembeddedcomputersystemshavebothRAMandROM,wespecifymemoryrequirementinglobalvariables,stackspace,fixedconstants,andprogramobjectcode.TheglobalvariablesplusmaximumstacksizemustfitintotheavailableRAM.Similarly,thefixedconstantsplusprogramsizemustfitintotheavailableROM.Wecanjudgeoursoftwaresystemaccordingtowhetherornotitsatisfiesgivenconstraints,likesoftwaredevelopmentcosts,memoryavailable,andtimetable.Manyofthesystemspecificationsarequantitative,andhenceareanappropriatemeasureofquality.

3.2.2.QualitativePerformanceMeasurements

Qualitativeperformancemeasurementsincludethoseparameterstowhichwecannotassignadirectnumericalvalue.Ofteninlifethemostimportantquestionsaretheeasiesttoask,butthehardesttoanswer.Suchisthecasewithsoftwarequality.Sothereforeweaskthefollowingqualitativequestions.Canweproveoursoftwareworks?Isoursoftwareeasytounderstand?Isoursoftwareeasytochange?Sincethereisnosingleapproachtowritingqualitysoftware,Icanonlyhopetopresentsometechniquesthatyoumaywishtointegrateintoyourownsoftwarestyle.Infact,wewilldevotemostthischaptertotheimportantissueofdevelopingqualitysoftware.Inparticular,wewillstudyself-documentedcode,abstraction,modularity,andlayeredsoftware.Theseparametersindeedplayaprofoundeffectonthebottom-linefinancialsuccessofourprojects.Althoughquitereal,becausethereisoftennotanimmediateanddirectrelationshipbetweensoftwarequalityandprofit,wemaybetemptedtodismissitsimportance.

Observation:Mostpeoplegetbetterwithpractice.Soifyouwishtobecomeabetterprogrammer,Isuggestyouwritegreatquantitiesofsoftware.

Togetabenchmarkonhowgoodaprogrammeryouare,Ichallengeyoutotwotests.Inthefirsttest,findamajorpieceofsoftwarethatyouhavewrittenover12monthsago,andthenseeifyoucanstillunderstanditenoughtomakeminorchangesinitsbehavior.Thesecondtestistoexchangewithapeeramajorpieceofsoftwarethatyouhavebothrecentlywritten(butnotwrittentogether),theninthesamemanner,ifyoucanmakeminorchangestoeachother’ssoftware.

Observation:Youcantellifyouareagoodprogrammerif1)youcanunderstandyourowncode12monthslater,and2)otherscanmakechangestoyourcode.

3.3.SoftwareStyleGuidelinesOneoftherecurringthemesofthissoftwarestylesectionisconsistency.Maintainingaconsistentstylewillhelpuslocateandunderstandthedifferentcomponentsofoursoftware,aswellaspreventusfromforgettingtoincludeacomponentorworseincludingittwice.

3.3.1.OrganizationofacodefileThefollowingregionsshouldoccurinthisorderineverycodefile(e.g.,file.c).

Openingcomments.Thefirstlineofeveryfileshouldcontainthefilename.Thisisbecausesomeprintersdonotautomaticallyprintthenameofthefile.Rememberthattheseopeningcommentswillbeduplicatedinthecorrespondingheaderfile(e.g.,file.h)andareintendedtobereadbytheclient,theonewhowillusetheseprograms.Ifmajorportionsofthissoftwarearecopiedfromcopyrightedsources,thenwemustsatisfythecopyrightrequirementsofthosesources.Therestoftheopeningcommentsshouldinclude

•Theoverallpurposeofthesoftwaremodule

•Thenamesoftheprogrammers

•Thecreation(optional)andlastupdatedates

•Thehardware/softwareconfigurationrequiredtousethemodule

•Copyrightinformation

Including.hfiles.Next,wewillplacethe #include statementsthataddthenecessaryheaderfiles.Addingothercodefiles,ifnecessary,willoccurattheendofthefile,buthereatthetopofthefileweincludejusttheheaderfiles.Normallytheorderdoesn’tmatter,sowewilllisttheincludefilesinahierarchicalfashionstartingwiththelowestlevelandendingatthehighesthigh.Iftheorderofthesestatementsisimportant,thenwriteacommentdescribingbothwhattheproperorderisandwhytheorderisimportant.Puttingthemtogetheratthetopwillhelpusdrawacallgraph,whichwillshowushowourmodulesareconnected.Inparticular,ifweconsidereachcodefiletobeaseparatemodule,thenthelistof#include statementsspecifieswhichothermodulescanbecalledfromthismodule.Ofcourseoneheaderfileisallowedtoincludeotherheaderfiles.However,weshouldavoidhavingoneheaderfileincludeotherheaderfiles.Thisrestrictionmakestheorganizationalstructureofthesoftwaresystemeasiertoobserve.Becarefultoincludeonlythosefilesthatareabsolutelynecessary.Addingunnecessaryincludestatementswillmakeoursystemseemmorecomplexthanitactuallyis.

externreferences.Afterincludingtheheaderfiles,wecandeclareanyexternalvariablesorfunctions.Externalreferenceswillberesolvedbythelinker,whenvariousmodulesarelinkedtogethertocreateasingleexecutableapplication.Placingthemtogetheratthetopofthefilewillhelpusseehowthissoftwaresystemfitstogether(i.e.,islinkedto)othersystems.

#definestatements.Afterexternalreferences,weshouldplacethe #define macros.Thesemacroscandefineoperationsorconstants.Sincethesedefinitionsarelocatedinthecodefile(e.g.,file.c),theywillbeprivate.Thismeanstheyareavailablewithinthisfileonly.Iftheclientdoesnotneedtouseorchangethemacrooperationorconstant,thenitshouldbemadeprivatebyplacingithereinthecodefile.Conversely,ifwewishtocreatepublicmacros,thenweplacethemintheheaderfileforthismodule.

structunionenumstatements.Afterthedefinestatements,weshouldcreatethenecessarydatastructuresusing struct union and enum .Again,sincethesedefinitionsarelocatedinthecodefile(e.g.,file.c),theywillbeprivate.

Globalvariablesandconstants.Afterthestructuredefinitions,weshouldincludetheglobalvariablesandconstants.Therearetwoaspectsofdatathatareimportant.First,wecanspecifywherethedataisallocated.Ifitisavariablethatneedstoexistpermanently,wewillplaceitinRAMasaglobalvariable.Ifitisaconstantthatneedstoexistpermanently,wewillplaceitinROMusing const .Ifthedataisneededtemporarily,wecandefineitasalocal.Thecompilerwillallocatelocalsinregistersoronthestackinwhicheverwayismostefficient.

int32_tPublicGlobal;//accessiblebyanymodule

staticint32_tPrivateGlobal;//accessibleinthisfileonly

constint32_tConstant=1234567;//inROM

voidfunction(void)

staticint32_tveryPrivateGlobal;//accessiblebythisfunctiononly

int32_tprivateLocal;//accessiblebythisfunctiononly

Wedefineaglobalvariableasonewithpermanentallocation.Intheaboveexamples, PublicGlobal PrivateGlobal and veryPrivateGlobal areglobal. ConstantwillbedefinedinROM,andcannotbechanged.Wedefinealocalvariableasonewithtemporaryallocation.ThevariableprivateLocal islocalandmayexistonthestackorinaregister.

Thesecondaspectofthedataisitsscope.Scopespecifieswhichsoftwarecanaccessthedata.Publicvariablescanbeaccessedbyanysoftware.Privatevariableshaverestrictedscope,whichcanbelimitedtotheonefile,theonefunction,oreventoone programblock.Ingeneral,wewishtominimizethescopeofourdata.Minimizingscopereducescomplexityandsimplifiestesting.Ifwespecifytheglobalwithstatic ,thenitwillbeprivateandcanonlybeaccessedbyprogramsinthisfile.Ifwedonotspecifytheglobalwithstatic thenitwillbepublic,andcanbeaccessedanyprogram.Forexample,the PublicGlobal variablecanbedefinedinothermodulesusingextern andthelinkerwillresolvethereference.However,the PrivateGlobal cannotbeaccessedfromsoftwareinanotherfile.Weputalltheglobalstogetherbeforeanyfunctiondefinitionstosymbolizethefactthatanyfunctioninthisfilehasaccesstotheseglobals.Ifwehaveapermanentvariablethatisonlyaccessbyonefunction,thenitshouldbedefinedinsidethefunctionwith static .Forexample,thevariable veryPrivateGlobal ispermanentlyallocatedinRAM,butcanonlybeaccessedbythefunction.

MaintenanceTip:Reducecomplexityinoursystembyrestrictingdirectaccesstoourdata.

Prototypesofprivatefunctions.Aftertheglobals,weshouldaddanynecessaryprototypes.Justlikeglobalvariables,wecanrestrictaccesstoprivatefunctionsbydefiningthemasstatic.Prototypesforthepublicfunctionswillbeincludedinthecorrespondingheaderfile.Ingeneral,wewillarrangethecodeimplementationsinatop-downfashion.Althoughnotnecessary,wewillincludetheparameternameswiththeprototypes.Descriptiveparameternameswillhelpdocumenttheusageofthefunction.Forexample,whichofthefollowingprototypesiseasiertounderstand?

staticvoidplot(int16_t,int16_t);

staticvoidplot(int16_ttime,int16_tpressure);

Implementationsofthefunctions.Theheartoftheimplementationfilewillbe,ofcourse,theimplementations.Again,privatefunctionsshouldbedefinedasstatic.Thefunctionsshouldbesequencedinalogicalmanner.Themosttypicalsequenceistop-down,meaningwebeginwiththehighestlevelandfinishwiththelowestlevel.Anotherappropriatesequencemirrorsthemannerinwhichthefunctionswillbeused.Forexample,startwiththeinitializationfunctions,followedbytheoperations,andendwiththeshutdownfunctions.Forexample:

OpenInputOutputClose

Including.cfiles.Ifthecompilerdoesnotsupportprojects,thenwewouldendthefilewith#include statementsthataddthenecessarycodefiles.Sincemostcompilerssupportprojects,weshoulduseitsorganizationalfeaturesandavoidincludingcodefiles.Theprojectsimplifiesthemanagementoflargesoftwaresystemsbyprovidingorganizationalstructuretothesoftwaresystem.Again,ifweuseprojects,thenincludingcodefileswillbeunnecessary,andhenceshouldbeavoided.

Employrun-timetesting.Ifourcompilersupports assert() functions,usethemliberally.Inparticular,placethematthebeginningoffunctionstotestthevalidityoftheinputparameters.Placethemaftercalculationstotestthevalidityoftheresults.Placetheminsideloopstoverifyindicesandpointersarevalid.Thereisasecondarybenefittousing assert() .The assert() statementsprovideinherentdocumentationoftheassumptions.

3.3.2.OrganizationofaheaderfileOnceagain,maintainingaconsistentstylefacilitatesunderstandingandhelpstoavoiderrorsofomission.Definitionsmadeintheheaderfilewillbepublic,i.e.,accessiblebyallmodules.Asstatedearlier,itisbettertomakeglobalvariablesprivateratherthanplacingthemintheheaderfile.Similarly,weshouldavoidplacingactualcodeinaheaderfile.

Therearetwotypesofheaderfiles.Thefirsttypeofheaderfilehasnocorrespondingcodefile.Inotherwords,thereisafile.h,butnofile.c.Inthistypeofheader,wecanlistglobalconstantsandhelpermacros.ExamplesofglobalconstantsareI/Oportaddresses(seelm3s1968.h),datatypes(seeinteger.h)andcalibrationcoefficients.Debuggingmacroscouldbegroupedtogetherandplacedinadebug.hfile.Wewillnotconsidersoftwareinthesetypesofheaderfilesasbelongingtoaparticularmodule.

Thesecondtypeofheaderfiledoeshaveacorrespondingcodefile.Thetwofiles,e.g.,file.h,andfile.c,formasoftwaremodule.Inthistypeofheader,wedefinetheprototypesforthepublicfunctionsofthemodule.Thefile.hcontainsthepolicies(behaviororwhatitdoes)andthefile.cfilecontainsthemechanisms(functionsorhowitworks.)Thefollowingregionsshouldoccurinthisorderineveryheaderfile.

Openingcomments.Thefirstlineofeveryfileshouldcontainthefilename.Thisisbecausesomeprintersdonotautomaticallyprintthenameofthefile.Rememberthattheseopeningcommentsshouldbeduplicatedinthecorrespondingcodefile(e.g.,file.c)andareintendedtobereadbytheclient,theonewhowillusetheseprograms.Weshouldrepeatcopyrightinformationasappropriate.Therestoftheopeningcommentsshouldinclude

•Theoverallpurposeofthesoftwaremodule

•Thenamesoftheprogrammers

•Thecreation(optional)andlastupdatedates

•Thehardware/softwareconfigurationrequiredtousethemodule

•Copyrightinformation

Including.hfiles.Nestedincludesintheheaderfileshouldbeavoided.Asstatedearlier,nestedincludesobscurethemannerinwhichthemodulesareinterconnected.

#definestatements.Publicconstantsandmacrosarenext.Specialcareisrequiredtodetermineifadefinitionshouldbemadeprivateorpublic.Oneapproachtothisquestionistobeginwitheverythingdefinedasprivate,andthenshiftdefinitionsintothepubliccategoryonlywhendeemednecessaryfortheclienttoaccessinordertousethemodule.Iftheparameterrelatestowhatthemoduledoesorhowtousethemodule,thenitshouldprobablybepublic.Ontheotherhand,ifitrelatestohowitworksorhowitisimplemented,itshouldprobablybeprivate.

structunionenumstatements.Thedefinitionsofpublicstructuresallowtheclientsoftwaretocreatedatastructuresspecificforthismodule.

Globalvariablesandconstants.Ifatallpossible,publicglobalvariablesshouldbeavoided.Publicconstantsfollowthesamerulesaspublicdefinitions.Iftheclientmusthaveaccesstoaconstanttousethemodule,thenitcouldbeplacedintheheaderfile.

Prototypesofpublicfunctions.Theprototypesforthepublicfunctionsarelast.Justliketheimplementationfile,wewillarrangethecodeimplementationsinatop-downfashion.Commentsshouldbedirectedtotheclient,andthesecommentsshouldclarifywhatthefunctiondoesandhowthefunctioncanbeused.Examplesofhowtousethemodulecouldbeincludedinthecomments.

Oftenwewishtoplacedefinitionsintheheaderfilethatmustbeincludedonlyonce.Ifmultiplefilesincludethesameheaderfile,thecompilerwillincludethedefinitionsmultipletimes.Somedefinitions,suchasfunctionprototypes,canbedefinedthenredefined.However,acommonapproachtoheaderfilesuses #ifndef conditionalcompilation.Iftheobjectisnotdefined,thenthecompilerwillincludeeverythingfromthe #ifndef untilthematching #endif .Insideofcourse,wedefinethatobjectsothattheheaderfileisskippedonsubsequentattemptstoincludeit.Eachheaderfilemusthaveauniqueobject.Onewaytoguaranteeuniquenessistousethenameoftheheaderfileitselfintheobjectname.

#ifndef__File_H__

#define__File_H__

structPosition

intbValid;//trueifpointisvalid

int16_tx;//incm

int16_ty;//incm

;

typedefstructPositionPositionType;

#endif

3.3.3.FormattingTherulessetoutinthissubsectionarenotnecessaryfortheprogramtocompileortorun.Rathertheintentoftherulesaretomakethesoftwareeasiertounderstand,easiertodebug,andeasiertochange.Justlikebeginninganexerciseprogram,theserulesmaybehardtofollowatfirst,butthedisciplinewillpaydividendsinthefuture.

Makethesoftwareeasytoread.Istronglyobjecttohardcopyprintoutsofcomputerprogramsduringthedevelopmentphaseofaproject.Atthistime,therearefrequentupdatesmadebymultiplemembersofthesoftwaredevelopmentteam.Becauseahardcopyprintoutwillbequicklyobsolete,weshoulddevelopanddebugsoftwarebyobservingitonthecomputerscreen.Inordertoeliminatehorizontalscrolling,nolineofcodeshouldbewiderthanthesizeoftheeditorscreen.Ifwedomakehardcopyprintoutsofthesoftwareattheendofaproject,thisrulewillresultinaprintoutthatiseasytoread.

Indentationshouldbesetat2spaces.Whentransportingcodefromonecomputertoanother,thetabsettingsmaybedifferent.So,whatlooksgoodononecomputermaylookuglyonanother.Forthisreason,weshouldavoidtabsandusejustspaces.Localvariabledefinitionscangoonthesamelineasthefunctiondefinition,orinthefirstcolumnonthenextline.

Beconsistentaboutwhereweputspaces.SimilartoEnglishpunctuation,thereshouldbenospacebeforeacommaorasemicolon,butthereshouldbeatleastonespaceoracarriagereturnafteracommaorasemicolon.Thereshouldbenospacebeforeorafteropenorcloseparentheses.Assignmentandcomparisonoperationsshouldhaveasinglespacebeforeandaftertheoperation.Oneexceptiontothesinglespaceruleisiftherearemultipleassignmentstatements,wecanlineuptheoperatorsandvalues.Forexample

data=1;

pressure=100;

voltage=5;

Beconsistentaboutwhereweputbraces .Misplacedbracescausebothsyntaxandsemanticerrors,soitiscriticaltomaintainaconsistentstyle.Placetheopeningbraceattheendofthelinethatopensthescopeofthemulti-stepstatement.Theonlycodethatcangoonthesamelineafteranopeningbraceisalocalvariabledeclarationoracomment.Placingtheopenbraceneartheendofthelineprovidesavisualcluethatanewcodeblockhasstarted.Placetheclosingbraceonaseparatelinetogiveaverticalseparationshowingtheendofthemulti-stepstatement.Thehorizontalplacementoftheclosebracegivesavisualcluethatthefollowingcodeisinadifferentblock.Forexample

voidmain(void)inti,j,k;

j=1;

if(sub0(j))

for(i=0;i<6;i++)

sub1(i);

k=sub2(i,j);

else

k=sub3();

Usebracesafterall if , else , for , do , while , case ,and switch commands,eveniftheblockisasinglecommand.Thisforcesustoconsiderthescopeoftheblockmakingiteasiertoreadandeasiertochange.Forexample,assumewestartwiththefollowingcode.

if(flag)

n=0;

Now,weaddasecondstatementthatwewanttoexecutealsoiftheflagistrue.Thefollowingerrormightoccurifwejustaddthenewstatement.

if(flag)

n=0;

c=0;

Ifallofourblocksareenclosedwithbraces,wewouldhavestartedwiththefollowing.

if(flag)

n=0;

Now,whenweaddasecondstatement,wegetthecorrectsoftware.

if(flag)

n=0;

c=0;

3.3.4.CodeStructure

Makethepresentationeasytoread.Wedefinepresentationasthelookandfeelofoursoftwareasdisplayedonthescreen.Ifatallpossible,thesizeofourfunctionsshouldbesmallenoughsothemajorityofa“singleidea”fitsonasinglecomputerscreen.Wemustconsiderthepresentationasatwo-dimensionalobject.Consequently,wecanreducethe2-Dareaofourfunctionsbyencapsulatingcomponentsanddefiningthemasprivatefunctions,orbycombiningmultiplestatementsonasingleline.Inthehorizontaldimension,weareallowedtogroupmultiplestatementsonasinglelineonlyifthecollectionmakessense.Weshouldlistmultiplestatementsonasingleline,ifwecandrawacirclearoundthestatementsandassignasimplecollectiveexplanationtothecode.

Observation:Mostprofessionalprogrammersdonotcreatehardcopyprintoutsofthesoftware.Rather,softwareisviewedonthecomputerscreen,anddevelopersuseacoderepositorylikeGitorSVNtostoreandsharetheirsoftware.

Anotherconsiderationrelatedtolistingmultiplestatementsonthesamelineisdebugging.Thecompileroftenplacesdebugginginformationoneachlineofcode.Breakpointsinsomesystemscanonlybeplacedatthebeginningofaline.Considerthefollowingthreepresentations.Sincethecompilergeneratesexactlythesamecodeineachcase,thecomputerexecutionwillbeidentical.Therefore,wewillfocusonthedifferencesinstyle.Thefirstexamplehasahorrificstyle.

voidtestFilter(int32_tstart,int32_tstop,int32_tstep)int32_tx,y;

initFilter();UART_OutString(“x(n)y(n)”);UART_OutChar(CR);

for(x=start;x<=stop;x=x+step)y=filter(x);UART_OutUDec(x);

UART_OutChar(SP);UART_OutUDec(y);UART_OutChar(CR);

Thesecondexampleplaceseachstatementonaseparateline.Althoughwritteninanadequatestyle,itisunnecessarilyvertical.

voidtestFilter(int32_tstart,int32_tstop,int32_tstep)

int32_tx;

int32_ty;

initFilter();

UART_OutString(“x(n)y(n)”);

UART_OutChar(CR);

for(x=start;x<=stop;x=x+step)

y=filter(x);

UART_OutUDec(x);

UART_OutChar(SP);

UART_OutUDec(y);

UART_OutChar(CR);

Thefollowingimplementationgroupsthetwovariabledefinitionstogetherbecausethecollectioncanbeconsideredasasingleobject.Thevariablesarerelatedtoeachother.Obviously, x and y arethesametype(32-bitsigned),butinaphysicalsense,theywouldhavethesameunits.Forexample,if x representsasignalinmV,then y isalsoasignalinmV.Similarly,theUARToutputsequencescausesimplewell-definedoperations.

voidtestFilter(int32_tstart,int32_tstop,int32_tstep)int32_tx,y;

initFilter();

UART_OutString(“x(n)y(n)”);UART_OutChar(CR);

for(x=start;x<=stop;x=x+step)

y=filter(x);

UART_OutUDec(x);UART_OutChar(SP);UART_OutUDec(y);UART_OutChar(CR);

The“makethepresentationeasytoread”guidelinesometimescomesinconflictwiththe“beconsistentwhereweplacebraces”guideline.Forexample,thefollowingexampleisobviouslyeasytoread,butviolatestheplacementofbracerule.

for(i=0;i<6;i++)dataBuf[i]=0;

Whenindoubt,wewillalwaysbeconsistentwhereweplacethebraces.Thecorrectstyleisalsoeasytoread.

for(i=0;i<6;i++)

dataBuf[i]=0;

Employmodularprogrammingtechniques.Complexfunctionsshouldbebrokenintosimplecomponents,sothatthedetailsofthelower-leveloperationsarehiddenfromtheoverallalgorithmsatthehigherlevels.Aninterestingquestionarises:Shouldasubfunctionbedefinedifitwillonlybecalledfromasingleplace?Theanswertothisquestion,infacttheanswertoallquestionsaboutsoftwarequality,isyesifitmakesthesoftwareeasiertounderstand,easiertodebug,andeasiertochange.

Minimizescope.Ingeneral,wehidetheimplementationofoursoftwarefromitsusage.Thescopeofavariableshouldbeconsistentwithhowthevariableisused.Inamilitarysense,weaskthequestion,“Whichsoftwarehastheneedtoknow?”Globalvariablesshouldbeusedonlywhenthelifetimeofthedataispermanent,orwhendataneedstobepassedfromonethreadtoanother.Otherwise,weshoulduselocalvariables.Whenonemodulecallsanother,weshouldpassdatausingthenormalparameter-passingmechanisms.Asmentionedearlier,weconsiderI/Oportsinamannersimilartoglobalvariables.ThereisnosyntacticmechanismtopreventamodulefromaccessinganI/Oport,sincetheportsareatfixedandknownabsoluteaddresses.ProcessorsusedtobuildgeneralpurposecomputershaveacomplexhardwaresystemtopreventunauthorizedsoftwarefromaccessingI/Oports,butthedetailsarebeyondthescopeofthisbook.Inmostembeddedsystems,however,wemustrelyonthedoes-accessratherthanthecan-accessmethodwhendealingwithI/Odevices.Inotherwords,wemusthavethedisciplinetorestrictI/Oportaccessonlyinthemodulethatisdesignedtoaccessit.Forsimilarreasons,weshouldconsidereachinterruptvectoraddressseparately,groupingitwiththecorrespondingI/Omodule.

Usetypes.Usinga typedef willclarifytheformatofavariable.Itisanotherexampleoftheseparationofmechanismandpolicy.Newdatatypesandstructureswillbeginwithanuppercaseletter.The typedef allowsustohidetherepresentationoftheobjectanduseanabstractconceptinstead.Forexample

typedefint16_tTemperature;

voidmain(void)TemperaturelowT,highT;

Thisallowsustochangetherepresentationoftemperaturewithouthavingtofindallthetemperaturevariablesinoursoftware.Noteverydatatyperequiresa typedef .Wewillusetypesforthoseobjectsoffundamentalimportancetooursoftware,andforthoseobjectsforwhichachangeinimplementationisanticipated.Asalways,thegoalistoclarify.Ifitdoesn’tmakeiteasiertounderstand,easiertodebug,oreasiertochange,don’tdoit.

Prototypeallfunctions.Publicfunctionsobviouslyrequireaprototypeintheheaderfile.Intheimplementationfile,wewillorganizethesoftwareinatop-downhierarchicalfashion.Sincethehighestlevelfunctionsgofirst,prototypesforthelower-levelprivatefunctionswillberequired.Groupingthelow-levelprototypesatthetopprovidesasummaryoverviewofthesoftwareinthismodule.Includeboththetypeandnameoftheinputparameters.Specifythefunctionasvoidevenifithasnoparameters.Theseprototypesareeasytounderstand:

voidstart(int32_tperiod,void(*functionPt)(void));

int16_tdivide(int16_tdividend,int16_tdivisor);

Theseprototypesarehardertounderstand:

start(int32_t,(*)());

int16_tdivide(int16_t,int16_t);

Declaredataandparametersasconstwheneverpossible.Declaringanobjectasconsthastwoadvantages.Thecompilercanproducemoreefficientcodewhendealingwithparametersthatdon’tchange.Thesecondadvantageistocatchsoftwarebugs,i.e.,situationswheretheprogramincorrectlyattemptstomodifydatathatitshouldnotmodify.

gotostatementsarenotallowed.Debuggingishardenoughwithoutaddingthecomplexitygeneratedwhenusing goto .Acorollarytothisruleiswhendevelopingassemblylanguagesoftware,weshouldrestrictthebranchingoperationstothesimplestructuresallowedinC.

++and—shouldnotappearincomplexstatements.Theseoperationsshouldonlyappearascommandsbythemselves.Again,thecompilerwillgeneratethesamecode,sotheissueisreadability.Thestatement

*(—pt)=buffer[n++];

shouldhavebeenwrittenas

—pt;

*(pt)=buffer[n];

n++;

Ifitmakessensetogroup,thenputthemonthesameline.Thefollowingcodeisallowed

buffer[n]=0;n++;

Beaparenthesiszealot.Whenmixingarithmetic,logical,andconditionaloperations,explicitlyspecifytheorderofoperations.Donotrelyontheorderofprecedence.Asalways,themajorstyleissueisclarity.Evenifthefollowingcodewereactuallytoperformtheintendedoperation(whichinfactitdoesnot),itwouldbepoorstyle.

if(x+1&0x0F==y|0x04)

Theprogrammerassignedtomodifyitinthefuturewillhaveabetterchanceifwehadwritten

if(((x+1)&0x0F)==(y|0x04))

Useenuminsteadof#defineorconst.Theuseofenumallowsforconsistencycheckingduringcompilation,andprovidesforeasytoreadsoftware.Agoodoptimizingcompilerwillcreateexactlythesameobjectcodeforthefollowingfourimplementationsofthesameoperation.Soonceagain,wefocusonstyle.Inthefirstimplementationweneededcommentstoexplaintheoperations.Inthesecondimplementationnocommentsareneededbecauseofthetwo#definestatements.

//implementation1

intMode;//0meanserror

voidfunction1(void)

Mode=1;//noerror

voidfunction2(void)

if(Mode==0)//error?

UART_OutString(“error”);

//implementation2

#defineNOERROR1

#defineERROR0

intMode;

voidfunction1(void)

Mode=NOERROR;

voidfunction2(void)

if(Mode==ERROR)

UART_OutString(“error”);

Inthethirdimplementation,shownbelowontheleft,thecompilerperformsatype-match,makingsureMode , NOERROR ,and ERROR arethesametype.Considerafourthimplementationthatusesenumerationtoprovideacheckofbothtypeandvalue.Wecanexplicitlysetthevaluesoftheenumeratedtypesifneeded.

//implementation3

constintNOERROR=1;

constintERROR=0;

intMode;

voidfunction1(void)

Mode=NOERROR;

voidfunction2(void)

if(Mode==ERROR)

UART_OutString(“error”);

//implementation4

enumMode_stateERROR,

NOERROR;

enumMode_stateMode;

voidfunction1(void)

Mode=NOERROR;

voidfunction2(void)

if(Mode==ERROR)

UART_OutString(“error”);

#define statements,ifusedproperly,canclarifyoursoftwareandmakeoursoftwareeasytochange.Itispropertousesizeinallplacesthatrefertothesizeofthedataarray.

#defineSIZE10

int16_tData[SIZE];

voidinitialize(void)int16_tj;

for(j=0;j<SIZE;j++)

Data[j]=0;

Don’tusebit-shiftforarithmeticoperations.Computerarchitecturesandcompilersusedtobesolimitedthatitmadesensetoperformmultiply/divideby2usingashiftoperation.Forexample,whenmultiplyinganumberby4,wemightbetemptedtowrite data<<2 .Thisiswrong;iftheoperationismultiply,weshouldwrite data*4 .Compileroptimizationhasdevelopedtothepointwherethecompilercanchoosetoimplement data*4 aseitherashiftormultiplydependingontheinstructionsetofthecomputer.Whenwewritemultiplywhenwemeanmultiply,andwriteshiftwhenwemeanshiftwecreatecodethatiseasytounderstand.

3.3.5.NamingconventionChoosingnamesforvariablesandfunctionsinvolvescreativethought,anditisintimatelyconnectedtohowwefeelaboutourselvesasprogrammers.Ofthepoliciespresentedinthissection,namingconventionsmaybethehardesthabitforustochange.Thedifficultyisthattherearemanyconventionsthatsatisfythe“easytounderstand”objective.Goodnamesreducetheneedfordocumentation.Poornamespromoteconfusion,ambiguity,andmistakes.Poornamescanoccurbecausecodehasbeencopiedfromadifferentsituationandinsertedintooursystemwithoutproperintegration(i.e.,changingthenamestobeconsistentwiththenewsituation.)Theycanalsooccurintheclutteredmindofasecond-rateprogrammer,whohurriestodeliversoftwarebeforeitisfinished.

Namesshouldhavemeaning.Ifweobserveanameoutofthecontextoftheplaceatwhichitwasdefined,themeaningoftheobjectshouldbeobvious.Theobject TxFifo isclearlyatransmitfirstinfirstoutcircularqueue.Thefunction UART_OutString willoutputastringtotheserialport.

Avoidambiguities.Don’tusevariablenamesinoursystemthatarevagueorhavemorethanonemeaning.Forexample,itisvaguetouse temp ,becausetherearemanypossibilitiesfortemporarydata,infact,itmightevenmeantemperature.Don’tusetwonamesthatlooksimilar,buthavedifferentmeanings.

Givehintsaboutthetype.Wecanfurtherclarifythemeaningofavariablebyincludingphrasesinthevariablenamethatspecifyitstype.Forexample, dataPt , timePt ,and putPt arepointers.Similarly, voltageBuf , timeBuf ,and pressureBuf aredatabuffers.Othergoodphrasesinclude FlagModeU16LIndexCnt ,whichrefertoBooleanflag,systemstate,unsigned16-bit,signed32-bit,indexintoanarray,andacounterrespectively.

Usethesamenametorefertothesametypeofobject.Forexample,everywhereweneedalocalvariabletostoreanASCIIcharacterwecouldusethename letter .Anothercommonexampleistousethenames i , j ,and k forindicesintoarrays.Thenames V1and R1 mightrefertoavoltageandaresistance.Theexactcorrespondenceisnotpartofthepoliciespresentedinthissection,justthefactthatacorrespondenceshouldexist.Onceanotherprogrammerlearnswhichnamesweuseforwhichobjecttypes,understandingourcodebecomeseasier.

Useaprefixtoidentifypublicobjects.Inthisstylepolicy,anunderlinecharacterwillseparatethemodulenamefromthefunctionname.Asanexceptiontothisrule,wecanusetheunderlinetodelimitwordsinallupper-casename(e.g., #defineMIN_PRESSURE10 ).Functionsthatcanbeaccessedoutsidethescopeofamodulewillbeginwithaprefixspecifyingthemoduletowhichitbelongs.Itispoorstyletocreatepublicvariables,butiftheyneedtoexist,theytoowouldbeginwiththemoduleprefix.Theprefixmatchesthefilenamecontainingtheobject.Forexample,ifweseeafunctioncall, UART_OutString(“Helloworld”); weknowthispublicfunctionbelongstotheUARTmodule,wherethepoliciesaredefinedinUART.handtheimplementationinUART.c.Noticethesimilaritybetweenthissyntax(e.g., UART_Init() )andthecorrespondingsyntaxwewoulduseifprogrammingthemoduleasaclassinobject-orientedlanguagelikeC++orJava(e.g., UART.Init() ).Usingthisconvention,wecaneasilydistinguishpublicandprivateobjects.

Useupperandlowercasetospecifytheallocationofanobject.WewilldefineI/Oportsandconstantsusingnolower-caseletters,liketypingwithcaps-lockon.Inotherwords,nameswithoutlower-caselettersrefertoobjectswithfixedvalues. TRUE , FALSE ,and NULL aregoodexamplesoffixed-valuedobjects.Asmentionedearlier,constantnamesformedfrommultiplewordswilluseanunderlinecharactertodelimittheindividualwords.E.g.,MAX_VOLTAGE , UPPER_BOUND ,and FIFO_SIZE .Permanently-allocatedglobalswillbeginwithacapitalletter,butincludesomelower-caseletters.Localvariableswillbeginwithalower-caseletter,andmayormaynotincludeuppercaseletters.Sinceallfunctionsarepermanentlyallocated,wecanstartfunctionnameswitheitheranupper-caseorlower-caseletter.Usingthisconvention,wecandistinguishconstants,globalsandlocals.Anobject’sproperties(public/private,local/global,constant/variable)arealwaysperfectlyclearattheplacewheretheobjectisdefined.Theimportanceofthenamingpolicyistoextendthatclarityalsototheplaceswheretheobjectisused.

Usecapitalizationtodelimitwords.Namesthatcontainmultiplewordsshouldbedefinedusingacapitallettertosignifythefirstletteroftheword.CreatingasinglenameoutputofmultiplewordsbycapitalizingthemiddlewordsandsqueezingoutthespacesiscalledCamelCase.Recallthatthecaseofthefirstletterspecifieswhetheristhelocalorglobal.Someprogrammersusetheunderlineasaword-delimiter,butexceptforconstants,wewillreserveunderlinetoseparatethemodulenamefromthevariablename.Table3.1presentsexamplesofthenamingconventionusedinthisbook.

Type Examples

Constants

CRSAFE_TO_RUNPORTASTACK_SIZESTART_OF_RAM

Localvariables maxTemperaturelastCharTypederrorCnt

Privateglobalvariable

MaxTemperatureLastCharTypedErrorCnt

Publicglobalvariable

DAC_MaxVoltageKey_LastCharTypedNetwork_ErrorCnt

Privatefunction ClearTimewrapPointerInChar

Publicfunction Timer_ClearTimeRxFifo_PutKey_InChar

Table3.1.Examplesofnames.Useunderlinetodefinethemodulename.Useuppercaseforconstants.UseCamelCaseforvariablesandfunctions.

Checkpoint3.2:Howcanyoutellifafunctionisprivateorpublic?

Checkpoint3.3:Howcanyoutellifavariableislocalorglobal?

3.3.6.CommentsDiscussionaboutcommentswasleftforlast,becausetheyaretheleastimportantaspectinvolvedinwritingqualitysoftware.Itismuchbettertowritewell-organizedsoftwarewithsimpleinterfaceshavingoperationssoeasytounderstandthatcommentsarenotnecessary.Thegoalofthissectionistopresentideasconcerningsoftwaredocumentationingeneral,andwritingcommentsinparticular.Becausemaintenanceisthemostimportantphaseofsoftwaredevelopment,documentationshouldassistsoftwaremaintenance.Inmanysituationsthesoftwareisnotstatic,butcontinuouslyundergoingchanges.Becauseofthisliquidity,Ibelievethatflowchartandsoftwaremanualsarenotgoodmechanismsfordocumentingprogramsbecauseitisdifficulttokeepthesetypesofdocumentationuptodatewhenmodificationsaremade.Therefore,thetermdocumentationinthisbookrefersalmostexclusivelytocommentsthatareincludedinthesoftwareitself.

Thebeginningofeveryfileshouldincludethefilename,purpose,hardwareconnections,programmer,date,andcopyright.Forexample,wecouldwrite:

//filenameadtest.c

//TestofTM4C123ADC

//1HzsamplingonPD3andoutputtotheserialport

//Lastmodified6/21/14byJonathanW.Valvano

//Copyright2014byJonathanW.Valvano

//Youmayuse,edit,runordistributethisfile

//aslongastheabovecopyrightnoticeremains

Thebeginningofeveryfunctionshouldincludealinedelimitingthestartofthefunction,purpose,inputparameters,outputparameters,andspecialconditionsthatapply.Thecommentsatthebeginningofthefunctionexplainthepolicies(e.g.,howtousethefunction.)Thesecomments,whicharesimilartothecommentsfortheprototypesintheheaderfile,areintendedtobereadbytheclient.Forexample,wecouldexplainafunctionthisway:

//––-––––UART_InUDec–––––––-

//InUDecacceptsASCIIinputinunsigneddecimal

//andconvertstoa32-bitunsignednumber

//validrangeis0to4294967295

//Input:none

//Output:32-bitunsignednumber

//Ifyouenteranumberabove2^32-1,itwilltruncate

//Backspacewillremovelastdigittyped

Commentscanbeaddedtoavariableorconstantdefinitiontoclarifytheusage.Inparticular,commentscanspecifytheunitsofthevariableorconstant.Forcomplicatedsituations,wecanuseadditionallinesandincludeexamples.E.g.,

int16_tV1;//voltageatnode1inmV,

//range-5000mVto+5000mV

uint16_tFs;//samplingrateinHz

intFoundFlag;//0ifkeywordnotyetfound,

//1iffound

uint16_tMode;//determinessystemaction,

//asoneofthefollowingthreecases

#defineIDLE0

#defineCOLLECT1

#defineTRANSMIT2

Commentscanbeusedtodescribecomplexalgorithms.Thesetypesofcommentsareintendedtobereadbyourcoworkers.Thepurposeofthesecommentsistoassistinchangingthecodeinthefuture,orapplyingthiscodeintoasimilarbutslightlydifferentapplication.Commentsthatrestatethefunctionprovidenoadditionalinformation,andactuallymakethecodehardertoread.Examplesofbadcommentsinclude:

time++;//addonetotime

mode=0;//setmodetozero

Goodcommentsexplainwhytheoperationisperformed,andwhatitmeans:

time++;//maintainelapsedtimeinmsec

mode=0;//switchtoidlemodebecausenodata

Wecanaddspacessothecommentfieldslineup.Asstatedearlier,weavoidtabsbecausetheyoftendonottranslatefromonesystemtoanother.Inthisway,thesoftwareisontheleftandthecommentscanbereadontheright.

MaintenanceTip:Ifitisnotwrittendown,itdoesn’texist.

Assoftwaredevelopers,ourgoalistoproducecodethatnotonlysolvesourcurrentproblembutcanalsoserveasthebasisofourfuturesolutions.Inordertoreusesoftwarewemustleaveourcodeinaconditionsuchthatfutureprogrammers(includingourselves)caneasilyunderstanditspurpose,constraints,andimplementation.Documentationisnotsomethingtackedontosoftwareafteritisdone,butratheritisadisciplinebuiltintoitateachstageofthedevelopment.Writingcommentsaswedevelopthesoftwareforcesustothinkaboutwhatthesoftwareisdoingandmoreimportantlywhywearedoingit.Therefore,weshouldcarefullydevelopaprogrammingstylethatprovidesappropriatecomments.Ifeelacommentthattellsuswhyweperformcertainfunctionsismoreinformativethancommentsthattelluswhatthefunctionsare.

Commonerror:Acommentthatsimplyrestatestheoperationdoesnotaddtotheoverallunderstanding.

Commonerror:Puttingacommentoneverylineofsoftwareoftenhidestheimportantinformation.

Goodcommentsassistusnowwhilewearedebuggingandwillassistuslaterwhenwearemodifyingthesoftware,addingnewfeatures,orusingthecodeinadifferentcontext.Whenavariableisdefined,weshouldaddcommentstoexplainhowthevariableisused.Ifthevariablehasunitsthenitisappropriatetoincludetheminthecomments.Itmayberelevanttospecifytheminimumandmaximumvalues.Atypicalvalueandwhatitmeansoftenwillclarifytheusageofthevariable.Forexample:

int16_tSetPoint;

//Thedesiredtemperatureforthecontrolsystem

//16-bitsignedtemperaturewithresolutionof0.5C,

//Therangeis-55Cto+125C

//Avalueof25means12.5C,

//Avalueof-25means-12.5C

Whenaconstantisused,wecouldaddcommentstoexplainwhattheconstantmeans.Ifthenumberhasunitsthenitisappropriatetoincludetheminthecomments.Forexample:

V=999;//999mVisthemaximumvoltage

Err=1;//errorcodeof1meansoutofrange

Therearetwotypesofreadersofourcomments.Ourclientissomeonewhowilluseoursoftwareincorporatingitintoalargersystem.Clientcommentsfocusonthepoliciesofthesoftware.Whatarethepossiblevalidinputs?Whataretheresultingoutputs?Whataretheerrorconditions?Justlikeavariable,itmayberelevanttospecifytheminimumandmaximumvaluesfortheinput/outputparameters.Typicalinput/outputvaluesandwhattheymeanoftenwillclarifytheusageofthefunction.Oftenwegiveentiresoftwareexamplesshowinghowthefunctionscouldbeused.

Thesecondtypeofcommentsisdirectedtotheprogrammerresponsiblefordebuggingandsoftwaremaintenance(coworker).Coworkercommentsfocusonthemechanismsofthesoftware.Thesecommentsexplainhowthefunctionworks.

Generallyweseparatethesecommentsfromtheonesintendedfortheuserofthefunction.Thisseparationisthefirstofmanyexamplesinthisbookoftheconcept“separationofpoliciesfrommechanisms”.Thepolicyiswhatthefunctiondoes,andthemechanismishowitworks.Specifically,weplacethissecondtypeofcommentswithinthebodyofthefunction.IfwearedevelopinginC,thenthesecommentsshouldbeincludedinthe*.cfilealongwiththefunctionimplementation.

Self-documentingcodeissoftwarewritteninasimpleandobviousway,suchthatitspurposeandfunctionareself-apparent.Descriptivenamesforvariables,constants,andfunctionswillgoalongwaytoclarifytheirusage.Towritewonderfulcodelikethis,wefirstmustformulatetheproblembyorganizingitintoclearwell-definedsubproblems.Howwebreakacomplexproblemintosmallpartsgoesalongwaytowardmakingthesoftwareself-documenting.Theconceptsofabstraction,modularity,andlayeredsoftware,allpresentedlaterinthischapter,addressthisimportantissueofsoftwareorganization.

Observation:Thepurposeofacommentistoassistindebuggingandmaintenance.

Weshouldusecarefulindentinganddescriptivenamesforvariables,functions,labels,andI/Oports.Liberaluseof #define provideexplanationofsoftwarefunctionwithoutcostofexecutionspeedormemoryrequirements.Adisciplinedapproachtoprogrammingistodeveloppatternsofwritingthatyouconsistentlyfollow.Softwaredevelopersareunlikeshortstorywriters.Whenwritingsoftwareitisagooddesignpracticetousethesamefunctionoutlineoverandoveragain.

Observation:Itisbettertowriteclearandsimplesoftwarethatiseasytounderstandwithoutcommentsthantowritecomplexsoftwarethatrequiresalotofextraexplanationtounderstand.

3.4.ModularSoftwareInthissectionweintroducetheconceptofmodularprogramminganddemonstratethatitisaneffectivewaytoorganizeoursoftwareprojects.Therearethreereasonsforformingmodules.First,functionalabstractionallowsustoreuseasoftwaremodulefrommultiplelocations.Second,complexityabstractionallowsustodivideahighlycomplexsystemintosmallerlesscomplicatedcomponents.Thethirdreasonisportability.IfwecreatemodulesfortheI/Odevicesthenwecanisolatetherestofthesystemfromthehardwaredetails.Portabilitywillbeenhancedwhenwecreateadevicedriverorboardsupportpackage.

3.4.1.VariablesVariablesareanimportantcomponentofsoftwaredesign,andtherearemanyfactorstoconsiderwhencreatingvariables.Someoftheobviousconsiderationsaretheallocation,size,andformatofthedata.However,animportantfactorinvolvingmodularsoftwareisscope.Thescopeofavariabledefineswhichsoftwaremodulescanaccessthedata.Variableswitharestrictedaccessareclassifiedasprivate,andvariablessharedbetweenmultiplemodulesarepublic.Wecanrestrictthescopetoasinglefile,asinglefunction,orevenasingleprogramblockwithinamatchingpairofbraces, .Ingeneral,whenwelimitthescopeofourvariablesasystemiseasiertodesign(becausethemodulesaresmallerandsimpler),easiertochange(becausecodecanbereused),andeasiertoverify(becauseinteractionsbetweenmodulesarewell-defined).However,sincemodulesarenotcompletelyindependentweneedamechanismtotransferinformationfromonetoanother.Theallocationofavariablespecifieswhereorhowitexists.Becausetheircontentsareallowedtochange,allvariablesmustbeallocatedinregistersorinRAM,butnotinROM.ConstantscanandshouldbeallocatedinROM.GlobalvariablescontaininformationthatispermanentandareusuallyassignedafixedlocationinRAM.Ontheotherhand,localvariablescontaintemporaryinformationandarestoredinaregisterorallocatedonthestack.Insummary,therearethreetypesofvariables:publicglobals(sharedpermanent),privateglobals(unsharedpermanent),andprivatelocals(unsharedtemporary).WewilllearnlaterinSection3.7howtocreatetemporaryvariablesontheheap,whichcanbepublicorprivateasneeded.Becausethereisnoappropriatewaytocreateapubliclocalvariable,weusuallyrefertoprivatelocalvariablessimplyaslocalvariables,andthefactthattheyareprivateisunderstood.

Alocalvariablehastemporaryallocationbecausewecreatelocalvariablesonthestackorinregisters.Becausethestackandregistersareuniquetoeachfunction,thisinformationcannotbesharedwithothersoftwaremodules.Therefore,undermostsituations,wecanfurtherclassifythesevariablesasprivate.Localvariablesareallocated,used,andthendeallocated,inthisspecificorder.Forspeedreasons,wewishtoassignlocalvariablestoaregister.Whenweassignlocalvariabletoaregister,wecandosoinaformalmanner.Therewillbeacertainlineintheassemblysoftwareatwhichtheregisterbeginstocontainthevariable(allocation),followedbylineswheretheregistercontainstheinformation(accessorusage),andacertainlineinthesoftwareafterwhichtheregisternolongercontainstheinformation(deallocation).InC,wedefinelocalvariablesafteranopeningbrace.

voidMyFunction(void)uint16_ti;//iisalocal

for(i=0;i<10;i++)uint32_tj;//jisalocal

j=i+100;

UART_OutUDec(j);

Theinformationstoredinalocalvariableisnotpermanent.Thismeansifwestoreavalueintoalocalvariableduringoneexecutionofthemodule,thenexttimethatmoduleisexecutedthepreviousvalueisnotavailable.Examplesincludeloopcountersandtemporarysums.Weusealocalvariabletostoredatathataretemporaryinnature.Wecanimplementalocalvariableusingthestackorregisters.Somereasonswhywechooselocalvariablesoverglobalvariables:

•Dynamicallocation/releaseallowsforreuseofRAM

•Limitedscopeofaccess(makingitprivate)providesfordataprotection

Onlytheprogramthatcreatedthelocalvariablecanaccessit

•Sinceaninterruptwillsaveregistersandcreateitsownstackframe

Workscorrectlyifcalledfrommultipleconcurrentthreads(reentrant)

•Sinceabsoluteaddressingisnotused,thecodeisrelocatable

AglobalvariableisallocatedatapermanentandfixedlocationinRAM.Apublicglobalvariablecontainsinformationthatissharedbymorethanoneprogrammodule.Wemustuseglobalvariablestopassdatabetweenthemainprogram(i.e.,foregroundthread)andanISR(i.e.,backgroundthread).IfafunctioncalledfromtheforegroundbelongstothesamemoduleastheISR,thenaglobalvariableusedtopassdatabetweenthefunctionandtheISRisclassifiedasaprivateglobal(assumingsoftwareoutsidethemoduledoesnotdirectlyaccessthedata).Globalvariablesareallocatedatcompiletimeandneverdeallocated.AllocationofaglobalvariablemeansthecompilerassignsthevariableafixedlocationinRAM.Theinformationtheystoreispermanent.Examplesincludetime

ofday,date,calibrationtables,username,temperature,FIFOqueues,andmessageboards.Whendealingwithcomplexdatastructures,pointerstothedatastructuresareshared.Ingeneral,itisapoordesignpracticetoemploypublicglobalvariables.Ontheotherhand,privateglobalvariablesarenecessarytostoreinformationthatispermanentinnature.InC,wedefineglobalvariablesoutsideofthefunction.

int32_tCount=0;//Countisaglobalvariable

voidMyFunction(void)

Count++;//numberoftimesfunctionwascalled

Checkpoint3.4:HowdoyoucreatealocalvariableinC?

Sometimeswestoretemporaryinformationinglobalvariablesoutoflaziness.Thispracticeistobediscouragedbecauseitwastesmemoryandmaycausethemoduletoworkincorrectlyifcalledfrommultipleconcurrentthreads(non-reentrant).Non-reentrantprogramscanproduceverysneakybugs,sincetheymightonlycrashinraresituationswhenthesamecodecalledfromdifferentthreadswhenthefirstthreadisinaparticularcriticalsection.Suchabugisdifficulttoreproduceanddiagnose.Ingeneral,itisgooddesigntolimitthescopeofavariableasmuchaspossible.

Checkpoint3.5:HowdoyoucreateaglobalvariableinC?

InC,a static localhaspermanentallocation,whichmeansitmaintainsitsvaluefromonecalltothenext.Itisstilllocalinscope,meaningitisonlyaccessiblefromwithinthefunction.I.e.,modifyingalocalvariablewith static changesitsallocation(itisnowpermanent),butdoesn’tchangeitsscope(itisstillprivate).Inthefollowingexample, count containsthenumberoftimesMyFunction iscalled.Theinitializationofastaticlocaloccursjustonce,duringstartup.

voidMyFunction(void)staticint32_tcount=0;

count++;//numberoftimesfunctionwascalled

InC,wecreateaprivateglobalvariableusingthe static modifier.Modifyingaglobalvariablewith static doesnotchangeitsallocation(itisstillpermanent),butdoesreduceitsscope.Regularglobalscanbeaccessedfromanyfunctioninthesystem(public),whereasa static globalcanonlybeaccessedbyfunctionswithinthesamefile. Staticglobalsareprivatetothatparticularfile.Functionscanbestaticalso,meaningtheycanbecalledonlyfromotherfunctionsinthefile.E.g.,

staticint16_tmyPrivateGlobalVariable;//thisfileonly

voidstaticMyPrivateFunction(void)

InC,a const globalisread-only.ItisallocatedintheROM.Constants,ofcourse,mustbeinitializedatcompiletime.E.g.,

constint16_tSlope=21;

constuint8_tSinTable[8]=0,50,98,142,180,212,236,250;

Checkpoint3.6:Howdoesthe static modifieraffectlocals,globals,andfunctionsinC?

Checkpoint3.7:Howdoesthe const modifieraffectaglobalvariableinC?

Commonerror:Ifyouleaveoffthe const modifierinthe SinTable example,thetablewillbeallocatedtwice,onceinROMcontainingtheinitialvalues,andonceinRAMcontainingdatatobeusedatruntime.Uponstartup,thesystemcopiestheROM-versionintotheRAM-version.

MaintenanceTip:Itisgoodpracticetospecifytheunitsofavariable(e.g.,volts,cmetc.).

Commonerror:InC,globalvariablesareinitializedtozerobydefault,butlocalvariablesarenotinitialized.

3.4.2.DividingtasksintosubtasksThekeytocompletinganycomplextaskistobreakitdownintomanageablesubtasks.Modularprogrammingisastyleofsoftwaredevelopmentthatdividesthesoftwareproblemintodistinctandindependentmodules.Thepartsareassmallaspossible,yetrelativelyindependent.Complexsystemsdesignedinamodularfashionareeasiertodebugbecauseeachmodulecanbetestedseparately.Industryexpertsestimatethat50to90%ofsoftwaredevelopmentcostisspentinmaintenance.Allfiveaspectsofsoftwaremaintenancearesimplifiedbyorganizingthesoftwaresystemintomodules.

•Correctingmistakes

•Addingnewfeatures

•Optimizingforexecutionspeedorprogramsize

•Portingtonewcomputersoroperatingsystems

•Reconfiguringthesoftwaretosolvesimilarrelatedprograms

Theapproachisparticularlyusefulwhenataskislargeenoughtorequireseveralprogrammers.

Aprogrammoduleisaself-containedsoftwaretaskwithclearentryandexitpoints.WemakethedistinctionbetweenmoduleandaClanguagefunction.Amodulecanbeacollectionoffunctionsthatinitsentiretyperformsawell-definedsetoftasks.ThecollectionofserialportI/Ofunctionspresentedlaterinsection3.4.4canbeconsideredonemodule.Acollectionof32‑bitmathoperationsisanotherexampleofamodule.Themainprogramandotherhigh-levelfunctionsmayconstituteamodule,butusuallyasetoffunctionsthatperformawell-definedtaskcanalsobewrittenasmodules.Modularprogramminginvolvesboththespecificationoftheindividualmodulesandtheconnectionschemebywhichthemodulesareconnectedtogethertoformthesoftwaresystem.Whilethemodulemaybecalledfrommanylocationsthroughoutthesystem,thereshouldbewell-definedentrypoints.

Theoverallgoalofmodularprogrammingistoenhanceclarity.Thesmallerthetask,theeasieritwillbetounderstand.Couplingisdefinedastheinfluenceonemodule’sbehaviorhasonanothermodule.Inordertomakemodulesmoreindependentwestrivetominimizecoupling.Obviousandappropriateexamplesofcouplingaretheinput/outputparametersexplicitlypassedfromonemoduletoanother.Ontheotherhand,informationstoredinsharedglobalvariablescanbequitedifficulttotrack.InasimilarwaysharedaccessestoI/Oportscanalsointroduceunnecessarycomplexity.Globalvariablescausecouplingbetweenmodulesthatcomplicatethedebuggingprocessbecausenowthemodulesmaynotbeabletobeseparatelytested.Ontheotherhand,wemustuseglobalvariablestopassinformationintoandoutofaninterruptserviceroutine,andfromonecalltoaninterruptserviceroutinetothenextcall.

Anotherproblemspecifictoembeddedsystemsistheneedforfastexecution.ForthisreasontheARMarchitecturehasenoughregisterssothatsomecanbeusedtostorelocalvariables.Allocatinglocalvariablesinregistersproducesshorterandfastercodeascomparedtoglobalsandstack-basedlocals.Whenpassinginformationthroughglobalvariablesisrequired,itisbettertouseawell-definedabstracttechniquelikeamailboxorfirst-in-first-out(FIFO)queue.

Assignalogicallycompletetasktoeachmodule.Themoduleislogicallycompletewhenitcanbeseparatedfromtherestofthesystemandplacedintoanotherapplication.Theinterfacesareextremelyimportant.Theinterfacesdeterminethepoliciesofourmodules.Inotherwords,theinterfacesdefinetheoperationsofoursoftwaresystem.Theinterfacesalsorepresentthecouplingbetweenmodules.Ingeneralwewishtominimizetheamountofinformationpassingbetweenthemodulesyetmaximizethenumberofmodules.Ofthefollowingthreeobjectiveswhendividingasoftwareprojectintosubtasks,itisreallyonlythefirstonethatmatters.

•Makethesoftwareprojecteasiertounderstand

•Increasethenumberofmodules

•Decreasetheinterdependency(minimizecoupling)

Wecandevelopandconnectmodulesinahierarchicalmanner.Constructnewmodulesbycombiningexistingmodules.Inahierarchicalsystemthemodulesareorganizedintoatree-structuredcallgraph.Inthecallgraph,anarrowpointsfromthecallingroutinetothemoduleitcalls.TheI/Oportsareorganizedintogroups(e.g.,alltheserialportI/Oregistersareinonegroup).Thecallgraphallowsustoseetheorganizationoftheproject.Tomakesimplercallgraphsonlargeprojectswecancombinemultiplerelatedfunctionsintoasinglemodule.ThemainprogramisatthetopandtheI/Oportsareatthebottom.Inahierarchicalsystemthemodulesareorganizedbothinahorizontalfashion(groupedtogetherbyfunction)andinaverticalfashion(overallpoliciesdecisionsatthetopandimplementationdetailsatthebottom).Sinceoneoftheadvantagesofbreakingalargesoftwareprojectintosubtasksisconcurrentdevelopment,itmakessensetoconsiderconcurrencywhendividingthetasks.Inotherwords,themodulesshouldbepartitionedinsuchawaythatmultipleprogrammerscandevelopthesubtasksasindependentlyaspossible.Ontheotherhandcarefulandconstantsupervisionisrequiredasmodulesareconnectedtogetherandtested.

Observation:IfmoduleAcallsmoduleB,andmoduleBcallsmoduleA,thenyouhavecreatedaspecialsituationthatmustaccountforthesemutualcalls.

Therearetwoapproachestohierarchicalprogramming.Thetop-downapproachstartswithageneraloverview,likeanoutlineofapaper,andbuildsrefinementintosubsequentlayers.Atop-downprogrammerwasoncequotedassaying,

“Writenosoftwareuntileverydetailisspecified”

Itprovidesabetterglobalapproachtotheproblem.Managersliketop-downbecauseitgivesthemtightercontrolovertheirworkers.Thetop-downapproachworkwellwhenanexistingoperationalsystemisbeingupgradedorrewritten.Ontheotherhandthebottom-upapproachstartswiththesmallestdetail,buildsupthesystem“onebrickatatime.”Thebottom-upapproachprovidesarealisticappreciationoftheproblembecauseweoftencannotappreciatethedifficultyorthesimplicityofaproblemuntilwehavetriedit.Itallowsprogrammerstostartimmediatelycoding,andgivesprogrammersmoreinputintothedesign.Forexample,alow-levelprogrammermaybeabletopointoutfeaturesthatarenotpossibleandsuggestotherfeaturesthatareevenbetter.Somesoftwareprojectsareflawedfromtheirconception.Withbottom-updesign,theobviousflawssurfaceearlyinthedevelopmentcycle.

Ibelievebottom-upisbetterwhendesigningacomplexsystemandspecificationsareopen-ended.Ontheotherhand,top-downisbetterwhenyouhaveaveryclearunderstandingoftheproblemspecificationsandtheconstraintsofyourcomputersystem.ThebestsoftwareIhaveeverproducedwasactuallywrittentwice.Thefirstpasswasprogrammedbottomupandservedonlytoprovideaclearunderstandingoftheproblem,clarificationofthefeaturesIwanted,andthelimitationsofmyhardware.Iliterallythrewallthesourcecodeinthetrash,andprogrammedthesecondpassinatop-downmanner.

ArthurC.Clarke’sThirdLaw:Anysufficientlyadvancedtechnologyisindistinguishablefrommagic.

J.PorterClark’sLaw:Sufficientlyadvancedincompetenceisindistinguishablefrommalice.

OneofthebiggestmistakesbeginningprogrammersmakeistheinappropriateusageofI/Ocalls(e.g.,screenoutputandkeyboardinput).Anexplanationfortheirfoolishbehavioristhattheyhaven’thadtheexperienceyetoftryingtoreusesoftwaretheyhavewrittenforoneprojectinanotherproject.Softwareportabilityisdiminishedwhenitislitteredwithuserinput/output.ToreusesoftwarewithuserI/Oinanothersituation,youwillalmostcertainlyhavetoremovetheinput/outputstatements.Ingeneral,weavoidinteractiveI/Oatthelowestlevelsofthehierarchy,ratherreturndataandflagsandletthehigherlevelprogramdotheinteractiveI/O.Oftenweaddkeyboardinputandscreenoutputcallswhentestingoursoftware.ItisimportanttoremovetheI/Othatnotdirectlynecessaryaspartofthemodulefunction.Thisallowsyoutoreusethesefunctionsinsituationswherescreenoutputisnotavailableorappropriate.Obviouslyscreenoutputisallowedifthatisthepurposeoftheroutine.

CommonError:PerformingunnecessaryI/Oinasubroutinemakesithardertoreuseatalatertime.

Fromaformalperspective,I/Odevicesareconsideredasglobal.ThisisbecauseI/Odevicesresidepermanentlyatfixedaddresses.FromasyntacticviewpointanymodulehasaccesstoanyI/Odevice.InordertoreducethecomplexityofthesystemwewillrestrictthenumberofmodulesthatactuallydoaccesstheI/Odevice.ItwillbeimportanttoclarifywhichmoduleshaveaccesstoI/Odevicesandwhentheyareallowedtoaccessit.WhenmorethanonemoduleaccessesanI/Odevice,thenitisimportanttodevelopwaystoarbitrate(whichmodulegoesfirstiftwoormorewanttoaccesssimultaneously)orsynchronize(makeasecondmodulewaituntilthefirstisfinished.)ThesearbitrationissueswillbepresentedinChapters4and5.

Informationhidingissimilartominimizingcoupling.Itisbettertoseparatethemechanismsofsoftwarefromitspolicies.Weshouldseparatewhatthefunctiondoes(therelationshipbetweenitsinputsandoutputs)fromhowitdoesit.Itisgoodtohidecertaininnerworkingsofamodule,andsimplyinterfacewiththeothermodulesthroughthewell-definedinput/outputparameters.ForexamplewecouldimplementaFIFObymaintainingthecurrentnumberofelementsinaglobalvariable, Count .Agoodmodulewillhidehow Count isimplementedfromitsusers.IftheuserwantstoknowhowmanyelementsareintheFIFO,itcallsa TxFifo_Size() routinethatreturnsthevalueof Count .Abadlywrittenmodulewillnothide Count fromitsusers.Theusersimplyaccessestheglobalvariable Count .IfweupdatetheFIFOroutines,makingthemfasterorbetter,wemighthavetoupdatealltheprogramsthataccess Count too.Theobject-orientedprogrammingenvironmentsprovidewell-definedmechanismstosupportinformationhiding.Thisseparationofpoliciesfrommechanismscanbeseenalsoinlayeredsoftware.

TheKeepItSimpleStupidapproachtriestogeneralizetheproblemsothatitfitsanabstractmodel.Unfortunately,thepersonwhodefinesthesoftwarespecificationsmaynotunderstandtheimplicationsandalternatives.Sometimeswecanrestatetheproblemtoallowforasimpler(andpossiblymorepowerful)solution.Asasoftwaredeveloper,wealwaysaskourselvesthesequestions:

“Howimportantisthisfeature?”

“Whatalternativewayscouldthissystembestructured?”

“HowcanIredefinetheproblemtomakeitsimpler?”

Wecanclassifythecouplingbetweenmodulesashighlycoupled,looselycoupled,oruncoupled.Ahighly-coupledsystemisnotdesirable,becausethereisagreatdealofinterdependencebetweenmodules.Aloosely-coupledsystemisoptimal,becausethereissomedependencebutinterconnectionsareweak.Anuncoupledsystem,onewithnointerconnectionsatall,istypicallyinappropriateinanembeddedsystem,becauseallcomponentsshouldbeactingtowardsacommonobjective.Therearethreewaysinwhichmodulescanbecoupled.Thenaturalwayinwhichmodulesarecouplediswhereonemodulecallsorinvokesafunctioninasecondmodule.Thistypeofcoupling,calledinvocationcoupling,canbevisualizedwithacallgraph.Asecondwaymodulescanbecoupledisbydatatransfer.Ifinformationflowsfromonemoduletoanother,weclassifythisasbandwidthcoupling.Bandwidth,whichistheinformationtransferrate,isaquantitativemeasureofcoupling.Bandwidthcouplingcanbevisualizedwithadataflowgraph.Thethirdtypeofcoupling,calledcontrolcoupling,occurswhenactionsinonemoduleaffectthecontrolpathwithinanothermodule.Forexample,ifModuleAsetsaglobalflagandModuleBusestheglobalflagtodecideitsexecutionpath.Controlcouplingishardtovisualizeandhardtodebug.Therefore,itisapoordesigntoemploythistypeofcoupling.

Anotherwaytocategorizecouplingistoexaminehowinformationispassedorsharedbetweenmodules.Wewilllistthemechanismsfrompoortoexcellent.ItisextremelypoordesigntoallowModuleAdirectlymodifydataorflagswithinModuleB.Similarlypoordesignistoorganizeimportantdataintoacommonsharedglobalspaceandallowmodulestoreadandwritethesedata.ItisoktoallowModuleAtocallModuleBandpassitacontrolflag.ThiscontrolflagwillinturnaffecttheexecutionwithinModuleB.Itisgooddesigntohaveonemodulepassdatatoanothermodule.Astampisdefinedasstructureddatapassedfromonemoduletoanother.Primitivedatapassedbetweenmodulesisunstructured.

Couplingisawaytodescribehowmodulesconnectwitheachother,butitisalsoimportanttoanalyzehowvariouscomponentswithinonemoduleinteractwitheachother.Cohesionisthedegreeofinterrelatednessofinternalpartswithinthemodule.Ingeneral,wewishtomaximizecohesion.Acohesivemodulehasallcomponentsofthemodulearedirectedtowardsandessentialforthesametask.Itisalsoimportanttoanalyzehowcomponentsarerelatedaswedesignmodules.Coincidentalcohesionoccurswhencomponentsofthemoduleareunrelated,resultingpoordesign.Examplesofcoincidentalcohesionwouldbeacollectionoffrequentlyusedroutines,acollectionofroutineswrittenbyasingleprogrammer,oracollectionofroutineswrittenduringacertaintimeinterval.

Logicalcohesionisagroupingofcomponentsintoasinglemodule(becausetheyperformsimilarfunctions).Anexampleoflogicalcohesionisacollectionofserialoutput,LCDoutput,andprinteroutputroutinesintoonemodule(becauseallroutinesperformoutput).Organizingmodulesinthisfashionisapoordesignandresultsinmodulesthatarehardtoreuse.

Temporalcohesioncombinescomponentsiftheyareconnectedintimesequence.Ifwearebakingbread,weactivatetheyeastinwarmwaterinonebowl,andthenwecombinetheflour,sugar,andspicesinanotherbowl.Thesetwostepsareconnectedonlyinasensethatwefirstdoone,andthenwedoanotherwhenmakingbread.Ifweweremakingcookies,wewouldneedtheflour,sugar,andspicesbutnottheyeast.Wewanttomixandmatchexistingmodulestocreatenewdesigns,assuch,weexpectthesequenceofmoduleexecutiontochange.

Anotherpoordesign,calledproceduralcohesion,groupsfunctionstogetherinordertoensuremandatoryordering.Forexample,anembeddedsystemmighthaveaninputport,anoutputport,andatimermodule.Inordertoworkproperly,allthreemustbeinitialized.Itwouldbehardtoreusecodeifweplacedallthreeinitializationroutinesintoonemodule.

Wenextpresentappropriatereasonstogroupcomponentsintoonemodule.Communicationalcohesionexistswhencomponentsoperateonthesamedata.Anexampleofcommunicationalcohesionwouldbeacollectionofroutinesthatfilterandextractfeaturesfromthedata.

Sequentialcohesionoccurswhencomponentsaregroupedintoonemodule,becausetheoutputfromonecomponentistheinputtoanothercomponent.Sequentialcohesionisanaturalconsequenceofminimizingbandwidthbetweenmodules.Anexampleofsequentialcohesionisafuzzylogiccontroller.Thiscontrollerhasfivestages:crispinput,fuzzification,rules,defuzzification,andcrispoutput.Theoutputofeachstageistheinputtothenextstage.Theinputbandwidthtothecontrollerandtheoutputbandwidthfromthecontrollercanbequitelow,buttheamountofinformationtransferredbetweenstagescanbethousandsoftimeslarger.

Thebestkindofcohesionisfunctionalcohesion,whereallcomponentscombinetoimplementasingleobjective,andeachcomponenthasanecessarycontributiontotheobjective.I/Odevicedrivers,whichareacollectionofroutinesforasingleI/Odevice,exhibitfunctionalcohesion.

Anotherwaytoclassifygoodandbadmodularityistoobservefaninandfanoutbehavior.Inadataflowgraph,thetailofanarrowdefinesadataoutput,andtheheadofanarrowdefinesadatainput.Thefaninofamoduleisthenumberofothermodulesthathavedirectcontrolonthatparticularmodule.Fanincanbevisualizedbycountingthenumberofarrowheadsthatterminateonthemoduleinthedataflowgraph,shownpreviouslyinFigure1.9.Thefanoutofamoduleisnumberofothermodulesdirectlycontrolledbythismodule.Fanoutcanbevisualizedbycountingthenumberoftailsofarrowsthatoriginateonthemoduleinthedataflowgraph.Ingeneral,asystemwithhighfanoutispoorlydesigned,becausethatonemodulemayconstituteabottleneckoracriticalsafetypath.Inotherwords,themodulewithhighfanoutisprobablydoingtoomuch,performingthetasksthatshouldbedistributedtoothermodules.Highfaninisnotnecessarilyapoordesign,dependingontheapplication.

3.4.3.DeviceDriversandBoardSupportPackage

Asthesizeandcomplexityofoursoftwaresystemsincrease,welearntoanticipatethechangesthatoursoftwaremustundergointhefuture.Inparticular,wecanexpecttoredesignoursystemtorunonnewandmorepowerfulhardwareplatforms.Asimilarexpectationisthatbetteralgorithmsmaybecomeavailable.Theobjectiveofthissectionistousealayeredsoftwareapproachtofacilitatethesetypesofchanges.

WecanusethecallgraphliketheonedrawninFigure3.1tovisualizesoftwarelayers.Thearrowspointfromthecallingfunctiontothefunctionitcalls.Figure3.1showsonlyonemoduleateachlayer,butacomplexsystemmighthavemultiplemodulesateachlayer.Afunctioninalayercancallafunctionwithinthesamemodule,oritcancallapublicfunctioninamoduleofthesameorlowerlayer.Somelayeredsystemsrestrictthecallsonlytomodulesinthemostadjacentlayerbelowit.IfweplaceallthefunctionsthataccesstheI/Ohardwareinthebottommostlayer,wecancallthislayerahardwareabstractionlayer(HAL).Thisbottom-mostlayercanalsobecalledaboardsupportpackage(BSP)ifI/Odevicesarereferencedinanabstractmanner.Eachmiddlelayerofmodulesonlycallslowerlevelmodules,butnotmodulesatahigherlevel.Usuallythetoplayerconsistsofthemainprogram.Inamulti-threadedenvironmenttherecanbemultiplemainprogramsatthetop-mostlevel,butfornowassumethereisonlyonemainprogram.

AnexampleofalayeredsystemisTransmissionControlProtocol/InternetProtocol(TCP/IP),whichconsistsofatleastfourdistinctlayers:application(http,telnet,SMTP,FTP),transport(UDP,TCP),internet(IP,ICMP,IGMP),andnetworklayers(Ethernet).

Figure3.1.Alayeredapproachtointerfacingaprinter.ThebottomlayeristheBSP.

Todevelopalayeredsoftwaresystemwebeginwithamodularsystem.Themainadvantageoflayeredsoftwareistheabilitytoseparatethemodulesintogroupsorlayerssuchthatonelayermaybereplacedwithoutaffectingtheotherlayers.Forexample,youcouldchangewhichmicrocontrolleryouareusing,bymodifyingthelowlevelwithoutanychangestotheotherlevels.Figure3.1depictsalayeredimplementationofaprinterinterface.Inasimilarway,youcouldreplacetheprinterwithasolidstatediskbyreplacingjustthemiddleandlowerlayers.Ifweweretoemploybufferingand/ordatacompressiontoenhancecommunicationbandwidth,thenthesealgorithmswouldbeaddedtothemiddlelevel.Alayeredsystemshouldallowyoutochangetheimplementationofonelayerwithoutrequiringredesignoftheotherlayers.

Agateisusedtoconnectonelayertothenext.AnothernameforthisgateisapplicationprograminterfaceorAPI.Thegatesprovideamechanismtolinkthelayers.Becausethesizeofthesoftwareonanembeddedsystemissmall,itispossibleandappropriatetoimplementalayeredsystemusingstandardfunctioncallsbysimplycompilingandlinkingallsoftwaretogether.Wewillseeinthenextsectionthatthegatecanbeimplementedbycreatingaheaderfilewithprototypestopublicfunctions.Thefollowingrulesapplytolayeredsoftwaresystems:

1.Amodulemaymakeasimplecalltoothermodulesinthesamelayer.

2.Amodulemaymakeacalltoalowerlevelmoduleonlybyusingthegate.

3.Amodulemaynotdirectlyaccessanyfunctionorvariableinanotherlayer

withoutgoingthroughthegate.

4.Amodulemaynotcallahigherlevelroutine.

5.Amodulemaynotmodifythevectoraddressofanotherlevel’shandler(s).

6.(optional)Amodulemaynotcallfartherdownthantheimmediately

adjacentlowerlevel.

7.(optional)AllI/Ohardwareaccessisgroupedinthelowestlevel.

8.(optional)AlluserinterfaceI/Oisgroupedinthehighestlevel

unlessitisthepurposeofthemoduleitselftodosuchI/O.

Thepurposeofrule6istoallowmodificationsatthelowlayertonotaffectoperationatthehighestlayer.Ontheotherhand,forefficiencyreasonsyoumaywishtoallowmodulecallsfurtherdownthantheimmediatelyadjacentlowerlayer.Togetthefulladvantageoflayeredsoftware,itiscriticaltodesignfunctionallycompleteinterfacesbetweenthelayers.Theinterfaceshouldsupportallcurrentfunctionsaswellasprovideforfutureexpansions.

AdevicedriverconsistsofthesoftwareroutinesthatprovidethefunctionalityofanI/Odevice.AdevicedriverusuallydoesnothidewhattypeofI/Omoduleitis.E.g.,inthenextsection,weconsideradevicedriverforaserialport.Aboardsupportpackageis

similartoadevicedriver,exceptthatthereismoreofanattempttohidewhattheI/Odeviceactuallyis.Aboardsupportpackageprovidesahigherlevelofabstractionthanaregulardevicedriver.Thedriverconsistsoftheinterfaceroutinesthattheoperatingsystemorsoftwaredeveloper’sprogramcallstoperformI/Ooperationsaswellasthelow-levelroutinesthatconfiguretheI/Odeviceandperformtheactualinput/output.Theissueoftheseparationofpoliciesfrommechanismsisveryimportantindevicedriverdesign.Thepoliciesofadriverincludethelistoffunctionsandtheoverallexpectedresults.Inparticular,thepoliciescanbesummarizedbytheinterfaceroutinesthattheOSorsoftwaredevelopercancalltoaccessthedevice.Themechanismsofthedevicedriverincludethespecifichardwareandlow-levelsoftwarethatactuallyperformtheI/O.Asanexample,considerthewidevarietyofmassstoragedevicesthatareavailable.Floppydisk,RAMdisks,integrateddeviceelectronics(IDE)harddrive,SerialAdvancedTechnologyAttachment(SATA)harddrive,flashEEPROMdrive,andevenanetworkcanbeusedtosaveandrecalldatafiles.AsimplemassstoragesystemmighthavethefollowingC-levelinterfacefunctions,asexplainedinthefollowingprototypes(ineachcasethefunctionsreturn0ifsuccessfulandanerrorcodeiftheoperationfails:

inteFile_Init(void);//initializefilesystem

inteFile_Create(charname[]);//createnewfile,makeitempty

inteFile_WOpen(charname[]);//openafileforwriting

inteFile_Write(int8_tdata);//streamdataintoopenfile

inteFile_WClose(void);//closethefileforwriting

inteFile_ROpen(charname[]);//openafileforreading

inteFile_ReadNext(int8_t*pt);//streamdataoutofopenfile

inteFile_RClose(void);//closethefileforreading

inteFile_Delete(charname[]);//removethisfile

Buildingahardwareabstractionlayer(HAL)isthesameideaasseparationofpoliciesfrommechanisms.AdiagramofthislayeredconceptwasshowninFigure3.1.Intheabovefileexample,aHALorBSPwouldtreatallthepotentialmassstoragedevicesthroughthesamesoftwareinterface.Anotherexampleofthisabstractionisthewaysomecomputerstreatpicturesonthevideoscreenandpicturesprintedontheprinter.Withtheabstractionlayer,thesoftwaredeveloper’sprogramdrawslinesandcolorsbypassingthedatainastandardformattothedevicedriver,andtheOSredirectstheinformationtothevideographicsboardorcolorLaserWriterasappropriate.Thislayeredapproachallowsonetomixandmatchhardwareandsoftwarecomponentsbutdoessuffersomeoverheadandinefficiency.

Low-leveldevicedriversnormallyexistintheBasicInput/OutputSystem(BIOS)ROMandhavedirectaccesstothehardware.Theyprovidetheinterfacebetweenthehardwareandtherestofthesoftware.Goodlow-leveldevicedriversallow:

1.Newhardwaretobeinstalled;

2.Newalgorithmstobeimplemented

a.Synchronizationwithbusywait,interrupts,orDMA

b.Errordetectionandrecoverymethods

c.Enhancementslikeautomaticdatacompression

3.Higherlevelfeaturestobebuiltontopofthelowlevel

a.OSfeatureslikeblockingsemaphores

b.Additionalfeatureslikefunctionkeys

andstillmaintainthesamesoftwareinterface.Inlargersystemslikethepersonalcomputer(PC),thelow-levelI/OsoftwareiscompiledandburnedinROMseparatefromthecodethatwillcallit,itmakessensetoimplementthedevicedriversassoftwareinterrupts(sometimescalledtraps)andspecifythecallingsequencelanguage-independent.Wedefinethe“clientprogrammer”asthesoftwaredeveloperthatwillusethedevicedriver.Inembeddedsystemslikeweuse,itisappropriatetoprovidedevice.handdevice.cfilesthattheclientprogrammercancompilewiththeirapplication.Inacommercialsetting,youmaybeabletodelivertotheclientonlythedevice.htogetherwiththeobjectfile.Linkingistheprocessofresolvingaddressestocodeandprogramsthathavebeencompliedseparately.Inthisway,theroutinescanbecalledfromanyprogramwithoutrequiringcomplicatedlinking.Inotherwords,whenthedevicedriverisimplementedwithasoftwareinterrupt,thelinkingoccursatruntimethroughthevectoraddressofthesoftwareinterrupt.Inourembeddedsystemhowever,thelinkingwillbestaticoccurringatthetimeofcompilation.

3.4.4.SerialPortDriverTheconceptofadevicedrivercanbeillustratedwiththefollowingdesignofaserialportdevicedriver.Inthissection,thecontentsoftheheaderfile(UART.h)willbepresented,andtheimplementationswillbedevelopedinthenextchapter.Thedevicedriversoftwareisgroupedintofourcategories.Protecteditemscanonlybedirectlyaccessedbythedevicedriveritself,andpublicitemscanbeaccessedbyothermodules.

1.Datastructures:global(private)Thefirstcomponentofadevicedriverincludesprivateglobaldatastructures.Tobeprivateglobalmeansonlyprogramswithinthedriveritselfmaydirectlyaccessthesevariables.Iftheuserofthedevicedriver(e.g.,aclient)needstoreadorwritetothesevariables,thenthedriverwillincludepublicfunctionsthatallowappropriateread/writefunctions.Oneexampleofaprivateglobalvariablemightbean OpenFlag ,whichistrueiftheserialporthasbeenproperlyinitialized.TheimplementationdevelopedinChapter4willhavenoprivateglobalvariables,buttheUARTimplementationdevelopedinChapter5willincludeaprivateFIFOqueue.

intOpenFlag=0;//trueifdriverhasbeeninitialized

2.Initializationroutines(public,calledbytheclientonceinthebeginning)Thesecondcomponentofadevicedriverincludesthepublicfunctionsusedtoinitializethedevice.Tobepublicmeanstheuserofthisdrivercancallthesefunctionsdirectly.Aprototypetopublicfunctionswillbeincludedintheheaderfile(UART.h).Thenamesofpublicfunctionswillbeginwith UART_ .ThepurposeofthisfunctionistoinitializetheUARThardware.

//––––UART_Init––––

//InitializeSerialportUART

//Input:none

//Output:none

voidUART_Init(void);

3.RegularI/Ocalls(public,calledbyclienttoperformI/O)Thethirdcomponentofadevicedriverconsistsofthepublicfunctionsusedtoperforminput/outputwiththedevice.Becausethesefunctionsarepublic,prototypeswillbeincludedintheheaderfile(UART.h).Theinputfunctionsaregrouped,followedbytheoutputfunctions.

//––––UART_InChar––––

//Waitfornewserialportinput

//Input:none

//Output:ASCIIcodeforkeytyped

charUART_InChar(void);

//––––UART_InString––––

//Waitforasequenceofserialportinput

//Input:maxSizeisthemaximumnumberofcharacterstolookfor

//Output:Null-terminatedstringinbuffer

voidUART_InString(char*buffer,uint16_tmaxSize);

//––––UART_InUDec––––

//InUDecacceptsASCIIinputinunsigneddecimalformat

//andconvertstoa32-bitunsignednumber(0to4294967295)

//Input:none

//Output:32-bitunsignednumber

uint32_tUART_InUDec(void);

//––––UART_OutChar––––

//Output8-bittoserialport

//Input:letterisan8-bitASCIIcharactertobetransferred

//Output:none

voidUART_OutChar(charletter);

//––––UART_OutString––––

//OutputString(NULLtermination)

//Input:pointertoaNULL-terminatedstringtobetransferred

//Output:none

voidUART_OutString(char*buffer);

//––––UART_OutUDec––––

//Outputa32-bitnumberinunsigneddecimalformat

//Input:32-bitnumbertobetransferred

//Output:none

//Variableformat1-10digitswithnospacebeforeorafter

voidUART_OutUDec(uint32_tnumber);

4.Supportsoftware(private)Thelastcomponentofadevicedriverconsistsofprivatefunctions.Becausethesefunctionsareprivate,prototypeswillnotbeincludedintheheaderfile(UART.h).Weplacehelperfunctionsandinterruptserviceroutinesinthecategory.

NoticethatthisUARTexampleimplementsalayeredapproach,similartoFigure3.1.Thelow-levelfunctionsprovidethemechanismsandareprotected(hidden)fromtheclientprogrammer.Thehigh-levelfunctionsprovidethepoliciesandareaccessible(public)totheclient.WhenthedevicedriversoftwareisseparatedintoUART.handUART.cfiles,youneedtopaycarefulattentionastohowmanydetailsyouplaceintheUART.hfile.Agooddevicedriverseparatesthepolicy(overalloperation,howitiscalled,whatitreturns,whatitdoes,etc.)fromtheimplementation(accesstohardware,howitworks,etc.)Ingeneral,youplacethepoliciesintheUART.hfile(tobereadbytheclient)andtheimplementationsintheUART.cfile(tobereadbyyouandyourcoworkers).Thinkofitthisway:ifyouweretowritecommercialsoftwarethatyouwishedtosellforprofitandyoudeliveredtheUART.hfileanditscompiledobjectfile,howlittleinformationcouldyouplaceintheUART.hfileandstillhavethesoftwaresystembefullyfunctional.Insummary,thepolicieswillbepublic,andtheimplementationswillbeprivate.

Observation:AlayeredapproachtoI/Oprogrammingmakesiteasierforyoutoupgradetonewertechnology.

Observation:AlayeredapproachtoI/Oprogrammingallowsyoutodoconcurrentdevelopment.

3.4.5.AbstractOutputDeviceDriverIntheUARTdrivershownintheprevioussection,theroutinesclearlyinvolveaUART.AnotherapproachtoI/Oistoprovideahigh-levelabstractioninsuchawaythattheI/Odeviceitselfishiddenfromtheuser.Therearemultipleprojectsonthebook’swebsitethatimplementthisabstraction.Theoverallpurposeoftheseexamplesistoprovideanoutputstreamforthestandard printf featuretowhichmostCprogrammersareaccustomed.FortheTM4C123andTM4C1293LaunchPadboards,wecansendoutputtothePCusingUART0,toaST7735colorgraphicsLCD,ortoalow-costNokia5110graphicsLCD.TheimplementationsfortheLM3SStellaris ® kitsusetheorganiclightemittingdiode(OLED)display.Eventhoughallthesedisplaysarequitedifferent,theyallbehaveinasimilarfashion.

InC,wecanspecifytheoutputstreamusedby printf bywritinga fputc function.The fputc functionisaprivateandimplementedinsidethedriver.Itsendscharacterstothedisplayandmanagesthecursor,tab,linefeedandcarriagereturnfunctionalities.Theusercontrolsthedisplaybycallingthefollowingfivepublicfunctions.

voidOutput_Init(void);//Initializesthedisplayinterface.

voidOutput_Clear(void);//Clearsthedisplay

voidOutput_Off(void);//Turnsoffthedisplay

voidOutput_On(void);//Turnsonthedisplay

voidOutput_Color(uint32_tnewColor);//Setcoloroffutureoutput

Theuserperformsoutputbycalling printf .Thisabstractionclearlyseparateswhatitdoes(outputinformation)fromhowitworks(sendspixeldatatothedisplayoverUART,SSI,orI2C).Intheseexamplesalloutputissenttothedisplay;however,wecouldmodifythe fputc functionandredirecttheoutputstreamtootherdevicessuchastheUSB,Ethernet,ordisk.FortheLM3Sboards,thisexampleiscalledOLED_xxx.zip.FortheTM4Cboards,thisexamplecanbefoundasST7735_xxx.zip,Printf_Nokia5110_xxx.zip,andPrintf_UART_xxx.zip.

3.5.FiniteStateMachinesSoftwareabstractioniswhenwedefineacomplexproblemwithasetofbasicabstractprinciples.Ifwecanconstructoursoftwaresystemusingthesebuildingblocks,thenwehaveabetterunderstandingoftheproblem.Thisisbecausewecanseparatewhatwearedoingfromthedetailsofhowwearegettingitdone.Thisseparationalsomakesiteasiertooptimize.Itprovidesforaproofofcorrectfunctionandsimplifiesbothextensionsandcustomization.AgoodexampleofabstractionistheFiniteStateMachine(FSM)implementations.TheabstractprinciplesofFSMdevelopmentaretheinputs,outputs,states,andstatetransitions.IfwecantakeacomplexproblemandmapitintoaFSMmodel,thenwecansolveitwithasimpleFSMsoftwaretools.OurFSMsoftwareimplementationwillbeeasytounderstand,debug,andmodify.OtherexamplesofsoftwareabstractionincludeProportionalIntegralDerivativedigitalcontrollers,fuzzylogicdigitalcontrollers,neuralnetworks,andlinearsystemsofdifferentialequations(e.g.,PSPICE.)Ineachcase,theproblemismappedintowell-definedmodelwithasetofabstractyetpowerfulrules.Then,thesoftwaresolutionisamatterofimplementingtherulesofthemodel.

Linkedlistsarelistsornodeswhereoneormoreoftheentriesisa(link)toothernodesofsimilarstructure.Wecanhavestatically-allocatedfixed-sizelinkedliststhataredefinedatassembleorcompiletimeandexistthroughoutthelifeofthesoftware.Ontheotherhand,weimplementdynamically-allocatedvariable-sizelinkedliststhatareconstructedatruntimeandcangrowandshrinkinsize.Wewilluseadatastructuresimilartoalinkedlistcalledalinkedstructuretobuildafinitestatemachinecontroller.Linkedstructuresareveryflexibleandprovideamechanismtoimplementabstractions.

Awell-definedmodelorframeworkisusedtosolveourproblem(implementedwithalinkedstructure).Thethreeadvantagesofabstractionare1)itcanbefastertodevelopbecausealotofthebuildingblockspreexist;2)itiseasiertodebug(provecorrect)becauseitseparatesconceptualissuesfromimplementation;and3)itiseasiertochange.

Animportantfactorwhenimplementingfinitestatemachinesusinglinkedstructuresisthatthereshouldbeaclearandone-to-onemappingbetweenthefinitestatemachineandthelinkedstructure.I.e.,thereshouldbeonestructureforeachstate.

Wewillpresenttwotypesoffinitestatemachines.TheMooreFSMhasanoutputthatdependsonlyonthestate,andthenextstatedependsonboththeinputandcurrentstate.WewilluseaMooreimplementationifthereisanassociationbetweenastateandanoutput.Therecanbemultiplestateswiththesameoutput,buttheoutputdefinesinpartwhatitmeanstobeinthatstate.Forexample,inatrafficlightcontroller,thestateofgreenlightontheNorthroad(redlightontheEastroad)iscausedbyoutputtingaspecificpatterntothetrafficlight.

Ontheotherhand,theMealyFSMhasanoutputthatdependsonboththeinputandthestate,andthenextstatealsodependsoninputandcurrentstate.WewilluseaMealyimplementationiftheoutputcausesthestatetochange.Inthissituation,wedonotneedaspecificoutputtobeinthatstate;rathertheoutputsarerequiredtocausethestatetransition.Forexample,tomakearobotstandup,weperformaseriesofoutputscausingthestatetochangefromsittingtostanding.AlthoughwecanrewriteanyMealymachineasaMooremachineandviceversa,itisbettertoimplementtheformatthatismorenaturalfortheparticularproblem.Inthiswaythestategraphwillbeeasiertounderstand.

Checkpoint3.8:WhatarethedifferencesbetweenaMealyandMoorefinitestatemachine?

Oneofthecommonfeaturesinmanyfinitestatemachinesisatimedelay.WewilllearnveryelaboratemechanismstohandletimeinChapters5-6,butinthissectionwewillusetheSysTickdelayfunctionspresentedinProgram2.11.

Example3.1.Designaline-trackingrobotthathastwodrivewheelsandtwolinesensorsusingaFSM.Thegoalistodrivetherobotalongalineplacedinthecenteroftheroad.Therobothastwodrivewheelsandathirdfreeturningbalancewheel.Figure3.2showsthatPF1drivestheleftwheelandPF2drivestherightwheel.Ifbothmotorsareon(PF2–1=11),therobotgoesstraight.Ifjusttheleftmotorison(PF2–1=01),therobotwillturnright.Ifjusttherightmotorison(PF2–1=10),therobotwillturnleft.Thelinesensorsareundertherobotandcandetectwhetherornottheyseetheline.ThetwosensorsareconnectedtoPortF,suchthat:

PF4,PF0equalto0,0meanswearelost,wayofftotherightorwayofftotheleft.

PF4,PF0equalto0,1meansweareoffjustalittlebittotheright.

PF4,PF0equalto1,0meansweareoffjustalittlebittotheleft.

PF4,PF0equalto1,1meansweareonline.

Figure3.2.Robotwithtwodrivewheelsandtwolinesensors(seesections6.5and8.2forcircuitdetails).Solution:ThefocusofthisexampleistheFSM,buttheQRB1134sensorinterfacewillbedescribedlaterinexample6.1,andtheDCmotorinterfacewillbedescribedinSection6.5.ThefirststepindesigningaFSMistocreatesomestates.TheoutputsofaMooreFSMareonlyafunctionofthecurrentstate.AMooreimplementationwaschosenbecausewedefineourstatesbywhatbelievetobetrueandwewillhaveoneaction(output)thatdependsonthestate.Eachstateisgivenasymbolicnamewherethestatenameeitherdescribes“whatweknow”or“whatwearedoing”.Wecouldhavedifferentiatedbetweenalittleofftotheleftandwayofftotheleft,butthissolutioncreatesasimplesolutionwith3states.

Thefinitestatemachineimplementsthisline-trackingalgorithm .Eachstatehasa2-bitoutputvalue,andfournextstatepointers.Thestrategywillbeto:

Gostraightifweareontheline.

Turnrightifweareofftotheleft.

Turnleftifweareofftotheright.

Finally,weimplementtheheuristicsbydefiningthestatetransitions,asillustratedinFigure3.3andTable3.2.Ifweusedtobeintheleftstateandcompletelyloosetheline(input00),thenweknowweareleftoftheline.Similarlyifweusedtobeintherightstateandcompletelyloosetheline(input00),thenweknowwearerightoftheline.However,weusedtobeonthecenterofthelineandthencompletelyloosetheline(input00),wedonotknowwearerightorleftoftheline.Themachinewillguesswewentrightoftheline.Inthisimplementationweputaconstantdelayof10msineachstate.Weputthetimetowaitintothemachineasaparameterforeachstatetoprovideforclarityofhowitworksandsimplifypossiblechangesinthefuture.Ifweareofftotheright(input10),thenitwilloscillatebetweenCenterandRightstates,makingaslowturnleft.Ifweareofftotheleft(input01),thenitwilloscillatebetweenCenterandLeftstates,makingaslowturnright.

Figure3.3.GraphicalformofaMooreFSMthatimplementsalinetrackingrobot.

Input

State Motor Delay 00 01 10 11

Center 1,1 1 Right Left Right Center

Left 1,0 1 Left Center Right Center

Right 0,1 1 Right Left Center Center

Table3.2.TabularformofaMooreFSMthatimplementsalinetrackingrobot.

Thefirststepindesigningthesoftwareistodecideonthesequenceofoperations.

1)Initializetimeranddirectionsregisters

2)Specifyinitialstate

3)PerformFSMcontroller

a)OutputtoDCmotors,whichdependsonthestate

b)Delay,whichdependsonthestate

c)Inputfromlinesensors

d)Changestates,whichdependsonthestateandtheinput

ThesecondstepistodefinetheFSMgraphusingadatastructure.Program3.1showsatableimplementationoftheMooreFSM.Thisimplementationusesatabledatastructure,whereeachstateisanentryinthetable,andstatetransitionsaredefinedasindicesintothistable.Thefour Next parametersdefinetheinput-dependentstatetransitions.Thewaittimesaredefinedinthesoftwareasfixed-pointdecimalnumberswithunitsof0.01s.ThelabelCenter ismoredescriptivethanthestatenumber0.Noticethe1-1correspondencebetweenthetabularforminTable3.2andthesoftwarespecificationof fsm[3] .This1-1correspondencemakesitpossibletoprovethesoftwareexactlyexecutestheFSMasdescribedinthetable.

structState

uint32_tOut;//2-bitoutput

uint32_tDelay;//timein10ms

uint8_tNext[4];;

typedefconststructStateSTyp;

#defineCenter0

#defineLeft1

#defineRight2

StateTypefsm[3]=

0x03,1,Right,Left,Right,Center,//Center

0x02,1,Left,Center,Right,Center,//Left

0x01,1,Right,Left,Center,Center//Right

;

#definePF21(*((volatileunsignedlong*)0x40025018))

#definePF4(*((volatileunsignedlong*)0x40025040))

#definePF0(*((volatileunsignedlong*)0x40025004))

intmain(void)uint32_tS;//indextothecurrentstate

uint32_tinput,output;//stateI/O

Robot_Init();//InitializePortF,SysTick

S=Center;//initialstate

while(1)

output=fsm[S].Out;//setoutputfromFSM

PF21=output<<1;//dooutputtotwomotors

SysTick_Wait10ms(fsm[S].Delay);//wait

input=PF0+(PF4>>3);//readsensors

S=fsm[S].Next[input];//nextdependsoninputandstate

voidRobot_Init(void)

SYSCTL_RCGCGPIO_R|=0x20;//1)activateclockforPortF

SysTick_Init();//initializeSysTick(program2.11)

GPIO_PORTF_LOCK_R=0x4C4F434B;//2)unlockGPIOPortF

GPIO_PORTF_CR_R=0x1F;//allowchangestoPF4-0

GPIO_PORTF_AMSEL_R=0x00;//3)disableanalogonPF

GPIO_PORTF_PCTL_R=0x00000000;//4)PCTLGPIOonPF4-0

GPIO_PORTF_DIR_R=0x0E;//5)PF4,PF0in,PF3-1out

GPIO_PORTF_AFSEL_R=0x00;//6)disablealtfunctonPF7-0

GPIO_PORTF_DEN_R=0x1F;//7)enabledigitalI/OonPF4-0

Program3.1.TableimplementationofaMooreFSM(LineTrackerTable_xxx.zip).Program3.2usesalinkedstructure,whereeachstateisanode,andstatetransitionsaredefinedaspointerstoothernodes.Again,noticethe1-1correspondencebetweenTable3.2andthesoftwarespecificationof fsm[3] .

structState

uint32_tOut;//2-bitoutput

uint32_tDelay;//timein10ms

conststructState*Next[4];;

typedefconststructStateSTyp;

#defineCenter&fsm[0]

#defineLeft&fsm[1]

#defineRight&fsm[2]

StateTypefsm[3]=

0x03,1,Right,Left,Right,Center,//Center

0x02,1,Left,Center,Right,Center,//Left

0x01,1,Right,Left,Center,Center//Right

;

intmain(void)STyp*pt;//statepointer

uint32_tinput,output;//stateI/O

Robot_Init();//InitializePortF,SysTick

pt=Center;//initialstate

while(1)

output=pt->Out;//setoutputfromFSM

PF21=output<<1;//dooutputtotwomotors

SysTick_Wait10ms(pt->Delay);//wait

input=PF0+(PF4>>3);//readsensors

pt=pt->Next[input];//nextdependsoninputandstate

Program3.2.PointerimplementationofaMooreFSM(LineTrackerLinked_xxx.zip).

YoucanfindatrafficlightcontrollerinVolume1andonthebookwebsiteasTableTrafficLight_xxx.zipPointerTrafficLight_xxx.zipandPortableTrafficLight_xxx.zip,wherexxxreferstothespecificmicrocontrolleronwhichtheexamplewastested.

Observation:ThetableimplementationrequireslessmemoryspacefortheFSMdatastructure,butthepointerimplementationwillrunfaster.

SomemicrocontrollershaveROMthatisone-timeprogrammedatthefactory.TheseROMscannotbeerasedandrewritten.OnmicrocontrollersthathavebothROMandEEPROMwecanplacetheFSMdatastructureinEEPROMandtheprograminROM.Thisallowsustomakeminormodificationstothefinitestatemachine(add/deletestates,changeinput/outputvalues)bychangingthelinkedstructureinEEPROMwithoutmodifyingtheprograminROM.InthiswaysmallmodificationstothefinitestatemachinecanbemadebyreprogrammingtheEEPROMwithouthavingtoproducenewmicrocontrollerchips.

ThepurposeofaboardsupportpackageistohideasmuchoftheI/Odetailsaspossible.WeimplementaBSPwhenweexpectthehigh-levelsystemwillbedeployedontomanylow-levelplatforms.ThesolutioninProgram3.3canbequicklyadaptedtoanyLM3S/LM4F/TM4Cusinganyportandanycontiguoussetofbitssimplybychangingthe #define statements.

#defineBSP_InPortGPIO_PORTB_DATA_R

#defineBSP_InPort_DIRGPIO_PORTB_DIR_R

#defineBSP_InPort_DENGPIO_PORTB_DEN_R

#defineBSP_OutPortGPIO_PORTD_DATA_R

#defineBSP_OutPort_DIRGPIO_PORTD_DIR_R

#defineBSP_OutPort_DENGPIO_PORTD_DEN_R

#defineBSP_GPIO_ENSYSCTL_RCGCGPIO_R

#defineBSP_InPort_Mask0x00000008//bitmaskforPortD

#defineBSP_In_M0x00000003//bitmaskforpins1,0

#defineBSP_In_Shift0x00000000//shiftvalueforInputpins

#defineBSP_OutPort_Mask0x00000002//bitmaskforPortB

#defineBSP_Out_M0x0000003F//bitmaskforpins5-0

#defineBSP_Out_Shift0x00000000//shiftvalueforOutputpins

structState

uint32_tOut;

uint32_tTime;

conststructState*Next[4];;

typedefconststructStateSTyp;

#definegoN&FSM[0]

#definewaitN&FSM[1]

#definegoE&FSM[2]

#definewaitE&FSM[3]

STypFSM[4]=

0x21,3000,goN,waitN,goN,waitN,

0x22,500,goE,goE,goE,goE,

0x0C,3000,goE,goE,waitE,waitE,

0x14,500,goN,goN,goN,goN;

intmain(void)STyp*pt;//statepointer

uint32_tinput;//activateclocksoninputandoutputports

BSP_GPIO_EN|=BSP_InPort_Mask|BSP_OutPort_Mask;

SysTick_Init();//initializeSysTicktimer,program2.11

BSP_InPort_DIR&=~BSP_In_M;//makeInPortpinsinputs

BSP_InPort_DEN|=BSP_In_M;//enabledigitalI/OonInPort

BSP_OutPort_DIR|=BSP_Out_M;//makeOutPortpinsout

BSP_OutPort_DEN|=BSP_Out_M;//enabledigitalI/OonOutPort

pt=goN;

while(1)

BSP_OutPort=(BSP_OutPort&(~BSP_Out_M))|((pt->Out)>>BSP_Out_Shift);

SysTick_Wait10ms(pt->Time);

input=(BSP_InPort&BSP_In_M)>>BSP_In_Shift;//00,01,10,11

pt=pt->Next[input];

Program3.3.EnhancedCimplementationofaTrafficLightFSM(inputonPB1-0,outputPD5-0).Checkpoint3.9:ChangeProgram3.3toplacetheinputonPA5-4andtheoutputonPB6-1.

TheFSMapproachmakesiteasytochange.Tochangethewaittimeforastate,wesimplychangethevalueinthedatastructure.Toaddmorestates,wesimplyincreasethesizeofthe fsm[] structureanddefinethe Out , Time ,and Next fieldsforthesenewstates.

Toaddmoreoutputsignals,wesimplyincreasetheprecisionofthe Out field.Toaddmoreinputlines,weincreasethesizeofthenextfield.Ifthereareninputbits,thenthesizeofthenextfieldwillbe2n.Forexample,iftherearefourinputlines,thenthereare16possiblecombinations,whereeachinputpossibilityrequiresa Next valuespecifyingwheretogoifthiscombinationoccurs.

Example3.2.Thegoalistodesignafinitestatemachinerobotcontroller,asillustratedinFigure3.4.Becausetheoutputscausetherobottochangestates,wewilluseaMealyimplementation.TheoutputsofaMealyFSMdependonboththeinputandthecurrentstate.ThisrobothasmoodsensorsthatareinterfacedtoPortB.Therobothasfourpossibleconditions:

00 OK ,therobotisfeelingfine

01 Tired ,therobotenergylevelsarelow10 Curious ,therobotsensesactivityaroundit11 Anxious ,therobotsensesdangerTherearefouractionsthisrobotcanperform,whicharetriggeredbypulsing(makehigh,thenmakelow)oneofthefoursignalsinterfacedtoPortD.

PD3 SitDown ,assumingitisstanding,itwillperformmovestositdown

PD2 StandUp ,assumingitissitting,itwillperformmovestostandup

PD1 LieDown ,assumingitissitting,itwillperformmovestoliedown

PD0 SitUp ,assumingitissleeping,itwillperformmovestositupSolution:Forthisdesignwecanlistheuristicsdescribinghowtherobotistooperate:

IftherobotisOK,itwillstayinitscurrentstate.

Iftherobot’senergylevelsarelow,itwillgotosleep.

Iftherobotsensesactivityaroundit,itwillawakenfromsleep.

Iftherobotsensesdanger,itwillstandup.

Figure3.4.Robotinterface.

Theserulesareconvertedintoafinitestatemachinegraph,asshowninFigure3.5.Eacharrowspecifiesbothaninputandanoutput.Forexample,the“ Tired/SitDown ”arrowfrom Stand to Sit statesmeansifweareinthe Stand stateandtheinputis Tired ,thenwewilloutputthe SitDown commandandgotothe Sit state.Mealymachinescanhavetimedelays,butthisexamplejustdidn’thavetimedelays.

Figure3.5.MealyFSMforarobotcontroller.

Thefirststepindesigningthesoftwareistodecideonthesequenceofoperations.

1)Initializedirectionsregisters

2)Specifyinitialstate

3)PerformFSMcontroller

a)Inputfromsensors

b)Outputtotherobot,whichdependsonthestateandtheinput

c)Changestates,whichdependsonthestateandtheinput

ThesecondstepistodefinetheFSMgraphusingalinkeddatastructure.TwopossibleimplementationsoftheMealyFSMarepresented.TheimplementationinProgram3.4definestheoutputsassimplenumbers,whereeachpulseisdefinedasthebitmaskrequiredtocausethataction.Thefour Next parametersdefinetheinput-dependentstatetransitions.

structState

uint32_tOut[4];//outputs

conststructState*Next[4];//next

;

typedefconststructStateStateType;

#defineStand&FSM[0]

#defineSit&FSM[1]

#defineSleep&FSM[2]

#defineNone0x00

#defineSitDown0x08//pulseonPD3

#defineStandUp0x04//pulseonPD2

#defineLieDown0x02//pulseonPD1

#defineSitUp0x01//pulseonPD0

StateTypeFSM[3]=

None,SitDown,None,None,//Standing

Stand,Sit,Stand,Stand,

None,LieDown,None,StandUp,//Sitting

Sit,Sleep,Sit,Stand,

None,None,SitUp,SitUp,//Sleeping

Sleep,Sleep,Sit,Sit

;

intmain(void)StateType*pt;//currentstate

uint32_tinput;

SYSCTL_RCGCGPIO_R|=0x0000000A;//clockonPortsBandD

pt=Stand;//initialstate

GPIO_PORTB_DIR_R&=~0x03;//makePB1-0inputfrommoodsensor

GPIO_PORTB_AMSEL_R&=~0x03;//disableanalogonPB

GPIO_PORTB_PCTL_R&=~0x000000FF;//PCTLGPIOonPB1-0

GPIO_PORTB_DEN_R|=0x03;//enabledigitalI/OonPB1-0

GPIO_PORTD_DIR_R|=0x0F;//makePD3-0outputtorobot

GPIO_PORTD_AMSEL_R&=~0x0F;//disableanalogonPD

GPIO_PORTD_PCTL_R&=~0x0000FFFF;//PCTLGPIOonPD3-0

GPIO_PORTD_DEN_R|=0x0F;//enabledigitalI/OonPD3-0

while(1)

input=GPIO_PORTB_DATA_R&0x03;//input=0-3

GPIO_PORTD_DATA_R|=pt->Out[Input];//pulse

GPIO_PORTD_DATA_R&=~0x0F;

pt=pt->Next[Input];//nextstate

Program3.4.OutputsdefinedasnumbersforaMealyFiniteState

Machine(PointerRobot_xxx.zip).

Program3.5usesfunctionstoaffecttheoutput.Althoughthefunctionsinthissolutionperformsimpleoutput,thisimplementationcouldbeusedwhentheoutputoperationsarecomplex.Againpropermemoryallocationisrequiredifwewishtoimplementastand-aloneorembeddedsystem.The const qualifierisusedtoplacetheFSMdatastructureinEEPROM.Bit-specificoutputsareimplementedonPortD.

structState

void*CmdPt[4];//outputsarefunctionpointers

conststructState*Next[4];//next

;

typedefconststructStateStateType;

#defineStand&FSM[0]

#defineSit&FSM[1]

#defineSleep&FSM[2]

voidNone(void);

#defineGPIO_PORTD0(*((volatileuint32_t*)0x40007004))

#defineGPIO_PORTD1(*((volatileuint32_t*)0x40007008))

#defineGPIO_PORTD2(*((volatileuint32_t*)0x40007010))

#defineGPIO_PORTD3(*((volatileuint32_t*)0x40007020))

voidSitDown(void)

GPIO_PORTD3=0x08;

GPIO_PORTD3=0x00;//pulseonPD3

voidStandUp(void)

GPIO_PORTD2=0x04;

GPIO_PORTD2=0x00;//pulseonPD2

voidLieDown(void)

GPIO_PORTD1=0x02;

GPIO_PORTD1=0x00;//pulseonPD1

voidSitUp(void)

GPIO_PORTD0=0x01;

GPIO_PORTD0=0x00;//pulseonPD0

StateTypeFSM[3]=

(void*)&None,(void*)&SitDown,(void*)&None,(void*)&None,//Standing

Stand,Sit,Stand,Stand,

(void*)&None,(void*)LieDown,(void*)&None,(void*)&StandUp,//Sitting

Sit,Sleep,Sit,Stand,

(void*)&None,(void*)&None,(void*)&SitUp,(void*)&SitUp,//Sleeping

Sleep,Sleep,Sit,Sit

;

intmain(void)StateType*pt;//currentstate

uint32_tinput;

SYSCTL_RCGCGPIO_R|=0x0000000A;//clockonPortsBandD

pt=Stand;//initialstate

GPIO_PORTB_DIR_R&=~0x03;//makePB1-0inputfrommoodsensor

GPIO_PORTB_AMSEL_R&=~0x03;//disableanalogonPB

GPIO_PORTB_PCTL_R&=~0x000000FF;//PCTLGPIOonPB1-0

GPIO_PORTB_DEN_R|=0x03;//enabledigitalI/OonPB1-0

GPIO_PORTD_DIR_R|=0x0F;//makePD3-0outputtorobot

GPIO_PORTD_AMSEL_R&=~0x0F;//disableanalogonPD

GPIO_PORTD_PCTL_R&=~0x0000FFFF;//PCTLGPIOonPD3-0

GPIO_PORTD_DEN_R|=0x0F;//enabledigitalI/OonPD3-0

while(1)

input=GPIO_PORTB_DATA_R&0x03;//input=0-3

((void(*)(void))pt->CmdPt[Input])();//function

pt=pt->Next[input];//nextstate

Program3.5.OutputsdefinedasfunctionsforaMealyFiniteStateMachine(FunctionRobot_xxx.zip).

Observation:InordertomaketheFSMrespondquicker,wecouldimplementatimedelayfunctionthatreturnsimmediatelyifanalarmconditionoccurs.Ifnoalarmexists,itwaitsthespecifieddelay.

Checkpoint3.10:Whathappensiftherobotissleepingthenbecomesanxious?

3.6.ThreadsSoftware(e.g.,program,code,module,procedure,function,subroutineetc.)isalistofinstructionsforthecomputertoexecute.Athreadontheotherhandisdefinedasthepathofactionofsoftwareasitexecutes.Theexpression“thread”comesfromtheanalogyshowninFigure3.6.Thissimpleprogramprintsthe8-bitnumbers000001002…Ifweconnectthestatementsofourexecutingprogramwithacontinuousline(thethread)wecanvisualizethedynamicbehaviorofoursoftware.

Figure3.6.Illustrationofthedefinitionofathread.Theexecutionofthemainprogramiscalledtheforegroundthread.Inmostembeddedapplications,theforegroundthreadexecutesaloopthatneverends.Wewilllearnlater,thatthisthreadcanbebroken(executionsuspended,thenrestarted)byinterruptsanddirectmemoryaccess.

Withinterruptswecancreatemultiplethreads.Somethreadswillbecreatedstatically,meaningtheyexistthroughoutthelifeofthesoftware,whileotherswillbecreatedanddestroyeddynamically.Therewillusuallybeoneforegroundthreadrunningthemainprogramliketheaboveexample.Inadditiontothisforegroundthread,eachinterruptsourcehasitsownbackgroundthread,whichisstartedwhenevertheinterruptisrequested.Figure3.7showsasoftwaresystemwithoneforegroundthreadandtwobackgroundthreads.The“Key”threadisinvokedwheneverakeyistouchedonthekeyboardandthe“Time”threadisinvokedevery1msinaperiodicfashion.

Figure3.7.Interruptsallowustohavemultiplebackgroundthreads.Becausethereisbutoneprocessor,thecurrentlyrunningthreadmustbesuspendedinordertoexecuteanotherthread.Intheabovefigure,thesuspensionofthemainprogramisillustratedbythetwobreaksintheforegroundthread.Whenakeyistouched,themainprogramissuspended,anda Keyhandler threadiscreatedwithan“empty”stackanduninitializedregisters.Whenthe Keyhandler isdoneitexecutesareturnfrominterrupttorelinquishcontrolbacktothemainprogram.Theoriginalstackandregistersofthemainprogramwillberestoredtothestatebeforetheinterrupt.Inasimilarway,whenthe1mstimeroccurs,themainprogramissuspendedagain,anda Timehandler threadiscreatedwithitsown“empty”stackanduninitializedregisters.Wecanthinkofeachthreadashavingitsownregistersanditsownstackarea.InChapter5,wewilldiscussindetailthisapproachtomultithreadedprogramming.Inareal-timeoperatingsystem(RTOS)thereisapreemptivethreadschedulerthatallowsoursoftwaretohavemultipleforegroundthreadsandmultiplestacks.ThefocusofVolume3willbethedesignandanalysisofreal-timeoperatingsystems.

Parallelprogrammingallowsthecomputertoexecutemultiplethreadsatthesametime.State-of-the-artmulti-coreprocessorscanexecuteaseparateprogramineachofitscores.Forkandjoinarethefundamentalbuildingblocksofparallelprogramming.Afterafork,twoormoresoftwarethreadswillberuninparallel,i.e.,thethreadswillrunsimultaneouslyonseparateprocessors.Twoormoresimultaneoussoftwarethreadscanbecombinedintooneusingajoin.TheflowchartsymbolsforforkandjoinareshowninFigure3.8.

Softwareexecutionafterthejoinwillwaituntilallthreadsabovethejoinarecomplete.Asananalogyofparallelexecution,whenafarmerwantstobuildabarn,heinviteshisthreeneighborsoverandgiveseveryoneahammer.Theforkoperationchangesthesituationfromthefarmerworkingalonetofourpeoplereadytobuild.Thefourpeoplenowworkinparalleltoaccomplishthesinglegoalofbuildingthebarn.Whentheoveralltaskiscomplete,thejoinoperationcausestheneighborstogohome,andthefarmerisworkingaloneagain.

Parallelprogrammingisadifficultconceptformanysoftwaredevelopers,becausewehavebeenclassicallytrainedtothinkofcomputerexecutionasasingletime-linearthread.However,therearenumerousreal-worldscenariosfromwhichtolearntheartofparallelprogramming.Amanagerofabusiness,anArmygeneral,andairtrafficcontrolareobviousexamplesthatemployparalleloperations.Fromtheseillustrationsweobservehierarchicaldecisionmaking,delegationofresponsibilities,andhavinganelaboratesystemofchecksandbalances.Alloftheseconceptstranslateintocomplexsoftwaresystemsinvolvingparallelexecution.

Toimplementparallelexecutionweneedacomputerthatcanexecutemorethanoneinstructionatatime.Amulti-coreprocessorhastwoormoreindependentcentralprocessingunits,calledcores.Thecoressharedsomememorybutalsohavesomeprivatestorage.Thecorescanfetchandexecuteinstructionsatthesametime,thusincreasingoverallperformance.TheforkoperationinFigure3.8willactivatethreecoreslaunchingsoftwaretobeexecutedonthosethreenewcores.Thejoinoperationwillwaituntilallfourbrancheshavecompleted,anddeactivatethreeofthecores.

Figure3.8.Flowchartsymbolstodescribeparallelandconcurrentprogramming.Concurrentprogrammingallowsthecomputertoexecutemultiplethreads,butonlyonerunsatatime.Interruptsareonemechanismtoimplementconcurrencyonreal-timesystems.Interruptshaveahardwaretriggerandasoftwareaction.Aninterruptisaparameter-lesssubroutinecall,triggeredbyahardwareevent.TheflowchartsymbolsforinterruptsarealsoshowninFigure3.8.Thetriggerisahardwareeventsignalingitistimetodosomething.Examplesofinterrupttriggerswewillseeinthisbookincludenewinputdatahasarrived,outputdeviceisidle,andperiodicevent.Thesecondcomponentofaninterrupt-drivensystemisthesoftwareactioncalledaninterruptserviceroutine(ISR).Theforegroundthreadisdefinedastheexecutionofthemainprogram,andthebackgroundthreadsareexecutionsoftheISRs.Considertheanalogyofsittinginacomfychairreadingabook.Readingabookislikeexecutingthemainprogramintheforeground.Youstartreadingatthebeginningofthebookandbasicallyreadonepageattimeinasequentialfashion.Youmightjumptothebackandlooksomethingupintheglossary,thenjumpbacktowhereyouwhere,whichisanalogoustoafunctioncall.Similarly,youmightreadthesamepageafewtimes,whichisanalogoustoaprogramloop.Eventhoughyouskiparoundalittle,theorderofpagesyoureadfollowsalogicalandwell-definedsequence.Conversely,ifthetelephonerings,youplaceabookmarkinthebookandanswerthephone.Whenyouarefinishedwiththephoneconversation,youhangupthephoneandcontinuereadinginthebookwhereyouleftoff.TheringingphoneisanalogoustohardwaretriggerandthephoneconversationislikeexecutingtheISR.

Aprogramsegmentisreentrantifitcanbeconcurrentlyexecutedbytwo(ormore)threads.InFigure3.7wecanconceiveofthesituationwherethemainprogramstartsexecutingafunction,isinterrupted,andthebackgroundthreadcallsthatsamefunction.Inorderfortwothreadstoshareafunction,thefunctionmustbereentrant.Toimplementreentrantsoftware,placelocalvariablesonthestack,andavoidstoringintoI/Odevicesandglobalmemoryvariables.TheissueofreentrancywillbecoveredindetaillaterinChapter5.

3.7.FirstInFirstOutQueue

3.7.1.ClassicaldefinitionofaFIFOThefirstinfirstoutcircularqueue(FIFO)isquiteusefulforimplementingabufferedI/Ointerface(Figure3.9).Itcanbeusedforbothbufferedinputandbufferedoutput.Theorderpreservingdatastructuretemporarilysavesdatacreatedbythesource(producer)beforeitisprocessedbythesink(consumer).TheclassofFIFOsstudiedinthissectionwillbestaticallyallocatedglobalstructures.Becausetheyareglobalvariables,itmeanstheywillexistpermanentlyandcanbecarefullysharedbymorethanoneprogram.TheadvantageofusingaFIFOstructureforadataflowproblemisthatwecandecoupletheproducerandconsumerthreads.WithouttheFIFOwewouldhavetoproduce1pieceofdata,thenprocessit,produceanotherpieceofdata,thenprocessit.WiththeFIFO,theproducerthreadcancontinuetoproducedatawithouthavingtowaitfortheconsumertofinishprocessingthepreviousdata.Thisdecouplingcansignificantlyimprovesystemperformance.

Figure3.9.TheFIFOisusedtobufferdatabetweentheproducerandconsumer.YouhaveprobablyalreadyexperiencedtheconvenienceofFIFOs.Forexample,aFIFOisusedwhilestreamingaudiofromtheInternet.AssounddataarereceivedfromtheInternettheyareput(calls Fifo_Put )inaFIFO.Whenthesoundboardneedsdataitcalls Fifo_Get .AslongastheFIFOnevercomesfullorempty,thesoundisplayedinacontinuousmanner.AFIFOisalsousedwhenyouaskthecomputertoprintafile.Ratherthanwaitingfortheactualprintingtooccurcharacterbycharacter,theprintcommandwillputthedatainaFIFO.Whenevertheprinterisfree,itwillgetdatafromtheFIFO.TheadvantageoftheFIFOisitallowsyoutocontinuetouseyourcomputerwhiletheprintingoccursinthebackground.Toimplementthismagicofbackgroundprintingwewillneedinterrupts.Therearemanyproducer/consumerapplications.InTable3.3theprocessesontheleftareproducersthatcreateorinputdata,whiletheprocessesontherightareconsumerswhichprocessoroutputdata.

Source/Producer Sink/Consumer

Keyboardinput Programthatinterprets

Programwithdata Printeroutput

Programsendsmessage Programreceivesmessage

MicrophoneandADC Programthatsavessounddata

Programthathassounddata

DACandspeaker

Table3.3.Producerconsumerexamples.

TheproducerputsdataintotheFIFO.The Fifo_Put operationdoesnotdiscardinformationalreadyintheFIFO.IftheFIFOisfullandtheusercalls Fifo_Put ,the Fifo_Put routinewillreturnafullerrorsignifyingthelast(newest)datawasnotproperlysaved.ThesinkprocessremovesdatafromtheFIFO.The Fifo_Get routinewillmodifytheFIFO.Afteraget,theparticularinformationreturnedfromthegetroutineisnolongersavedontheFIFO.IftheFIFOisemptyandtheusertriestoget,the Fifo_Get routinewillreturnanemptyerrorsignifyingnodatacouldberetrieved.TheFIFOisorderpreserving,suchthattheinformationisreturnedbyrepeatedcallsof Fifo_Get inthesameorderasthedatawassavedbyrepeatedcallsof Fifo_Put .

Therearemanywaystoimplementastatically-allocatedFIFO.WecanuseeitherapointeroranindextoaccessthedataintheFIFO.Wecanuseeithertwopointers(ortwoindices)ortwopointers(ortwoindices)andacounter.ThecounterspecifieshowmanyentriesarecurrentlystoredintheFIFO.ThereareevenhardwareimplementationsofFIFOqueues.Webeginwiththetwo-pointerimplementation.Itisalittlehardertoimplement,butitdoeshavesomeadvantagesovertheotherimplementations.

3.7.2.Two-pointerFIFOimplementationThetwo-pointerimplementationhas,ofcourse,twopointers.Ifweweretohaveinfinitememory,aFIFOimplementationiseasy(Figure3.10). GetPt pointstothedatathatwillberemovedbythenextcallto Fifo_Get ,and PutPtpointstotheemptyspacewherethedatawillstoredbythenextcallto Fifo_Put ,seeProgram3.6.

Figure3.10.TheFIFOimplementationwithinfinitememory.int8_tstaticvolatile*PutPt;//putnext

int8_tstaticvolatile*GetPt;//getnext

intFifo_Put(int8_tdata)//callbyvalue

*PutPt=data;//Put

PutPt++;//next

return(1);//trueifsuccess

intFifo_Get(int8_t*datapt)

*datapt=*GetPt;//returnbyreference

GetPt++;//next

return(1);//trueifsuccess

Program3.6.CodefragmentsshowingthebasicideaofaFIFO.Therearefourmodificationsthatarerequiredtotheabovesubroutines.IftheFIFOisfullwhen Fifo_Put iscalledthenthefunctionshouldreturnafullerror.Similarly,iftheFIFOisemptywhen Fifo_Get iscalled,thenthefunctionshouldreturnanemptyerror. PutPtmustbewrappedbackuptothetopwhenitreachesthebottom(Figure3.11).

Figure3.11.TheFIFOFifo_Put operationshowingthepointerwrap.The GetPt mustalsobewrappedbackuptothetopwhenitreachesthebottom(Figure3.12).

Figure3.12.TheFIFOFifo_Get operationshowingthepointerwrap.TherearetwomechanismstodeterminewhethertheFIFOisemptyorfull.AsimplemethodistoimplementacountercontainingthenumberofbytescurrentlystoredintheFIFO. Fifo_Get woulddecrementthecounterand Fifo_Put wouldincrementthecounter.Wewillnotimplementacounterbecauseincrementinganddecrementingacountercausesaracecondition,meaningthecountercouldbecomeincorrectwhensharedinamultithreadedenvironment.RaceconditionsandcriticalsectionswillbepresentedinChapter5.

ThesecondmethodistopreventtheFIFOfrombeingcompletelyfull.TheimplementationofthisFIFOmoduleisshowninProgram3.7.YoucanfindalltheFIFOsofthissectiononthebookwebsiteasFIFO_xxx.zip,wherexxxreferstothespecificmicrocontrolleronwhichtheexamplewastested.

#defineFIFOSIZE10//canbeanysize

#defineFIFOSUCCESS1

#defineFIFOFAIL0

typedefint8_tDataType;

DataTypevolatile*PutPt;//putnext

DataTypevolatile*GetPt;//getnext

DataTypestaticFifo[FIFOSIZE];

//initializeFIFO

voidFifo_Init(void)

PutPt=GetPt=&Fifo[0];//Empty

//addelementtoFIFO

intFifo_Put(DataTypedata)

DataTypevolatile*nextPutPt;

nextPutPt=PutPt+1;

if(nextPutPt==&Fifo[FIFOSIZE])

nextPutPt=&Fifo[0];//wrap

if(nextPutPt==GetPt)

return(FIFOFAIL);//Failed,FIFOfull

else

*(PutPt)=data;//Put

PutPt=nextPutPt;//Success,update

return(FIFOSUCCESS);

//removeelementfromFIFO

intFifo_Get(DataType*datapt)

if(PutPt==GetPt)

return(FIFOFAIL);//EmptyifPutPt=GetPt

*datapt=*(GetPt++);

if(GetPt==&Fifo[FIFOSIZE])

GetPt=&Fifo[0];//wrap

return(FIFOSUCCESS);

Program3.7.Two-pointerimplementationofaFIFO(FIFO_xxx.zip).Forexample,iftheFIFOhad10bytesallocated,thenthe Fifo_Put subroutinewouldallowamaximumof9bytestobestored.Iftherewerealready9bytesintheFIFOandanother Fifo_Put werecalled,thentheFIFOwouldnotbemodifiedandafullerrorwouldbereturned.SeeFigure3.13.Inthiswayif PutPt equals GetPt atthebeginningof Fifo_Get ,thentheFIFOisempty.Similarly,if PutPt+1 equals GetPt atthebeginningof Fifo_Put ,thentheFIFOisfull.Becarefultowrapthe PutPt+1 beforecomparingitto Fifo_Get .Thismethoddoesnotrequirethelengthtobestoredorcalculated.

Figure3.13.FlowchartsofthepointerimplementationoftheFIFOqueue.TocheckforFIFOfull,thefollowing Fifo_Put routineattemptstoputusingatemporary PutPt .IfputtingmakestheFIFOlookempty,thenthetemporary PutPt isdiscardedandtheroutineisexitedwithoutsavingthedata.ThisiswhyaFIFOwith10allocatedbytescanonlyhold9datapoints.Ifputtingdoesn’tmaketheFIFOlookempty,thenthetemporary PutPt isstoredintotheactual PutPt savingthedataasdesired.

TocheckforFIFOempty,the Fifo_Get routineinProgram3.7simplycheckstoseeif GetPt equals PutPt .Iftheymatchatthestartoftheroutine, thenFifo_Get returnswiththe“empty”conditionsignified.

Since Fifo_Put and Fifo_Get havereadmodifywriteaccessestoglobalvariablestheyarethemselvesnotreentrant.Similarly Fifo_Init hasamultiplestepwriteaccesstoglobalvariables.Therefore Fifo_Init isnotreentrant.

Oneadvantageofthispointerimplementationisthatifyouhaveasinglethreadthatcallsthe Fifo_Get (e.g.,themainprogram)andasinglethreadthatcallsthe Fifo_Put (e.g.,theserialportreceiveinterrupthandler),thenthis Fifo_Put functioncaninterruptthis Fifo_Get functionwithoutlossofdata.Sointhisparticularsituation,interruptswouldnothavetobedisabled.Itwouldalsooperateproperlyiftherewereasingleinterruptthreadcalling Fifo_Get (e.g.,theserialporttransmitinterrupthandler)andasinglethreadcalling Fifo_Put (e.g.,themainprogram.)Ontheotherhand,ifthesituationismoregeneral,andmultiplethreadscouldcall Fifo_Put ormultiplethreadscouldcall Fifo_Get ,thentheinterruptswouldhavetobetemporarilydisabled.

3.7.3.TwoindexFIFOimplementationTheothermethodtoimplementaFIFOistouseindicesratherthanpointers.ThisFIFOhastherestrictionthatthesizemustbeapowerof2.InProgram3.8, FIFOSIZE is16andthelogic PutI&(FIFOSIZE-1) returnsthebottomfourbitsoftheputindex.Similarly,thelogic GetI&(FIFOSIZE-1) returnsthebottomfourbitsofthegetindex.Usingthebottombitsoftheindexremovesthenecessarytocheckforoutofboundsandwrapping.

//Two-indeximplementationofthetransmitFIFO

//canhold0toFIFOSIZEelements

#defineFIFOSIZE16//mustbeapowerof2

#defineFIFOSUCCESS1

#defineFIFOFAIL0

typedefint8_tDataType;

uint32_tvolatilePutI;//putnext

uint32_tvolatileGetI;//getnext

DataTypestaticFifo[FIFOSIZE];

//initializeindexFIFO

voidFifo_Init(void)

PutI=GetI=0;//Empty

//addelementtoendofindexFIFO

intFifo_Put(DataTypedata)

if((PutI-GetI)&~(FIFOSIZE-1))

return(FIFOFAIL);//Failed,fifofull

Fifo[PutI&(FIFOSIZE-1)]=data;//put

PutI++;//Success,update

return(FIFOSUCCESS);

//removeelementfromfrontofindexFIFO

intFifo_Get(DataType*datapt)

if(PutI==GetI)

return(FIFOFAIL);//EmptyifPutI=GetI

*datapt=Fifo[GetI&(FIFOSIZE-1)];

GetI++;//Success,update

return(FIFOSUCCESS);

Program3.8.Implementationofatwo-indexFIFO.Thesizemustbeapoweroftwo(FIFO_xxx.zip).IftheFIFOisfull,then (PutI-GetI) willequal16,meaningallelementsofthebufferhavedata.Theexpression ~(FIFOSIZE-1) yieldstheconstant0xFFFFFFF0.Forallsizesthatareapowerof2,theifstatementinputwillbenonzeroifthereare FIFOSIZEelementsintheFIFO.WiththisimplementationaFIFOwith16allocatedbytescanactuallyhold16datapoints.TheFIFOisemptyif PutI equals GetI .Ifempty,theFifo_Get functionreturnswiththe FIFOFAIL condition.

3.7.4.FIFObuildmacrosWhenweneedmultipleFIFOsinoursystem,wecouldswitchovertoC++anddefinetheFIFOasaclass,andtheninstantiatemultipleobjectstocreatetheFIFOs.Asecondapproachwouldbetouseatexteditor,openthesourcecodecontainingProgram3.7or3.8,copy/pasteit,andthenchangenamessothefunctionsareunique.AthirdapproachisshowninPrograms3.9and3.10,whichdefinesmacrosallowingustocreateasmanyFIFOsasweneed.

//macrotocreateapointerFIFO

#defineAddPointerFifo(NAME,SIZE,TYPE,SUCCESS,FAIL)\

TYPEvolatile*NAME##PutPt;\

TYPEvolatile*NAME##GetPt;\

TYPEstaticNAME##Fifo[SIZE];\

voidNAME##Fifo_Init(void)\

NAME##PutPt=NAME##GetPt=&NAME##Fifo[0];\

\

intNAME##Fifo_Put(TYPEdata)\

TYPEvolatile*nextPutPt;\

nextPutPt=NAME##PutPt+1;\

if(nextPutPt==&NAME##Fifo[SIZE])\

nextPutPt=&NAME##Fifo[0];\

\

if(nextPutPt==NAME##GetPt)\

return(FAIL);\

\

else\

*(NAME##PutPt)=data;\

NAME##PutPt=nextPutPt;\

return(SUCCESS);\

\

\

intNAME##Fifo_Get(TYPE*datapt)\

if(NAME##PutPt==NAME##GetPt)\

return(FAIL);\

\

*datapt=*(NAME##GetPt##++);\

if(NAME##GetPt==&NAME##Fifo[SIZE])\

NAME##GetPt=&NAME##Fifo[0];\

\

return(SUCCESS);\

Program3.9.Two-pointermacroimplementationofaFIFO(FIFO_xxx.zip).

Tocreatea20-elementFIFOstoringunsigned16-bitnumbersthatreturns1onsuccessand0onfailureweinvoke

AddPointerFifo(Rx,20,uint16_t,1,0)

creatingthethreefunctionsRxFifo_Init() , RxFifo_Get() ,and RxFifo_Put() .

Program3.10isamacroallowingustocreatetwo-indexFIFOssimilartoProgram3.8.

//macrotocreateanindexFIFO

#defineAddIndexFifo(NAME,SIZE,TYPE,SUCCESS,FAIL)\

uint32_tvolatileNAME##PutI;\

uint32_tvolatileNAME##GetI;\

TYPEstaticNAME##Fifo[SIZE];\

voidNAME##Fifo_Init(void)\

NAME##PutI=NAME##GetI=0;\

\

intNAME##Fifo_Put(TYPEdata)\

if((NAME##PutI-NAME##GetI)&~(SIZE-1))\

return(FAIL);\

\

NAME##Fifo[NAME##PutI&(SIZE-1)]=data;\

NAME##PutI##++;\

return(SUCCESS);\

\

intNAME##Fifo_Get(TYPE*datapt)\

if(NAME##PutI==NAME##GetI)\

return(FAIL);\

\

*datapt=NAME##Fifo[NAME##GetI&(SIZE-1)];\

NAME##GetI##++;\

return(SUCCESS);\

\

uint16_tNAME##Fifo_Size(void)\

return((uint16_t)(NAME##PutI-NAME##GetI));\

Program3.10.Macroimplementationofatwo-indexFIFO.Thesizemustbeapoweroftwo(FIFO_xxx.zip).Tocreatea32-elementFIFOstoringsigned32-bitnumbersthatreturns0onsuccessand1onfailureweinvoke

AddIndexFifo(Tx,32,int32_t,0,1)

creatingthethreefunctionsTxFifo_Init() , TxFifo_Get() ,and TxFifo_Put() .

Checkpoint3.11:ShowCcodetocreatethreeFIFOscalledCAN1CAN2andCAN3.EachFIFOstores8-bitbytesandmustbeabletostoreupto99elements.

Checkpoint3.12:ShowCcodetocreatetwoFIFOscalledF1andF2.EachFIFOstores16-bithalfwordsandmustbeabletostoreupto256elements.

3.8.MemoryManagementandtheHeapSofar,wehaveseentwotypesofallocation:permanentallocationinglobalvariablesandtemporaryallocationinlocalvariables.Whenweallocatelocalvariablesinregistersoronthestackthesevariablesmustbeprivatetothefunctionandcannotbesharedwithotherfunctions.Furthermore,eachtimethefunctionisinvokednewlocalvariablesarecreated,anddatafrompreviousinstantiationsarenotavailable.Thisbehaviorisusuallyexactlywhatwewanttohappenwithlocalvariables.However,wecanusetheheap(ormemorymanager)tohavetemporaryallocationinawaythatismuchmoreflexible.Inparticular,wewillbeabletoexplicitlydefinewhendataareallocatedandwhentheyaredeallocatedwiththeonlyrestrictionbeingwefirstallocate,nextweuse,andthenwedeallocate.Furthermore,wecancontrolthescopeofthedatainaflexiblemanner.

Theuseoftheheapinvolvestwosystemfunctions: malloc and free .Whenwewishtoallocatespacewecallmalloc andspecifyhowmanybytesweneed. malloc willreturnapointertothenewobject,whichwemuststoreinapointervariable.Iftheheaphasnomorespace,malloc willreturna0,whichmeansnullpointer.Theheapimplementstemporaryallocation,sowhenwearedonewiththedata,wereturnittotheheapbycalling free .Considerthefollowingsimpleexamplewiththreefunctions.

int32_t*Pt;

voidBegin(void)

Pt=(*int32_t)malloc(4*20);//allocate20words

voidUse(void)inti;

for(i=0;i<20;i++)

Pt[i]=i;//putdataintoarray

voidEnd(void)

free(Pt);

Thepointer Pt ispermanentlyallocated.TheleftsideofFigure3.14showsthatinitially,eventhoughthepointerexists,itdoesnotpointtoanything.Morespecifically,thecompilerwillinitializeitto0;this0isdefinedasanullpointer,meaningitisnotvalid.Whenmalloc iscalledthepointerisnowvalidandpointstoa20-wordarray.Thearrayisinsidetheheapand Pt pointstoit.Anytimeaftermalloc iscalledandbefore free iscalledthearrayexistsandcanbeaccessedviathepointer Pt .Afteryoucallfree,thepointerhasthesamevalueasbefore.However,thearrayitselfdoesnotexist.I.e.,these80bytesdonotbelongtoyourprogramanymore.Inparticular,afteryoucallfreetheheapisallowedtoallocatethesebytestosomeotherprogram.Weirdandcrazyerrorswilloccurifyouattempttodereferencethepointerbeforethearrayisallocated,orafteritisreleased.

Figure3.14.Theheapisusedtodynamicallyallocatememory.Thisarrayexistsandthepointerisvalidfromwhenyoucall malloc upuntilthetimeyoucall free .InC,theheapdoesnotmanagethepointerstoallocatedblock;yourprogrammust.Ifyoucallmalloc tentimesinarow,youmustkeeptrackofthetenpointersyoureceived.Thescopeofthisarrayisdeterminedbythescopeofthepointer, Pt .If Pt ispublic,thenthearrayispublic.Ifstaticweretobeaddedtothedefinitionof Pt ,thenthescopeofthearrayisrestrictedtosoftwarewithinthisfile.Inthefollowingexample,thescopeofthearrayisrestrictedtotheonefunction.Withinoneexecutionofthefunction,thearrayisallocated,used,andthendeallocated,justlikealocalvariable.

voidFunction(void)inti;

int32_t*pt;

pt=(*int32_t)malloc(4*20);//allocate20words

for(i=0;i<20;i++)

pt[i]=i;//putdataintoarray

free(pt);

Amemoryleakoccursifsoftwareusestheheaptoallocatespacebutforgetstodeallocatethespacewhenitisfinished.Thefollowingisanexampleofamemoryleak.Eachtimethefunctioniscalled,ablockofmemoryisallocated.Thepointertotheblockisstoredinalocalvariable.Whenthefunctionreturns,thepointernolongerexists.Thismeanstheallocatedblockintheheapexists,buttheprogramhasnopointertoit.Inotherwords,eachtimethisfunctionreturns80bytesfromtheheaparepermanentlylost.

voidLeakyFunction(void)inti;

int32_t*pt;

pt=(*int32_t)malloc(4*20);//allocate20words

for(i=0;i<20;i++)

pt[i]=i;//putdataintoarray

Ingeneral,theheapmanagerallowstheprogramtoallocateavariableblocksize,butinthissectionwewilldevelopasimplifiedheapmanagerhandlesjustfixedsizeblocks.Inthisexample,theblocksizeisspecifiedbytheconstantSIZE .Theinitializationwillcreatealinkedlistofallthefreeblocks(Figure3.15).

Figure3.15.Theinitialstateoftheheaphasallofthefreeblockslinkedinalist.Program3.11ashowstheglobalstructuresfortheheap.TheseentriesaredefinedinRAM. SIZE isthenumberof8-bitbytesineachblock.Allblocksallocatedandreleasedwiththismemorymanagerwillbeofthisfixedsize. NUM isthenumberofblockstobemanaged. FreePt pointstothefirstfreeblock.

#defineSIZE80

#defineNUM5

#defineNULL0//emptypointer

int8_t*FreePt;

int8_tHeap[SIZE*NUM];

Program3.11a.Privateglobalstructuresforthefixed-blockmemorymanager.Initializationmustbeperformedbeforetheheapcanbeused.Program3.11bshowsthesoftwarethatpartitionstheheapintoblocksandlinksthemtogether. FreePt pointstoalinearlinkedlistoffreeblocks.Initiallythesefreeblocksarecontiguousandinorder,butasthemanagerisusedthepositionsandorderofthefreeblockscanvary.Itwillbethepointersthatwillthreadthefreeblockstogether.

voidHeap_Init(void)

int8_t*pt;

FreePt=&Heap[0];

for(pt=&Heap[0];

pt!=&Heap[SIZE*(NUM-1)];

pt=pt+SIZE)

*(int32_t*)pt=(int32_t)(pt+SIZE);

*(int32_t*)pt=NULL;

Program3.11b.Functionstoinitializetheheap.Toallocateablocktomanagerjustremovesoneblockfromthefreelist.Program3.11cshowstheallocateandreleasefunctions.The Heap_Allocate functionwillfailandreturnanullpointerwhentheheapbecomesempty.The Heap_Release returnsablocktothefreelist.Thissystemdoesnotchecktoverifyareleasedblockactuallywaspreviouslyallocated.

void*Heap_Allocate(void)

int8_t*pt;

pt=FreePt;

if(pt!=NULL)

FreePt=(char*)*(char**)pt;

return(pt);

voidHeap_Release(void*pt)

int8_t*oldFreePt;

oldFreePt=FreePt;

FreePt=(char*)pt;

*(int32_t*)pt=(int32_t)oldFreePt;

Program3.11c.Functionstoallocateandreleasememoryblocks(HeapFixedBlock_xxx.zip).Checkpoint3.13:Thereare5blocksinthissimpleheap.HowcouldthememorymanagerdetermineifblockI(where0≤I≤4)isallocatedorfree?

Checkpoint3.14:Usingthismemorymanager,writeamallocandfreefunctionssuchthatthesizeisrestrictedtoamaximumof100bytes.I.e.,youmayassumetheuserneverasksformorethan100bytesatatime.

3.9.IntroductiontoDebuggingEveryprogrammerisfacedwiththeneedtodebugandverifythecorrectnessoftheirsoftware.Inthissectionwewillstudyhardwarelevelprobesliketheoscilloscope,logicanalyzer,JTAG,andin-circuit-emulator(ICE);softwareleveltoolslikesimulators,monitors,andprofilers;andmanualtoolslikeinspectionandprintstatements.

3.9.1.DebuggingToolsMicrocontroller-relatedproblemsoftenrequiretheuseofspecializedequipmenttodebugthesystemhardwareandsoftware.Usefulhardwaretoolsincludealogicprobe,anoscilloscope,alogicanalyzer,andaJTAGdebugger.AlogicprobeisahandhelddevicewithanLEDorbuzzer.YouplacetheprobeonyourdigitalcircuitandLED/buzzerwillindicatewhetherthesignalishighorlow.Anoscilloscope,orscope,graphicallydisplaysinformationaboutanelectroniccircuit,wherethevoltageamplitudeversustimeisdisplayed.Ascopehasoneortwochannels,withmanywaystotriggerorcapturedata.AscopeisparticularlyusefulwheninterfacinganalogsignalsusinganADCorDAC.ThePicoScope2104(fromhttp://www.picotech.com/)isalow-costbuteffectivetoolfordebuggingmicrocontrollercircuits.Alogicanalyzerisessentiallyamultiplechanneldigitalstoragescopewithmanywaystotrigger.AsshowninFigure3.16,wecanconnectthelogicanalyzertodigitalsignalsthatarepartofthesystem,orwecanconnectthelogicanalyzerchannelstounusedmicrocontrollerpinsandaddsoftwaretotogglethosepinsatstrategictimes/places.Asatroubleshootingaid,itallowstheexperimentertoobservenumerousdigitalsignalsatvariouspointsintimeandthusmakedecisionsbaseduponsuchobservations.Oneproblemwithlogicanalyzersisthemassiveamountofinformationthatitgenerates.Touseananalyzereffectivelyonemustlearnpropertriggeringmechanismstocapturedataatappropriatetimeseliminatingtheneedtosiftthroughvolumesofoutput.ThelogicanalyzerfiguresinthisbookwerecollectedwithalogicanalyzerUSBee(fromhttp://www.usbee.com).SomeofthemodelsfromUSBeecombinealogicanalyzerwithanoscilloscope,creatinganextremelyeffectivedebuggingtool.

MaintenanceTip:First,findthethingsthatwillbreakyou.Second,breakthem.

Commonerror:Sometimestheoriginalsystemoperatesproperly,andthedebuggingcodehasmistakes.

Figure3.16.Alogicanalyzerandexampleoutput.

Figure3.17showsalogicanalyzeroutput,wheresignalsRxCANandTxCANaredigitalinput/output,butsignalsPB3,PB1,andPB5aredebuggingoutputstomeasuringtimingrelationshipsbetweensoftwareexecutionanddigitalI/O.TherisingedgeofPB3isusedtotriggerthedatacollection.

Figure3.17.USBeeSXlogicanalyzeroutput(alsoconsiderAnalogDiscoveryfromwww.digilentinc.com).Anemulatorisahardwaredebuggingtoolthatrecreatestheinput/outputsignalsoftheprocessorchip.Touseanemulator,weremovetheprocessorchipandinserttheemulatorcableintothechipsocket.Inmostcases,theemulator/computersystemoperatesatfullspeed.Theemulatorallowstheprogrammertoobserveandmodifyinternalregistersoftheprocessor.Emulatorsareoftenintegratedintoapersonalcomputer,sothatitseditor,harddrive,andprinterareavailableforthedebuggingprocess.

Theonlydisadvantageofthein-circuitemulatorisitscost.Toprovidesomeofthebenefitsofthishigh-priceddebuggingequipment,manymicrocontrollersuseaJTAGdebugger.TheJTAGhardwareexistsbothonthemicrocontrollerchipitselfandasanexternalinterfacetoapersonalcomputer.AlthoughnotasflexibleasanICE,theJTAGcanprovidetheabilitytoobservesoftwareexecutioninreal-time,theabilitytosetbreakpoints,theabilitytostopthecomputer,andtheabilitytoreadandwriteregisters,I/Oportsandmemory.

3.9.2.DebuggingTheory

Debuggingisanessentialcomponentofembeddedsystemdesign.Weneedtoconsiderdebuggingduringallphasesofthedesigncycle.Itisimportanttodevelopastructureormethodwhenverifyingsystemperformance.Thissectionwillpresentanumberoftoolswecanusewhendebugging.Termssuchasprogramtesting,diagnostics,performancedebugging,functionaldebugging,tracing,profiling,instrumentation,visualization,optimization,verification,performancemeasurement,andexecutionmeasurementhavespecializedmeanings,buttheyarealsousedinterchangeably,andtheyoftendescribeoverlappingfunctions.Forexample,thetermsprofiling,tracing,performancemeasurement,orexecutionmeasurementmaybeusedtodescribetheprocessofexaminingaprogramfromatimeviewpoint.But,tracingisalsoatermthatmaybeusedtodescribetheprocessofmonitoringaprogramstateorhistoryforfunctionalerrors,ortodescribetheprocessofsteppingthroughaprogramwithadebugger.Usageofthesetermsamongresearchersandusersvary.

Black-boxtestingissimplyobservingtheinputsandoutputswithoutlookinginside.Black-boxtestinghasanimportantplaceindebuggingamoduleforitsfunctionality.Ontheotherhand,white-boxtestingallowsyoutocontrolandobservetheinternalworkingsofasystem.Acommonmistakemadebynewengineersistojustperformblackboxtesting.Effectivedebuggingusesboth.Onemustalwaysstartwithblack-boxtestingbysubjectingahardwareorsoftwaremoduletoappropriatetest-cases.Oncewedocumentthefailedtest-cases,wecanusethemtoaidusineffectivelyperformingthetaskofwhite-boxtesting.

Wedefineadebugginginstrumentassoftwarecodethatisaddedtotheprogramforthepurposeofdebugging.Aprintstatementisacommonexampleofaninstrument.Usingtheeditor,weaddprintstatementstoourcodethateitherverifyproperoperationordisplayrun-timeerrors.Akeytowritinggooddebugginginstrumentsistoprovideforamechanismtoreliablyandefficientlyremoveallthemwhenthedebuggingisdone.Considerthefollowingmechanismsasyoudevelopyourownuniquedebuggingstyle.

•Placeallprintstatementsinauniquecolumn(e.g.,firstcolumn.),sothattheonlycodethatexistsinthiscolumnwillbedebugginginstruments.

•Definealldebugginginstrumentsasfunctionsthatallhaveaspecificpatternintheirnames(e.g.,beginwith Debug_ ).Inthisway,thefind/replacemechanismoftheeditorcanbeusedtofindallthecallstotheinstruments.

•Definetheinstrumentssothattheytestaruntimeglobalflag.Whenthisflagisturnedoff,theinstrumentsperformnofunction.Noticethatthismethodleavesapermanentcopyofthedebuggingcodeinthefinalsystem,causingittosufferaruntimeoverhead,butthedebuggingcodecanbeactivateddynamicallywithoutrecompiling.Manycommercialsoftwareapplicationsutilizethismethodbecauseitsimplifies“on-site”customersupport.

•Useconditionalcompilation(orconditionalassembly)toturnonandofftheinstrumentswhenthesoftwareiscompiled.Whenthecompilersupportsthisfeature,itcanprovidebothperformanceandeffectiveness.

Somecompilerssupportaconfigurationmodethatcanbesettodebugorrelease.Indebugmode,debugginginstrumentsareadded.Inreleasemode,theinstrumentsareremoved.

Checkpoint3.15:Considerthedifferencebetweenaruntimeflagthatactivatesadebuggingcommandversusacompile-timeflag.Inbothcasesitiseasytoactivate/deactivatethedebuggingstatements.Listonefactorforwhicheachmethodissuperiortotheother.

Checkpoint3.16:Whatistheadvantageofleavingdebugginginstrumentsinafinaldeliveredproduct?

Nonintrusivenessisthecharacteristicorqualityofadebuggerthatallowsthesoftware/hardwaresystemtooperatenormallyasifthedebuggerdidnotexist.Intrusivenessisusedasameasureofthedegreeofperturbationcausedinprogramperformancebythedebugginginstrumentitself.Lettbethetimerequiredtoexecutetheinstrument,andlet tbetheaveragetimeinbetweenexecutionsoftheinstrument.Onequantitativemeasureofintrusivenessist/ t,whichisthefractionofavailableprocessortimeusedbythedebugger.Forexample,aprintstatementaddedtoyoursourcecodemaybeveryintrusivebecauseitmightsignificantlyaffectthereal-timeinteractionofthehardwareandsoftware.Observingsignalsthatalreadyexistaspartofthesystemwithanoscilloscopeorlogicanalyzerisnonintrusive.Adebugginginstrumentisclassifiedasminimallyintrusiveifithasanegligibleeffectonthesystembeingdebugged.Inarealmicrocontrollersystem,breakpointsandsingle-steppingarealsointrusive,becausetherealhardwarecontinuestochangewhilethesoftwarehasstopped.Whenaprograminteractswithreal-timeevents,theperformancecanbesignificantlyalteredwhenusingintrusivedebuggingtools.Tobeeffectivewemustemploynonintrusiveorminimallyintrusivemethods.

Checkpoint3.17:Whatdoesitmeanforadebugginginstrumenttobeminimallyintrusive?Givebothageneralanswerandaspecificcriterion.

Although,awidevarietyofprogrammonitoringanddebuggingtoolsareavailabletoday,inpracticeitisfoundthatanoverwhelmingmajorityofuserseitherstillpreferorrelymainlyupon“roughandready”manualmethodsforlocatingandcorrectingprogramerrors.Thesemethodsincludedesk-checking,dumps,andprintstatements,withprintstatementsbeingoneofthemostpopularmanualmethods.Manualmethodsareusefulbecausetheyarereadilyavailable,andtheyarerelativelysimpletouse.But,theusefulnessofmanualmethodsislimited:theytendtobehighlyintrusive,andtheydonotprovideadequatecontroloverrepeatability,eventselection,oreventisolation.Areal-timesystem,wheresoftwareexecutiontimingiscritical,usuallycannotbedebuggedwithsimpleprintstatements,becausetheprintstatementitselfwillrequiretoomuchtimetoexecute.

Thefirststepofdebuggingistostabilizethesystem.Inthedebuggingcontext,westabilizetheproblembycreatingatestroutinethatfixes(orstabilizes)alltheinputs.Inthisway,wecanreproducetheexactinputsoverandoveragain.Oncestabilized,ifwemodifytheprogram,wearesurethatthechangeinouroutputsisafunctionofthemodificationwemadeinoursoftwareandnotduetoachangeintheinputparameters.

Accelerationmeanswewillspeedupthetestingprocess.Whenwearetestingonemodulewecanincreasehowfastthefunctionsarecalledinanattempttoexposepossiblefaults.Furthermore,sincewecancontrolthetestenvironment,wewillvarythetestconditionsoverawiderangeofpossibleconditions.Stresstestingmeanswerunthesystembeyondtherequirementstoseeatwhatpointitbreaksdown.

Whenasystemhasasmallnumberofpossibleinputs(e.g.,lessthanamillion),itmakessensetotestthemall.Whenthenumberofpossibleinputsislargeweneedtochooseasetofinputs.Coveragedefinesthesubsetofpossibleinputsselectedfortesting.Acornercaseisdefinedasasituationattheboundarywheremultipleinputsareattheirmaximum,likethecornerofa3-Dcube.Atthecornersmallchangesininputmaycauselotsofinternalandexternalchanges.Inparticular,weneedtotestthecaseswethinkmightbedifficult(e.g.,theclockoutputincrementsonesecondfrom11:59:59PMDecember31,1999.)Therearemanywaystodecideonthecoverage.Wecanselectvalues:

•Neartheextremesandinthemiddle

•Mosttypicalofhowourclientswillproperlyusethesystem

•Mosttypicalofhowourclientswillimproperlyusethesystem

•Thatdifferbyone

•Youknowyoursystemwillfinddifficult

•UsingarandomnumbergeneratorTostabilizethesystemwedefineafixedsetofinputstotest,runthesystemontheseinputs,andrecordtheoutputs.Debuggingisaprocessoffindingpatternsinthedifferencesbetweenrecordedbehaviorandexpectedresults.Theadvantageofmodularprogrammingisthatwecanperformmodulardebugging.Wemakealistofmodulesthatmightbecausingthebug.Wecanthencreatenewtestroutinestostabilizethesemodulesanddebugthemoneatatime.Unfortunately,sometimesallthemodulesseemtowork,butthecombinationofmodulesdoesnot.Inthiscasewestudytheinterfacesbetweenthemodules,lookingforintendedandunintended(e.g.,unfriendlycode)interactions.

Theemergenceofconcurrentsystems(e.g.,distributednetworksofmicrocontrollers),optimizingarchitectures(e.g.,pipelines,cache,branchprediction,outoforderexecution,conditionalexecution,andmulti-coreprocessors),andtheincreasingneedforsecurityandreliablyplacefurtherdemandsondebuggers.Thecomplexitiesintroducedbytheinteractionofmultipleeventsortimedependentprocessesaremuchmoredifficulttodebugthanerrorsassociatedwithsequentialprograms.Thebehaviorofnon-real-timesequentialprogramsisreproducible:foragivensetofinputstheiroutputsremainthesame.Inthecaseofconcurrentorreal-timeprogramsthisdoesnotholdtrue.Controloverrepeatability,eventselection,andeventisolationisevenmoreimportantforconcurrentorreal-timeenvironments.

Sometimes,themeaningandscopeofthetermdebuggingitselfisnotclear.Weholdtheviewthatthegoalofdebuggingistomaintainandimprovesoftware,andtheroleofadebuggeristosupportthisendeavor.Wedefinethedebuggingprocessastesting,stabilizing,localizing,andcorrectingerrors.Andinouropinion,althoughtesting,stabilizing,andlocalizingerrorsareimportantandessentialtodebugging,theyareauxiliaryprocesses:theprimarygoalofdebuggingistoremedyfaultsandverifythesystemisoperatingwithinspecifications.

3.9.3.FunctionalDebuggingFunctionaldebugginginvolvestheverificationofinput/outputparameters.Itisastaticprocesswhereinputsaresupplied,thesystemisrun,andtheoutputsarecomparedagainsttheexpectedresults.Wewillpresentsevenmethodsoffunctionaldebugging.

1.SingleSteppingorTrace.Manydebuggersallowyoutosettheprogramcountertoaspecificaddressthenexecuteoneinstructionatatime.StepOverwillexecuteoneinstruction,unlessthatinstructionisasubroutinecall,inwhichcasethesimulatorwillexecutetheentiresubroutineandstopattheinstructionfollowingthesubroutinecall.StepOutassumestheexecutionhasalreadyenteredafunctionandwillfinishexecutionofthefunctionandstopattheinstructionfollowingthefunctioncall.

2.Breakpointswithoutfiltering.Thefirststepofdebuggingistostabilizethesystemwiththebug.Inthedebuggingcontext,westabilizetheproblembycreatingatestroutinethatfixes(orstabilizes)alltheinputs.Inthisway,wecanreproducetheexactinputsoverandoveragain.Oncestabilized,ifwemodifytheprogram,wearesurethatthechangeinouroutputsisafunctionofthemodificationwemadeinoursoftwareandnotduetoachangeintheinputparameters.Abreakpointisamechanismtotagplacesinoursoftware,whichwhenexecutedwillcausethesoftwaretostop.

3.Conditionalbreakpoints.Oneoftheproblemswithbreakpointsisthatsometimeswehavetoobservemanybreakpointsbeforetheerroroccurs.Onewaytodealwiththisproblemistheconditionalbreakpoint.Addaglobalvariablecalled count andinitializeittozerointheritual.Addthefollowingconditionalbreakpointtotheappropriatelocation,andrunthesystemagain(youcanchangethe32tomatchthesituationthatcausestheerror).

if(++count==32)

breakpoint();//<=placebreakpointhere

Noticethatthebreakpointoccursonlyonthe32ndtimethebreakisencountered.Anyappropriateconditioncanbesubstituted.

4.Instrumentation:printstatements.Theuseofprintstatementsisapopularandeffectivemeansforfunctionaldebugging.Thedifficultywithprintstatementsinembeddedsystemsisthatastandard“printer”maynotbeavailable.Anotherproblemwithprintingisthatmostembeddedsystemsinvolvetime-dependentinteractionswithitsexternalenvironment.Theprintstatementitselfmaysoslowthatthedebugginginstrumentitselfcausesthesystemtofail.Therefore,theprintstatementisusuallyintrusive.Oneexceptiontothisruleisiftheprintingchanneloccursinthebackgroundusinginterrupts,andthetimebetweenprintstatements(t2)islargecomparedtothetimetoexecutiononeprint(t1),thentheprintstatementswillbeminimallyintrusive.Nevertheless,thisbookwillfocusondebuggingmethodsthatdonotrelyontheavailabilityofaprinter.

5.Instrumentation:dumpintoarraywithoutfiltering.Oneofthedifficultieswithprintstatementsisthattheycansignificantlyslowdowntheexecutionspeedinreal-timesystems.Manytimesthebandwidthoftheprintfunctionscannotkeeppacewithdatabeinggeneratedbythedebuggingprocess.Forexample,oursystemmaywishtocallafunction1000timesasecond(orevery1ms).Ifweaddprintstatementstoitthatrequire50mstoperform,thepresenceoftheprintstatementswillsignificantlyaffectthesystemoperation.Inthissituation,theprintstatementswouldbeconsideredextremelyintrusive.Anotherproblemwithprintstatementsoccurswhenthesystemisusingthesameoutputhardwareforitsnormaloperation,asisrequiredtoperformtheprintfunction.Inthissituation,debuggeroutputandnormalsystemoutputareintertwined.

Tosolveboththesesituations,wecanaddadebuggerinstrumentthatdumpsstrategicinformationintoarraysatruntime.Wecanthenobservethecontentsofthearrayatalatertime.OneoftheadvantagesofdumpingisthattheJTAGdebuggingallowsyoutovisualizememoryevenwhentheprogramisrunning.Program3.12showsadumpinstrument.

#defineSIZE100

uint32_tDebug_Buffer[SIZE][2];

unsignedintDebug_Cnt=0;

voidDebug_Dump(void)//dumpHappyandSad

if(Debug_Cnt<SIZE)

Debug_Buffer[Debug_Cnt][0]=Happy;

Debug_Buffer[Debug_Cnt][1]=Sad;

Debug_Cnt++;

Program3.12.Instrumentationdumpwithoutfiltering.Assume Happy and Sad arestrategic32-bitvariables.ThefirststepwheninstrumentingadumpistodefineabufferinRAMtosavethedebuggingmeasurements.The Debug_Cnt willbeusedtoindexintothebuffers. Debug_Cnt mustbeinitializedtozero,beforethedebuggingbegins.Thedebugginginstrument,showninProgram3.13,savesthestrategicvariablesintothebuffer.

Next,youadd Debug_Dump(); statementsatstrategicplaceswithinthesystem.Youcaneitherusethedebuggertodisplaytheresultsoraddsoftwarethatprintstheresultsaftertheprogramhasrunandstopped.Inthisway,youcancollectinformationintheexactsamemanneryouwouldifyouwereusingprintstatements.

6.Instrumentation:dumpintoarraywithfiltering.Oneproblemwithdumpsisthattheycangenerateatremendousamountofinformation.Ifyoususpectacertainsituationiscausingtheerror,youcanaddafiltertotheinstrument.Afilterisasoftware/hardwareconditionthatmustbetrueinordertoplacedataintothearray.Inthissituation,ifwesuspecttheerroroccurswhenthepointernearstheendofthebuffer,wecouldaddafilterthatsavesinthearrayonlywhenthepointerisaboveacertainvalue.IntheexampleshowninProgram3.13,theinstrumentsavesthestrategicvariablesintothebufferonlywhen Sad isgreaterthan100.

#defineSIZE100

uint32_tDebug_Buffer[SIZE][2];

unsignedintDebug_Cnt=0;

voidDebug_FilteredDump(void)//dumpHappyandSad

if((Sad>100)&&(Debug_Cnt<SIZE))

Debug_Buffer[Debug_Cnt][0]=Happy;

Debug_Buffer[Debug_Cnt][1]=Sad;

Debug_Cnt++;

Program3.13.Instrumentationdumpwithfilter.7.MonitorusingtheLEDheartbeat.Anothertoolthatworkswellforreal-timeapplicationsisthemonitor.Amonitorisanindependentoutputprocess,somewhatsimilartotheprintstatement,butonethatexecutesmuchfasterandthusismuchlessintrusive.TheOLEDorLCDcanbeaneffectivemonitorforsmallamountsofinformationifthetimebetweenoutputsismuchlargerthanthetimetooutput.AnotherpopularmonitoristheLED.YoucanplaceoneormoreLEDsonindividualotherwiseunusedoutputbits.SoftwaretogglestheseLEDstoletyouknowwhatpartsoftheprogramarerunning.AnLEDisanexampleofaBooleanmonitororheartbeat.AssumeanLEDisattachedtoPortDbit1.Program3.14willtoggletheLED.

#definePD1(*((volatileuint32_t*)0x40007008))

#defineDebug_HeartBeat()(PD1^=0x02)

Program3.14.AnLEDmonitor.Next,youadd Debug_HeartBeat(); statementsatstrategicplaceswithinthesystem.PortDmustbeinitializedsothatbit1isanoutputbeforethedebuggingbegins.YoucaneitherobservetheLEDdirectlyorlookattheLEDcontrolsignalswithahigh-speedoscilloscopeorlogicanalyzer.WhenusingLEDmonitorsitisbettertomodifyjusttheonebit,leavingtheother7asis.Inthisway,youcanhavemultiplemonitorsononeport.

Checkpoint3.18:WriteadebugginginstrumentthattogglesPortAbit3.

3.9.4.PerformanceDebuggingPerformancedebugginginvolvestheverificationoftimingbehaviorofoursystem.Itisadynamicprocesswherethesystemisrun,andthedynamicbehaviorofthesystemiscomparedagainsttheexpectedresults.Wewillpresentthreemethodsofperformancedebugging,thenapplythetechniquestomeasureexecutionspeed.

1.Countingbuscycles.Forsimpleprogramswithlittleandnobranchingandforsimplemicrocontrollers,wecanestimatetheexecutionspeedbylookingattheassemblycodeandaddingupthetimetoexecuteeachinstruction.

2.Instrumentationmeasuringwithanindependentcounter.SysTickisa24-bitcounterdecrementedeverybusclock.Itautomaticallyrollsoverwhenitgetsto0.Ifwearesuretheexecutionspeedofourfunctionislessthan224buscycles,wecanusethistimertocollecttiminginformationwithonlyaminimalamountofintrusiveness.

3.InstrumentationOutputPort.Anothermethodtomeasurereal-timeexecutioninvolvesanoutputportandanoscilloscope.Connectamicrocontrolleroutputbittoyourscope.Adddebugginginstrumentsthatset/cleartheseoutputbitsatstrategicplaces.Remembertosettheport’sdirectionregisterto1.AssumeanoscilloscopeisattachedtoPortDbit1.Program3.15canbeusedtosetandclearthebit.

#definePD1(*((volatileuint32_t*)0x40007008))

#defineDebug_Set()(PD1=0x02)

#defineDebug_Clear()(PD1=0x00)

Program3.15.Instrumentationoutputport.Next,youadd Debug_Set(); and Debug_Clear(); statementsbeforeandafterthecodeyouwishtomeasure.PortDmustbeinitializedsothatbit1isanoutputbeforethedebuggingbegins.Youcanobservethesignalwithahigh-speedoscilloscopeorlogicanalyzer.

Debug_Set();

Stuff();//Usercodetobemeasured

Debug_Clear();

Toillustratethesethreemethods,wewillconsidermeasuringtheexecutiontimeofanintegersquarerootfunctionaspresentedProgram3.16.

Thefirstmethodistocountbuscyclesusingtheassemblylisting.Thisapproachisonlyappropriateforveryshortprograms,andbecomesdifficultforlongprogramswithmanyconditionalbranchinstructions.ThetimetoexecuteeachassemblyinstructioncanbefoundintheCortexTM-MTechnicalReferenceManual.BecauseofthecomplexityoftheARM ® CortexTM-M,thismethodisonlyapproximate.Forexamplethetimetoexecuteadividedependsonthedata,andthetimetoexecuteabranchdependsonthealignmentoftheinstructionpipeline.AportionoftheassemblyoutputgeneratedbytheARMKeilTMuVision®compilerispresentedontheleftsideofProgram3.16.Noticethatthetotalcyclecountforcouldrangefrom155to353cycles.At8MHztheexecutiontimecouldrangefrom19.4to44.1µs.Formostprogramsitisactuallyverydifficulttogetanaccuratetimemeasurementusingthistechnique.

sqrtMOVr1,r0[1]

MOVSr3,#0x01[1]

ADDr0,r3,r1,LSR#4[1]

MOVSr2,#0x10[1]

Bchck[2-4]

loopMLAr3,r0,r0,r1[2]*16

UDIVr3,r3,r0[2-12]*16

LSRSr0,r3,#1[1]*16

SUBSr2,r2,#1[1]*16

chckCMPr2,#0x00[1]*17

BNEloop[2-4]*17

BXlr[2-4]

//Newton’smethod

//sisaninteger

//sqrt(s)isaninteger

uint32_tsqrt(uint32_ts)

uint32_tt;//t*tbecomess

intn;//loopcounter

t=s/10+1;//initialguess

for(n=16;n;—n)//willfinish

t=((t*t+s)/t)/2;

returnt;

Program3.16.AssemblylistingandCcodeforasqrtfunction.

ThesecondmethodusesaninternaltimercalledSysTick.TheARM ® CortexTM-Mmicrocontrollersprovidethe24-bitSysTickregister( NVIC_ST_CURRENT_R )thatisautomaticallydecrementedatthebusfrequency.Whenthecounterhitszero,itisreloadedto0xFFFFFFandcontinuestocountdown.Ifwearesurethefunctionwillcompleteinatimelessthan224buscycles,thentheinternaltimercanbeusedtomeasureexecutionspeedempirically.ThecodeinProgram3.17firstreadstheSysTickcounter,executesthefunction,andthenreadstheSysTickcounteragain.Theelapsedtimeisthedifferenceinthecounterbeforeandafter.Sincetheexecutionspeedmaybedependentontheinputdata,itisoftenwisetomeasuretheexecutionspeedforawiderangeofinputparameters.Thereisaslightoverheadinthemeasurementprocessitself.Tobeaccurate,youcouldmeasurethisoverheadandsubtractitoffyourmeasurements.Inthiscase,aconstant7issubtractedsothatifthecalltothefunctionwerecompletelyremovedtheelapsedtimewouldreturn0.Noticethatinthisexample,thetotaltimeincludingparameterpassingismeasured.Experimentalresultsshowthisfunctionexecutesin238buscycles.At8MHz,thiscorrespondsto29.8µs.

uint32_tBefore,Elapsed;

voidmain(void)volatileuint32_tOut;

SysTick_Init();//Program2.11

Before=NVIC_ST_CURRENT_R;

Out=sqrt(1000);

Elapsed=(Before-NVIC_ST_CURRENT_R–7)&0x00FFFFFF;

Program3.17:Empiricalmeasurementofdynamicefficiency.Thethirdtechniquecanbeusedinsituationswhereatimerisunavailableorwheretheexecutiontimemightbelargerthan224counts.Inthisempiricaltechniqueweattachanunusedoutputpintoanoscilloscopeortoalogicanalyzer.Wewillsetthepinhighbeforethecalltothefunctionandsetthepinlowafterthefunctioncall.Inthiswayapulseiscreatedonthedigitaloutputwithadurationequaltotheexecutiontimeofthefunction.WeassumePortDisavailable,andbit0isconnectedtothescope.Byplacingthefunctioncallinaloop,thescopecanbetriggered.Withastoragescopeorlogicanalyzer,thefunctionneedbecalledonlyonce.Togetherwithanoscilloscopeorlogicanalyzer,Program3.18measurestheexecutiontimeofthefunction sqrt (Figure3.18).Westabilizethesystembycallingitoverandover.Usingthescope,wecanmeasurethewidthofthepulseonPD1,whichwillbeexecutiontimeofthefunction sqrt .Runningat8MHz,theresultsinFigure3.18showittakes30.5 stoexecute sqrt(1000) ,whichis244buscycles.

intmain(void)uint32_tOut;

PortD_Init();//Program2.6

while(1)

Debug_Set();//Program3.15

Out=sqrt(1000);

Debug_Clear();//Program3.15

Program3.18.Anotherempiricalmeasurementofdynamicefficiency.

Figure3.18.LogicanalyzeroutputmeasuredfromProgram3.18usinganUSBeeAX.Checkpoint3.19:IfyouweretoremovetheOut=sqrt(1000);lineinProgram3.18,whatwouldyouexpectthepulsewidthonPD0tobe?WhydoesProgram3.17yieldaresultsmallerthanProgram3.18?

3.9.5.ProfilingProfilingisatypeofperformancedebuggingthatcollectsthetimehistoryofprogramexecution.Profilingmeasureswhereandwhenoursoftwareexecutes.Itcouldalsoincludewhatdataisbeingprocessed.Forexampleifwecouldcollectthetime-dependentbehavioroftheprogramcounter,thenwecouldseetheexecutionpatternsofoursoftware.Wecanprofiletheexecutionofamultiplethreadsoftwaresystemtodetectreentrantactivity.

Profilingusingasoftwaredumptostudyexecutionpattern.Inthissection,wewilldiscusssoftwareinstrumentsthatstudytheexecutionpatternofoursoftware.Inordertocollectinformationconcerningexecutionwewilladddebugginginstrumentsthatsavethetimeandlocationinarrays(Program3.19).Byobservingthesedata,wecandeterminebothatimeprofile(when)andanexecutionprofile(where)ofthesoftwareexecution.Runningthisprofilerevealedthesequenceofplacesas0,1,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,and3.Eachcallto Debug_Profilerequires32cyclestoexecute.Therefore,thisinstrumentisalotlessintrusivethanaprintstatement.

uint32_tDebug_time[20];

uint8_tDebug_place[20];

uint32_tn;

voidDebug_Profile(uint8_tp)

if(n<20)

Debug_time[n]=NVIC_ST_CURRENT_R;//recordcurrenttime

Debug_place[n]=p;

n++;

uint32_tsqrt(uint32_ts)

uint32_tt;//t*tbecomess

intn;//loopcounter

Debug_Profile(0);

t=s/10+1;//initialguess

Debug_Profile(1);

for(n=16;n;—n)//willfinish

Debug_Profile(2);

t=((t*t+s)/t)/2;

Debug_Profile(3);

returnt;

Program3.19:Atime/positionprofiledumpingintoadataarray.

ProfilingusinganOutputPort.Inthissection,wewilldiscussahardware/softwarecombinationtovisualizeprogramactivity.OurdebugginginstrumentwillsetoutputportbitsD3–D0(Program3.20).Wewillplacetheseinstrumentsatstrategicplacesinthesoftware.Inparticular,wewilloutput1,2,4,or8toPortD,whereeachbituniquelyspecifieswhereintheprogramweareexecuting(Figure3.19).Weconnectthefouroutputpinstoalogicanalyzerandobservetheprogramactivity.Eachdebugginginstrumentrequiresonly4cyclestoexecute.SotheprofileinProgram3.20islessintrusivethantheoneinProgram3.19.

#definePROFILE(*((volatileuint32_t*)0x4000703C))

uint32_tsqrt(uint32_ts)

uint32_tt;//t*tbecomess

intn;//loopcounter

PROFILE=1;

t=s/10+1;//initialguess

PROFILE=2;

for(n=16;n;—n)//willfinish

PROFILE=4;

t=((t*t+s)/t)/2;

PROFILE=8;

PROFILE=0;

returnt;

Program3.20:Atime/positionprofileusingfouroutputbits.

Figure3.19.LogicanalyzeroutputmeasuredfromProgram3.20usinganUSBeeAX.ThreadProfile.Whenmorethanoneprogram(multiplethreads)isrunning,youcouldusethetechniqueinProgram3.20tovisualizethethreadthatiscurrentlyactive(theonerunning).Foreachthread,weassignoneoutputpin.Thedebugginginstrumentwouldsetthecorrespondingbithighwhenthethreadstartsandclearthebitwhenthethreadstops.Wewouldthenconnecttheoutputpinstoalogicanalyzertovisualizeinrealtimethethreadthatiscurrentlyrunning.

3.10.Exercises3.1List3factorsthatwecanusetoevaluatethe“quality”ofaprogram.

3.2In32wordsorless,describethemeaningofeachofthefollowingterms.

a)Dynamicefficiencyb)Staticefficiencyc)Scoped)Cohesion

3.3Considerthereasonswhyonechooseswhichtechniquetocreateavariable.

a)Listthreereasonswhyonewouldimplementavariableusingaregister.

b)ListthreereasonswhyonewouldimplementavariableonthestackandaccessitusingSPindexedmodeaddressing.

c)ListthreereasonswhyonewouldimplementavariableinRAMandaccessitusingextendedmodeaddressing.

3.4In32wordsorless,giveanexampleofeachofthefollowingterms.

a)Invocationcouplingb)Bandwidthcouplingc)Controlcoupling3.5Foreachtermspecifyifitisaconstantoravariable.Forthevariablesspecifyifitispermanentortemporaryandifitispublicorprivate.

a)FIFO_SIZE

b)Fifo_Size

c)FifoSize

d)fifoSize

3.6In32wordsorless,giveanexampleofeachofthefollowingterms.

a)Logicalcohesionb)Temporalcohesionc)Proceduralcohesiond)Communicationalcohesione)Sequentialcohesionf)Functionalcohesion

3.7Describehowyoucouldcreateanarraythatistemporaryinallocationbutpublicinscope.

3.8In32wordsorless,explainthedifferencesbetweenaMealyandMooreFSM.ForwhichtypesofproblemsshouldyouimplementwithMealy?ForwhichtypesofproblemsshouldyouimplementwithMoore?

3.9In32wordsorless,describethemeaningofeachofthefollowingterms.

a)Module

b)BSPc)HALd)Devicedriver

3.10Giveaquantitativemeasureofmodularity.E.g.,systemAismoremodularthansystemBif…

3.11In32wordsorless,describethemeaningofeachofthefollowingdebuggingterms.

a)Profileb)Intrusivec)Stabilized)Heartbeate)Monitor

f)Dump

g)Logicanalyzerh)Filter

D3.12WritesoftwaretoimplementtheMooreFSMshowninFigure3.20.IncludetheFSMstatemachine,portinitialization,timerinitializationandtheFSMcontroller.Thecommandsequencewillbeoutput,waitthespecifiedtimeinms,input,thenbranchtonextstate.The2-bitinputisonPortG(PG1andPG0)andthe3-bitoutputisalsoonPortG(PG7,PG6,PG5).

Figure3.20.FSMforExercise3.12.D3.13WritesoftwaretoimplementtheMealyFSMshowninFigure3.21.IncludetheFSMstatemachine,portinitialization,timerinitializationandtheFSMcontroller.Thecommandsequencewillbeinput,output,wait10ms,input,andthenbranchtonextstate.The1-bitinputisonPortG(PG0)andthe3-bitoutputisonPortF(PF3,PF2,PF1).

Figure3.21.FSMforExercise3.13.D3.14Rewritethe SysTick_Wait functioninProgram2.11,sothatitcontinuouslychecksanalarminputonPA7.AslongasPA7islow(normal),itwillwaittheprescribedtime.But,ifPA7goeshigh(alarm),thewaitfunctionreturns.

3.11.LabAssignmentsLab3.1Theoverallobjectiveistocreatea4-keydigitallock.Thesystemhasfourdigitalinputsandonedigitaloutput.TheLEDwillbeinitiallyon,signifyingthedoorislocked.Definetwoseparatekeycodes,onetolockandonetounlockthedoor.Forexample,ifthekeysarenumbers1,2,3and4,onepossiblekeycodeis23.Thismeansifyoupushboththe2and3keys(notpushingthe1,4keys)thedoorwillunlock.Implementthedesignsuchthattheunlockfunctionoccursinthesoftwareofthemicrocontroller.

Lab3.2Theoverallobjectiveistocreatealinetrackingrobot.Thesystemhastwodigitalinputsandtwodigitaloutputs.YoucansimulatethesystemwithtwoswitchesandtwoLEDs,orbuildarobotwithtwoDCmotorsandtwoopticalreflectancesensors.Bothsensorinputswillbeonifthemachineiscompletelyontheline.Onesensorinputwillbeonandtheotheroffifthemachineisjustgoingoffthetrack.Ifthemachineistotallyofftheline,thenbothsensorinputswillbeoff.Implementthecontrollerusingafinitestatemachine.ChooseaMooreorMealyformatasappropriate.

Lab3.3Theoverallobjectiveistocreateanenhancedtrafficlightcontroller.Thesystemhasthreedigitalinputsandsevendigitaloutputs.YoucansimulatethesystemwiththreeswitchesandsevenLEDs.TheinputsareNorth,East,andWalk.Theoutputsaresixforthetrafficlightandoneforawalksignal.Implementthecontrollerusingafinitestatemachine.ChooseaMooreorMealyformatasappropriate.

Lab3.4Theoverallobjectiveistocreatean8-keydigitallock.Thesystemhaseightdigitalinputsandonedigitaloutput.TheLEDwillbeinitiallyoff,signifyingthedoorislocked.Defineakeysequencetounlockthedoor.Forexample,ifthekeysarenumbers1,2,…and8,onepossiblekeycodeis556.Thismeansifyoupushthe5,releasethe5,pushthe5,releasethe5andpushthe6,thenthedoorwillunlock.TheunlockoperationwillbeatwosecondpulseontheLED.

Lab3.5Theoverallobjectiveistodesignavendingmachinecontroller.Thesystemhasfivedigitalinputsandthreedigitaloutputs.YoucansimulatethesystemwithfiveswitchesandthreeLEDs.Theinputsare quarter , dime , nickel , soda ,and diet .The quarter inputwillgohigh,thengolowwhena25¢coinisaddedtothemachine.The dime and nickel inputsworkinasimilarmannerforthe10¢and5¢coins.Thesodascost35¢each.Theuserpressesthe soda buttontoselectaregularsodaandthe diet buttontoselectadietsoda.The GiveSoda outputwillreleasearegularsodaifpulsedhigh,thenlow.Similarly,the GiveDiet outputwillreleaseadietsodaifpulsedhigh,thenlow.The Change outputwillreleasea5¢coinifpulsedhigh,thenlow.Implementthecontrollerusingafinitestatemachine.ChooseaMooreorMealyformatasappropriate.Sincetherearesomanyinputsandatmostoneisactiveatatime,youmaywishtoimplementaFSMwithadifferentformatthantheexamplesinthebook.

4.Hardware-SoftwareSynchronizationChapter4objectivesareto:•IntroducebasicperformancemeasuresforI/Ointerfacing

•Formalizetimingusingequationsandgraphicaldiagrams

•PresentPetriNetsasawaytodescribesynchronization

•IntroduceKahnProcessNetworks(KPN)todescribedistributedsystems

•Compareandcontrastpossibleapproachestosynchronization

•Presentthebasichardware/softwareforaparallelportLCDinterface

•Interfaceasteppermotorusingblind-cyclesynchronization

•Discussthebasicconceptsofbusy-waitsynchronization

•Introducethegeneralconceptofahandshakeinterface

•Implementabusy-waitserialportdevicedriver

•Interfaceakeypadusingbusy-waitsynchronization

Oneofthefactorsthatmakeembeddedsystemsdifferentfromregularcomputersisthespecializedinput/outputdevicesweattachtoourembeddedsystems.Whiletheentirebookaddressesthedesignandanalysisofembeddedsystems,thischapterservesasanintroductiontothecriticaltaskofI/Ointerfacing.Interfacingincludesboththephysicalconnectionsofthehardwaredevicesandthesoftwareroutinesthataffectinformationexchange.Keytothistaskistheneedtosynchronizethesoftwareandhardwarecomponents.Thechapterbeginswithperformancemeasurestoevaluatetheeffectivenessofoursystem(latency,bandwidth,priority).Asengineerswearenotsimplyaskedtodesignandbuilddevices,butwearealsorequiredtoevaluateourproducts.Latencyandbandwidtharetwoquantitativeperformanceparameterswecanmeasureonourreal-timeembeddedsystem.Anumberofformaltoolswillbepresentedincludingtimingequations,timingdiagrams,PetriNets,andKahnProcessNetworks(KPN).Theedge-triggeredI/OportsontheLM3S/LM4F/TM4Cmicrocontrollerswillbepresented.Next,fivebasicapproachestohardware/softwaresynchronizationarepresented,whichincludeblindcycle,busywait,interrupts,periodicpolling,anddirectmemoryaccess.AmorethoroughtreatmentofinterruptswillbepresentedinChapter5.However,thediscussionsinthischapterwillpointtosituationsthatrequireinterruptsynchronization.Therestofchapterpresentssimpleexamplestoillustratethe“blind-cycle”and“busy-wait”approachestointerfacing.

4.1.Introduction

4.1.1.PerformanceMeasuresLatencyisthetimebetweenwhentheI/Odeviceindicatedserviceisrequiredandwhenserviceisinitiated.Latencyincludeshardwaredelaysinthedigitalgatespluscomputerhardwaredelays.Latencyalsoincludessoftwaredelays.Foraninputdevice,softwarelatency(orsoftwareresponsetime)isthetimebetweennewinputdatareadyandthesoftwarereadingthedata.Foranoutputdevice,latencyisthedelayfromoutputdeviceidleandthesoftwaregivingthedevicenewdatatooutput.Inthisbook,wewillalsohaveperiodicevents.Forexample,inourdataacquisitionsystems,wewishtoinvoketheADCatafixedtimeinterval.Inthiswaywecancollectasequenceofdigitalvaluesthatapproximatethecontinuousanalogsignal.SoftwarelatencyinthiscaseisthetimebetweenwhentheADCissupposedtobestartedandwhenitisactuallystarted.Themicrocontroller-basedcontrolsystemalsoemploysperiodicsoftwareprocessing.Similartothedataacquisitionsystem,thelatencyinacontrolsystemisthetimebetweenwhenthecontrolsoftwareissupposedtoberunandwhenitisactuallyrun.Areal-timesystemisonethatcanguaranteeaworst-caselatency.Inotherwords,thereisanupperboundonthesoftwareresponsetime.Throughputorbandwidthisthemaximumdataflow(bytespersecond)thatcanbeprocessedbythesystem.SometimesthebandwidthislimitedbytheI/Odevice,whileothertimesitislimitedbycomputersoftware.Bandwidthcanbereportedasanoverallaverageorashort-termmaximum.Prioritydeterminestheorderofservicewhentwoormorerequestsaremadesimultaneously.Priorityalsodeterminesifahigh-priorityrequestshouldbeallowedtosuspendalow-priorityrequestthatiscurrentlybeingprocessed.Wemayalsowishtoimplementequalprioritysothatnoonedevicecanmonopolizethecomputer.Insomecomputerliterature,thetermsoftreal-timeisusedtodescribeasystemthatsupportspriority.

4.1.2.SynchronizingthesoftwarewiththestateoftheI/OOnecanthinkofthehardwareasbeinginoneofthreestates.Theidlestateoccurswhenthedeviceisdisabledorinactive.NoI/Ooccursintheidlestate.Whenactive(notidle),thehardwaretogglesbetweenthebusyanddonestates,asillustratedinFigure4.1.Foraninputdevice,astatusflagissetwhennewinputdataareavailable.Thebusy-to-donestatetransitionwillcauseabusy-waitloop(gadflyloop)tocomplete.Thisbusy-to-donetransitioncouldalsotriggeraninterrupt.Oncethesoftwarerecognizesthattheinputdevicehasnewdata,itwillreadthedataandasktheinputdevicetocreatemoredata.Itisthebusy-to-donestatetransitionthatsignalstothecomputerthatserviceisrequired.Whenthehardwareisinthedonestate,theI/Otransactioniscomplete.Oftenthesimpleprocessofreadingthedatawillcleartheflagandrequestanotherinput.

Foranoutputdevice,astatusflagissetwhentheoutputisidleandreadytoacceptmoredata.The“busytodone”statetransitioncausesabusy-waitlooptocomplete.Oncethesoftwarerecognizestheoutputisidle,itgivestheoutputdeviceanotherpieceofdatatooutput.Itwillbeimportanttomakesurethesoftwareclearstheflageachtimenewoutputisstarted.

Figure4.1.Initiallythedeviceisidle.Whileactive,itmaybeinthebusyordonestate.Theinputdevicesetsaflagwhenithasnewdata.Theoutputdevicesetsaflagwhenithasfinishedoutputtingthelastdata.TheproblemwithI/Odevicesisthattheyareusuallymuchslowerthansoftwareexecution.Therefore,weneedsynchronization,whichistheprocessofthehardwareandsoftwarewaitingforeachotherinamannersuchthatdataisproperlytransmitted.Awaytovisualizethissynchronizationistodrawastateversustimeplotoftheactivitiesofthehardwareandsoftware.Foraninputdevice,thesoftwarebeginsbywaitingfornewinput(Figure4.2).Whentheinputdeviceisbusy,itisintheprocessofcreatingnewinput.Whentheinputdeviceisdone,newdataareavailable.Whentheinputdevicemakesthetransitionfrombusytodone,itreleasesthesoftwaretogoforward.Inasimilarway,whenthesoftwareacceptstheinput,itcanreleasetheinputdevicehardware.Thearrowsfromonegraphtotheotherrepresentthesynchronizingevents.Inthisexample,thetimeforthesoftwaretoreadandprocessthedataislessthanthetimefortheinputdevicetocreatenewinput.ThissituationiscalledI/Obound.

Figure4.2.Thesoftwaremustwaitfortheinputdevicetobeready.Iftheinputdevicewerefasterthanthesoftware,asituationcalledCPUbound,thenthesoftwarewaitingtimewouldbezero.Ingeneral,thebandwidthdependsonboththehardwareandthesoftware.Inagivensystem,iftheproducerandconsumerratesvary,thesystemmayoscillatebetweenI/OboundandCPUbound.

Thisconfigurationisalsolabeledasunbufferedbecausethehardwareandsoftwaremustwaitforeachotherduringthetransmissionofeachpieceofdata.Abufferedsystemallowstheinputdevicetoruncontinuously,fillingaFIFOasfastasitcan.Inthesameway,thesoftwarecanemptytheFIFOwheneveritisreadyandwheneverthereisdatainthebuffer.WewillimplementbufferedinterfacesinChapter5usinginterrupts.

Figure4.3containsastateversustimeplotoftheactivitiesoftheoutputdevicehardwareandsoftware.Foranoutputdevice,thesoftwarebeginsbygeneratingdatathensendingittotheoutputdevice.Whentheoutputdeviceisbusy,itisprocessingthedata.Normallywhenthesoftwarewritesdatatoanoutputport,thatonlystartstheoutputprocess.Thetimeittakesanoutputdevicetoprocessdataisusuallylongerthanthesoftwareexecutiontime.Whentheoutputdeviceisdone,itisreadyfornewdata.Whentheoutputdevicemakesthetransitionfrombusytodone,itreleasesthesoftwaretogoforward.Inasimilarway,whenthesoftwarewritesdatatotheoutput,itreleasestheoutputdevicehardware.TheoutputinterfaceillustratedinFigure4.3isalsoI/Oboundbecausethetimefortheoutputdevicetoprocessdataislongerthanthetimeforthesoftwaretogenerateandwriteit.

Figure4.3.Thesoftwaremustwaitfortheoutputdevicetofinishthepreviousoperation.Thisoutputinterfaceisalsounbuffered,becausewhenthehardwareisdone,itwillwaitforthesoftware,andafterthesoftwaregeneratesdata,itwaitsforthehardware.Abufferedsystemwouldallowthesoftwaretoruncontinuously,fillingaFIFOasfastasitwishes.Inthesameway,thehardwarecanemptytheFIFOwheneveritisreadyandwheneverthereisdatainthebuffer.WewillimplementbufferedinterfacesinChapter5usinginterrupts.

ThepurposeofourinterfaceistoallowthemicroprocessortointeractwithitsexternalI/Odevice.TherearefivemechanismstosynchronizethemicroprocessorwiththeI/Odevice.EachmechanismsynchronizestheI/Odatatransfertothebusytodonetransition.Thefivemethodsarediscussedinthefollowingparagraphs.

BlindcycleisamethodwherethesoftwaresimplywaitsafixedamountoftimeandassumestheI/Owillcompleteafterthatfixeddelay.Foraninputdevice,thesoftwaretriggers(starts)theexternalinputhardware,waitaspecifiedtime,thenreadsdatafromthedevice(leftsideofFigure4.4.)Foranoutputdevice,thesoftwarewritesdatatotheoutputdevice,triggers(starts)thedevice,thenwaitsaspecifiedtime(leftsideofFigure4.5.)Wecallthismethodblind,becausethereisnostatusinformationabouttheI/Odevicereportedtothesoftware.ThismethodisappropriateforsituationswheretheI/Ospeedisshortandpredictable.

Figure4.4.Flowchartsshowinganinputinterfaceusingblind-cycle,busywaitandinterrupts.

Figure4.5.Flowchartsshowinganoutputinterfaceusingblind-cycle,busywaitandinterrupts.BusywaitorgadflyisasoftwareloopthatcheckstheI/Ostatuswaitingforthedonestate.Foraninputdevice,thesoftwarewaitsuntiltheinputdevicehasnewdata,andthenreadsitfromtheinputdevice(middleofFigure4.4.)Foranoutputdevice,thesoftwarewritesdata,triggerstheoutputdevicethenwaitsuntilthedeviceisfinished.Anotherapproachtooutputdeviceinterfacingisforthesoftwaretowaituntiltheoutputdevicehasfinishedthepreviousoutput,writedata,andthentriggerthedevice(middleofFigure4.5.)Wewilldiscussthesetwoapproachestooutputdeviceinterfacinglaterinthechapter.Busy-waitsynchronizationwillbeusedinsituationswherethesoftwaresystemisrelativelysimpleandreal-timeresponseisnotimportant.

Aninterruptuseshardwaretocausespecialsoftwareexecution.Withaninputdevice,thehardwarewillrequestaninterruptwheninputdevicehasnewdata.Thesoftwareinterruptservicewillreadfromtheinputdeviceandsaveinaglobalstructure(rightsideofFigure4.4).Withanoutputdevice,thehardwarewillrequestaninterruptwhentheoutputdeviceisidle.Thesoftwareinterruptservicewillgetdatafromaglobalstructure,andthenwritetothedevice(rightsideofFigure4.5).

Sometimesweconfigurethehardwaretimertorequestinterruptsonaperiodicbasis.Thesoftwareinterruptservicewillperformaspecialfunction.AdataacquisitionsystemneedstoreadtheADCataregularrate.DetailsofdataacquisitionsystemscanbefoundinChapter10.Somecomputerscanbeconfiguredtorequestaninterruptonanaccesstoanillegaladdressoradividebyzero.Interruptsynchronizationwillbeusedinsituationswherethesoftwaresystemisfairlycomplexorwhenreal-timeresponseisimportant.

PeriodicpollingusesaclockinterrupttoperiodicallychecktheI/Ostatus.Withaninputdevice,areadyflagissetwhentheinputdevicehasnewdata.Atthenextperiodicinterrupt,thesoftwarewillreadthedataandsavetheminaglobalstructure.Withanoutputdevice,areadyflagissetwhentheoutputdeviceisidle.Atthenextperiodicinterrupt,thesoftwarewillgetdatafromaglobalstructureandwriteit.Periodicpollingwillbeusedinsituationsthatrequireinterrupts,buttheI/Odevicedoesnotsupportinterruptrequests.

DMA,ordirectmemoryaccessisaninterfacingapproachthattransfersdatadirectlyto/frommemory.Withaninputdevice,thehardwarewillrequestaDMAtransferwhentheinputdevicehasnewdata.Withoutthesoftware’sknowledgeorpermissiontheDMAcontrollerwillreadfromtheinputdeviceandsaveinmemory.Withanoutputdevice,thehardwarewillrequestaDMAtransferwhentheoutputdeviceisidle.TheDMAcontrollerwillgetdatafrommemory,andthenwritetothedevice.SometimesweconfigurethehardwaretimertorequestDMAtransfersonaperiodicbasis.DMAcanbeusedtoimplementahigh-speeddataacquisitionsystem.DMAsynchronizationwillbeusedwhenbandwidthandlatencyareimportant.

4.1.3.VarietyofAvailableI/OPortsMicrocontrollersperformdigitalI/Ousingtheirports.Inthischapterwewillfocusontheinputandoutputofdigitalsignals.Microcontrollershaveawidevarietyofconfigurations,onlyfewofwhichareillustratedinTable4.1.EachmicrocontrollermanufacturerhasmultiplefamiliesconsistingofawiderangeofpartswithvaryingnumbersandtypesofI/Odevices.Forexample,theTM4C123GH6ZRBhas120GPIOpins.ItistypicalforportpinstobeprogrammedviasoftwareforalternativefunctionsotherthanparallelI/O.Eachmicrocontrollerfamilycomeswithamechanismtodownloadcodeanddebug.The9S12familyincludesabackgrounddebugmodule.TheTexasInstrumentsMSP430,Stellaris ®andTiva ® familiesincludeeitheraJTagorSpy-Bi-Wiredebugginginterface.TheMicrochipPICfamilydebugsinavarietyofwaysincludingIn-CircuitSerialProgramming,In-CircuitEmulator,PICSTARTandPROMATE.TheAtmelfamilycanbedevelopedwithaJTagdebugger.Basically,wefirstchoosetheprocessortype(e.g.,PIC,MSP430,9S12,LM3S,orLM4F)dependingonoursoftwareprocessingneeds.Next,wechoosethefamilydependingonourI/Orequirements.Lastly,wechoosetheparticularpartdependingonourmemoryrequirements.

Portpins

Alternativefunctions.

PIC12F629 6 Verylowcost,ADC,timer

MSP430F2013 10 Verylowpower,ADC,SCI,SPI,I2C,andtimer

MC9S12C32 60 Serial,timer,ADC,SPI,CAN

AT91RM Upto122

ARMThumb,ADC,serial,DMA,USB,Ethernet,Smartcard

LM3S/LM4F/TM4C Upto120

Cortex-M,ADC,serial,DMA,USB,Ethernet,LCD,CAN,QEI

Table4.1.ThenumberofI/Oportsandalternativefunction.

Thecurrenttrendinthecomputerindustryiscustomerspecificintegratedcircuits(CSIC).Asimilartermforthisdevelopmentprocessisapplicationspecificintegratedcircuits(ASIC).Withtheseapproaches,thedesignengineers(customer)firstevaluatetheneedsoftheirproject.Thedesignengineersworkingcloselywiththecomputermanufacturermakealistoffeaturesthemicrocontrollerrequires.Forexample

CPUtypeCISC,RISC,multiplebuses,multiplecores

CoprocessorsFloatingpoint,DMA,graphics

MemoryRAM,EEPROM,FlashROM,OTPROM,ROM

PowerPLL,sleepstates,variablesupply,reducedoutputdrive

Analog8to16-bitADC,8to12-bitDAC,analogcomparators

TimerPulsewidthmodulation,Inputcapture,Outputcompare

ParallelPortsEdgetriggerinterrupts,pull-up,pull-down,opencollector

SerialAsynchronous(UART),synchronous(SPI),peripheral(I2C)

SoundIntegratedInterchipSound(I2S)

MotorQuadratureEncoderInterface(QEI)

NetworksUSB,CAN,Ethernet,wireless

Thedesignengineerseitherchooseamicrocontrollerfromexistingproductsthatmeetstheirneeds,ortheengineerscontractwiththemanufacturertoproduceamicrocontrollerwiththeexactspecificationsforthatproject.Manymanufacturersdistributestartercode,referencedesigns,orwhitepapersshowingcompleteimplementationsusingthatparticularmicrocontrollertosolveactualproblems.Theavailabilityofsuchsolutionswillbeextremelyhelpful,eveniftheapplicationsarejustremotelysimilartoyourproblem.

4.2.Timing

4.2.1.TimingEquationsWheninterfacingdevices,itisimportanttomanagewheneventsoccur.Typicaleventsincludetheriseorfallofcontrolsignals,whendatapinsneedtobecorrect,andwhendatapinsactuallycontainthepropervalues.Inthisbook,wewillusetwomechanismstodescribethetimingofevents.Inthissection,wepresentaformalsyntaxcalledtimingequations,whicharealgebraicmechanismstodescribetime.Inthenextsection,wewillpresentgraphicalmechanismscalledtimingdiagrams.

Whenusingatimingequation,weneedtodefineazero-timereference.Forsynchronoussystems,whicharesystemsbasedonaglobalclock,wecandefineoneedgeoftheclockastime=0.Timingequationscancontainnumberconstantstypicallygiveninns,variables,andedges.Forexample,↓AmeansthetimewhensignalAfalls,and↑Ameansthetimewhenitrises.Tospecifyanintervaloftime,wegiveitsstartandstoptimesbetweenparenthesesseparatedbyacomma.Forexample,(400,520)meansthetimeintervalbeginsat400nsandendsat520ns.Thesetwonumbersarerelativetothezero-timereference.

Wecanusealgebraicvariables,edges,andexpressionstodescribecomplexbehaviors.Sometimingintervalsarenotdependentonthezero-timereference.Forexample,(↑A-10,↑A+t)meansthetimeintervalbegins10nsbeforetherisingedgeofsignalAandendsattimetafterthatsamerisingedge.Sometimingvariablesweseefrequentlyindatasheetsinclude

tpdpropagationdelayfromachangeininputtoachangeinoutput

tpHLpropagationdelayfrominputtooutput,astheoutputgoesfromhightolow

tpLHpropagationdelayfrominputtooutput,astheoutputgoesfromlowtohigh

tpZLpropagationdelayfromcontroltooutput,astheoutputgoesfromfloatingtolow

tpZHpropagationdelayfromcontroltooutput,astheoutputgoesfromfloatingtohigh

tpLZpropagationdelayfromcontroltooutput,astheoutputgoesfromlowtofloating

tpHZpropagationdelayfromcontroltooutput,astheoutputgoesfromhightofloating

tenpropagationdelayfromfloatingtodriveneitherhighorlow,sameastpZLandtpZHtdispropagationdelayfromdrivenhigh/lowtofloating,sameastpLZandtpHZtsusetuptime,thetimebeforeaclockinputdatamustbevalid

thholdtime,thetimeafteraclockinputdatamustcontinuetobevalid

Sometimeswearenotquitesureexactlywhenaneventstartsorstops,butwecangiveupperandlowerbounds.Wewillusebracketstospecifythistiminguncertainty.Forexample,assumeweknowtheintervalstartssomewherebetween400and430ns,andstopssomewherebetween520and530ns,wewouldthenwrite([400,430],[520,530]).

Asexamples,wewillconsiderthetimingofanotgate,atristatedriver,andanoctalDflip-flop,asshowninFigure4.6.Iftheinputtothe74HC04islow,itsoutputwillbehigh.Conversely,iftheinputtothe74HC04ishigh,itsoutputwillbelow.Thereareeightdatainputstothe74HC244,labeledasA.ItseightdataoutputsarelabeledY.The74HC244tristatedriverhastwomodes.Whentheoutputenable,OE*,islow,theoutputYequalstheinputA.WhenOE*ishigh,theoutputYfloats,meaningitisnotdrivenhighorlow.Theslashwithan8overtopmeansthereareeightsignalsthatalloperateinasimilarorcombinedfashion.The74HC374octalDflip-flophaseightdatainputs(D)andeightdataoutputs(Q).ADflip-flipwillstoreorlatchitsDinputsontherisingedgeofitsClk.TheOE*signalonthe74HC374worksinamannersimilartothe74HC244.WhenOE*islow,thestoredvaluesintheflip-flopareavailableatitsQoutputs.WhenOE*ishigh,theQoutputsfloat.ThemakingOE*gohighorlowdoesnotchangetheinternalstoredvalues.OE*onlyaffectswhetherornotthestoredvaluesaredrivenontheQoutputs.

Positivelogicmeansthetrueorassertedstateisahighervoltagethanthefalseornotassertedstate.Negativelogicmeansthetrueorassertedstateisalowervoltagethanthefalseornotassertedstate.The*inthenameOE*meansnegativelogic.Othersyntaxstylesthatmeannegativelogicincludeaslashbeforethesymbol(e.g.,\OE),theletterninthename(OEn),oralineoverthetop(e.g., ).

Figure4.6.ANOTgate,atristatedriver,andanoctalDflip-flop.Wewillbeginwiththetimingofthe74HC04notgate.Thetypicalpropagationdelaytime(tpd)forthisnotgateis8ns.Consideringjustthetypicaldelay,wespecifytimewhenYrisesintermsofthetimewhenAfalls.Thatis

↑Y=↓A+tpd=↓A+8

Fromthe74HC04datasheet,weseethemaximumpropagationdelayis15ns,andnominimumisgiven.Sincethedelaycannotbenegative,wesettheminimumtozeroandwrite

↑Y=[↓A,↓A+15]=↓A+[0,15]

WespecifythetimeintervalwhenYishighas

(↑Y,↓Y)=([↓A,↓A+15],[↑A,↑A+15])=(↓A+[0,15],↑A+[0,15])

Whendataistransferredfromonelocation(thesource)andstoredintoanother(thedestination),therearetwotimeintervalsthatwilldetermineifthetransferwillbesuccessful.Thedataavailableintervalspecifieswhenthedatadrivenbythesourceisvalid.Thedatarequiredintervalspecifieswhenthedatatobestoredintothedestinationmustbevalid.Forasuccessfultransferthedataavailableintervalmustoverlap(startbeforeandendafter)thedatarequiredinterval.Leta,b,c,dbetimesrelativetothesamezero-timereference,letthedataavailableintervalbe(a,d),andletthedatarequiredintervalbe(b,c),asshowninFigure4.7.Thedatawillbesuccessfullytransferredif

a≤bandc≤d

Figure4.7.Thedataavailableintervalshouldoverlapthedatarequiredinterval.TheexampleshowninFigure4.8illustratesthefundamentalconceptoftimingforadigitalinterface.Theobjectiveistotransferthedatafromtheinput,In,totheoutput,Out.First,weassumethesignalattheIninputofthe74HC244isalwaysvalid.Whenthetristatecontrol,G*,islowthentheIniscopiedtotheBus.OntherisingedgeofC,the74HC374Dflip-flopwillcopythisdatatotheoutputOut.

Figure4.8.Simplecircuittoillustratethatthedataavailableintervalshouldoverlapthedatarequiredinterval.ThedataavailableintervaldefineswhenthesignalBuscontainsvaliddataandisdeterminedbythetimingofthe74HC244.Fromitsdatasheet,theoutputofthe74HC244isvalidbetween0and38nsafterthefallofG*.Itwillremainvaliduntil0to38nsaftertheriseofG*.Thedataavailableintervalis

DA=(↓G*+ten,↑G*+tdis)=(↓G*+[0,38],↑G*+[0,38])

Thedatarequiredintervalisdeterminedbythetimingofthe74HC374.The74HC374input,Bus,mustbevalidfrom25nsbeforetheriseofCandremainvaliduntil5nsafterthatsameriseofC.Thetimebeforetheclockthedatamustbevalidiscalledthesetuptime.Thesetuptimeforthe74HC374is25ns.Thetimeaftertheclockthedatamustcontinuetobevalidiscalledtheholdtime.Theholdtimeforthe74HC374is5ns.Thedatarequiredintervalis

DR=(↑C-tsu,↑C+th)=(↑C-25,↑C+5)

Sincetheobjectiveistomakethedataavailableintervaloverlapthedatarequiredwindow,theworstcasesituationwillbetheshortestdataavailableandthelongestdatarequiredintervals.Withoutlossofinformation,wecanwritetheshortestdataavailableintervalas

DA=(↓G*+38,↑G*)

Thusthedatawillbeproperlytransferredifthefollowingaretrue:

↓G*+38≤↑C-25and↑C+5≤↑G*

NoticeinFigure4.8,thesignalbetweenthe74HC244and74HC374islabeledBus.Abusisacollectionofsignalsthatfacilitatethetransferofinformationfromonepartofthecircuittoanother.Considerasystemwithmultiple74HC244’sandmultiple74HC374’s.TheYoutputsofallthe74HC244’sandtheDinputsofallthe74HC374’sareconnectedtothisbus.Ifthesystemwishedtotransferfrominput6tooutput5,itwouldclearG6*low,makeC5rise,andthensetG6*high.AtsomepointC5mustfall,buttheexacttimeisnotcritical.Oneoftheproblemswithsharedbuswillbebusarbitration,whichisamechanismtohandlesimultaneousrequests.

4.2.2.TimingDiagramsAnalternativemechanismfordescribingwheneventsoccurusesvoltageversustimegraphs,calledtimingdiagrams.Itisveryintuitivetodescribetimingeventsusinggraphsbecauseitiseasytovisuallysorteventsintotheirpropertimesequence.Figure4.9definesthesymbolswewillusetodrawtimingdiagramsinthisbook.Arrowswillbeaddedtodescribethecausalrelationsinourinterface.Numbersorvariablescanbeincludedthatdefinehowfaraparteventswillbeorshouldbe.Itisimportanttohaveitclearinourmindswhetherwearedrawinganinputoranoutputsignal,becausewhatasymbolmeansdependsonwhetherwearedrawingthetimingofaninputoranoutputsignal.Manydatasheetsusethetristatesymbolwhendrawinganinputsignaltomean“don’tcare”.

Figure4.9.Nomenclaturefordrawingtimingdiagrams.

Toillustratethegraphicalrelationshipofdynamicdigitalsignals,wewilldrawtimingdiagramsforthethreedevicespresentedinthelastsection,seeFigure4.10.Thearrowsinthe74HC04timingdiagramdescribethecausalbehavior.Iftheinputweretorise,thentheoutputwillfalltpHLtimelater.ThesubscriptHLreferstotheoutputchangingfromhightolow.Similarly,iftheinputweretofall,thentheoutputwillrisetpLHtimelater.

Figure4.10.TimingdiagramsforthecircuitsinFigure4.6.

Thearrowsinthe74HC244timingdiagramalsodescribethecausalbehavior.IftheinputAisvalidandiftheOE*weretofall,thentheoutputwillgofromfloatingtoproperlydrivententimelater.IftheOE*islowandiftheinputAweretochange,thentheoutputwillchangetpdtimelater.IftheOE*weretorise,thentheoutputwillgofromdriventofloatingtdistimelater.

TheparallellinesontheDtimingofthe74HC374meantheinputmustbevalid.“Mustbevalid”meanstheDinputcouldbehighorlow,butitmustbecorrectandnotchanging.Ingeneral,arrowsrepresentcausalrelationships(i.e.,“this”causes“that”).Hence,arrowsshouldbedrawnpointingtotheright,towardsincreasingtime.Thesetuptimearrowisanexceptiontothe“arrowspointtotheright”rule.Thesetuparrow(labeledwithtsu)defineshowlongbeforeanedgemusttheinputbestable.Theholdarrow(labeledwithth)defineshowlongafterthatsameedgetheinputmustcontinuetobestable.

Thetimingofthe74HC244mimicsthebehaviorofdevicesonthecomputerbusduringareadcycle,andthetimingofthe74HC374clockmimicsthebehaviorofdevicesduringawritecycle.Figure4.11showsthetimingdiagramfortheinterfaceproblempresentedinFigure4.8.AgainweassumetheinputInisvalidatalltimes.Thedataavailable(DA)anddatarequired(DR)intervalsrefertodataontheBus.Inthistimingdiagram,weseegraphicallythesamedesignconstraintdevelopedwithtimingequations.↓G*+38mustbelessthanorequalto↑C‑25and↑C+5 mustbelessthanorequalto↑G*.Oneoftheconfusingpartsaboutatimingdiagramisthatitcontainsmoreinformationthanactuallymatters.Forexample,noticethatthefallofCisdrawnbeforetheriseofG*.Inthisinterface,therelativetimingof↑G*and↓Cdoesnotmatter.However,wedraw↓CsothatwecanspecifythewidthoftheCpulsemustbeatleast20ns.

Figure4.11.TimingdiagramoftheinterfaceshowninFigure4.8.

4.3.PetriNetsInthelastchapter,wepresentedfinitestatemachinesasaformalmechanismtodescribesystemswithinputsandoutputs.Inthischapter,wepresenttwomethodstodescribesynchronizationincomplexsystems:PetriNetsandKahnProcessNetworks.PetriNetscanbeusedtostudythedynamicconcurrentbehaviorofnetwork-basedsystemswherethereisdiscreteflow,suchaspacketsofdata.APetriNetiscomprisedofPlaces,Transition,andArcs.Places,drawnascirclesinFigure4.12,cancontainzero,one,ormoretokens.Considerplacesasvariables(orbuffers)andtokensasdiscretepacketsofdata.Tokensaredrawninthenetasdotswitheachdotrepresentingonetoken.Formally,thetokensneednotcomprisedataandcouldsimplyrepresenttheexistenceofanevent.Transitions,drawnasverticalbars,representsynchronizingactions.Considertransitionsassoftwarethatperformsworkforthesystem.Fromaformalperspective,aPetriNetdoesnotmodeltimedelay.But,fromapracticalviewpointweknowexecutingsoftwaremustconsumetime.Thearcs,drawnasarrows,connectplacestotransitions.Anarcfromaplacetoatransitionisaninputtothetransition,andanarcfromatransitiontoaplaceisanoutputofthetransition.

Figure4.12.PetriNetsarebuiltwithplaces,transitionsandarcs.Placescanholdtokens.Forexample,aninputswitchcouldbemodeledasadevicethatinsertstokensintoaplace.Thenumberoftokenswouldthenrepresentthenumberoftimestheswitchhasbeenpressed.Analphanumerickeyboardcouldalsobemodeledasaninputdevicethatinsertstokensintoaplace.However,wemightwishtoassignanASCIIstringtothetokengeneratedbyakeyboarddevice.AnoutputdeviceinaPetriNetcouldbemodeledasatransitionwithonlyinputarcsbutnooutputarcs.Anoutputdeviceconsumestokens(data)fromthenet.

Arcsareneverdrawnfromplacetoplace,norfromtransitiontotransition.Transitionnodeisreadytofireifandonlyifthereisatleastonetokenateachofitsinputplaces.Conversely,atransitionwillnotfireifoneormoreinputplacesisempty.Firingatransitionproducessoftwareaction(ataskisperformed).Formally,firingatransitionwillconsumeonetokenfromeachofitsinputplacesandgenerateonetokenforeachofitsoutputplaces.Figure4.13illustratesanexamplefiring.Inthiscase,thetransitionwillwaitfortheretobeatleastonetokeninbothitsinputplaces.Whenitfiresitwillconsumetwotokensandmergethemintoonetokenaddedtoitsoutputplace.Ingeneral,onceatransitionisreadytofire,thereisnoguaranteewhenitwillfire.OneusefulextensionofthePetriNetassignsaminimumandmaximumtimedelayfrominputtooutputforeachtransitionthatisreadytofire.

Figure4.13.Firingatransitionconsumesonetokenateachinputandproducesonetokenateachoutput.Figure4.14illustratesasequentialoperation.Thethreetransitionswillfireinastrictlyorderedsequence:firstt1,nextt2,andthent3.

Figure4.14.APetriNetusedtodescribeasequentialoperation.Figure4.15illustratesconcurrentoperation.Oncetransitiont1fires,transitionst2andt3arerunningatthesametime.Onadistributedsystemt2andt3mayberunninginparallelonseparatecomputers.Onasystemwithoneprocessor,twooperationsaresaidtoberunningconcurrentlyiftheyarebothreadytorun.Becausethereisasingleprocessor,thetasksmustrunoneatatime.

Figure4.15.APetriNetusedtodescribeconcurrentoperations.Figure4.16demonstratesaconflictorracecondition.Botht1andt2arereadytofire,butthefiringofoneleadstothedisablingoftheother.Itwouldbeamistaketofirethemboth.Agoodsolutionwouldbetotaketurnsinsomefairmanner(flipacoinoralternate).Adeterministicmodelwillalwaysproducethesameoutputfromagivenstartingconditionorinitialstate.Becauseoftheuncertaintywhenorifatransitionwillfire,asystemdescribedwithaPetriNetisnotdeterministic.

Figure4.16.Ift1weretofire,itwoulddisablet2.Ift2weretofire,itwoulddisablet1.

Figure4.17describesanassemblylineonamanufacturingplant.Therearetworobots.ThefirstrobotpicksuponePart1andonePart2,placingthepartstogether.Aftertherobotplacesthetwoparts,itdrillsaholethroughthecombination,andthenplacesthepartialassemblyintothePartialbin.ThesecondrobotfirstcombinesPart3withthepartialassemblyandscrewsthemtogether.ThefinishedproductisplacedintotheDonebin.Thetokensrepresentthestateofthesystem,andtransitionsareactionsthatcausethestatetochange.

Thethreesupplytransitionsareinputmachinesthatplacepartsintotheirrespectivepartsbins.ThetokensinplacesPart1,Part2,Part3,Partial,andDonerepresentthenumberofcomponentsintheirrespectivebins.Thefirstrobotperformstwooperationsbutcanonlyperformoneatatime.TheRdy-to-P&Pplacehasatokenifthefirstrobotisidleandreadytopickandplace.TheRdy-to-drillplacehasatokenifthefirstrobotisholdingtwopartsandisreadytodrill.ThePick&Placetransitionistheactioncausedbythefirstrobotasitpicksuptwopartsplacingthemtogether.TheDrilltransitionistheactioncausedbythefirstrobotasitdrillsaholeandplacesthepartialassemblyintothePartialbin.

Figure4.17.APetriNetusedtodescribeanassemblyline.Thefirstrobotperformstwooperationsbutcanonlyperformoneatatime.TheRdy-to-P&Pplacehasatokenifthefirstrobotisidleandreadytopickandplace.TheRdy-to-drillplacehasatokenifthefirstrobotisholdingtwopartsandisreadytodrill.ThePick&Placetransitionistheactioncausedbythefirstrobotasitpicksuptwopartsplacingthemtogether.TheDrilltransitionistheactioncausedbythefirstrobotasitdrillsaholeandplacesthepartialassemblyintothePartialbin.

Thesecondrobotperformstwooperations.TheRdy-to-combineplacehasatokenifthesecondrobotisidleandreadytocombine.TheRdy-to-screwplacehasatokenifthesecondrobotisholdingtwopartsandisreadytoscrew.TheCombinetransitionistheactioncausedbythesecondrobotasitpicksupaPart3andaPartialcombiningthemtogether.TheScrewtransitionistheactioncausedbythesecondrobotasitscrewsittogetherandplacesthecompletedassemblyintotheDonebin.TheShiptransitionisanoutputmachinethatsendscompletedassembliestotheirproperdestination.

Checkpoint4.1:Assumingnoadditionalinputmachinesarefired,runthePetriNetshowninFigure4.17untilitstalls.Howmanycompetedassembliesareshipped?

4.4.KahnProcessNetworksGillesKahnfirstintroducedtheKahnProcessNetwork(KPN).WeuseKPNstomodeldistributedsystemsaswellassignalprocessingsystems.EachnoderepresentsacomputationblockcommunicatingwithothernodesthroughunboundedFIFOchannels.ThecirclesinFigure4.18arecomputationalblocksandthearrowsareFIFOqueues.Theresultingprocessnetworkexhibitsdeterministicbehaviorthatdoesnotdependonthevariouscomputationorcommunicationdelays.Assuch,KPNshavefoundmanyapplicationsinmodelingembeddedsystems,high-performancecomputingsystems,andcomputationaltasks.

Figure4.18.AKahnProcessNetworkconsistsofprocessnodeslinkedbyunboundedFIFOqueues.ForeachFIFO,onlyoneprocessputs,andonlyoneprocessgets.Figure4.18showsaKPNwithfourprocessesandthreeedges(communicationchannels).ProcessesP1andP2areproducers,generatingdataintochannelsAandBrespectively.ProcessP3consumesonetokenfromchannelAandanotherfromchannelB(ineitherorder)andthenproducesonetokenintochannelC.ProcessP4isaconsumerbecauseitconsumestokens.

WecanuseaKPNtodescribesignalprocessingsystemswhereinfinitestreamsofdataaretransformedbyprocessesexecutinginsequenceorparallel.Streamingdatameansweinput/analyze/outputonedatapacketatatimewithoutthedesiretoseetheentirecollectionofdataallatonce.Despiteparallelprocesses,multitaskingorparallelismarenotrequiredforexecutingthismodel.InaKPN,processescommunicateviaunboundedFIFOchannels.Processesreadandwriteatomicdataelements,oralternativelycalledtokens,fromandtochannels.ThereadtokenisequivalenttoaFIFOgetandthewritetokenisaFIFOput.InaKPN,writingtoachannelisnon-blocking.ThismeansweexpecttheputFIFOcommandtoalwayssucceed.Inotherwords,theFIFOneverbecomesfull.Fromapracticalperspective,wecanuseKPNmodelingforsituationswheretheFIFOsneveractuallydobecomefull.Furthermore,theapproximatebehaviorofasystemcanbestillbedeemedforsystemswhereFIFOfullerrorsareinfrequent.FortheseapproximationswecoulddiscarddatawiththeFIFObecomesfullonaputinsteadofwaitingfortheretobefreespaceintheFIFO.

Ontheotherhandreadingfromachannelrequiresblocking.Aprocessthatreadsfromanemptychannelwillstallandcanonlycontinuewhenthechannelcontainssufficientdataitems(tokens).Processesarenotallowedtotestaninputchannelforexistenceoftokenswithoutconsumingthem.Givenaspecificinput(token)historyforaprocess,theprocessmustbedeterministicsothatitalwaysproducesthesameoutputs(tokens).Timingorexecutionorderofprocessesmustnotaffecttheresultandthereforetestinginputchannelsfortokensisforbidden.

InordertooptimizeexecutionsomeKPNsdoallowtestinginputchannelsforemptinessaslongasitdoesnotaffectoutputs.Itcanbebeneficialand/orpossibletodosomethinginadvanceratherthanwaitforachannel.IntheexampleshowninFigure4.18,processP3mustgetfrombothchannelAandchannelB.TheleftsideofProgram4.1showstheprocessstallsiftheAFifoisempty(evenifthereisdataintheBFifo).IfthefirstFIFOisempty,itmightbeefficienttoseeifthereisdataintheotherFIFOtosavetime(rightsideofProgram4.1).

ProcessesofaKPNaredeterministic.Forthesameinputhistorytheymustalwaysproduceexactlythesameoutput.Processescanbemodeledassequentialprogramsthatdoreadsandwritestoportsinanyorderorquantityaslongasthedeterminismpropertyispreserved.

KPNprocessesaremonotonic,whichmeansthattheyonlyneedpartialinformationoftheinputstreaminordertoproducepartialinformationoftheoutputstream.Monotonicityallowsparallelism.InaKPNthereisatotalorderofeventsinsideasignal.However,thereisnoorderrelationbetweeneventsindifferentsignals.Thus,KPNsareonlypartiallyordered,whichclassifiesthemasanuntimedmodel.

voidProcess3(void)

int32_tinA,inB,out;

while(1)

while(AFifo_Get(&inA));

while(BFifo_Get(&inB));

out=compute(inA,inB);

CFifo_Put(out);

voidProcess3(void)

int32_tinA,inB,out;

while(1)

if(AFifo_Size()==0)

while(BFifo_Get(&inB));

while(AFifo_Get(&inA));

else

while(AFifo_Get(&inA));

while(BFifo_Get(&inB));

out=compute(inA,inB);

CFifo_Put(out);

Program4.1.TwoCimplementationsofaprocessonaKPN.Theoneontherightisoptimized.

4.5.Edge-triggeredInterfacingSynchronizingsoftwaretohardwareeventsrequiresthesoftwaretorecognizewhenthehardwarechangesstatesfrombusytodone.Manytimesthebusytodonestatetransitionissignifiedbyarising(orfalling)edgeonastatussignalinthehardware.Forthesesituations,weconnectthisstatussignaltoaninputofthemicrocontroller,andweuseedge-triggeredinterfacingtoconfiguretheinterfacetosetaflagontherising(orfalling)edgeoftheinput.Usingedge-triggeredinterfacingallowsthesoftwaretorespondquicklytochangesintheexternalworld.Ifweareusingbusy-waitsynchronization,thesoftwarewaitsfortheflag.Ifweareusinginterruptsynchronization,weconfiguretheflagtorequestaninterruptwhenset.EachofthedigitalI/OpinsontheLM3S/LM4F/TM4Cfamilycanbeconfiguredforedgetriggering.Table2.18listedsomeoftheI/OregistersassociatedwiththeLM3S1968andTM4C123.Table4.2expandsthislisttoincludealltheregistersavailableforPortA.ThedifferencesbetweenmembersoftheLM3S/LM4F/TM4Cfamilyincludethenumberofports(e.g.,theLM3S1968hasportsA–H,whiletheTM4C123hasportsA–F)andthenumberofpinsineachport(e.g.,theLM3S1968haspins7–0inPortF,whiletheTM4C123haspins4–0inPortF).Formoredetails,refertothedatasheetforyourspecificmicrocontroller.AnyorallofdigitalI/Opinscanbeconfiguredasanedge-triggeredinput.WhenwritingCcodeusingtheseregisters,includetheheaderfileforyourparticularmicrocontroller(e.g.,tm4c123gh6pm.h).

TouseanyofthefeaturesforadigitalI/Oport,wefirstenableitsclockintheSYSCTL_RCGCGPIO_R.ForeachbitwewishtousewemustsetthecorrespondingDEN(DigitalEnable)bit.Touseapinasregulardigitalinputoroutput,weclearitsAFSEL(AlternateFunctionSelect)bit.SettingtheAFSELwillactivatethepin’sspecialfunction(e.g.,UART,I2C,CANetc.)Forregulardigitalinput/output,weclearDIR(Direction)bitstomaketheminput,andwesetDIRbitstomakethemoutput.

TherearefouradditionalregistersfortheLM4F/TM4C.WeclearbitsintheAMSELregistertousetheportfordigitalI/O.AMSELbitsexistforthosepinswhichhaveanalogfunctionality.WhichpinshavewhichfunctionalitywasshowninTables2.7and2.8.WesetthealternativefunctionusingbothAFSELandPCTLregisters.WeneedtounlockPD7andPF0ifwewishtousethem.BecausePC3-0implementstheJTAGdebugger,wewillneverunlockthesepins.PinsPC3-0,PD7andPF0aretheonlyonesthatimplementtheCRbitsintheircommitregisters,where0meansthepinislockedand1meansthepinisunlocked.Tounlockapin,wefirstwrite0x4C4F434BtotheLOCKregister,andthenwewritezerostotheCRregister.

Address 7 6 5 4 3 2 1 0 Name

$4000.43FC DATA DATA DATA DATA DATA DATA DATA DATA GPIO_PORTA_DATA_R

$4000.4400 DIR DIR DIR DIR DIR DIR DIR DIR GPIO_PORTA_DIR_R

$4000.4404 IS IS IS IS IS IS IS IS GPIO_PORTA_IS_R

$4000.4408 IBE IBE IBE IBE IBE IBE IBE IBE GPIO_PORTA_IBE_R

$4000.440C IEV IEV IEV IEV IEV IEV IEV IEV GPIO_PORTA_IEV_R

$4000.4410 IME IME IME IME IME IME IME IME GPIO_PORTA_IM_R

$4000.4414 RIS RIS RIS RIS RIS RIS RIS RIS GPIO_PORTA_RIS_R

$4000.4418 MIS MIS MIS MIS MIS MIS MIS MIS GPIO_PORTA_MIS_R

$4000.441C ICR ICR ICR ICR ICR ICR ICR ICR GPIO_PORTA_ICR_R

$4000.4420 SEL SEL SEL SEL SEL SEL SEL SEL GPIO_PORTA_AFSEL_R

$4000.4500 DRV2 DRV2 DRV2 DRV2 DRV2 DRV2 DRV2 DRV2 GPIO_PORTA_DR2R_R

$4000.4504 DRV4 DRV4 DRV4 DRV4 DRV4 DRV4 DRV4 DRV4 GPIO_PORTA_DR4R_R

$4000.4508 DRV8 DRV8 DRV8 DRV8 DRV8 DRV8 DRV8 DRV8 GPIO_PORTA_DR8R_R

$4000.450C ODE ODE ODE ODE ODE ODE ODE ODE GPIO_PORTA_ODR_R

$4000.4510 PUE PUE PUE PUE PUE PUE PUE PUE GPIO_PORTA_PUR_R

$4000.4514 PDE PDE PDE PDE PDE PDE PDE PDE GPIO_PORTA_PDR_R

$4000.4518 SLR SLR SLR SLR SLR SLR SLR SLR GPIO_PORTA_SLR_R

$4000.451C DEN DEN DEN DEN DEN DEN DEN DEN GPIO_PORTA_DEN_R

$4000.4524 CR CR CR CR CR CR CR CR GPIO_PORTA_CR_R

$4000.4528 AMSEL AMSEL AMSEL AMSEL AMSEL AMSEL AMSEL AMSEL GPIO_PORTA_AMSEL_R

31-28 27-24 23-20 19-16 15-12 11-8 7-4 3-0

$4000.452C PMC7 PMC6 PMC5 PMC4 PMC3 PMC2 PMC1 PMC0 GPIO_PORTA_PCTL_R

$4000.4520 LOCK(write0x4C4F434Btounlock,otherlocks)(reads1iflocked,0ifunlocked) GPIO_PORTA_LOCK_R

Table4.2.PortAregisters.CRAMSELLOCKandPCTLonlyonLM4F/TM4C

Toconfigureanedge-triggeredpin,wefirstenabletheclockontheportandconfigurethepinasaregulardigitalinput.ClearingtheIS(InterruptSense)bitconfiguresthebitforedgetriggering.IftheISbitweretobeset,thetriggeroccursonthelevelofthepin.Sincemostbusytodoneconditionsaresignifiedbyedges,wetypicallytriggeronedgesratherthanlevels.NextwewritetotheIBE(InterruptBothEdges)andIEV(InterruptEvent)bitstodefinetheactiveedge.Wecantriggerontherising,falling,orbothedges,aslistedinTable4.3.WecleartheIME(InterruptMaskEnable)bitsifweareusingbusy-waitsynchronization,andwesettheIMEbitstouseinterruptsynchronization.

DIR AFSEL IS IBE IEV IME Portmode

0 0 0 0 0 0 Input,fallingedgetrigger,busywait

0 0 0 0 1 0 Input,risingedgetrigger,busywait

0 0 0 1 - 0 Input,bothedgestrigger,busywait

0 0 0 0 0 1 Input,fallingedgetrigger,interrupt

0 0 0 0 1 1 Input,risingedgetrigger,interrupt

0 0 0 1 - 1 Input,bothedgestrigger,interrupt

Table4.3.Edge-triggeredmodes.

ThehardwaresetsanRIS(RawInterruptStatus)bit(calledthetrigger)andthesoftwareclearsit(calledtheacknowledgement).ThetriggeringeventlistedinTable4.3willsetthecorrespondingRISbitinthe GPIO_PORTA_RIS_R registerregardlessofwhetherornotthatbitisallowedtorequestacontrollerinterrupt.Inotherwords,clearinganIMEbitdisablesthecorrespondingpin’sinterrupt,butitwillstillsetthecorrespondingRISbitwhentheinterruptwouldhaveoccurred.ThesoftwarecanacknowledgetheeventbywritingonestothecorrespondingIC(InterruptClear)bitinthe GPIO_PORTA_IC_Rregister.TheRISbitsarereadonly,meaningifthesoftwareweretowritetothisregisters,itwouldhavenoeffect.Forexample,toclearbits2,1,and0inthe GPIO_PORTA_RIS_R register,wewritea0x07tothe GPIO_PORTA_IC_Rregister.WritingzerosintoICbitswillnotaffecttheRISbits.

Forinputsignalswehavetheoptionofaddingeitherapull-upresistororapull-downresistor.IfwesetthecorrespondingPUE(Pull-UpEnable)bitonaninputpin,theequivalentofa50to110k resistorto+3.3Vpowerisinternallyconnectedtothepin.Similarly,ifwesetthecorrespondingPDE(Pull-DownEnable)bitonaninputpin,theequivalentofa55to180k resistortogroundisinternallyconnectedtothepin.Wecannothavebothpull-upandapull-downresistor,sosettingabitinoneregisterautomaticallyclearsthecorrespondingbitintheotherregister.

Atypicalapplicationofpull-upandpull-downmodeistheinterfaceofsimpleswitches.Usingthesemodeseliminatestheneedforanexternalresistorwheninterfacingaswitch.ComparetheinterfacesonPortAtotheinterfacesonPortBillustratedinFigure4.19.ThePortAinterfacesemploysoftware-configuredinternalresistors,whilethePortBinterfacesrequireactualresistors.ThePA2andPB2interfacesinFigure4.19a)implementnegativelogicswitchinputs,andthePA3andPB3interfacesinFigure4.19b)implementpositivelogicswitchinputs.

Checkpoint4.2:Whatdonegativelogicandpositivelogicmeaninthiscontext?

Figure4.19.Edge-triggeredinterfacescangenerateinterruptsonaswitchtouch.Checkpoint4.3:WhatvaluestoyouwriteintoDIR,AFSEL,PUE,andPDEtoconfiguretheswitchinterfacesofPA2andPA3inFigure4.19?

Usingedgetriggeringtosynchronizesoftwaretohardwarecentersaroundtheoperationofthetriggerflags,RIS.Abusy-waitinterfacewillreadtheappropriateRISbitoverandover,untilitisset.WhentheRISbitisset,thesoftwarewillcleartheRISbit(bywritingaonetothecorrespondingICbit)andperformthedesiredfunction.Withinterruptsynchronization,theinitializationphasewillarmthetriggerflagbysettingthecorrespondingIMEbit.Inthisway,theactiveedgeofthepinwillsettheRISandrequestaninterrupt.Theinterruptwillsuspendthemainprogramandrunaspecialinterruptserviceroutine(ISR).ThisISRwillcleartheRISbitandperformthedesiredfunction.AttheendoftheISRitwillreturn,causingthemainprogramtoresume.Inparticular,fiveconditionsmustbesimultaneouslytrueforanedge-triggeredinterrupttoberequested:

•Thetriggerflagbitisset(RIS)

•Thearmbitisset(IME)

•Theleveloftheedge-triggeredinterruptmustbelessthanBASEPRI

•Theedge-triggeredinterruptmustbeenabledintheNVIC_EN0_R

•Bit0ofthespecialregisterPRIMASKis0

Inthischapterwewilldevelopblind-cycleandbusy-waitsolutions,andtheninthenextchapterwewillredesignthesystemsusinginterruptsynchronization.

4.6.ConfiguringDigitalOutputPinsTouseadigitalport,wemustfirstenableitsclockinthe SYSCTL_RCGCGPIO_Rregister.Similartousinganinputpin,wemustsettheDENbitsandcleartheAFSELbits.Tomakeapinanoutput,wesetthecorrespondingDIRbit.Thereareanumberofchoiceswhenconfiguringdigitaloutputpins.TheregistersforPortAarelistedinTable4.2.TheavailableIOHandIOLforadigitaloutputcanbespecifiedwithitsDRV2,DRV4,orDRV8bits.Foreachbit,exactlyoneoftheDRV2,DRV4,orDRV8bitsisset,specifyingtheoutputdrivecurrenttobe2mA,4mA,or8mArespectively.Youcansavepowerbychoosingthesmallestcurrentrequiredtodrivetheinterface.Inparticular,determinetheIIHandIILofthedeviceconnectedtotheoutputpinandmake|IOH|>|IIH|and|IOL|>|IIL|.TheTM4C1294microcontrolleraddsa12mAoutputmode.Toactivate12mAoutputonPA2ontheTM4C1294,weexecute

GPIO_PORTA_PC_R=(GPIO_PORTA_PC_R&0xFFFFFFCF)+0x0030;

GPIO_PORTA_DR4R_R|=0x02;//2mA

GPIO_PORTA_DR8R_R|=0x02;//+4mAmore

GPIO_PORTA_DR12R_R|=0x02;//+4mAmore

Anoutputpincanbeconfiguredasopendrain,whichissimilartoopencollector,bysettingtheODE(OpenDrainEnable)bitforthepin.Inopendrainmode,theoutputstatesarezeroandoff.Inparticular,ifweoutputa0,thepinwillgolow(VOL,IOL).Ifweoutputa1,thepinwillfloat,whichisneitherhighnorlow.Inthefloatingstate,theoutputwillnotsourceorsinkanycurrent.WecanuseopendrainmodeforinterfacingnegativelogicLEDsandtocreateamulti-dropnetwork.

Anadditionalconfigurationbitforoutputpinsistheslewratecontrol.TheslewrateofanoutputsignalisdefinedasthemaximumdV/dtoccurringwhentheoutputisswitching.Formostdigitalinterfaces,wewantthefastestpossibleslewratesothedigitaloutputspendsaslittletimeinthetransitionrangeaspossible.However,forsomenetworkinterfaceswewillwanttolimittheslewratetopreventnoisecouplingacrossthewiresofacable.Whentheoutputpinisconfiguredat8mAdrive,wecansetSRL(SlewRateLimit)bitsinthe GPIO_PORTA_SRL_R registertolimittheslewrateonthepin.WhentheSRLbitis0,an8-mAoutputwillrisefrom20%to80%ofitsvoltagein[6,9ns]andwillfallfrom80%to20%in[6,10ns].WhentheSRLbitis1,theriseandfalltimesincreaseto[10,12ns]and[11,13ns]respectively.

4.7.Blind-cycleInterfacingThebasicapproachforblind-cyclesynchronizationistoissueanI/Ocommandthenwaitafixedtimedelayfortheoperationtocomplete.Itsadvantageissimplicity.ItisappropriateforI/Odevicesthatarefastandpredicable.Itisrobustbecauseitcannotcrash(i.e.,neverreturning).Unfortunately,thereareseveraldisadvantagesofblind-cyclesynchronization.Iftheoutputrateisvariable(likea“carriagereturn”,“tab”,“graphics”,or“formfeed”)thenthistechniqueisawkward.Iftheinputrateisunknown(likeakeyboard)thistechniqueisinappropriate.Thedelayrepresentswastedtime.Forasimplesystem,thiswasteusuallydoesn’tmatter.Butforamorecomplexsystem,thistimecouldbeusedbythesoftwaretoperformotheroperations.

4.7.1.HD44780-controlledLCDBecausethereisnostatusfeedbackfromthedevice,ifthedeviceismissingorbrokenthesoftwarewillnotknow.Nevertheless,blindcyclecountingcanbeappropriateforsimplehigh-speedinterfaces.Inthissection,wewillusetheblindcyclemethodtointerfaceanLCDbecausemostoperationswillcompleteinlessthan40 s.Liquidcrystaldisplaysoperateatlowpowerandhaveflexiblegraphics.Many5V-poweredLCDsuseanindustrystandardHD44780Scontroller.TheHD44780UandST7066ULCDdriversshowninFigure4.20workinasimilarwaybutoperateatasupplyvoltageof3.3V.Thelow-levelsoftwareinitializesandoutputstotheLCDcontroller.ThemicrocontrollersimplywritesASCIIcharacterstotheLCDcontroller.EachASCIIcharacterismappedintoa5by8bitpixelimage,calledafont.A1by8LCDis40pixelswideby8pixelstall,andtheLCDcontrollerisresponsibleforrefreshingthepixelsinaraster-scannedmannersimilartomaintaininganimageonaTVscreen.

TherearefourtypesofaccesscyclestotheLCDcontrollerdependingonRSandR/WasshowninTable4.4.Normally,youwriteASCIIcharactersintothedatabuffer(calledDDRAMinthedatasheets)tohavethemdisplayedonthescreen.However,youcancreateupto8newcharactersbywritingtothecharactergeneratorRAM(CGRAM).ThesenewcharactersexistasASCIIdata0to7.

RS R/W Operation

0 0 WriteacommandtotheLCDinstructionregister

0 1 ReadBusyFlag(bit7)

1 0 WritedatatotheLCDdatabuffer

1 1 ReaddatafromtheLCDtothemicrocontroller

Table4.4.TwocontrolsignalsspecifythetypeofaccesstotheLCD.

Wecoulduseeitherblind-cycleorbusy-waitsynchronization.Mostoperationsrequire37µstocompletewhilesomerequire1.52ms.Programs4.2and4.3usetheSysTicktimertocreatetheblind-cyclewait.Busy-waitsynchronizationwouldhaveprovidedfeedbacktodetectafaultyinterface,butbusywaithastheproblemofcreatingasoftwarecrashiftheLCDneverfinishes.Abetterinterfacewouldhaveutilizedbothbusywaitandblind-cycle,sothatthesoftwarecanreturnwithanerrorcodeifadisplayoperationdoesnotfinishontime(duetoabrokendisplay).Withblind-cyclesynchronizationweonlyusethewritecommandandwritedataoperations.Consequently,wecoulddisconnectthePG1pinandgroundtheLCDR/Wline.

Figure4.20.InterfaceofanLCD.First,wepresentalow-levelprivatehelperfunction,seeProgram4.2.ThisfunctionwouldnothaveaprototypeintheLCD.hfile.Thedefinemacrosspecifythemappingfromthelogicnametothephysicaloutputpin.AsshowninTable4.4,anoutputcommandiscreatedwithR/W=0andRS=0.TooutputacommandtotheLCD,wefirstwritethe8-bitcommandtothedatalines,thenwepulsetheEpin(E=1,E=0.)

#defineE0x80//onPA7

#defineRS0x40//onPA6

#defineLCDDATA(*((volatileuint32_t*)0x400053FC))//PORTB

#defineLCDCMD(*((volatileuint32_t*)0x40004300))//PA7-PA6

#defineBusFreq50//assuminga50MHzbusclock

#defineT500nsBusFreq/2//500ns

#defineT40us40*BusFreq//40us

#defineT160us160*BusFreq//160us

#defineT1600us1600*BusFreq//1.60ms

#defineT5ms5000*BusFreq//5ms

#defineT15ms15000*BusFreq//15ms

//sendacommandbytetotheLCD

voidOutCmd(uint8_tcommand)

LCDDATA=command;

LCDCMD=0;//E=0,R/W=0,RS=0

SysTick_Wait(T500ns);//wait500ns

LCDCMD=E;//E=1,R/W=0,RS=0

SysTick_Wait(T500ns);//wait500ns

LCDCMD=0;//E=0,R/W=0,RS=0

SysTick_Wait(T40us);//wait40us

Program4.2.PrivatefunctionsforanLCD.(LCD_xxx.zip).Figure4.21showsaroughsketchoftheE,RS,R/Wanddatasignalsasthe OutCmdfunctionisexecuted.Astimeadvances,theprogramexecutesfromtoptobottom,andtheoutputsignalsinthetimingdiagramprogresslefttoright.The500-nswaitsaretosatisfythesetupandholdtimesfortheLCD,andthe40- swaitistheblind-cycledelaytoallowthecommandtocomplete.Theclearandhomecommandswillrequirea1.6-msblindwait.

Figure4.21.TimingdiagramoftheLCDsignalsasacommandissenttotheLCD.//InitializeLCD

voidLCD_Init(void)

SYSCTL_RCGCGPIO_R|=0x03;//1)activateclockforPortsAandB

while((SYSCTL_PRGPIO_R&0x03)!=0x03);//ready?

GPIO_PORTB_AMSEL_R&=~0xFF;//3)disableanalogfunctiononPB7-0

GPIO_PORTA_AMSEL_R&=~0xC0;//disableanalogfunctiononPA7-6

GPIO_PORTB_PCTL_R=0x00000000;//4)configurePB7-0asGPIO

GPIO_PORTA_PCTL_R&=~0xFF000000;//configurePA7-6asGPIO

GPIO_PORTB_DIR_R=0xFF;//5)setdirectionregister

GPIO_PORTA_DIR_R|=0xC0;

GPIO_PORTB_AFSEL_R=0x00;//6)regularportfunction

GPIO_PORTA_AFSEL_R&=~0xC0;

GPIO_PORTB_DEN_R=0xFF;//7)enabledigitalport

GPIO_PORTA_DEN_R|=0xC0;

GPIO_PORTB_DR8R_R=0xFF;//enable8mAdrive

GPIO_PORTA_DR8R_R|=0xC0;

SysTick_Init();//Program2.11

LCDCMD=0;//E=0,R/W=0,RS=0

SysTick_Wait(T15ms);//seedatasheetforspecificwaittime

OutCmd(0x30);//command0x30=Wakeup

SysTick_Wait(T5ms);//mustwait5ms,busyflagnotavailable

OutCmd(0x30);//command0x30=Wakeup#2

SysTick_Wait(T160us);//mustwait160us,busyflagnotavailable

OutCmd(0x30);//command0x30=Wakeup#3

SysTick_Wait(T160us);//mustwait160us,busyflagnotavailable

OutCmd(0x38);//Functionset:8-bit/2-line

OutCmd(0x10);//Setcursor

OutCmd(0x0C);//DisplayON;CursorON

OutCmd(0x06);//Entrymodeset

//Inputs:letterisASCIIcharacter,0to0x7FOutputs:none

voidLCD_OutChar(charletter)

LCDDATA=letter;

LCDCMD=RS;//E=0,R/W=0,RS=1

SysTick_Wait(T500ns);//wait500ns

LCDCMD=E+RS;//E=1,R/W=0,RS=1

SysTick_Wait(T500ns);//wait500ns

LCDCMD=RS;//E=0,R/W=0,RS=1

SysTick_Wait(T40us);//wait40us

//Inputs:noneOutputs:none

voidLCD_Clear(void)

OutCmd(0x01);//ClearDisplay

SysTick_Wait(T1600us);//wait1.6ms

OutCmd(0x02);//Cursortohome

SysTick_Wait(T1600us);//wait1.6ms

Program4.3.PublicfunctionsfortheLCD(LCD_xxx.zip).Thehigh-levelpublicfunctionsareshowninProgram4.3.ThesefunctionswouldhaveprototypesintheLCD.hfile.TheinitializationsequenceiscopiedfromthedatasheetoftheLCDcontroller.Figure4.22showsaroughsketchoftheE,RS,R/Wanddatasignalsasthe LCD_OutChar functionisexecuted.ThisinterfacecanoperatewithanLCDpoweredwith+5V.ForsomeLCDs,theVOHoftheLM3S/LM4F/TM4CisnotquitehighenoughfortheVIHoftheLCDwhentheLCDispoweredwith+5V.Inthesecases,the T500ns delaysneedtobeincreasedbyafactorof10.

Figure4.22.TimingdiagramoftheLCDsignalsasdataissenttotheLCD.

4.7.2.StepperMotorInterfaceSteppermotorsareverypopularformicrocontroller-controlledmachinesbecauseoftheirinherentdigitalinterface.Itiseasyforamicrocontrollertocontrolboththepositionandvelocityofasteppermotorinanopen-loopfashion.AlthoughthecostofasteppermotoristypicallyhigherthananequivalentDCpermanentmagneticfieldmotor,theoverallsystemcostisreducedbecausesteppermotorsmaynotrequirefeedbacksensors.Forthesereasons,theyareusedinmanycomputerperipheralssuchasharddiskdrives,scanners,andprinters.Figure4.23showsasteppermotorismadewithEMcoilsandpermanentmagnets(teeth).

Figure4.23.Steppermotorshavepermanentmagnetsontherotorandelectromagneticsaroundthestator.Figure4.24showsasimplifiedsteppermotor.Thepermanentmagnetstepperhasarotorandastator.Therotorismanufacturedfromagear-shapedpermanentmagnet.ThissimplerotorhasoneNorthtoothandoneSouthtooth.NorthandSouthteethareequallyspacedandoffsetfromeachotherbyhalfthetoothpitchasillustratedbytherotorwith12teeth.Thestatorconsistsofmultipleiron-coreelectromagnetswhosepolesarealsoequallyspaced.Thestatorofthissimplesteppermotorhasfourelectromagnetsandfourpoles.ThesteppermotorsinFigure4.23have50Northteethand50Southteeth,resultinginmotorswith200stepsperrevolution.Thestatorofasteppermotorwith200stepsperrevolutionhaseightelectromagnetseachwithfivepoles,makingatotalof40poles.

Figure4.24.Simplesteppermotorwith4steps/revolutionandarotorwith12teeth.TheoperationofthissimplesteppermotorisillustratedinFigure4.25.Ingeneral,iftherearenNorthteethandnSouthteeth,theshaftwillrotate360°/(4∙n)perstep.ForthissimplemotorinFigure4.25,eachstepcauses90°rotation.

Figure4.25.Thefull-stepsequencetorotateasteppermotor.

Output=0101.Assumetheinitialpositionistheoneontheleftwiththeoutputequalto0101.TherearestrongattractiveforcesbetweenNorthandSouthmagnets.ThisisastablestatebecausetheNorthtoothisequallypositionedbetweenthetwoSouthelectromagnets,andtheSouthtoothisequallypositionedbetweenthetwoNorthelectromagnets.Thereisnonettorqueontheshaft,sothemotorwillstayfixedatthisangle.Infact,ifthereisanattempttorotatetheshaftwithanexternaltorque,thesteppermotorwillopposethatrotationandtrytomaintaintheshaftanglefixedatthisposition.Infact,steppermotorsareratedaccordingtotheirholdingtorque.Typicalholdingtorquesrangefrom10to300oz·in.

Output=0110.Whenthesoftwarechangestheoutputto0110,thepolarityofPhaseBisreversed.Therotorisinanunstablestate,becausetheNorthtoothisneartheNorthelectromagnetonthetop,andtheSouthtoothisneartheSouthelectromagnetonthebottom.Therotorwillmovebecausetherearestrongrepulsiveforcesfromthetopandbottompoles.Byobservingtheleftandrightpoles,thecloseststablestateoccursiftherotorrotatesclockwise,resultinginthestablestateillustratedasthepictureinFigure4.25labeled“Output=0110”.The“Output=0110”stateisexactly90°clockwisefromthe“Output=0101”state.Now,onceagaintheNorthtoothisneartheSouthpolesandtheSouthtoothisneartheNorthpoles.Thisnewpositionhasstrongattractiveforcesfromallfourpoles,holdingtherotoratthisnewposition.

Output=1010.Next,thesoftwareoutputs1010,causingthepolarityofPhaseAtobereversed.Thistime,therotorisinanunstablestatebecausetherearestrongrepulsiveforcesontheleftandrightpoles.Thecloseststablestateoccursiftherotorrotatesclockwise,resultinginthestablestateillustratedinthepictureFigure4.25labeledas“Output=1010”.Thisnewstateisexactly90°clockwisefromthelaststate,movingtopositiontheNorthtoothneartheSouthpolesandtheSouthtoothneartheNorthpoles.

Output=1001.Whenthesoftwareoutputs1001,thepolarityofPhaseBisreversed.Thiscausesarepulsiveforceonthetopandbottompolesandtherotorrotatesclockwiseagainby90°,resultinginthestablestateshownasthepicturelabeled“Output=1001”.Aftereachchangeinsoftware,therearetwopolesthatrepelandtwopolesthatattract,causingtheshafttorotate.TherotormovesuntilitreachesanewstablestatewiththeNorthtoothclosetoSouthpolesandtheSouthtoothclosetoNorthpoles.Whenthesoftwareoutputsa0101,itwillrotate90°resultinginapositionsimilartotheoriginal“Output=0101”state.Ifthesoftwareoutputsanewvaluefromthe5,6,10,9sequenceevery250ms,themotorwillspinclockwiseat1rps.Therotorwillspininacounterclockwisedirectionifthesequenceisreversed.

Thereisaneight-numbersequencecalledhalf-stepping.Infullstepping,thedirectionofcurrentinoneofthecoilsisreversedineachstep.Inhalf-stepping,thecoilgoesthroughano-currentstatebetweenreversals.Thehalf-steppingsequenceis0101,0100,0110,00101010,1000,1001,and0001.Ifacoilisdrivenwiththe00command,itisnotenergized,andtheelectromagnetappliesnoforcetotherotor.Amotorthatrequires200fullstepstorotateoncewillrequire400half-stepstorotateonce.Inotherwords,thehalf-stepangleis½ofafull-stepangle.

Inafour-wire(orbipolar)steppermotor,theelectromagnetsarewiredtogether,creatingtwophases.Thefive-andsix-wire(orunipolar)steppermotorsalsohavetwophases,buteachiscenter-tappedtosimplifythedrivecircuitry.Inabipolarstepperallcopperinthewindingscarriescurrentatalltimes;whereasinaunipolarstepper,onlyhalfthecopperinthewindingsisusedatanyonetime.

Tospinthesteppermotorataconstantspeedthesoftwareoutputsthe5–6–10–9sequenceseparatedbyafixedtimebetweenoutputs.Thisisanexampleofblind-cycleinterfacing,becausethereisnofeedbackfromthemotortothesoftwaregivingtheactualspeedand/orpositionofthemotor.If tisthetimebetweenoutputsinseconds,andthemotorhasnstepsperrevolution,themotorspeedwillbe60/(n* t)inRPM.

Thetimebetweenstatesdeterminestherotationalspeedofthemotor.Let∆tbethetimebetweensteps,andletθbethestepanglethentherotationalvelocity,v,isθ/∆t.Aslongastheloadontheshaftisbelowtheholdingtorqueofthemotor,thepositionandspeedcanbereliablymaintainedwithanopenloopsoftwarecontrolalgorithm.Inordertopreventslips(digitalcommandsthatproducenorotormotion)itisimportanttolimitthechangeinaccelerationorjerk.Let∆t(n-2),∆t(n-1),∆t(n)bethediscretesequenceoftimesbetweensteps.Theinstantaneousrotationalvelocityisgivenby

v(n)=θ/∆t

Theaccelerationisgivenby

a(n)=(v(n)-v(n-1))/∆t(n)=(θ/∆t(n)-θ/∆t(n-1))/∆t(n)=θ/∆t(n)2-θ/(∆t(n-1)∆t(n))

Thechangeinacceleration,orjerk,isgivenby

b(n)=(a(n)-a(n-1))/∆t(n)

Forexampleifthetimebetweenstepsistobeincreasedfrom1000to2000µs,anineffectiveapproachasshowninTable4.5wouldbesimplytogodirectlyfrom1000to2000.Thisproducesaverylargejerkthatmaycausethemotortoslip.

n ∆t(µs) v(n)(°/sec)

a(n)(°/sec2) b(n)(°/sec3)

1 1000 1800

2 1000 1800 0.00E+00

3 2000 900 -2.50E+05 0.00E+00

4 2000 900 0.00E+00 -2.25E+08

5 2000 900 0.00E+00 2.25E+08

6 2000 900 0.00E+00 0.00E+00

Table4.5.Anineffectiveapproachtochangingmotorspeed.

Table4.6showsthatamoregradualchangefrom1000to2000producesa10timessmallerjerk,reducingthepossibilityofslips.Theoptimalsolution(theonewiththesmallestjerk)occurswhenv(t)hasaquadraticshape.Thiswillmakea(t)linear,andb(t)aconstant.Limitingthejerkisparticularlyimportantwhenstartingtomoveastoppedmotor.

n ∆t(µs) v(n)(°/sec)

a(n)(°/sec2) b(n)(°/sec3)

1 1000 1800

2 1000 1800 0.00E+00

3 1000 1800 0.00E+00 0.00E+00

4 1008 1786 -1.39E+04 -1.38E+07

5 1032 1744 -4.11E+04 -2.64E+07

6 1077 1671 -6.77E+04 -2.47E+07

7 1152 1563 -9.37E+04 -2.26E+07

8 1275 1411 -1.19E+05 -1.96E+07

9 1500 1200 -1.41E+05 -1.48E+07

10 1725 1044 -9.06E+04 2.91E+07

11 1848 974 -3.78E+04 2.86E+07

12 1923 936 -1.96E+04 9.44E+06

13 1968 915 -1.09E+04 4.43E+06

14 1992 904 -5.65E+03 2.63E+06

15 2000 900 -1.77E+03 1.94E+06

16 2000 900 0.00E+00 8.85E+05

17 2000 900 0.00E+00 0.00E+00

Table4.6.Aneffectiveapproachtochangingmotorspeed.

ThebipolarsteppermotorcanbecontrolledwithtwoH-bridgedrivers.WecoulddesigntwoH-bridgedriversusingindividualtransistors,butusinganintegrateddriverusuallyprovidesbetterperformanceatlowercost.Inthissection,wewillpresentsteppermotorinterfacesthatemployintegratedcircuitstocontrolthesteppermotor.Inadditiontothedevicespresentedinthissection,Allegro,TexasInstruments,andSTMicroelectronicsofferavarietyofsteppermotorcontrollers,e.g.,A3972,A3980,UCN5804B,andL6203.TheL293isapopularICforinterfacingsteppermotors.ItusesDarlingtontransistorsinadoubleH-bridgeconfiguration(asshowninFigure4.26),whichcanhandleupto1Aperchannelandvoltagesfrom4to36V.AsimilarH-bridgeinterfacechipthatcandrive5AistheL6203fromSTMicroelectronics.

Figure4.26.BipolarsteppermotorinterfaceusingaL293driver.The1N914snubberdiodesprotecttheelectronicsfromthebackEMFgeneratedwhencurrentsareswitchedonandoff.TheL293Dhasinternalsnubberdiodes,butcanhandleonly600mA.Figure4.26showsfourdigitaloutputsfromthemicrocontrollerconnectedtothe1A,2A,3A,4Ainputs.Thesoftwarerotatesthesteppermotorusingeitherthestandardfull-step(5–6–10–9…)orhalf-step(5–4–6–2–10–8–9–1…)sequence.

Theunipolarstepperarchitectureprovidesforbi-directionalcurrentsbyusingacentertaponeachphase,asshowninFigure4.27.Thecentertapisconnectedtothe+VpowersourceandthefourendsofthephasesarecontrolledwithdriversintheL293.Onlyhalfoftheelectromagnetsareenergizedatonetime.TheL293providesupto1Acurrent.

Figure4.27.Unipolarsteppermotorinterface.Checkpoint4.4:Whatchangescouldyoumaketoasteppermotorsystemtoincreasetorque,increasingtheprobabilitythatastepcommandactuallyrotatestheshaft?

Checkpoint4.5:Doyouneedasensorfeedbacktomeasuretheshaftpositionwhenusingasteppermotor?

Example4.1:Interfacea5-V,200-mAunipolarsteppermotor.Themotorhas200stepsperrevolution.Writesoftwarethatcansetthemotorangle.

Solution:Thecomputercanmakethemotorspinbyoutputtingthesequence…,10,9,5,6,10,9,5,6,…overandover.Foramotorwith200steps/revolutioneachnewoutputwillcausethemotortorotate1.8°.Ifthetimeinbetweenoutputsisfixedat∆tseconds,thentheshaftrotationspeedwillbe0.005/∆tinrps.Ineachsystem,wewillconnectthesteppermotortotheleastsignificantbitsofoutputPortDusingFigure4.27.The1N914diodeswillprotecttheL293fromthebackEMFthatwilldevelopacrossthecoilwhenthecurrentisshutoff.Wedefinetheactivestateofthecoilwhencurrentisflowing.ThebasicoperationissummarizedinTable4.7.

PortDoutput

A1 A2 B1 B2

10 Activate Deactivate Activate Deactivate

9 Activate Deactivate Deactivate Activate

5 Deactivate Activate Deactivate Activate

6 Deactivate Activate Activate Deactivate

Table4.7.Steppermotorsequence.

Wewillimplementalinked-listdatastructuretoholdtheoutputpatterns(Figure4.28andProgram4.4).Thisapproachyieldsasolutionthatiseasytounderstandandchange.Ifthecomputeroutputsthesequencebackwardsthenthemotorwillspinintheotherdirection.Inensureproperoperation,this…,10,9,5,6,10,9,5,6,…sequencemustbefollowed.Forexample,assumethecomputeroutputs…,9,5,6,10,and9.Nowitwishestoreversedirection,sincetheoutputisalreadyat9,thenitshouldbeginat10,andcontinuewith6,5,9,…Inotherwordsifthecurrentoutputis“9”thentheonlytwovalidnextoutputswouldbe“5”ifitwantedtospinclockwiseor“10”ifitwantedtospincounterclockwise.Maintainingthispropersequencewillbesimplifiedbyimplementingadoublecircularlinked-list.Foreachnodeinthelinked-listtherearetwovalidnextstatesdependinguponwhetherthecomputerwishestospinclockwiseorcounterclockwise.

Figure4.28.Adoublecircularlinked-listusedtocontrolthesteppermotor.

Aslipiswhenthecomputerissuesasequencechange,butthemotordoesnotmove.Aslipcanoccuriftheloadontheshaftexceedstheavailabletorqueofthemotor.Aslipcanalsooccurifthecomputertriestochangetheoutputstoofast.Ifthesystemknowstheinitialshaftangle,andthemotorneverslips,thenthecomputercancontrolboththeshaftspeedandanglewithoutapositionsensor.TheroutinesCWandCCWwillstepthemotoronceintheclockwiseandcounterclockwisedirectionsrespectively.IfeverytimethecomputercallsCWorCCWitweretowaitfor5ms,thenthemotorwouldspinat1rps.ThelinkeddatastructurewillbestoredinflashROM.ThevariablesareallocatedintoRAMandinitializedatruntimeintheritual.

structState

uint8_tOut;//Output

conststructState*Next[2];//CW/CCW

;

typedefconststructStateStateType;

typedefStateType*StatePtr;

#defineclockwise0//Nextindex

#definecounterclockwise1//Nextindex

StateTypefsm[4]=

10,&fsm[1],&fsm[3],

9,&fsm[2],&fsm[0],

5,&fsm[3],&fsm[1],

6,&fsm[0],&fsm[2]

;

uint8_tPos;//between0and199

conststructState*Pt;//CurrentState

Program4.4.Adoublecircularlinkedlistusedtocontrolthesteppermotor.

Theprogramsthatstepthemotoralsomaintainthepositionintheglobal, Pos .Ifthemotorslips,thenthesoftwarevariablewillbeinerror.Alsoitisassumedthemotorisinitiallyinposition0atthetimeoftheinitialization(Program4.5).

#defineSTEPPER(*((volatileuint32_t*)0x4000703C))

voidStepper_CW(uint32_tdelay)//Move1.8degreesclockwise

Pt=Pt->Next[clockwise];//circular

STEPPER=Pt->Out;//stepmotor

if(Pos==199)//shaftangle

Pos=0;//reset

else

Pos++;//CW

SysTick_Wait(delay);

voidStepper_CCW(uint32_tdelay)//Move1.8degcounterclockwise

Pt=Pt->Next[counterclockwise];//circular

STEPPER=Pt->Out;//stepmotor

if(Pos==0)//shaftangle

Pos=199;//reset

else

Pos—;//CCW

SysTick_Wait(delay);//blind-cyclewait

voidStepper_Init(void)//InitializeStepperinterface

SYSCTL_RCGCGPIO_R|=0x08;//activateportD

SysTick_Init();//program2.11

Pos=0;Pt=&fsm[0];

GPIO_PORTD_AFSEL_R&=~0x0F;//GPIOfunctiononPD3-0

GPIO_PORTD_AMSEL_R&=~0x0F;//disableanalogfunctiononPD3-0

GPIO_PORTD_PCTL_R&=~0x0000FFFF;//configurePD3-0asGPIO

GPIO_PORTD_DIR_R|=0x0F;//makePD3-0out

GPIO_PORTD_DEN_R|=0x0F;//enabledigitalI/OonPD3-0

GPIO_PORTD_DR8R_R|=0x0F;//enable8mAdrive

Program4.5.Helperandinitializationfunctionsusedtocontrolthesteppermotor.

InProgram4.6,thesoftwarewillstepthemotortothedesiredposition.Wecanusethecurrentposition, pos ,todetermineifitwouldbefastertogoCWorCCW. CWsteps iscalculatedasthenumberofstepsfrom pos todesiredifthemotorwheretospinclockwise.Ifitisgreaterthan100,thenitwouldbefastertogettheregoingcounterclockwise.

//Turnsteppermotortodesiredposition(0<=desired<=199)

//timeisthenumberofbuscyclestowaitaftereachstep

voidStepper_Seek(uint8_tdesired,uint32_ttime)

int16_tCWsteps;

if((CWsteps=(desired-Pos))<0)

CWsteps+=200;

//CWstepsis0to199

if(CWsteps>100)

while(desired!=Pos)

Stepper_CCW(time);

else

while(desired!=Pos)

Stepper_CW(time);

Program4.6.High-levelfunctiontocontrolthesteppermotor(Stepper_xxx.zip).

Therearenineconsiderationswhenselectingasteppermotor:speed,torque,holdingtorque,bipolar/unipolar,voltage,current,steps/rotation,size,andweight.Thefirsttwoparametersarespeedandtorque.Speedistherateinrotationsperminute(RPM)thatthemotorwillspin,andtorqueistheavailableforcetimesdistancethesteppermotorcanprovideatthatspeed.Steppermotorsalsohaveaholdingtorque,whichistheforcetimesdistancethatthemotorwillremainstoppedwhentheinputpatternisconstant.Weselectthemotorvoltagetomatchtheavailablepowersupply.UnlikeLEDs,weMUSTnotusearesistorinserieswithamotortoreducethevoltage.Ingeneral,themotorvoltagematchesthepowersupplyvoltage.Wheninterfacingwewillneedtoknowmaximumcurrent.Wechooseabipolarstepperforsituationswherespeed,torque,andefficiencyareimportant.Unipolarsteppersareappropriateforlow-costsystems.Ifwearetryingtocontrolshaftangleitwillbebettertohaveamotorwithmorestepsperrotation.

4.8.Busy-WaitSynchronizationTosynchronizethesoftwarewiththeI/Odevice,themicrocontrollermustbeabletorecognizethebusytodonetransition.Withbusy-waitsynchronization,thesoftwarechecksastatusbitintheI/Odeviceandloopsbackuntilthedeviceisready.Anothernameforbusy-waitisgadfly.Thebusy-waitloopmustprecedethedatatransferforaninputdevice,butforanoutputdevicethebusy-waitloopcanbeeitherbeforeorafterthedatatransfer(Figure4.29).

Figure4.29.Softwareflowchartsforbusy-waitI/O.Thetermgadflywaschosenasanalternatetobusywait,becauseithasanegativecontextoutsidethecomputerfield.PlatoinhisbookApologydescribesSocratesasagadfly,meaningheisaconstantandannoyingpesttotheAthenianpoliticalscene.Inbiology,agadflyisalargebuginthehorse-flyfamily.Asocialgadflyisanirritatingpersonwhoconstantlyupsetsthestatusquo.Aswewilllearninthisbook,interruptsynchronizationwillprovidemoreelegantandeffectivesolutionsovergadflysynchronizationforcomplexsystemswhenconsideringissuessuchaslatency,bandwidth,priority,andlow-power.

Observation:Busy-waitisappropriateforsimplesystemsthathaveonlyafewtasks.

Toperformanoutputthesoftwaremusttransferthedataandwaitforcompletion.Thetransferusuallyexecutesinashortamountoftimebecauseitinvolvesjustafewinstructionswithnobackwardjumps.Ontheotherhand,thetimewaitinguntiltheoutputdeviceisreadyisusuallylongcomparedtothedatatransfer.TheinterfaceisI/Oboundwhenthetimeforthehardwaretoperformtheoutputislongcomparedtothetimeforthesoftwaretoproducethedata.Thesetwostepscanbeperformedineitherorder,aslongasthatorderisconsistentlymaintained,andweassumethedeviceisinitiallyready.Pollingbeforetheoutput,allowsthecomputertoperformadditionaltaskswhiletheoutputisoccurring.Therefore,pollingbeforetheoutputwillhaveahigherbandwidththanpollingaftertheoutput.Ontheotherhand,pollingaftertheoutput,allowsthecomputertoknowexactlywhentheoutputhasbeencompleted.

Toillustratethedifferencesbetweenpollingbeforeandafterthe“writedata”operation,considerasystemwith3printers(Figure4.30).Eachprintercanprintacharacterin1ms.Inotherwords,aprinterwillbeready1msafterthe“writedata”operation.Wewillalsoassumeallthreeprintersareinitiallyready.Sincetheexecutionspeedofthemicrocontrollerisfastcomparedtothe1msittakestoprintacharacter,wewillneglectthesoftwareexecutiontime(I/Obound).Inthe“busywaitbeforeoutput”system,allthree

outputsarestartedtogetherandwilloperateconcurrently.Inthe“busywaitafteroutput”system,thesoftwarewaitsfortheoutputonprinter1tofinishbeforestartingtheoutputonprinter2.Inthissystem,thethreeoutputsareperformedsequentially,thatisaboutthreetimesslowerthanthefirstcase.

Time(ms) Busywaitbeforeoutput

Busywaitafteroutput

0 Start1,2,3 Start1

from0to1

Waitfor1 Waitfor1

1 Start1,2,3 Start2

from1to2

Waitfor1 Waitfor2

2 Start1,2,3 Start3

from2to3

Waitfor1 Waitfor3

3 Start1,2,3 Start1

from3to4

Waitfor1 Waitfor1

4 Start1,2,3 Start2

from4to5

Waitfor1 Waitfor2

Figure4.30.Twosoftwareflowchartsformultiplebusy-waitoutputs.PerformanceTip:WheneverwecanestablishconcurrentI/Ooperations,wecanexpectanimprovementintheoverallsystembandwidth.

Toimplementbusy-waitsynchronizationwithmultipleI/Odevices,simplypolltheminsequenceandperformserviceasrequired.TheexampleinFigure4.31implementsafixedpollingorder,anddoesnotallowhighprioritydevicestosuspendtheserviceoflowerprioritydevices.Considertheinterfacelatencyfordevice1,whichisthetimebetweenwhenStatus1becomereadyuntilthetimethesoftwareperformstheinput/outputserviceondata1.TheworstcasewouldbefortheStatus1signaltobecomereadyrightafterthesoftwarepolledit.Forthisworstcasescenario,thesoftwaremighthavetoservicedevice2,servicedevice3,andexecuteotherfunctionsbeforepollingdevice1again.WhentherearemultipleI/Odevices,theinterfacelatencywithbusy-waitsynchronizationwillbepoor.

Figure4.31.Asoftwareflowchartformultiplebusy-waitinputsandoutputs.Aparallelinterfaceencodestheinformationasseparatebinarybitsonindividualportpins,andtheinformationisavailablesimultaneously.ForexampleifwearetransmittingASCIIcharacters,thentheASCIIcodeisrepresentedbydigitalvoltageson7(or8)digitalsignals.Wheninterfacingadevicetoamicrocontroller,ahandshakedinterfaceprovidesamechanismforthedevicetowaitforthemicrocontrollerandforthemicrocontrollertowaitforthedevice.

Youcanfindthethreeexamplesthatimplementbusy-waitonthebookwebsiteasParallelKeypad_xxx.zip,Handshake_xxx.zip,andParallelPrinter_xxx.zip.Thebookwebsiteisathttp://users.ece.utexas.edu/~valvano/arm/.

Example4.2:Designaone-directionalcommunicationchannelbetweentwomicrocontrollersusingahandshakedprotocol.

Solution:Handshakingallowseachdevicetowaitfortheother.Thisisparticularlyimportantwhencommunicatingbetweentwocomputers.TherightsideofFigure4.32showsthetimingasonebyteistransferredfromtransmittertoreceiver.Thekeytodesigningahandshakedprotocolistorequireeachsidetowaituntiltheothersideisready.ThetransmitterwillbegincommunicationbyputtingnewdataontheDatalinesandissuingarisingedgeonReady.Furthermore,noticethedataisproperlydrivenwheneverReadyishigh.Next,thereceiversignalsitisabouttoreadthedatabymakingitsacknowledge,calledAck,low.ThereceiverthenreadstheresultwhileAckislow.Finally,thereceiveracknowledgesacceptanceofthedatabyoutputtingarisingedgeonAck,signalingitisreadytoacceptanother.ThehandshakeendswithafallingedgeonReady.

Figure4.32.Handshakedinterfacebetweentwomicrocontrollers(LM3S1968solution).

TheinitializationforthetransmitterisgiveninProgram4.7.PG1isconfiguredasaninput.PG0andPortFaredigitaloutputs.TheinitializationforthereceiverisgiveninProgram4.8.PG1andPortFareconfiguredasinputs.PG0isasimpledigitaloutput.

#defineReady(*((volatileuint32_t*)0x40026004))//PG0

#defineAck(*((volatileuint32_t*)0x40026008))//PG1

#defineData(*((volatileuint32_t*)0x400253FC))//PortF

voidXmt_Init(void)volatileuint32_tdelay;

SYSCTL_RCGCGPIO_R|=0x60;//activateclockforPortsF,G

while((SYSCTL_PRGPIO_R&0x60)!=0x60);//ready?

Ready=0;

GPIO_PORTG_DIR_R|=0x01;//PG0isReadyout

GPIO_PORTG_DIR_R&=~0x02;//PG1isAckin

GPIO_PORTG_DEN_R|=0x03;//enabledigitalI/OonPG1-0

GPIO_PORTF_DIR_R=0xFF;//makePF7-0outdata

GPIO_PORTF_DEN_R=0xFF;//enabledigitalI/OonPF7-0

Program4.7.Initializationofthetransmitter.

#defineAck(*((volatileuint32_t*)0x40026004))//PG0

#defineReady(*((volatileuint32_t*)0x40026008))//PG1

#defineData(*((volatileuint32_t*)0x400253FC))//PortF

voidRcv_Init(void)

SYSCTL_RCGCGPIO_R|=0x60;//activateclockforPortsF,G

Ack=1;//allowtimetofinishactivating

GPIO_PORTG_DIR_R|=0x01;//PG0isAckout

GPIO_PORTG_DIR_R&=~0x02;//PG1isReadyin

GPIO_PORTG_DEN_R|=0x03;//enabledigitalI/OonPG1-0

GPIO_PORTF_DIR_R=0x00;//makePF7-0inputdata

GPIO_PORTF_DEN_R=0xFF;//enabledigitalI/OonPF7-0

Program4.8.Initializationofthereceiver.

Thisinterfaceiscalledhandshakedorinterlockedbecauseeachevent(1to7)followsinsequenceoneeventaftertheother(Program4.9).ThearrowsinFigure4.32representcausalevents.Inahandshakedinterface,oneeventcausesthenexttooccur,andthearrowscreatea“headtotail”sequence.Thereisnospecificminimumormaximumtimedelayforthesecausalevents,excepttheymustoccurinsequence.

1TransmitteroutputsnewData2TransmittermakesarisingedgeofReadysignifyingnewDataavailable3ReceivermakesafallingedgeofAcksignifyingitisstartingtoprocessdata4ThereceiverreadstheData5ReceivermakesarisingedgeonAcksignifyingithascapturedtheData6TransmittermakesafallingedgeonReadymeaningDataisnotvalid7TransmitternolongerneedstomaintainDataonitsoutputs

//Transmitter

voidOut(uint8_tdata)

Data=data;//1)

Ready=1;//2)

while(Ack);//3)

while(Ack==0);//5)

//Receiver

uint8_tIn(void)

uint8_tdata;

while(Ready==0);//2)

Ack=0;//3)

data=Data;//4)

Ready=0;//6)

Ack=1;//5)

while(Ready);//6)

return(data);

Program4.9.Handshakingroutinestoinitializeandtransferdata.

Oneoftheissuesinvolvedinhandshakedinterfacesiswhetherthereceivershouldwaitforbothsteps2and6orjuststep2.Similarly,shouldthetransmitterwaitforbothsteps3and5?Itisamorerobustdesigntoaffectallfourofthesewaits.Ifthereceiverdidnotwaitforstep6(Ready=0),thenasubsequentcallto In mayfindReadystillhighfromthelastcall,andreturnthesameDataasecondtime.Wecouldhaveemployededgetriggering,butsincetheinterfaceisinterlocked,thereisnoneedtocapturetheedges.Edge-triggeredmodeonthePG1wouldberequiredifinterruptsynchronizationweredesired.Program4.9showsthesolutionusingbusy-waitsynchronization.

Checkpoint4.6:AssumebothcomputersinProgram4.9haveexecutedtheinitialization.AssumeOutiscalledbeforeIn.Listthetime-sequencedexecutioninbothcomputers.RepeatforthecasethatIniscalledbeforeOut.

Observation:Programswrittenforembeddedcomputersaretightlycoupled(dependhighly)onthehardware,thereforeitisgoodprogrammingpracticetodocumentthehardwareconfigurationinthesoftware.

Performancetip:Wheninitializingoutputpins,itisbettertofirstwritethedesiredinitialoutputvalue,andthensetthedirectionregistertooutput.Thiswaythepingoesfrominputtooutputofthecorrectvalue,ratherthanfrominputtooutputofthewrongvalue,andthenoutputwiththecorrectvalue.

Handshakingisaveryreliablesynchronizationmethodwhenconnectingdevicesfromdifferentmanufacturersanddifferentspeeds.Italsoallowsyoutoupgradeonedevice(e.g.,getanewerandfastersensor)withoutredesigningbothsidesoftheinterface.HandshakingisusedfortheSmallComputerSystemsInterface(SCSI)andtheIEEE488instrumentationbus.

4.9.UARTInterfaceInthissectionwewilldevelopasimpledevicedriverusingtheUniversalAsynchronousReceiver/Transmitter(UART).Thisserialportallowsthemicrocontrollertocommunicatewithdevicessuchasothercomputers,printers,inputsensors,andLCDs.Serialtransmissioninvolvessendingonebitatime,suchthatthedataisspreadoutovertime.Thetotalnumberofbitstransmittedpersecondiscalledthebaudrate.Thereciprocalofthebaudrateisthebittime,whichisthetimetosendonebit.MostmicrocontrollershaveatleastoneUART.BeforediscussingthedetailedoperationontheLM3S/LM4F/TM4C,wewillbeginwithgeneralfeaturescommontoalldevices.EachUARTwillhaveabaudratecontrolregister,whichweusetoselectthetransmissionrate.Eachdeviceiscapableofcreatingitsownserialclockwithatransmissionfrequencyapproximatelyequaltotheserialclockinthecomputerwithwhichitiscommunicating.Aframeisthesmallestcompleteunitofserialtransmission.Figure4.33plotsthesignalversustimeonaserialport,showingasingleframe,whichincludesastartbit(whichis0),8bitsofdata(leastsignificantbitfirst),andastopbit(whichis1).Thereisalwaysonlyonestartbit,buttheStellaris/TivaUARTsallowustoselectthe5to8databitsand1or2stopbits.TheUARTcanaddeven,odd,ornoparitybit.However,wewillemploythetypicalprotocolof1startbit,8databits,noparity,and1stopbit.Thisprotocolisusedforbothtransmittingandreceiving.Theinformationrate,orbandwidth,isdefinedastheamountofdataorusefulinformationtransmittedpersecond.FromFigure4.33,weseethat10bitsaresentforeverybyteofusualdata.Therefore,thebandwidthoftheserialchannel(inbytes/second)isthebaudrate(inbits/sec)dividedby10.

Figure4.33.Aserialdataframewith8-bitdata,1startbit,1stopbit,andnoparitybit.CommonError:Ifyouchangethebusclockfrequencywithoutchangingthebaudrateregister,theUARTwilloperateatanincorrectbaudrate.

Checkpoint4.7:AssumingtheprotocoldrawninFigure4.33andabaudrateof9600bits/sec,whatisthebandwidthinbytes/sec?

Table4.8showsthethreemostcommonlyusedRS232signals.TheRS232standardusesaDB25connectorthathas25pins.TheEIA-574standardusesRS232voltagelevelsandaDB9connectorthathasonly9pins.ThemostcommonlyusedsignalsofthefullRS232standardareavailablewiththeEIA-574protocols.OnlyTxD,RxD,andSGarerequiredtoimplementasimplebidirectionalserialchannel,thustheothersignalsarenotshown(Figure4.34).Wedefinethedataterminalequipment(DTE)asthecomputeroraterminalandthedatacommunicationequipment(DCE)asthemodemorprinter.

DB25Pin

RS232Name

DB9Pin

EIA-574

Signal Description True DTE DCE

Name

2 BA 3 103 TxD TransmitData

-12V out in

3 BB 2 104 RxD ReceiveData

-12V in out

7 AB 5 102 SG SignalGround

Table4.8.Thecommonly-usedsignalsontheRS232andEIA-574protocols.

Figure4.34.HardwareinterfaceimplementinganasynchronousRS232channel.TheLM3S1968hasthreeUARTports.TheLM4F120andTM4C123haveeightUARTports.Observation:MostLM3S/LM4F/TM4CdevelopmentkitssendoneUARTchannelthroughtheUSBcable,sothecircuitshowninFigure4.34willnotbeneeded.OnthePCsideofthecable,theserialchannelbecomesavirtualCOMport.Afteryouhaveinstalledthedriversforyourdevelopmentkit,lookintheWindowsDeviceManagertoseewhichCOMportitis.

RS232isanon-return-to-zero(NRZ)protocolwithtruesignifiedasavoltagebetween-5and‑15V.Falseissignifiedbyavoltagebetween+5and+15V.AMAX3232converterchipisusedtotranslatebetweenthe+5.5/-5.5VRS232levelsandthe0/+3.3Vdigitallevels,asshowninFigure4.34.Thecapacitorsinthiscircuitareimportant,becausetheyformachargepumpusedtocreatethe±5.5voltagesfromthe+3.3Vsupply.TheRS232timingisgeneratedautomaticallybytheUART.Duringtransmission,theMaximchiptranslatesadigitalhighonmicrocontrollersideto-5.5VontheRS232/EIA‑574cable,andadigitallowistranslatedto+5.5V.Duringreceiving,theMaximchiptranslatesnegativevoltagesonRS232/EIA‑574cabletoadigitalhighonthemicrocontrollerside,andapositivevoltageistranslatedtoadigitallow.ThecomputerisclassifiedasDTE,soitsserialoutputispin3intheEIA‑574cable,anditsserialinputispin2intheEIA‑574cable.WhenconnectingaDTEtoanotherDTE,weuseacablewithpins2and3crossed.I.e.,pin2ononeDTEisconnectedtopin3ontheotherDTEandpin3ononeDTEisconnectedtopin2ontheotherDTE.WhenconnectingaDTEtoaDCE,thenthecablepassesthesignalsstraightacross.Inallsituations,thegroundsareconnectedtogetherusingtheSGwireinthecable.Thischannelisclassifiedasfullduplex,becausetransmissioncanoccurinbothdirectionssimultaneously.

4.9.1.TransmittinginasynchronousmodeWewillbeginwithtransmission,becauseitissimple.ThetransmitterportionoftheUARTincludesadataoutputpin,withdigitallogiclevelsasdrawninFigure4.33.Thetransmitterhasa16-elementFIFOanda10-bitshiftregister,whichcannotbedirectlyaccessedbytheprogrammer(Figure4.35).TheFIFOandshiftregisterinthetransmitterareseparatefromtheFIFOandshiftregisterassociatedwiththereceiver.TooutputdatausingtheUART,thesoftwarewillfirstchecktomakesurethetransmitFIFOisnotfull(itwillwaitifTXFFis1)andthenwritetothetransmitdataregister(e.g., UART0_DR_R ).Thebitsareshiftedoutinthisorder:start,b0,b1,b2,b3,b4,b5,b6,b7,andthenstop,whereb0istheLSBandb7istheMSB.Thetransmitdataregisteriswriteonly,whichmeansthesoftwarecanwritetoit(tostartanewtransmission)butcannotreadfromit.Eventhoughthetransmitdataregisterisatthesameaddressasthereceivedataregister,thetransmitandreceivedataregistersaretwoseparateregisters.

Figure4.35.Dataandshiftregistersimplementtheserialtransmission.

WhenanewbyteiswrittentoUART0_DR_R ,itisputintothetransmitFIFO.Bytebybyte,theUARTgetsdatafromtheFIFOandloadsthemintothe10-bittransmitshiftregister.The10-bitshiftregisterincludesastartbit,8databits,and1stopbit.Then,theframeisshiftedoutonebitatatimeataratespecifiedbythebaudrateregister.IftherearealreadydataintheFIFOorintheshiftregisterwhenthe UART0_DR_R iswritten,thenewframewillwaituntilthepreviousframeshavebeentransmitted,beforeittooistransmitted.TheFIFOguaranteesthedataaretransmittedintheordertheywerewritten.Theserialporthardwareisactuallycontrolledbyaclockthatis16timesfasterthanthebaudrate,referredtointhedatasheetasBaud16.Whenthedataarebeingshiftedout,thedigitalhardwareintheUARTcounts16timesinbetweenchangestotheU0Txoutputline.

Thesoftwarecanactuallywrite16bytestothe UART0_DR_R ,andthehardwarewillsendthemalloneatatimeintheproperorder.ThisFIFOreducesthesoftwareresponsetimerequirementsoftheoperatingsystemtoservicetheserialporthardware.Unfortunately,itdoescomplicatethehardware/softwaretiming.At9600bits/sec,ittakes1.04mstosendaframe.Therefore,therewillbeadelayrangingfrom1.04and16.7msbetweenwritingtothedataregisterandthecompletionofthedatatransmission.ThisdelaydependsonhowmuchdataarealreadyintheFIFOatthetimethesoftwarewritesto UART0_DR_R .

4.9.2.ReceivinginasynchronousmodeReceivingdataframesisalittletrickierthantransmissionbecausewehavetosynchronizethereceiveshiftregisterwiththeincomingdata.ThereceiverportionoftheUARTincludesanU0Rxdatainputpinwithdigitallogiclevels.Attheinputofthemicrocontroller,trueis3.3Vandfalseis0V.Thereisalsoa16-elementFIFOanda10-bitshiftregister,whichcannotbedirectlyaccessedbytheprogrammer(Figure4.36).AgainthereceiveshiftregisterandreceiveFIFOareseparatefromthoseinthetransmitter.Thereceivedataregister, UART0_DR_R ,isreadonly,whichmeanswriteoperationstothisaddresshavenoeffectonthisregister(recallwriteoperationsactivatethetransmitter).Thereceiverobviouslycannotstartatransmission,butitrecognizesanewframebyitsstartbit.Thebitsareshiftedinusingthesameorderasthetransmittershiftedthemout:start(0),b0,b1,b2,b3,b4,b5,b6,b7,andthenstop(1).

Therearesixstatusbitsgeneratedbyreceiveractivity.TheReceiveFIFOemptyflag,RXFE,isclearwhennewinputdataareinthereceiveFIFO.Whenthesoftwarereadsfrom UART0_DR_R ,dataareremovedfromtheFIFO.WhentheFIFObecomesempty,theRXFEflagwillbeset,meaningtherearenomoreinputdata.Thereareotherflagsassociatedwiththereceiver.ThereisaReceiveFIFOfullflagRXFF,whichissetwhentheFIFOisfull.Therearefourstatusbitsassociatedwitheachbyteofdata.Forthisreason,thereceiveFIFOis12bitswide.Theoverrunerror,OE,issetwheninputdataarelostbecausetheFIFOisfullandmoreinputframesarearrivingatthereceiver.Anoverrunerroriscausedwhenthereceiverinterfacelatencyistoolarge.Thebreakerror,BE,issetwhentheinputisheldlowformorethanaframe.ThePEbitissetonaparityerror.Becausetheerrorrateissolow,mostsystemsdonotimplementparity.Theframingerror,FE,issetwhenthestopbitisincorrect.Framingerrorsareprobablycausedbyamismatchinbaudrate.

Figure4.36.Dataregistershiftregistersimplementthereceiveserialinterface.Thereceiverwaitsforthe1to0edgesignifyingastartbit,thenshiftsin10bitsofdataoneatatimefromtheU0Rxline.TheinternalBaud16clockis16timesfasterthanthebaudrate.Afterthe1to0edge,thereceiverwaits8Baud16clocksandsamplesthestartbit.16Baud16clockslateritsamplesb0.Every16Baud16clocksitsamplesanotherbituntilitreachesthestopbit.TheUARTneedsaninternalclockfasterthanthebaudratesoitcanwaitthehalfabittimebetweenthe1to0edgebeginningthestartbitandthemiddleofthebitwindowneededforsampling.Thestartandstopbitsareremoved(checkedforframingerrors),the8bitsofdataand4bitsofstatusareputintothereceiveFIFO.TheFIFOimplementshardwarebufferingsodatacanbesafelystoredifthesoftwareisperformingothertasks.

Observation:IfthereceivingUARTdevicehasabaudratemismatchofmorethan5%,thenaframingerrorcanoccurwhenthestopbitisincorrectlycaptured.

Anoverrunoccurswhenthereare16elementsinthereceiveFIFO,anda17thframecomesintothereceiver.Inordertoavoidoverrun,wecandesignareal-timesystem,i.e.,onewithamaximumlatency.ThelatencyofaUARTreceiveristhedelaybetweenthetimewhennewdataarrivesinthereceiver(RXFE=0)andthetimethesoftwarereadsthedataregister.Ifthelatencyisalwayslessthan160bittimes,thenoverrunwillneveroccur.

Observation:Withaserialportthathasashiftregisterandonedataregister(noFIFObuffering),thelatencyrequirementoftheinputinterfaceisthetimeittakestotransmitonedataframe.

IntheexampleillustratedinFigure4.37,assumetheUARTreceiveshiftregisterandreceiveFIFOareinitiallyempty(RXFE=1).17incomingserialframesoccuronerightafteranother(lettersA–Q),butthesoftwaredoesnotrespond.Attheendofthefirstframe,the0x41goesintothereceiveFIFO,andtheRXFEflagiscleared.Normally,the UART_InChar functionwouldrespondtoRXFEbeingclearandreadthedatafromtheUART.Inthisscenariohowever,thesoftwareisbusydoingotherthingsanddoesnotrespondtothepresenceofdatainthereceiveFIFO.Next,15moreframesareshiftedinandenteredintothereceiveFIFO.Attheendofthe16thframe,theFIFOisfull(RXFF=1).Ifthesoftwareweretorespondatthispoint,thenall16characterswouldbeproperlyreceived.Ifthe17thframeoccursbeforethefirstisreadbythesoftware,thenanoverrunerroroccurs,andaframeislost.WecanseefromthisworstcasescenariothatthesoftwaremustreadthedatafromtheUARTwithin160bittimesoftheclearingofRXFE.

Figure4.37.Seventeenreceivedataframesresultinanoverrun(OE)error.

4.9.3.LM3S/LM4F/TM4CUARTDetailsNextwewilloverviewthespecificUARTfunctionsontheLM3S/LM4F/TM4Cmicrocontrollers.ThissectionisintendedtosupplementratherthanreplacetheTexasInstrumentsmanuals.WhendesigningsystemswithanyI/Omodule,youmustalsorefertothereferencemanualofyourspecificmicrocontroller.Itisalsogooddesignpracticetoreviewtheerrataforyourmicrocontrollertoseeifanyquirks(mistakes)existinyourmicrocontrollerthatmightapplytothesystemyouaredesigning.

TheLM3S/LM4F/TM4CmicrocontrollershaveonetoeightUARTs.ThespecificportpinsusedtoimplementtheUARTsvaryfromonechiptothenext.Tofindwhichpinsyourmicrocontrolleruses,youwillneedtoconsultitsdatasheet.Table4.9showssomeoftheregistersfortheUART0.IfthemicrocontrollerhasmultipleUARTs,theregisternameswillreplacethe0witha1–7.Fortheexactregisteraddresses,youshouldincludetheappropriateheaderfile(e.g.,tm4c1294ncpdt.h).ToactivateaUARTyouwillneedtoturnontheUARTclockintheSYSCTL_RCGCUART_Rregister.YoushouldalsoturnontheclockforthedigitalportintheSYSCTL_RCGCGPIO_Rregister.Youneedtoenablethetransmitandreceivepinsasdigitalsignals.Thealternativefunctionforthesepinsmustalsobeselected.

TheOE,BE,PE,andFEareerrorflagsassociatedwiththereceiver.Youcanseetheseflagsintwoplaces:associatedwitheachdatabytein UART0_DR_R orasaseparateerrorregisterin UART0_RSR_R .Theoverrunerror(OE)issetifdatahasbeenlostbecausetheinputdriverlatencyistoolong.BEisabreakerror,meaningtheotherdevicehassentabreak.PEisaparityerror(however,wewillnotbeusingparity).Theframingerror(FE)willgetsetifthebaudratesdonotmatch.Thesoftwarecanclearthesefourerrorflagsbywritinganyvalueto UART0_RSR_R .

ThestatusofthetwoFIFOscanbeseeninthe UART0_FR_R register.TheBUSYflagissetwhilethetransmitterstillhasunsentbits.ItwillbecomezerowhenthetransmitFIFOisemptyandthelaststopbithasbeensent.Ifyouimplementbusy-waitoutputbyfirstoutputtingthenwaitingforBUSYtobecome0(middleflowchartofFigure4.29),thentheroutinewillwritenewdataandreturnafterthatparticulardatahasbeencompletelytransmitted.

The UART0_CTL_R controlregistercontainsthebitsthatturnontheUART.TXEistheTransmitterEnablebit,andRXEistheReceiverEnablebit.WesetTXE,RXE,andUARTENequalto1inordertoactivatetheUARTdevice.However,weshouldclearUARTENduringtheinitializationsequence.

31–12 11 10 9 8 7–0 Name

$4000.C000 OE BE PE FE DATA UART0_DR_R

31–3 3 2 1 0

$4000.C004 OE BE PE FE UART0_RSR_R

31–8 7 6 5 4 3 2–0

$4000.C018 TXFE RXFF TXFF RXFE BUSY UART0_FR_R

31–16 15–0

$4000.C024 DIVINT UART0_IBRD_R

31–6 5–0

$4000.C028 DIVFRAC UART0_FBRD_R

31–8 7 6–5 4 3 2 1 0

$4000.C02C SPS WLEN FEN STP2 EPS PEN BRK UART0_LCRH_R

31–10 9 8 7 6–3 2 1 0

$4000.C030 RXE TXE LBE SIRLP SIREN UARTEN UART0_CTL_R

31–6 5-3 2-0

$4000.C034 RXIFLSEL TXIFLSEL UART0_IFLS_R

31-11 10 9 8 7 6 5 4

$4000.C038 OEIM BEIM PEIM FEIM RTIM TXIM RXIM UART0_IM_R

$4000.C03C OERIS BERIS PERIS FERIS RTRIS TXRIS RXRIS UART0_RIS_R

$4000.C040 OEMIS BEMIS PEMIS FEMIS RTMIS TXMIS RXMIS UART0_MIS_R

$4000.C044 OEIC BEIC PEIC FEIC RTIC TXIC RXIC UART0_ICR_R

Table4.9.SomeUARTregisters.Eachregisteris32bitswide.Shadedbitsarezero.

The UART0_IBRD_R andUART0_FBRD_R registersspecifythebaudrate.Thebaudratedividerisa22-bitbinaryfixed-pointvaluewitharesolutionof2-6.TheBaud16clockiscreatedfromthesystembusclock,withafrequencyof(Busclockfrequency)/divider.Thebaudrateis16timesslowerthanBaud16

Baudrate=Baud16/16=(Busclockfrequency)/(16*divider)

Forexample,ifthebusclockis8MHzandthedesiredbaudrateis19200bits/sec,thenthedividershouldbe8,000,000/16/19200or26.04167.Asabinaryfixed-pointnumber,thisnumberisabout11010.000011.Wecanestablishthisbaudratebyputtingthe11010into UART0_IBRD_R andthe000011into UART0_FBRD_R .Inreality,11010.000011isequalto1667/64or26.046875.Thebaudratesinthetransmitterandreceivermustmatchwithin5%forthechanneltooperateproperly.Theerrorforthisexampleis0.02%.

Thethreeregisters UART0_LCRH_R , UART0_IBRD_R ,and UART0_FBRD_R formaninternal30-bitregister.Thisinternalregisterisonlyupdatedwhenawriteoperationto UART0_LCRH_R isperformed,soanychangestothebaud-ratedivisormustbefollowedbyawritetothe UART0_LCRH_R registerforthechangestotakeeffect.Outofreset,bothFIFOsaredisabledandactas1-byte-deepholdingregisters.TheFIFOsareenabledbysettingtheFENbitin UART0_LCRH_R .

Checkpoint4.8:Assumethebusclockis6MHz.WhatisthebaudrateifUART0_IBRD_Requals10andUART0_FBRD_Requals20?

Checkpoint4.9:Assumethebusclockis50MHz.WhatvaluesshouldyouputinUART0_IBRD_RandUART0_FBRD_Rtomakeabaudrateof38400bits/sec?

TouseinterruptswewillenabletheFIFOsbysettingtheFENbitinthe UART0_LCRH_R register.RXIFLSELspecifiesthereceiveFIFOlevelthatcausesaninterrupt.TXIFLSELspecifiesthetransmitFIFOlevelthatcausesaninterrupt.

RXIFLSELRXFIFOSetRXMISinterrupttriggerwhen

0x0≥⅛fullReceiveFIFOgoesfrom1to2characters

0x1≥¼fullReceiveFIFOgoesfrom3to4characters

0x2≥½fullReceiveFIFOgoesfrom7to8characters

0x3≥¾fullReceiveFIFOgoesfrom11to12characters

0x4≥⅞fullReceiveFIFOgoesfrom13to14characters

TXIFLSELTXFIFOSetTXMISinterrupttriggerwhen

0x0≤⅞emptyTransmitFIFOgoesfrom15to14characters

0x1≤¾emptyTransmitFIFOgoesfrom13to12characters

0x2≤½emptyTransmitFIFOgoesfrom9to8characters

0x3≤¼emptyTransmitFIFOgoesfrom5to4characters

0x4≤⅛emptyTransmitFIFOgoesfrom3to2characters

TherearesevenpossibleinterrupttriggerflagsthatareintheUART0_RIS_R register.ThesettingoftheTXRISandRXRISflagsisdefinedabove.TheOERISflagissetonanoverrun,newincomingframereceivedbutthereceiveFIFOisfull.TheBERISflagissetonabreakerror.ThePERISflagissetonaparityerror.TheFERISflagissetonaframingerror(stopbitisnothigh).TheRTRISissetonareceivertimeout,whichiswhenthereceiverFIFOisnotemptyandnoincomingframeshaveoccurredina32-bittimeperiod.Eachoftheseventriggerflagshasacorrespondingarmbitinthe UART0_IM_Rregister.AbitintheUART0_MIS_R registersetifthetriggerflagisbothsetandarmed.Toacknowledgeaninterrupt(makethetriggerflagbecomezero),softwarewritesa1tothecorrespondingbitin UART0_IC_R .

4.9.4.UARTdevicedriverSoftwarethatsendsandreceivesdatamustimplementamechanismtosynchronizethesoftwarewiththehardware.Inparticular,thesoftwareshouldreaddatafromtheinputdeviceonlywhendataisindeedready.Similarly,softwareshouldwritedatatoanoutputdeviceonlywhenthedeviceisreadytoacceptnewdata.Withbusy-waitsynchronization,

thesoftwarecontinuouslychecksthehardwarestatuswaitingforittobeready.Inthissection,wewillusebusy-waitsynchronizationtowriteI/OprogramsthatsendandreceivedatausingtheUART.Afteraframeisreceived,thereceiveFIFOwillbenotempty(RXFEbecomes0)andthe8-bitdataisavailabletoberead.Togetnewdatafromtheserialport,thesoftwarefirstwaitsforRXFEtobezero,thenreadstheresultfrom UART0_DR_R .Recallthatwhenthesoftwarereads UART0_DR_R itgetsdatafromthereceiveFIFO.ThisoperationisillustratedinFigure4.38andshowninProgram4.10.Inasimilarfashion,whenthesoftwarewishestooutputviatheserialport,itfirstwaitsforTXFFtobeclear,thenperformstheoutput.Whenthesoftwarewrites UART0_DR_R itputsdataintothetransmitFIFO.AninterruptsynchronizationmethodwillbepresentedinChapter5.

Figure4.38.FlowchartsofInCharandOutCharusingbusy-waitsynchronization.Theinitializationprogram, UART_Init ,enablestheUARTdeviceandselectsthebaudrate.TheinputroutinewaitsinaloopuntilRXFEis0(FIFOnotempty),thenreadsthedataregister.TheoutputroutinefirstwaitsinaloopuntilTXFFis0(FIFOnotfull),thenwritesdatatothedataregister.Pollingbeforewritingdataisanefficientwaytoperformoutput.

//Assumesan80MHzbusclock,creates115200baudrate

voidUART_Init(void)//shouldbecalledonlyonce

SYSCTL_RCGCUART_R|=0x0001;//activateUART0

SYSCTL_RCGCGPIO_R|=0x0001;//activateportA

UART0_CTL_R&=~0x0001;//disableUART

UART0_IBRD_R=43;//IBRD=int(80000000/(16*115,200))=int(43.40278)

UART0_FBRD_R=26;//FBRD=round(0.40278*64)=26

UART0_LCRH_R=0x0070;//8-bitwordlength,enableFIFO

UART0_CTL_R=0x0301;//enableRXE,TXEandUART

GPIO_PORTA_PCTL_R=(GPIO_PORTA_PCTL_R&0xFFFFFF00)+0x00000011;//UART

GPIO_PORTA_AMSEL_R&=~0x03;//TM4C,disableanalogonPA1-0

GPIO_PORTA_AFSEL_R|=0x03;//enablealtfunctonPA1-0

GPIO_PORTA_DEN_R|=0x03;//enabledigitalI/OonPA1-0

//Waitfornewinput,thenreturnASCIIcode

charUART_InChar(void)

while((UART0_FR_R&0x0010)!=0);//waituntilRXFEis0

return((char)(UART0_DR_R&0xFF));

//Waitforbuffertobenotfull,thenoutput

voidUART_OutChar(chardata)

while((UART0_FR_R&0x0020)!=0);//waituntilTXFFis0

UART0_DR_R=data;

Program4.10.DevicedriverfunctionsthatimplementserialI/O(UART_xxx.zip).Checkpoint4.10:HowdoesthesoftwareclearRXFE?

Checkpoint4.11:HowdoesthesoftwareclearTXFF?

Checkpoint4.12:Describewhathappensifthereceivingcomputerisoperatingonabaudratethatistwiceasfastasthetransmittingcomputer?

Checkpoint4.13:Describewhathappensifthetransmittingcomputerisoperatingonabaudratethatistwiceasfastasthereceivingcomputer?

4.10.KeyboardInterfaceInthissectionweattempttointerfaceswitchestodigitalI/Opinsandwillconsiderthreeinterfacingschemes,asshowninFigure4.39.Inadirectinterfaceweconnecteachswitchuptoaseparatemicrocontrollerinputpin.Forexampleusingjustone8-bitparallelport,wecanconnect8switchesusingthedirectscheme.Anadvantageofthisinterfacingapproachisthatthesoftwarecanrecognizeall256(28)possibleswitchpatterns.Iftheswitcheswereremotefromthemicrocontroller,wewouldneeda9-wirecabletoconnectittothemicrocontroller.Ingeneral,iftherearenswitches,wewouldneedn/8parallelportsandn+1wiresinthecable.Thismethodwillbeusedwhenthereareasmallnumberofswitches,orwhenwemustrecognizemultiplesimultaneouskeypresses.Wewillusethedirectapproachformusickeyboardsandformodifierkeyssuchasshift,control,andalt.

Figure4.39.Threeapproachestointerfacingmultiplekeys.Inascannedinterfacetheswitchesareplacedinarow/columnmatrix.The atthefouroutputssignifiesopendrain(anoutputwithtwostates:HiZandlow.)Thesoftwaredrivesonerowatatimetozero,whileleavingtheotherrowsatHiZ.Byreadingthecolumn,thesoftwarecandetectifakeyispressedinthatrow.Thesoftware“scans”thedevicebycheckingallrowsonebyone.TheTable4.10illustratesthesequencetoscanthe4rows.

Row Out3 Out2 Out1 Out0

3 0 HiZ HiZ HiZ

2 HiZ 0 HiZ HiZ

1 HiZ HiZ 0 HiZ

0 HiZ HiZ HiZ 0

Table4.10.Scanningpatternsfora4by4matrixkeyboard.

Forcomputerswithoutanopendrainoutputmode,thedirectionregistercanbetoggledtosimulatethetwooutputstates,HiZ/0,oropendrainlogic.ThismethodcaninterfacemanyswitcheswithasmallnumberofparallelI/Opins.Inourexamplesituation,thesingle8-bitI/Oportcanhandle16switcheswithonlyan8-wirecable.Thedisadvantageofthescannedapproachoverthedirectapproachisthatitcanonlyhandlesituationswhere0,1or2switchesaresimultaneouslypressed.Thismethodisusedformostoftheswitchesinourstandardcomputerkeyboard.Theshift,alt,andcontrolkeysareinterfacedwiththedirectmethod.Wecan“arm”thisinterfaceforinterruptsbydrivingalltherowstozero.Theedge-triggeredinputcanbeusedtogenerateinterruptsontouchandrelease.Becauseoftheswitchbounce,anedge-triggeredinterruptwilloccurwhenanyofthekeyschange.Inthissectionwewillinterfacethekeypadusingbusy-waitsynchronization.

Withascannedapproach,wegiveuptheabilitytodetectthreeormorekeyspressedsimultaneously.Ifthreekeysarepressedinan“L”shape,thenthefourthkeythatcompletestherectanglewillappeartobepressed.Therefore,specialkeysliketheshift,control,option,andaltarenotplacedinthescannedmatrix,butratherareinterfaceddirectly,eachtoaseparateinputport.Ingeneral,annbymmatrixkeypadhasn*mkeys,butrequiresonlyn+mI/Opins.Youcandetectany0,1,or2keycombinations,butithastroublewhen3ormorearepressed.Thescannedkeyboardoperatesproperlyif

1.Nokeyispressed2.Exactlyonekeyispressed3.Exactlytwokeysarepressed.

Inamultiplexedinterface,thecomputeroutputsthebinaryvaluedefiningtherownumber,andahardwaredecoder(ordemultiplexer)willoutputthezeroontheselectedrowandHiZ’sontheotherrows.Thedecodermusthaveopencollectoroutputs(illustratedagainbythe intheabovecircuit.)Thecomputersimplyoutputsthesequence0x00,0x10,0x20,0x30,…,0xF0toscanthe16rows,asshownintheTable4.11.

Computeroutput Decoder

Row Out3 Out2 Out1 Out0 15 14 … 0

15 1 1 1 1 0 HiZ HiZ

14 1 1 1 0 HiZ 0 HiZ

…

1 0 0 0 1 HiZ HiZ HiZ

0 0 0 0 0 HiZ HiZ 0

Table4.11.Scanningpatternsforamultiplexed16by16matrixkeyboard.

Inasimilarway,thecolumninformationispassedtoahardwareencoderthatcalculatesthecolumnpositionofanyzerofoundintheselectedrow.Oneadditionalsignalisnecessarytosignifytheconditionthatnokeysarepressedinthatrow.Sincethisinterfacehas16rowsand16columns,wecaninterfaceupto256keys!Wecouldsacrificeoneofthecolumnstodetectthenokeypressedinthisrowsituation.Inthisway,wecaninterface240(15•16)keysonthesingle8-bitparallelport.Ifmorethanonekeyispressedinthesamerow,thismethodwillonlydetectoneofthem.Thereforeweclassifythisschemeasonlybeingabletohandlezerooronekeypressed.

Applicationsthatcanutilizethisapproachincludetouchscreensandtouchpadsbecausetheyhavealotofswitchesbutareonlyinterestedinthe0or1touchsituation.Implementinganinterruptdriveninterfacewouldrequiretoomuchadditionalhardware.Inthiscase,periodicpollinginterruptsynchronizationwouldbeappropriate.Ingeneral,annbymmatrixkeypadhasn*mkeys,butrequiresonlyx+y+1I/Opins,where2x=nand2y=m.Theextrainputisusedtodetecttheconditionwhennokeyispressedinthat

Example4.3:Interfacea16-keymatrixkeyboard.Therewillbeeitheronekeytouchedornokeystouched.

Solution:Thismatrixkeyboarddividesthesixteenkeysintofourrowsandfourcolumns,asshowninFigure4.40.Eachkeyexistsatauniquerow/columnlocation.ItwilltakeeightI/Opinstointerfacetherowsandcolumns.AnyoutputportontheLM3S/LM4F/TM4Ccouldhavebeenusedtointerfacetherows.Toscanthematrix,thesoftwarewilldrivetherowsoneatatimewithopencollectorlogicthenreadthecolumns.Theopencollectorlogic,withoutputsHiZand0,willbecreatedbytogglingthedirectionregisteronthefourrows.Actual10k pull-upresistorswillbeplacedonthecolumninputs(PA5-PA2)ratherthanconfiguredinternally,becausetheinternalpull-upsarenotfastenoughtohandlethescanningprocedure.

Figure4.40.Amatrixkeyboardinterfacedtothemicrocontroller.

Program4.11showstheinitializationsoftware.Thedatastructurewillassistinthescanningalgorithm,anditprovidesavisualmappingfromthephysicallayoutofthekeystotheASCIIcodeproducedwhentouchingthatkey.Thestructurealsomakesiteasytoadaptthissolutiontootherkeyboardinterfaces.Aperiodicinterruptcanbeusedtodebouncetheswitches.Thekeytodebouncingistonotobservetheswitchesmorefrequentlythanonceevery10ms.

voidMatrixKeypad_Init(void)

SYSCTL_RCGCGPIO_R|=0x09;//1)activateclockforPortsAandD

while((SYSCTL_PRGPIO_R&0x09)!=0x09);//ready?

GPIO_PORTA_AFSEL_R&=~0x3C;//GPIOfunctiononPA5-2

GPIO_PORTA_AMSEL_R&=~0x3C;//disableanalogfunctiononPA5-2

GPIO_PORTA_PCTL_R&=~0x00FFFF00;//configurePA5-2asGPIO

GPIO_PORTA_DEN_R|=0x3C;//enabledigitalI/OonPA5-2

GPIO_PORTA_DIR_R&=~0x3C;//makePA5-2in(PA5-2columns)

GPIO_PORTD_AFSEL_R&=~0x0F;//GPIOfunctiononPD3-0

GPIO_PORTD_AMSEL_R&=~0x0F;//disableanalogfunctiononPD3-0

GPIO_PORTD_PCTL_R&=~0x0000FFFF;//configurePD3-0asGPIO

GPIO_PORTD_DATA_R&=~0x0F;//DIRn=0,OUTn=HiZ;DIRn=1,OUTn=0

GPIO_PORTD_DEN_R|=0x0F;//enabledigitalI/OonPD3-0

GPIO_PORTD_DIR_R&=~0x0F;//makePD3-0in(PD3-0rows)

GPIO_PORTD_DR8R_R|=0x0F;//enable8mAdrive

Program4.11.Initializationsoftwareforamatrixkeyboard.

Program4.12showsthescanningsoftware.ThescanningsequenceislistedinTable4.12.Therearetwostepstoscanaparticularrow:

1.Selectthatrowbydrivingitlow,whiletheotherrowsareHiZ,2.Readthecolumnstoseeifanykeysarepressedinthatrow,0meansthekeyispressed

1meansthekeyisnotpressed

Itisimportanttoobservecolumnandrowsignalsonadualtraceoscilloscopewhilerunningthesoftwareatfullspeed,becauseittakestimeforcorrectsignaltoappearonthecolumnaftertherowischanged.Insomecases,asoftwaredelayshouldbeinsertedbetweensettingtherowandreadingthecolumn.Thelengthofthedelayyouwillneeddependsonthesizeofthepull-upresistorandanystraycapacitancethatmayexistinyourcircuit.

direction PD0 PD1 PD2 PD3 PA2 PA3 PA4 PA5

0x01 0 HiZ HiZ HiZ 1 2 3 A

0x02 HiZ 0 HiZ HiZ 4 5 6 B

0x04 HiZ HiZ 0 HiZ 7 8 9 C

0x08 HiZ HiZ HiZ 0 * 0 # D

Table4.12.Patternsfora4by4matrixkeyboard.

structRow

uint8_tdirection;

charkeycode[4];;

typedefconststructRowRowType;

RowTypeScanTab[5]=

0x01,“123A”,//row0

0x02,“456B”,//row1

0x04,“789C”,//row2

0x08,“*0#D”,//row3

0x00,“”;

/*ReturnsASCIIcodeforkeypressed,

Numisthenumberofkeyspressed

bothequalzeroifnokeypressed*/

charMatrixKeypad_Scan(int32_t*Num)

RowType*pt;

uint32_tcolumn;charkey;

uint32_tj;

(*Num)=0;

key=0;//defaultvalues

pt=&ScanTab[0];

while(pt->direction)

GPIO_PORTD_DIR_R=pt->direction;//oneoutput

GPIO_PORTD_DATA_R&=~0x0F;//DIRn=0,OUTn=HiZ;DIRn=1,OUTn=0

for(j=1;j<=10;j++);//veryshortdelay

column=((GPIO_PORTA_DATA_R&0x3C)>>2);//readcolumns

for(j=0;j<=3;j++)

if((column&0x01)==0)

key=pt->keycode[j];

(*Num)++;

column>>=1;//shiftintoposition

pt++;

returnkey;

/*Waitsforakeybepressed,thenreleased

ReturnsASCIIcodeforkeypressed,

Numisthenumberofkeyspressed

bothequalzeroifnokeypressed*/

charMatrixKeypad_In(void)int32_tn;

charletter;

do

letter=MatrixKeypad_Scan(&n);

while(n!=1);//repeatuntilexactlyone

do

letter=MatrixKeypad_Scan(&n);

while(n!=0);//repeatuntilrelease

returnletter;

Program4.12.Scanningsoftwareforamatrixkeyboard(MatrixKeypad_xxx.zip).

Two-keyrolloveroccurswhentheoperatoristypingquickly.Forexample,iftheoperatoristypingtheA,B,thenC,he/shemighttypeA,AB,B,BC,C,andthenrelease.Withrollover,thekeyboarddoesnotgothroughano-keystateinbetweentyping.ThehardwareinterfaceinFigure4.40couldhandletwo-keyrollover,butthesoftwaresolutioninProgram4.12doesnot.AninterruptversionofthisinterfacewillbepresentedinChapter5.

4.11.Exercises4.1In16wordsorless,describethemeaningofeachofthefollowingterms.

a)Latency

b)Real-time

c)Bandwidth

d)I/Obound

e)CPUbound

f)Blind-cyclesynchronization

g)Busy-waitsynchronization

h)Interruptsynchronization

i)Monotonic

j)Non-blocking

k)Deterministic

l)Slewrate

m)Opendrain

n)Handshake

4.2In16wordsorless,describethemeaningofeachofthefollowingterms.

a)Frame

b)Baudrate

c)Bandwidth

d)Fullduplex

e)DTE

f)DCE

g)Framingerror

h)Overrunerror

i)Devicedriver

j)NRZ

k)Bittime

l)Startandstopbits

4.3ListfivedifferentmethodsforI/Osynchronization.In16wordsorless,describeeachmethod.

4.4Listoneadvantageofblind-cyclesynchronization.Listonedisadvantage.

4.5Whatisdifferencebetweenbusy-waitandgadflysynchronization?

4.6In16wordsorless,definesetuptime.In16wordsorless,defineholdtime.

4.7Simplify↓A=5+[10,20]-[5,15]

4.8ConsideracircuitlikeFigure4.8,exceptusing74LS244and74LS374.Reworkthetimingequationstofindtheappropriatetimingrelationshipbetween↓G*,↑C,↑Cand↑G*sotheinterfacestillworks.Lookupthedatasheetsonwww.ti.com.

4.9ConsideracircuitinFigure4.8runningatvoltagesupplyVCCof2Vat25C.Reworkthetimingequationstofindtheappropriatetimingrelationshipbetween↓G*,↑C,↑Cand↑G*sotheinterfacestillworks.Lookupthedatasheetsonwww.ti.com.

4.10HighspeedCMOSlogicwillrunatawidevarietyofsupplyvoltages.Lookinthedatasheetsforthe74HC0474HC244and74HC374.MakeageneralobservationabouttherelationshipbetweenhowfastthechipoperatesandthepowersupplyvoltageforHClogic.

4.11Whatistheeffectofcapacitanceaddedtoadigitalline?

4.12WritecodetoconfigurePortDbit5asarisingedge-triggeredinput.WritecodethatwaitsforarisingedgeonPD5.

4.13WritecodetoconfigurePortAbit2asafallingedgeedge-triggeredinput.WritecodethatwaitsforafallingedgeonPA2.

4.14WritecodetoinitializeallofPortDasoutputs,inpowersavingmode.

4.15InterfaceapositivelogicswitchtoPA5withoutanexternalresistor.WritecodetoconfigurePortAbit5asneeded.Writecodethatwaitsfortheswitchtobetouched,thenreleased.Youmayassumetheswitchdoesnotbounce.

4.16InterfaceanegativelogicswitchtoPB6withoutanexternalresistor.WritecodetoconfigurePortBbit6asneeded.Writecodethatwaitsfortheswitchtobetouched,thenreleased.Youmayassumetheswitchdoesnotbounce.

4.17AssumeyouareusingtheFIFOimplementationsinProgram3.7.WhathappensintheKPNofProgram4.1ifaFIFObecomesempty?WhathappensinProgram4.1ifaFIFObecomesfull?

4.18Assumingthebusclocktobe8MHz,writecodetomakethebaudrate19200bits/sec.

4.19Assumingthebusclocktobe24MHz,whatvaluegoesinto UART0_IBRD_R and UART0_FBRD_R tomakethebaudrate9600bits/sec?

4.20LookuptheUARTmoduleinthedatasheetanddescribethesimilaritiesanddifferencesbetweenBUSYandTXFF.

4.21LookuptheUARTmoduleinthedatasheetanddescribethetwobitsofWLEN.

4.22LookuptheUARTmoduleinthedatasheet.

a)RedrawFigure4.33with1start,6data,noparityand1stopbit.

b)RedrawFigure4.33with1start,8data,noparityand2stopbits.

c)RedrawFigure4.33with1start,8data,evenparityand1stopbit.

4.23AssumeyouaregivenaworkingsystemwithaUARTconnectedtoanexternaldevice.Tosavepower,youdecidetoslowdownthebusclockbyafactorof4.BrieflyexplainthechangesrequiredfortheUARTsoftwaredriver.

4.24LookupinthedatasheethowtheUARToperatedwhentheFIFOsaredisabled.WouldthesolutioninProgram4.10stillworkiftheFIFOenablebit( UART0_LCRH_R bit4)werecleared?Ifitdoeswork,inwhatwayisitlessefficient?

4.25IsthewriteaccesstoPortAinProgram4.2friendly?WhatwouldyouchangeifyouwishedtoswitchthetwopinsonPortAtoPortE?

D4.26DrawtheequivalentoftheleftsideoftheKPNinProgram4.1usingaPetriNet.

D4.27LookatthedatasheetfortheNewhavenDisplayNHD-0108BZ-RN-YBW-3V3.WriteasoftwarefunctionusingtheLCDhardwareinSection4.7.1thatreadstheLCDstatusbit.Usethisfunctiontoconvert LCD_OutChar fromblindcycletobusywait.

D4.28Writethe Stepper_Seek functioninProgram4.6tominimizejerk.Slowlyrampuptheaccelerationasthemotorgoesfromstoppedtofullspeed( time parameter).Similarly,slowlyrampdowntheaccelerationasthemotorgoesfromfullspeedtostopped.

D4.29RewritetheprogramsinExample4.2touseUART0insteadoftheparallelports.Assumethebusclockis50MHz.Doitissuchawaythatthefunctionprototypesareidentical.LookupthemaximumallowablebaudratefortheUART.OpenoneoftheprojectsforExample4.2(Handshake_xxx.zip)fromthewebsiteandcountthenumberofassemblyinstructionsinboth Out and In .Addthesetwocountstogether(becausetheywaitforeachother).Multiplythetotalnumberofinstructionsby2busperiods(40ns).Usethisnumbertoestimatehowlongittakestosendonebyte.Comparethebandwidthsoftheserialandparallelapproaches.Mosthighspeedinterfacesuseaserialprotocol(e.g.,SATA,Ethernet.)Mostprocessorscommunicatewithmemoryusingaparallelprotocol.Listthreereasonsotherthanbandwidthforusingaseriallinkoveraparallelportwhencommunicatingbetweentwocomputers.

D4.30Rewrite UART_OutChar inProgram4.10tousebusy-waitsynchronizationontheBUSYbit.UsethemiddleflowchartinFigure4.29sotheroutinereturnsafterthedatahasbeentransmitted.

D4.31RewritethematrixkeypaddriverinPrograms4.12and4.13touseopendrainmodeonpinsPD3–PD0.Inotherwords,thedirectionregisterissetonceduringinitialization,andthedataregisterismodifiedduringscanning.

4.12.LabAssignmentsLab4.1Theoverallobjectiveistocreateaserialportdevicedriverthatsupportsfixed-pointinput/output.Theformatwillbe32-bitunsigneddecimalfixed-pointwitharesolutionof0.001.Youwillbeabletofindontheimplementationsofaserialportdriverthatsupportscharacter,string,andintegerI/O,UART_xxx.zip.Inparticular,youwilldesign,implement,andtesttworoutinescalled UART_FixIn and UART_FixOut . UART_FixIn willacceptinputfromtheUARTsimilartothefunction UART_InUDec . UART_FixOut willtransmitoutputtotheUARTsimilartothefunction UART_OutUDec .Duringthedesignphaseofthislab,youshoulddefinetherangeoffeaturesavailable.Youshoulddesign,implement,andtestamainprogramthatillustratestherangeofcapabilities.

Lab4.2ThesameasLab4.1excepttheformatwillbe32-bitsignedbinaryfixed-pointwitharesolutionof2-8.

Lab4.3Designafourfunction(add,subtract,multiply,divide)calculatorusingfixed-pointmathandtheUARTforinput/output.Theformatshouldbe32-bitsigneddecimalfixed-pointwitharesolutionof0.01.Youarefreetodesignthesyntaxofthecalculatorhoweveryouwish.

Lab4.4Designafourfunction(add,subtract,multiply,divide)calculatorusingfixed-pointmath.TakeinputfromamatrixkeypadandputoutputonanLCDorOLED.Theformatshouldbe32-bitsigneddecimalfixed-pointwitharesolutionof0.001.Youarefreetodesignthesyntaxofthecalculatorhoweveryouwish.

Lab4.5Theoverallobjectiveofthislabistodesign,implementandtestaparallelportexpander.Usinglessthan16I/Opinsofyourmicrocontroller,youwilldesignhardwareandsoftwarethatsupportstwo8-bitlatchedinputportsandtwostrobedoutputports.Eachinputporthas8datalinesandonelatchsignal.Ontherisingeachofthelatch,yoursystemshouldcapture(latch),thedatalines.Eachoutputporthas8datalinesandonestrobesignal.Thehardware/softwaresystemshouldgenerateapulseoutonstrobewhenevernewoutputissent.Theoutputportsdonotneedtobereadable.

Lab4.6Theoverallobjectiveofthislabistodesign,implementandtestaparalleloutputportexpander.UsingjustthreeI/Opinsofyourmicrocontroller,youwilldesignhardwareandsoftwarethatsupportsfour8-bitoutputports.Theoutputportsdonotneedtobereadable.

Lab4.7Theoverallobjectiveofthislabistodesign,implementandtestaparallelinputportexpander.UsingjustthreeI/Opinsofyourmicrocontroller,youwilldesignhardwareandsoftwarethatsupportsfour8-bitinputports.Theinputportsdonotneedtobelatched.

Lab4.8TheoverallobjectiveistocreateaHD44780-controlledLCDdevicedriverthatsupportsfixed-pointoutput.Theformatwillbe32-bitunsigneddecimalfixed-pointwithresolutionsof0.1,0.01and0.001.YouwillbeabletofindonthebookwebsiteimplementationsofaHD44780-controlledLCDdevicedriverthatsupportscharacter,string,andintegerI/O.Inparticular,youwilldesign,implement,andtestthreeroutinescalled LCD_FixOut1 LCD_FixOut2 and LCD_FixOut3 ,implementingfixed-pointresolutions0.1,0.01and0.001respectively.Duringthedesignphaseofthislab,youshoulddefinetherangeoffeaturesavailable.Youshoulddesign,implement,andtestamainprogramthatillustratestherangeofcapabilities.

Lab4.9ThesameasLab4.8excepttheformatwillbe32-bitsigneddecimalfixed-pointwithresolutionsof0.1,0.01and0.001.Youwilldesign,implement,andtestthreeroutines.

Lab4.10ThesameasLab4.8excepttheformatwillbe32-bitsignedbinaryfixed-pointwithresolutionsof2-4,2-8and2-12.Youwilldesign,implement,andtestthreeroutines.

Lab4.11ThesameasLab4.8excepttheformatswillbe12,24and32-bitunsignedintegers.Youwilldesign,implement,andtestthreeroutines.

Lab4.12TheoverallobjectiveistocreateaHD44780-controlledLCDdevicedriverthatsupportsvoltageversustimegraphicaloutput.Thedisplaywillbe8pixelshighby40pixelswide,using8characterpositionsontheLCD.YouwillneedtocontinuouslywritetotheCGRAMcreatingnewfonts,thenoutputthenewfontasdatatocreatetheimages.Theinitializationroutinewillclearthegraphandsettheminimumandmaximumrangescaleonthevoltageaxis(y-axis).Theplotroutinetakesavoltagedatapointbetweenminimumandmaximumanddrawsonepixelonthe8by40display.Ittakes40callstoplottocompleteoneimageonthedisplayasillustratedinFigure4.41.

Figure4.41.Imageafter40callstoplotwiththeLCDisinitializedtoMax=32,Min=0.Thesoftwareoutputsthisrepeatingpattern0,4,8,12,16,20,24,28,0,4,8,12,16…

Subsequentcallstoplotshouldremovethepointfrom40callsagoanddrawthenewpoint.Iftheplotiscalledatafixedperiod,thedisplayshouldshowacontinuoussweepofthevoltageversustimedata,likeanuntriggeredoscilloscope.Duringthedesignphaseofthislab,youshoulddefinetherangeoffeaturesavailable.Youshoulddesign,implement,andtestamainprogramthatillustratestherangeofcapabilities.

Lab4.13YouwillbeabletofindontheimplementationsofaOLEDdriverthatsupportstheprintfI/O,OLED_xxx.zip.Youwilldesign,implement,andtestthreeroutinescalled OLED_FixOut1 , OLED_FixOut2 ,and UART_FixOut3 .Thesethreefunctionsshouldimplement16-bitunsignedfixed-pointresolutions0.1,0.01and0.001respectively.Createfunctionslike Fixed_uDecOut2s thatconverttheintegerportionofthefixed-pointnumberwith =0.01toanASCIIstring.Youshouldbeabletocontrolthepositionandcoloroftheseoutputsinaconvenientmanner.Duringthedesignphaseofthislab,youshoulddefinetherangeoffeaturesavailable.Youshoulddesign,implement,andtestamainprogramthatillustratestherangeofcapabilities.

Lab4.14ThesameasLab4.13excepttheformatwillbe16-bitsigneddecimalfixed-pointwithresolutionsof0.1,0.01and0.001.Createfunctionslike Fixed_sDecOut2s thatconverttheintegerportionofthefixed-pointnumberwith =0.01toanASCIIstring.Youwilldesign,implement,andtestthreeroutines.

Lab4.15ThesameasLab4.13excepttheformatwillbe16-bitunsignedbinaryfixed-pointwithresolutionsof2-4,2-8and2-12.Youwilldesign,implement,andtestthreeroutines.Forexamplethefunction, Fixed_uBinOut8s convertstheintegerportionoftheunsigned16-bitfixed-pointnumberwith =2-8toanASCIIstring.

Lab4.16ThesameasLab4.13excepttheoutputwillgothroughtheUARTtoaCOMportonthePC.RunatermimalprogramlikePuTTyonthePC.

Lab4.17ThesameasLab4.14excepttheoutputwillgothroughtheUARTtoaCOMportonthePC.RunatermimalprogramlikePuTTyonthePC.

Lab4.18ThesameasLab4.15excepttheoutputwillgothroughtheUARTtoaCOMportonthePC.RunatermimalprogramlikePuTTyonthePC.

5.InterruptSynchronizationChapter5objectivesareto:•Introducetheconceptofinterruptsynchronization

•Discusstheissuesinvolvedinreentrantprogramming

•UsethefirstinfirstoutcircularqueueforbufferedI/O

•DiscussthespecificdetailsofusinginterruptsonCortex-M

•Interfacedevicesusingedge-triggeredinterrupts

•CreateperiodicinterruptsusingSysTick

•ImplementbackgroundI/Oforsimpledevicesusingperiodicpolling

Therearemanyreasonstoconsiderinterruptsynchronization.Thefirstconsiderationisthatthesoftwareinareal-timesystemmustrespondtohardwareeventswithinaprescribedtime.Givenachangeininput,itisnotonlynecessarytogetthecorrectresponse,butitwillbenecessarytogetthecorrectresponseatthecorrecttime.Toillustratetheneedforinterrupts,considerakeyboardinterfacewherethetimebetweennewkeyboardinputsmightbeassmallas10ms.Inthissituation,thesoftwarelatencyisthetimefromwhenthenewkeyboardinputisreadyuntilthetimethesoftwarereadsthenewdata.Inordertopreventlossofdatainthiscase,thesoftwarelatencymustbelessthan10ms.Wecanimplementreal-timesoftwareusingbusy-waitpollingonlywhenthesizeandcomplexityofthesystemisverysmall.Interruptsareimportantforreal-timesystemsbecausetheyprovideamechanismtoguaranteeanupperboundonthesoftwareresponsetime.Interruptsalsogiveusawaytorespondtoinfrequentbutimportantevents.Alarmconditionslikelowbatterypoweranderrorconditionscanbehandledwithinterrupts.Periodicinterrupts,generatedbythetimerataregularrate,willbenecessarytoimplementdataacquisitionandcontrolsystems.Intheunbufferedinterfacesofthepreviouschapter,thehardwareandsoftwaretookturnswaitingforeachother.Interruptsprovideawaytobufferthedata,sothatthehardwareandsoftwarespendlesstimewaiting.Inparticular,thebufferwewilluseisafirstinfirstoutqueueplacedbetweentheinterruptroutineandthemainprogramtoincreasetheoverallbandwidth.Wewillbeginourdiscussionwithgeneralissues,thenpresentthespecificdetailsabouttheLM3S/LM4F/TM4Cmicrocontrollers.Afterthat,anumberofsimpleinterruptexampleswillbepresented,andattheendofthechapterwewilldiscusssomeadvancedconceptslikepriorityandperiodicpolling.

5.1.MultithreadingAninterruptistheautomatictransferofsoftwareexecutioninresponsetoahardwareeventthatisasynchronouswiththecurrentsoftwareexecution.Thishardwareeventiscalledatrigger.ThehardwareeventcaneitherbeabusytoreadytransitioninanexternalI/Odevice(liketheUARTinput/output)oraninternalevent(likebusfault,memoryfault,oraperiodictimer.)Whenthehardwareneedsservice,signifiedbyabusytoreadystatetransition,itwillrequestaninterruptbysettingitstriggerflag.Athreadisdefinedasthepathofactionofsoftwareasitexecutes.Theexecutionoftheinterruptserviceroutineiscalledabackgroundthread.Thisthreadiscreatedbythehardwareinterruptrequestandiskilledwhentheinterruptserviceroutinereturnsfrominterrupt(e.g.,executingaBXLR ).Anewthreadiscreatedforeachinterruptrequest.Itisimportanttoconsidereachindividualrequestasaseparatethreadbecauselocalvariablesandregistersusedintheinterruptserviceroutineareuniqueandseparatefromoneinterrupteventtothenextinterrupt.Inamultithreadedsystem,weconsiderthethreadsascooperatingtoperformanoveralltask.Consequentlywewilldevelopwaysforthethreadstocommunicate(e.g.,FIFO)andsynchronizewitheachother.Mostembeddedsystemshaveasinglecommonoverallgoal.Ontheotherhand,general-purposecomputerscanhavemultipleunrelatedfunctionstoperform.Aprocessisalsodefinedastheactionofsoftwareasitexecutes.Processesdonotnecessarilycooperatetowardsacommonsharedgoal.ThreadsshareaccesstoI/Odevices,systemresources,andglobalvariables,whileprocesseshaveseparateglobalvariablesandsystemresources.ProcessesdonotshareI/Odevices.

Therearenostandarddefinitionsforthetermsmask,enableandarmintheprofessional,ComputerScience,orComputerEngineeringcommunities.Nevertheless,inthisbookwewilladheretothefollowingspecificmeanings.Toarm(disarm)adevicemeanstoenable(shutoff)thesourceofinterrupts.Eachpotentialinterruptingdevicehasaseparatearmbit.Onearms(disarms)adeviceifoneis(isnot)interestedininterruptsfromthissource.Toenable(disable)meanstoallowinterruptsatthistime(postponinginterruptsuntilalatertime).OntheARM ® CortexTM-Mprocessorthereisoneinterruptenablebitfortheentireinterruptsystem.Wedisableinterruptsifitiscurrentlynotconvenienttoacceptinterrupts.Inparticular,todisableinterruptswesettheIbitinPRIMASK.

Thesoftwarehasdynamiccontroloversomeaspectsoftheinterruptrequestsequence.First,eachpotentialinterrupttriggerhasaseparatearmbitthatthesoftwarecanactivateordeactivate.Thesoftwarewillsetthearmbitsforthosedevicesitwishestoacceptinterruptsfrom,andwilldeactivatethearmbitswithinthosedevicesfromwhichinterruptsarenottobeallowed.Inotherwordsitusesthearmbitstoindividuallyselectwhichdeviceswillandwhichdeviceswillnotrequestinterrupts.Thesecondaspectthatthesoftwarecontrolsistheinterruptenablebit.Specifically,bit0ofthespecialregisterPRIMASKistheinterruptmaskbit,I.Ifthisbitis1mostinterruptsandexceptionsarenotallowed,wewilldefineasdisabled.Ifthebitis0,theninterruptsareallowed,wewilldefineasenabled.TheBASEPRIregisterpreventsinterruptswithlowerpriorityinterrupts,butallowshigherpriorityinterrupts.ForexampleifBASEPRIequals3,thenrequestswithlevel0,1,and2caninterrupt,whilerequestsatlevels3andhigherwillbepostponed.Thesoftwarecanspecifytheprioritylevelofeachinterruptrequest.IfBASEPRIiszero,thenthepriorityfeatureisdisabledandallinterruptsareallowed.Four

conditionsmustbetrueforaninterrupttobegenerated:arm,enable,level,andtrigger.Adevicemustbearmed;interruptsmustbeenabled(I=0);theleveloftherequestedinterruptmustbelessthanBASEPRI;andanexternaleventmustoccursettingatriggerflag(e.g.,newUARTinputisready).Aninterruptcausesthefollowingsequenceofevents.First,thecurrentinstructionisfinished.Second,theexecutionofthecurrentlyrunningprogramissuspended,pushingeightregistersonthestack(R0,R1,R2,R3,R12,LR,PC,andPSRwiththeR0ontop).Third,theLRissettoaspecificvaluesignifyinganinterruptserviceroutine(ISR)isbeingrun(bits[31:8]to0xFFFFFF,bits[7:1]specifythetypeofinterruptreturntoperform,bit0willalwaysbe1ontheCortex-MmeaningThumbmode).Fourth,theIPSRissettotheinterruptnumberbeingprocessed.Lastly,thePCisloadedwiththeaddressoftheISR(vector).Thesefivesteps,calledacontextswitch,occurautomaticallyinhardwareasthecontextisswitchedfromforegroundtobackground.Next,thesoftwareexecutestheISR.

Ifatriggerflagisset,buttheprocessorisdisabled,theinterruptlevelisnothighenough,ortheflagisdisarmed,therequestisnotdismissed.Rathertherequestisheldpending,postponeduntilalatertime,whenthesystemdeemsitconvenienttohandletherequests.Clearingatriggerflagiscalledacknowledgement,whichoccursonlybyspecificsoftwareaction.Eachtriggerflaghasaspecificactionsoftwaremustperformtoclearthatflag.Wewillpayspecialattentiontotheseenable/disablesoftwareactions.Inparticularwewillneedtodisableinterruptswhenexecutingnonreentrantcodebutdisablinginterruptswillhavetheeffectofincreasingtheresponsetimeofsoftwaretoexternalevents.TheSysTickperiodicinterruptwillbetheonlyexampleofanautomaticacknowledgement.ForSysTick,theperiodictimerrequestsaninterrupt,butthetriggerflagwillbeautomaticallycleared.Foralltheothertriggerflags,theISRmustexplicitlyexecutecodethatclearstheflag.

Theinterruptserviceroutine(ISR)isthesoftwaremodulethatisexecutedwhenthehardwarerequestsaninterrupt.TheremaybeonelargeISRthathandlesallrequests(polledinterrupts),ormanysmallISRsspecificforeachpotentialsourceofinterrupt(vectoredinterrupts).TheCortex-Mhasbothpolledandvectoredinterrupts.Thedesignoftheinterruptserviceroutinerequirescarefulconsiderationofmanyfactors.TheISRmustacknowledgethetriggerflagthatcausedtheinterrupt.AftertheISRprovidesthenecessaryservice,itwillexecute BXLR .BecauseLRcontainsaspecialvalue,thisinstructionpullsthe8registersfromthestack,whichreturnscontroltothemainprogram.Therearetwostackpointers:PSPandMSP.ThesoftwareinthisbookwillexclusivelyusetheMSP.ItisimperativethattheISRsoftwarebalancethestackbeforeexiting.Executionofthemainprogramwillthencontinuewiththeexactstackandregistervaluesthatexistedbeforetheinterrupt.Althoughinterrupthandlerscancreateanduselocalvariables,parameterpassingbetweenthreadsmustbeimplementedusingsharedglobalmemoryvariables.Aprivateglobalvariablecanbeusedifaninterruptthreadwishestopassinformationtoitself,e.g.,fromoneinterruptinstancetoanother.Theexecutionofthemainprogramiscalledtheforegroundthread,andtheexecutionsofthevariousinterruptserviceroutinesarecalledbackgroundthreads.

AnaxiomwithinterruptsynchronizationisthattheISRshouldexecuteasfastaspossible.Theinterruptshouldoccurwhenitistimetoperformaneededfunction,andtheinterruptserviceroutineshouldperformthatfunction,andreturnrightaway.Placingbackwardbranches(busy-waitloops,iterations)intheinterruptsoftwareshouldbeavoidedifpossible.ThepercentageoftimespentexecutinganyoneISRshouldbeminimized.Foraninputdevice,theinterfacelatencyisthetimebetweenwhennewinputisavailable,andthetimewhenthesoftwarereadstheinputdata.WecanalsodefinedevicelatencyastheresponsetimeoftheexternalI/Odevice.Forexample,ifwerequestthatacertainsectorbereadfromadisk,thenthedevicelatencyisthetimeittaketofindthecorrecttrackandspinthedisk(seek)sothepropersectorispositionedunderthereadhead.Foranoutputdevice,theinterfacelatencyisthetimebetweenwhentheoutputdeviceisidle,andthetimewhenthesoftwarewritesnewdata.Areal-timesystemisonethatcanguaranteeaworstcaseinterfacelatency.

Manyfactorsshouldbeconsideredwhendecidingthemostappropriatemechanismtosynchronizehardwareandsoftware.Oneshouldnotalwaysusebusywaitbecauseoneistoolazytoimplementthecomplexitiesofinterrupts.Ontheotherhand,oneshouldnotalwaysuseinterruptsbecausetheyarefunandexciting.Busy-waitsynchronizationisappropriatewhentheI/Otimingispredicable,andwhentheI/Ostructureissimpleandfixed.BusywaitshouldbeusedfordedicatedsinglethreadsystemswherethereisnothingelsetodowhiletheI/Oisbusy.InterruptsynchronizationisappropriatewhentheI/Otimingisvariable,andwhentheI/Ostructureiscomplex.Inparticular,interruptsareefficientwhenthereareI/Odeviceswithdifferentspeeds.Interruptsallowforquickresponsetimestoimportantevents.Inparticular,usinginterruptsisonemechanismtodesignreal-timesystems,wheretheinterfacelatencymustbeshortandbounded.

Interruptscanalsobeusedforinfrequentbutcriticaleventslikepowerfailure,memoryfaults,andmachineerrors.Periodicinterruptswillbeusefulforreal-timeclocks,dataacquisitionsystems,andcontrolsystems.Forextremelyhighbandwidthandlowlatencyinterfaces,DMAshouldbeused.

Anatomicoperationisasequencethatoncestartedwillalwaysfinish,andcannotbeinterrupted.AllinstructionsontheARM ® CortexTM-Mprocessorareatomicexceptstoreandloadmultiple, STMLDM .Ifwewishtomakeasectionofcodeatomic,wecanrunthatcodewithI=1.Inthisway,interruptswillnotbeabletobreakapartthesequence.Again,requestedthataretriggeredwhileI=1arenotdismissed,butsimplypostponeduntilI=0.Inparticular,toimplementanatomicoperationwewill1)savethecurrentvalueofthePRIMASK,2)disableinterrupts,3)executetheoperation,and4)restorethePRIMASKbacktoitspreviousvalue.

Checkpoint5.1:Whatfourconditionsmustbetrueforaninterrupttooccur?

Checkpoint5.2:Howdoyouenableinterrupts?

Checkpoint5.3:Whatarethestepsthatoccurwhenaninterruptisprocessed?

Asyoudevelopexperienceusinginterrupts,youwillcometonoticeafewcommonaspectsthatmostcomputersshare.Thefollowingparagraphsoutlinethreeessentialmechanismsthatareneededtoutilizeinterrupts.Althougheverycomputerthatusesinterruptsincludesallthreemechanismsthereareawidespectrumofimplementationmethods.

Allinterruptingsystemsmusthavetheabilityforthehardwaretorequestactionfromcomputer.Theinterruptrequestscanbegeneratedusingaseparateconnectiontoprocessorforeachdevice,orusingasharednegativelogicwire-orrequestsusingopencollectorlogic.TheLM3S/TM4Cmicrocontrollersuseseparateconnectionstorequestinterrupts.

Allinterruptingsystemsmusthavetheabilityforthecomputertodeterminethesource.Avectoredinterruptsystememploysseparateconnectionsforeachdevicesothatthecomputercangiveautomaticresolution.Youcanrecognizeavectoredsystembecauseeachdevicehasaseparateinterruptvectoraddress.Withapolledinterruptsystem,theinterruptsoftwaremustpolleachdevice,lookingforthedevicethatrequestedtheinterrupt.MostinterruptsontheLM3S/TM4Cmicrocontrollersarevectored,buttherearesometriggers,likeedge-triggeredinterruptsontheGPIOpins,thatsharethesamevector.FortheseinterruptstheISRmustpolltoseewhichtriggercausedtheinterrupt.

Thethirdnecessarycomponentoftheinterfaceistheabilityforthecomputertoacknowledgetheinterrupt.Normallythereisatriggerflagintheinterfacethatissetonthebusytoreadystatetransition,i.e.,whenthedeviceneedsservice.Inessencethistriggerflagisthecauseoftheinterrupt.Acknowledgingtheinterruptinvolvesclearingthisflag.Itisimportanttoshutofftherequest,sothatthecomputerwillnotmistakenlyrequestasecond(andinappropriate)interruptserviceforthesamecondition.ThefirstIntelx86processorsusedahardwareacknowledgmentthatautomaticallyclearstherequest.ExceptforperiodicSysTick,LM3S/TM4Cmicrocontrollersusesoftwareacknowledge.Sowhendesigninganinterruptinginterface,itwillbeimportanttoknowexactlywhathardwareconditionswillsetthetriggerflag(andrequestaninterrupt)andhowthesoftwarewillclearit(acknowledge)intheISR.

CommonError:Thesystemwillcrashiftheinterruptserviceroutinedoesn’teitheracknowledgeordisarmthedevicerequestingtheinterrupt.

CommonError:TheISRsoftwareshouldnotdisableinterruptsatthebeginningnorshoulditreenableinterruptsattheend.WhichinterruptsareallowedtorunisautomaticallycontrolledbytheNVIC.

5.2.InterthreadCommunicationandSynchronizationForregularfunctioncallsweusetheregistersandstacktopassparameters,butinterruptthreadshavelogicallyseparateresistersandstack.Inparticular,registersareautomaticallysavedbytheprocessorasitswitchesfrommainprogram(foregroundthread)tointerruptserviceroutine(backgroundthread).ExitinganISRwillrestoretheregistersbacktotheirpreviousvalues.Thus,allparameterpassingmustoccurthroughglobalmemory.Onecannotpassdatafromthemainprogramtotheinterruptserviceroutineusingregistersorthestack.

Inthischapter,multithreadingmeansonemainprogram(foregroundthread)andmultipleISRs(backgroundthreads).Anoperatingsystemallowsmultipleforegroundthreads.Synchronizingthreadsisacriticaltaskaffectingefficiencyandeffectivenessofsystemsusinginterrupts.Inthissection,wewillpresentingeneralformthreeconstructstosynchronizethreads:binarysemaphore,mailbox,andFIFOqueue.

Abinarysemaphoreissimplyasharedflag,asdescribedinFigure5.1.Therearetwooperationsonecanperformonasemaphore.Signalistheactionthatsetstheflag.Waitistheactionthatcheckstheflag,andiftheflagisset,theflagisclearedandimportantstuffisperformed.ThisflagmustexistasaprivateglobalvariablewithrestrictedaccesstoonlytheWaitandSignalfunctions.InC,weaddthequalifier static toaglobalvariabletorestrictaccesstosoftwarewithinthesamefile.Inordertoreducecomplexityofthesystem,itwillbeimportanttolimittheaccesstothisflagtoasfewmodulesaspossible.

Thehistoryofsemaphoresdatesbacktotheinventionoffire.Manhasusedopticaltelegraphssuchasfire,smoke,andflagstocommunicateovershortdistances.Thetransmitterencodestheinformationasrecognizableimages.Theobserveratadistantlocationsees(receives)thesignalsproducedbythetransmitter.Semaphoreswereusedintheearlyrailroadstoallowmultipletrainstoshareaccesstoacommontrack.Thepositionofthesemaphoreflagdescribedthestatusoftheupcomingtrack.ThebasicideaofthesephysicalsemaphoresisusedinI/Ointerfacing.Asharedflagdescribesthestatus,andthreadscanobserveorchangetheflag.

Figure5.1.Asemaphorecanbeusedtosynchronizethreads.

Withbinarysemaphores,theflaghastwostates:0and1.Itisgooddesigntoassignameaningtothisflag.Forexample,0mightmeantheswitchhasnotbeenpressed,and1mightmeantheswitchhasbeenpressed.Figure5.1showstwoexamplesofthebinarysemaphore.Thebigarrowsinthisfiguresignifythesynchronizationlinkbetweenthethreads.Intheexampleontheleft,theISRsignalsthesemaphoreandthemainprogramwaitsonthesemaphore.Noticethe“importantstuff”isrunintheforegroundonceperexecutionoftheISR.Intheexampleontheright,themainprogramsignalsthesemaphoreandtheISRwaits.ItisgooddesigntohaveNObackwardsjumpsinanISR.Inthisparticularapplication,iftheISRisrunningandthesemaphoreis0,theactionisjustskippedandthecomputerreturnsfromtheinterrupt.

Acountingsemaphoreisasharedcounter,representingthenumberofobjects.Whenanewobjectiscreated,thecounterisincremented,andwhenanobjectisdestroyedthecounterisdecremented.Acounterdescribesthestatus,andthreadscanobserve,incrementordecrement.

Thesecondinterthreadsynchronizationschemeisthemailbox.Themailboxisabinarysemaphorewithassociateddatavariable.Figure5.2illustratesaninputdeviceinterfacedusinginterruptsynchronization.

Figure5.2.Amailboxcanbeusedtopassdatabetweenthreads.ThebigarrowinFigure5.2signifiesthecommunicationandsynchronizationlinkbetweenthebackgroundandforeground.Themailboxstructureisimplementedwithtwosharedglobalvariables.Mail containsdata,and Status isabinarysemaphorespecifyingwhetherthemailboxisfullorempty.Theinterruptisrequestedonitshardwaretrigger,signifyingnewdataarereadyfromtheinputdevice.TheISRwillreadthedatafromtheinputdeviceandstoreitinthesharedglobalvariableMail ,thenupdateitsstatustofull.Themainprogramwillperformothercalculations,whileoccasionallycheckingthestatusofthemailbox.Whenthemailboxhasdata,themainprogramwillprocessit.Thisapproachisadequateforsituationswheretheinputbandwidthisslowcomparedtothesoftwareprocessingspeed.

Onewaytovisualizetheinterruptsynchronizationistodrawastateversustimeplotoftheactivitiesofthehardware,themailbox,andthetwosoftwarethreads.Figure5.3showsthatattime(a)themailboxisempty,theinputdeviceisidleandthemainprogramisperformingothertasks,becausemailboxisempty.Whennewinputdataareready,thetriggerflagwillbeset,andaninterruptwillberequested.Attime(b)theISRreadsdatafrominputdeviceandsavesitinMail ,andthenitsets Status tofull.Attime(c)themainprogramrecognizes Status isfull.Attime(d)themainprogramprocessesdatafromMail ,sets Status toempty.Noticethateventhoughtherearetwothreads,onlyoneisactiveatatime.TheinterrupthardwareswitchestheprocessorfromthemainprogramtotheISR,andthereturnfrominterruptswitchestheprocessorback.

Figure5.3.Hardware/softwaretimingofaninputinterfaceusingamailbox.ThethirdsynchronizationtechniqueistheFIFOqueue,showninFigure5.4.Withmailboxsynchronization,thethreadsexecuteinlock-step:one,theother,one,theother…However,withtheFIFOqueueexecutionofthethreadsismorelooselycoupled.Theclassicproducer/consumerproblemhastwothreads.Onethreadproducesdataandtheotherconsumesdata.Foraninputdevice,thebackgroundthreadistheproducerbecauseitgeneratesnewdata,andtheforegroundthreadistheconsumerbecauseitusesupthedata.Foranoutputdevice,thedataflowsintheotherdirectionsotheproducer/consumerrolesarereversed.ItisappropriatetopassdatafromtheproducerthreadtotheconsumerthreadusingaFIFOqueue.Figure5.4showsoneproducerlinkedtooneconsumer.However,itispossibletohavemultipleproducersconnectedtomultipleconsumers.AnexampleofmultipleproducersisaUSBhuborEthernetrouter,wherepacketscanarrivefrommultipleessentiallyequivalentports.Anothernameforthisisclient-server,wheretheserverproducesdataandtheclientconsumesdata.

Figure5.4.Inaproducer/consumersystem,FIFOqueuescanbeusedtopassdatabetweenthreads.Observation:Forsystemswithinterrupt-drivenI/Oonmultipledevices,therewillbeaseparateFIFOforeachdevice.

WecouldprocessthedatawithintheISRitself,andjustreporttheresultsoftheprocessingtothemainprogramusingthemailbox.ProcessingdataintheISRisusuallypoordesignbecausewetrytominimizethetimerunningintheISR,inordertominimizelatencyofotherinterrupts.

Aninputdeviceneedsservice(busytodonestatetransition)whennewdataareavailable,seeFigures5.4and5.5.Theinterruptserviceroutine(background)willacceptthedataandputitintoaFIFO.Typically,theISRwillrestarttheinputhardware,causingadonetobusytransition.

Anoutputdeviceneedsservice(busytodonestatetransition)whenthedeviceisidle,readytooutputmoredata.Theinterruptserviceroutine(background)willgetmoredatafromtheFIFOandoutputit.Theoutputfunctionwillrestartthehardwarecausingadonetobusytransition.Twoparticularproblemswithoutputdeviceinterruptsare

1.Howdoesonegeneratethefirstinterrupt?

Inotherwords,howdoesonestarttheoutputthread?and

2.Whatdoesonedoifanoutputinterruptoccurs(deviceisidle)

butthereisnomoredatacurrentlyavailable(e.g.,FIFOisempty)?

Theforegroundthread(mainprogram)executesaloop,andaccessestheappropriateFIFOwhenitneedstoinputoroutputdata.Thebackgroundthreads(interrupts)areexecutedwhenthehardwareneedsservice.

Figure5.5.Theinputdeviceinterruptswhenithasnewdata,andtheoutputdeviceinterruptswhenidle.Onewaytovisualizetheinterruptsynchronizationistodrawastateversustimeplotoftheactivitiesofthehardwareandthetwosoftwaremodules.Figure5.6isdrawntodescribeasituationwherethetimebetweeninputsisabouttwiceaslongasittakesthesoftwaretoprocessthedata.Forthisexample,themainthreadbeginsbywaitingbecausetheFIFOisempty(a).Whentheinputdeviceisbusyitisintheprocessofcreatingnewinput.Whentheinputdeviceisdone,newdataareavailableandaninterruptisrequested.TheinterruptserviceroutinewillreadthedataandputitintotheFIFO(b).OncedataareintheFIFO,themainprogramisreleasedtogoonbecausethegetfunctionwillreturnwithdata(c).Themainprogramprocessesthedata(d)andthenwaitsformoreinput(a).Thearrowsfromonegraphtotheotherrepresentthesynchronizingevents.Becausethetimeforthesoftwaretoreadandprocessthedataislessthanthetimefortheinputdevicetocreatenewinput,thissituationiscalledI/Obound.Inthissituation,theFIFOhaseither0or1entry,andtheuseofinterruptsdoesnotenhancethebandwidthoverthebusy-waitimplementationspresentedinthepreviouschapter.EvenwithanI/Obounddeviceitmaybemoreefficienttoutilizeinterruptsbecauseitprovidesastraight-forwardapproachtoservicingmultipledevices.

Figure5.6.Hardware/softwaretimingofanI/Oboundinputinterface.

Inthissecondexample,theinputdevicestartswithaburstofhighbandwidthactivity.Figure5.7isdrawntodescribeasituationwheretheinputrateistemporarilytwotothreetimesfasterthanthesoftwarecanhandle.Aslongastheinterruptserviceroutineisfastenoughtokeepupwiththeinputdevice,andaslongastheFIFOdoesnotbecomefullduringtheburst,nodataarelost.Thesoftwarewaitsforthefirstdata(a),butthendoesnothavetowaituntiltheburstisover.Inthissituation,theoverallbandwidthishigherthanitwouldbewithabusy-waitimplementation,becausetheinputdevicedoesnothavetowaitforeachdatabytetobeprocessed(b).Thisistheclassicexampleofa“buffered”input,becausetheISRputsdatatheFIFO.ThemainprogramgetsdatafromtheFIFO(c),andthenprocessesit(d).WhentheI/Odeviceisfasterthanthesoftware,thesystemiscalledCPUbound.Aswewillseelater,thissystemwillworkonlyiftheproducerratetemporarilyexceedstheconsumerrate(ashortburstofhighbandwidthinput).Iftheexternaldevicesustainedthehighbandwidthinputrate,thentheFIFOwouldbecomefullanddatawouldbelost.

Figure5.7.Hardware/softwaretimingofaninputinterfaceduringahighbandwidthburst.Foraninputdevice,iftheFIFOisusuallyempty,theinterfaceisI/Obound.Duringtimeswhentherearemanyelements,theinterfaceisCPUbound.

Foranoutputdevice,theinterruptisrequestedwhentheoutputisidleandreadytoacceptmoredata.The“busytodone”statetransitioncausesaninterrupt.Theinterruptserviceroutinegivestheoutputdeviceanotherpieceofdatatooutput.Again,wecanvisualizetheinterruptsynchronizationbydrawingastateversustimeplotoftheactivitiesofthehardwareandthetwosoftwaremodules.Figure5.8isdrawntodescribeasituationwherethetimebetweenoutputsisabouthalfaslongasittakesthesoftwaretogeneratenewdata.Foranoutputdeviceinterface,theoutputdeviceisinitiallydisarmedandtheFIFOisempty.Themainthreadbeginsbygeneratingnewdata(a).AfterthemainprogramputsthedataintotheFIFOitarmstheoutputinterrupts(b).ThisfirstinterruptoccursimmediatelyandtheISRgetssomedatafromtheFIFOandoutputsittotheexternaldevice(c).Theoutputdevicebecomesbusybecauseitisintheprocessofoutputtingdata.Itisimportanttorealizethatitonlytakesthesoftwareontheorderof1µsectowritedatatooneofitsoutputports,butusuallyittakestheoutputdevicemuchlongertofullyprocessthedata.Whentheoutputdeviceisdone,itisreadytoacceptmoredataandaninterruptisrequested.IftheFIFOisemptyatthispoint,theISRwilldisarmtheoutputdevice(d).IftheFIFOisnotempty,theinterruptserviceroutinewillgetfromtheFIFO,andwriteitouttotheoutputport.Oncedataarewrittentotheoutputport,theoutput

deviceisreleasedtogoon.Inthisfirstexample,thetimeforthesoftwaretogeneratedataislargerthanthetimefortheexternaldevicetooutputit.ThisisanexampleofaCPUboundsystem.Inthissituation,theFIFOhaseither0or1entry,andtheuseofinterruptsdoesnotenhancethebandwidthoverthebusy-waitimplementationspresentedinthepreviouschapter.Neverthelessinterruptsprovideawell-definedmechanismfordealingwithcomplexsystems.

Figure5.8.Hardware/softwaretimingofaCPUboundoutputinterface.Inthissecondoutputexample,thesoftwarestartswithaburstofhighbandwidthactivity.Figure5.9isdrawntodescribeasituationwherethesoftwareproducesdataataratethatistemporarilymuchfasterthanthehardwarecanhandle.AslongastheFIFOdoesnotbecomefull,nodataarelost.Inthissituation,theoverallbandwidthishigherthanitwouldbewithabusy-waitimplementation,becausethesoftwaredoesnothavetowaitforeachdatabytetobeprocessedbythehardware.Thesoftwaregeneratesdata(a)andputsitintotheFIFO(b).Whentheoutputisidle,itgeneratesaninterrupt.TheISRgetsdataandrestartstheoutputdevice(c).

Figure5.9.Hardware/softwaretimingofanI/Oboundoutputinterface.Thisistheclassicexampleofa“buffered”output,becausedataentersthesystem(viathemainprogram)istemporarilystoredinabuffer(putintotheFIFO)andthedataareprocessedlater(bytheISR,getfromtheFIFO,writetoexternaldevice.)WhentheI/Odeviceisslowerthanthesoftware,thesystemiscalledI/Obound.Justliketheinputscenario,theFIFOmightbecomefulliftheproducerrateistoohighfortoolong.

Thereareothertypesofinterruptthatarenotaninputoroutput.Forexamplewewillconfigurethecomputertorequestaninterruptonaperiodicbasis.Thismeansaninterrupthandlerwillbeexecutedatfixedtimeintervals.Thisperiodicinterruptwillbeessentialfortheimplementationofreal-timedataacquisitionandreal-timecontrolsystems.Forexampleifweareimplementingadigitalcontrollerthatexecutesacontrolalgorithm100timesasecond,thenwewillsetuptheinternaltimerhardwaretorequestaninterruptevery10ms.Theinterruptserviceroutinewillexecutethedigitalcontrolalgorithmandreturntothemainthread.WewilluseperiodicinterruptsinChapters10and12.

PerformanceTip:ItispoordesigntoemploybackwardjumpsinanISR,becausetheymayaffectthelatencyofotherinterruptrequests.Wheneveryouarethinkingaboutusingabackwardjump,considerredesigningthesystemwithmoreordifferenttriggerstoreducethenumberofbackwardjumps.

Asyourecall,theFIFOpassesthedatafromtheproducertotheconsumer.Ingeneral,theratesatwhichdataareproducedandconsumedcanvarydynamically.Humansdonotenterdataintoakeyboardataconstantrate.Evenprintersrequiremoretimetoprintcolorgraphicsversusblackandwhitetext.Lettpbethetime(insec)betweencallsto Fifo_Put ,andrpbethearrivalrate(producerrateinbytes/sec)intothesystem,sorp=1/tp.Similarly,lettgbethetime(insec)betweencallsto Fifo_Get ,andrgbetheservicerate(consumerrateinbytes/sec)outofthesystem,sorg=1/tg.

Iftheminimumtimebetweencallsto Fifo_Put isgreaterthanthemaximumtimebetweencallsto Fifo_Get ,thenaFIFOisnotnecessaryandthedataflowcouldbesolvedwithamailbox.I.e.,noFIFOisneededifmin(tp)≥max(tg).Ontheotherhand,ifthetimebetweencallsto Fifo_Put becomeslessthanthetimebetweencallsto Fifo_Get becauseeither

•Thearrivalratetemporarilyincreases

•Theserviceratetemporarilydecreases

theninformationwillbecollectedintheFIFO.Forexample,apersonmighttypeveryfastforawhile,followedbylongpause.TheFIFOcouldbeusedtocapturewithoutlossallthedataasitcomesinveryfast.Clearlyonaveragethesystemmustbeabletoprocessthedata(theconsumerthread)atleastasfastastheaveragerateatwhichthedataarrives(producerthread).Iftheaverageproducerrateislargerthantheaverageconsumerrate

Ave(rp)>Ave(rg)

thentheFIFOwilleventuallyoverflownomatterhowlargetheFIFO.Iftheproducerrateistemporarilyhigh,andthatcausestheFIFOtobecomefull,thenthisproblemcanbesolvedbyincreasingtheFIFOsize.

Thereisfundamentaldifferencebetweenanemptyerrorandafullerror.ConsidertheapplicationofusingaFIFObetweenyourcomputeranditsprinter.Thisisagoodideabecausethecomputercantemporarilygeneratedatatobeprintedataveryhighratefollowedbylongpauses.Theprinterislikeaturtle.Itcanprintataslowbutsteady

rate.ThecomputerwillputabyteintotheFIFOthatitwantsprinted.TheprinterwillgetabyteoutoftheFIFOwhenitisreadytoprintanothercharacter.Afullerroroccurswhenthecomputercalls Fifo_Put attoofastarate.Afullerrorisserious,becauseifignoreddatawillbelost.RecallthatoneofthedefinitionsofaKahnProcessNetworkisthattheFIFOsareneverfull.So,implementingthedataflowinsuchawaythattheFIFOsneverbecomefullallowsustomodelthesystemasaKPN.Ontheotherhand,anemptyerroroccurswhentheprinterisreadytoprintbutthecomputerhasnothinginmind.Anemptyerrorisnotserious,becauseinthiscasetheprinterjustsitstheredoingnothing.

Checkpoint5.4:IftheFIFObecomesfull,canthesituationalwaysbesolvedbyincreasingthesize?

ConsideraFIFOthathasafeaturewherewecandeterminethenumberofelementsbycalling Fifo_Size .Ifweplacethisdebugginginstrumentinsidetheproducer,wecanmeasureahistogramofFIFOsizestellingus1)iftheFIFOeverbecamefull;2)iftheinterfaceisCPUbound;or3)iftheinterfaceisI/Obound.

uint32_tHistogram[FIFOSIZE];

#defineCollect()(Histogram[Fifo_Size()]++;)

5.3.CriticalSectionsIngeneral,iftwothreadsaccessthesameglobalmemoryandoneoftheaccessesisawrite,thenthereisacausaldependencybetweenthethreads.Thismeans,theexecutionordermayaffecttheoutcome.Sharedglobalvariablesareveryimportantinmultithreadedsystemsbecausetheyarerequiredtopassdatabetweenthreads,buttheyarecomplicatedanditishardtofindbugsthatresultwiththeiruse.

Aprogramsegmentisreentrantifitcanbeconcurrentlyexecutedbytwo(ormore)threads.Toimplementreentrantsoftware,weplacevariablesinregistersoronthestack,andavoidstoringintoglobalmemoryvariables.Whenwritinginassembly,weuseregisters,orthestackforparameterpassingtocreatereentrantsubroutines.Typicallyeachthreadwillhaveitsownsetofregistersandstack.Anonreentrantsubroutinewillhaveasectionofcodecalledavulnerablewindoworcriticalsection.Anerroroccursif

1)Onethreadcallsthefunctioninquestion

2)Itisexecutinginthecriticalsectionwheninterruptedbyasecondthread

3)Thesecondthreadcallsthesamefunction.

Thereareanumberofscenariosthatcanhappennext.Inthemostcommonscenario,thesecondthreadisallowedtocompletetheexecutionofthefunction,controlisthenreturnedtothefirstthread,andthefirstthreadfinishesthefunction.Thisfirstscenarioistheusualcasewithinterruptprogramming.Inthesecondscenario,thesecondthreadexecutespartofthecriticalsection,isinterruptedandthenre-enteredbyathirdthread,thethirdthreadfinishes,thecontrolisreturnedtothesecondthreadanditfinishes,lastlythecontrolisreturnedtothefirstthreadanditfinishes.Thissecondscenariocanhappenininterruptprogrammingifthesecondinterrupthashigherprioritythanthefirst.Acriticalsectionmayexistwhentwodifferentfunctionsthataccessandmodifythesamememory-residentdatastructure.

Program5.1showsaCfunctionandtheassemblycodegeneratedbytheARMKeilTMuVision®compiler.Thefunctionisnonreentrantbecauseoftheread-modify-writenonatomicaccesstotheglobalvariable, num .

numSPACE4

CountLDRr0,[pc,#116];R0=&num

;*******startofcriticalsection***

LDRr0,[r0,#0x00];R0=num

;couldbebadifinterruptoccurshere

uint32_tvolatilenum;

voidCount(void)

num=num+1;

ADDSr0,r0,#1

;couldbebadifinterruptoccurshere

LDRr1,[pc,#108];R1=&num

;couldbebadifinterruptoccurshere

STRr0,[r1,#0x00];updatenum

;*******endofcriticalsection***

BXlr

ptrDCDnum

Program5.1.Thisfunctionisnonreentrantbecauseoftheread-modify-writeaccesstoaglobal.Assumetherearetwoconcurrentthreads(themainprogramandabackgroundISR)thatbothcallthisfunction.Concurrentmeansthatboththreadsarereadytorun.Becausethereisonlyonecomputer,exactlyonethreadwillberunningatatime.Typically,theoperatingsystemswitchesexecutioncontrolbackandforthusinginterrupts.TherearethreeplacesintheassemblycodeatwhichifaninterruptweretooccurandtheISRcalledthesamefunction,theendresultwouldbe num wouldbeincrementedonlyonce,eventhoughthefunctionwascalledtwice.Assumeforthisexample num isinitially100.Anerroroccursif:

1.Themainprogramcalls Count2.Themainexecutes LDR r0,[r0,#0x00]makingR0=100

3.TheOShaltsthemain(usinganinterrupt)andstartstheISR4.theISRcalls CountExecutesnum=num+1; makingequalto101

5.TheOSreturnscontrolbacktothemainprogramR0isbacktoitsoriginalvalueof100

6.Themainprogramfinishedthefunction(adding1toR0)Making num equalto101

Anatomicoperationisonethatoncestartedisguaranteedtofinish.Inmostcomputers,onceaninstructionhasbegun,theinstructionmustbefinishedbeforethecomputercanprocessaninterrupt.Ingeneral,nonreentrantcodecanbegroupedintothreecategoriesallinvolving1)nonatomicsequences,2)writesand3)globalvariables.WewillclassifyI/Oportsasglobalvariablesfortheconsiderationofcriticalsections.Wewillgroupregistersintothesamecategoryaslocalvariablesbecauseeachthreadwillhaveitsownregistersandstack.

Thefirstgroupistheread-modify-writesequence:

1.Thesoftwarereadstheglobalvariableproducingacopyofthedata2.Thesoftwaremodifiesthecopy(originalvariableisstillunmodified)3.Thesoftwarewritesthemodificationbackintotheglobalvariable.

Inthesecondgroup,wehaveawritefollowedbyread,wheretheglobalvariableisusedfortemporarystorage:

1.Thesoftwarewritestotheglobalvariable(onlycopyoftheinformation)2.Thesoftwarereadsfromtheglobalvariableexpectingtheoriginaldatatobethere.

Inthethirdgroup,wehaveanon-atomicmulti-stepwritetoaglobalvariable:

1.Thesoftwarewritespartofthenewvaluetoaglobalvariable2.Thesoftwarewritestherestofthenewvaluetoaglobalvariable.

Observation:WhenconsideringreentrantsoftwareandvulnerablewindowsweclassifyaccessestoI/Oportsthesameasaccessestoglobalvariables.

Observation:Sometimeswestoretemporaryinformationinglobalvariablesoutoflaziness.Thispracticeistobediscouragedbecauseitwastesmemoryandmaycausethemoduletonotbereentrant.

Sometimewecanhaveacriticalsectionbetweentwodifferentsoftwarefunctions(onefunctioncalledbyonethread,andanotherfunctioncalledbyadifferentthread).Inadditiontoabovethreecases,anon-atomicmulti-stepreadwillbecriticalwhenpairedwithamulti-stepwrite.Forexample,assumeadatastructurehasmultiplecomponents(e.g.,hours,minutes,andseconds).Inthiscase,thewritetothedatastructurewillbeatomicbecauseitoccursinahighpriorityISR.Thecriticalsectionexistsintheforegroundbetweensteps1and3.Inthiscase,acriticalsectionexistseventhoughnosoftwarehasactuallybeenreentered.

Foreground thread

1.Themainreadssomeofthedata

3.Themain reads the rest ofthedata

Background thread

2. ISR writes to the datastructure

In a similar case, a non-atomic multi-step write will be critical when paired with amulti-stepread. Again,assumeadatastructurehasmultiplecomponents. In thiscase,the read from thedatastructurewillbeatomicbecause itoccurs inahighpriority ISR.Thecriticalsectionexistsintheforegroundbetweensteps1and3.

Foreground thread

1.Themainwrites someof thedata

3. The main writes the rest ofthedata

Backgroundthread

2. ISR reads from the datastructure

Whenmultiplethreadsareactive,itispossiblefortwothreadstobeexecutingthesameprogram.Forexample,thesystemmayberunningintheforegroundandcalls Func .Partwaythroughexecutionthe Func ,aninterruptoccurs.IftheISRalsocalls Func ,twothreadsaresimultaneouslyexecutingthefunction.Toexperimentallydetermineifafunctionhasbeenreentered,wecouldusetwoflagsortwooutputpins.Setoneofthem(PD1, Entered )atthestartandclearitattheend.Thethreadhasbeenre-enteredifthisflagorpinissetatthestartofthefunction,asshowninProgram3.7.Inthisexample,PortDbits1,0arenotpartoftheoriginalcode,butratherusedjustforthepurposeofdebugging.PD1is1whenonethreadstartsexecutingthefunction.However,ifPD0becomes1,thenthefunctionhasbeenreentered.

#definePD0(*((volatileuint32_t*)0x40007004))

#definePD1(*((volatileuint32_t*)0x40007008))

//functiontobetested

volatileintEntered=0,Flag=0;

voidFunc(void)

//functiontobetested

voidFunc(void)

if(PD1)PD0=1;

if(Entered)Flag=1;

Entered=1;

//theregularfunction

Entered=0;

PD1=2;

//theregularfunction

PD1=0;

Program5.2.Detectionofre-entrantbehaviorusingtwoflagsortwooutputpins.Ifcriticalsectionsdoexist,wecaneithereliminateitbyremovingtheaccesstotheglobalvariableorimplementmutualexclusion,whichsimplymeansonlyonethreadatatimeisallowedtoexecuteinthecriticalsection.Ingeneral,ifwecaneliminatetheglobalvariables,thenthesubroutinebecomesreentrant.Withoutglobalvariablesthereareno“vulnerable”windowsbecauseeachthreadhasitsownregistersandstack.Sometimesonemustaccessglobalmemorytoimplementthedesiredfunction.RememberthatallI/Oportsareconsideredglobal.Furthermore,globalvariablesarenecessarytopassdatabetweenthreads.

;***********DisableInterrupts***************

;disableinterrupts

;inputs:none

;outputs:none

DisableInterrupts

CPSIDI

BXLR

;***********EnableInterrupts***************

;disableinterrupts

;inputs:none

;outputs:none

EnableInterrupts

CPSIEI

BXLR

;***********StartCritical************************

;makeacopyofpreviousIbit,disableinterrupts

;inputs:none

;outputs:previousIbit

StartCritical

MRSR0,PRIMASK;saveoldstatus

CPSIDI;maskall(exceptfaults)

BXLR

;***********EndCritical************************

;usingthecopyofpreviousIbit,restoreIbittopreviousvalue

;inputs:previousIbit

;outputs:none

EndCritical

MSRPRIMASK,R0

BXLR

Program5.3.Assemblyfunctionsneededforinterruptenablinganddisabling.Asimplewaytoimplementmutualexclusionistodisableinterruptswhileexecutingthecriticalsection.Itisimportanttodisableinterruptsforasshortatimeaspossible,soastominimizetheeffectonthedynamicperformanceoftheotherthreads.Whilewearerunningwithinterruptsdisabled,time-criticaleventslikepowerfailureanddangerwarningscannotbeprocessed.Noticealsothattheinterruptsarenotsimplydisabledthenenabled.Beforethecriticalsection,theinterruptstatusissaved,andtheinterruptsdisabled.Afterthecriticalsection,theinterruptstatusisrestored.Youcannotsavetheinterruptstatusinaglobalvariable,ratheryoushouldsaveiteitheronthestackorinaregister.WewilladdtheassemblycodeofProgram5.3totheStartup.sfileinourprojectsthatuseinterrupts.Program5.4illustrateshowtoimplementmutualexclusionandeliminatethecriticalsection.

uint32_tvolatilenum;

voidCount(void)int32_tsr;

sr=StartCritical();

num=num+1;

EndCritical(sr);

Program5.4.Thisfunctionisreentrantbecauseoftheread-modify-writeaccesstotheglobalisatomic.Checkpoint5.5:Considerthesituationofnestedcriticalsections.Forexample,afunctionwithacriticalsectioncallsanotherfunctionthatalsohasacriticalsection.Whatwouldhappenifyousimplyaddeddisableinterruptatthebeginningandareenableinterruptattheendofeachcriticalsection?

Anothercategoryoftiming-dependentbugs,similartocriticalsections,iscalledaracecondition.Araceconditionoccursinamulti-threadedenvironmentwhenthereisacausaldependencybetweentwoormorethreads.Inotherwords,differentbehavioroccursdependingontheorderofexecutionoftwothreads.Inthisexampleofaracecondition,Thread-1initializesPortBbits3–0tobeoutputusingGPIO_PORTB_DIR_R=0x0F;Thread-2initializesPortBbits6–4tobeoutputusing GPIO_PORTB_DIR_R=0x70;Inparticular,ifThread-1runsfirstandThread-2runssecond,thenPortBbits3–0willbesettoinputs.Conversely,ifThread-2runsfirstandThread-1runssecond,thenPortBbits6–4willbesettoinputs.Thisisaraceconditioncausedbyunfriendlycode.Thesolutiontothisproblemistowritethetwoinitializationsinafriendlymanner.

Inasecondexample,assumetwothreadsaretryingtogetdatafromthesameinputdevice.Bothcallthefunction UART_InChar giveninProgram4.10.Whendataarrivesattheinput,thethreadthatexecutesfirstwillcapturethedata.ThisexampleisequivalenttothePetriNetconflictdrawninFigure4.16.

5.4.NVIContheARM Cortex-MProcessorOntheARM ® CortexTM-Mprocessor,exceptionsincluderesets,softwareinterruptsandhardwareinterrupts.Eachexceptionhasanassociated32-bitvectorthatpointstothememorylocationwheretheISRthathandlestheexceptionislocated.VectorsarestoredinROMatthebeginningofmemory.Program5.5showsthefirstfewvectorsasdefinedintheStartup.sfile. DCD isanassemblerpseudo-opthatdefinesa32-bitconstant.ROMlocation0x0000.0000hastheinitialstackpointer,andlocation0x0000.0004containstheinitialprogramcounter,whichiscalledtheresetvector.Itpointstoafunctioncalledtheresethandler,whichisthefirstthingexecutedfollowingreset.Thereareupto240(77ontheTM4C123microcontroller)possibleinterruptsourcesandtheir32-bitvectorsarelistedinorderstartingwithlocation0x0000.0008.Fromaprogrammingperspective,wecanattachISRstointerruptsbywritingtheISRsasregularCfunctionswithnoinputoroutputparametersandeditingtheStartup.sfiletospecifythosefunctionsfortheappropriateinterrupt.Forexample,ifwewroteaPortCinterruptserviceroutinenamed PortCISR ,thenwewouldreplace GPIOPortC_Handler with PortCISR .Inthisbook,wewillwriteourISRsusingstandardfunctionnamessothattheStartup.sfileneednotbeedited.I.e.,wewillsimplycalltheISRforedge-triggeredinterruptsonPortCas GPIOPortC_Handler .FormoredetailsseetheStartup.sfileswithintheinterruptexamplespostedonthebookwebsite.

EXPORT__Vectors

__Vectors;addressISR

DCDStackMem+Stack;0x00000000TopofStack

DCDReset_Handler;0x00000004ResetHandler

DCDNMI_Handler;0x00000008NMIHandler

DCDHardFault_Handler;0x0000000CHardFaultHandler

DCDMemManage_Handler;0x00000010MPUFaultHandler

DCDBusFault_Handler;0x00000014BusFaultHandler

DCDUsageFault_Handler;0x00000018UsageFaultHandler

DCD0;0x0000001CReserved

DCD0;0x00000020Reserved

DCD0;0x00000024Reserved

DCD0;0x00000028Reserved

DCDSVC_Handler;0x0000002CSVCallHandler

DCDDebugMon_Handler;0x00000030DebugMonitorHandler

DCD0;0x00000034Reserved

DCDPendSV_Handler;0x00000038PendSVHandler

DCDSysTick_Handler;0x0000003CSysTickHandler

DCDGPIOPortA_Handler;0x00000040GPIOPortA

DCDGPIOPortB_Handler;0x00000044GPIOPortB

DCDGPIOPortC_Handler;0x00000048GPIOPortC

DCDGPIOPortD_Handler;0x0000004CGPIOPortD

DCDGPIOPortE_Handler;0x00000050GPIOPortE

DCDUART0_Handler;0x00000054UART0

DCDUART1_Handler;0x00000058UART1

DCDSSI0_Handler;0x0000005CSSI

DCDI2C0_Handler;0x00000060I2C

DCDPWMFault_Handler;0x00000064PWMFault

DCDPWM0_Handler;0x00000068PWMGenerator0

Program5.5.SoftwaresyntaxtosettheinterruptvectorsfortheLM3S/LM4F/TM4C.Table5.1explainswheretofindtheprioritybitsforsomeoftheinterruptsontheLM3S/TM4C.Inparticular,thistableshowsthevectoraddress,interruptnumber,IRQnumber,ISRnameasdefinedinthefileStartup.s,whichregistercontainsprioritybitsandwhichbitstomodifywhenconfiguringpriority.Eachprocessorisalittledifferentsocheckthedatasheet.

Vectoraddress Number IRQ ISRnameinStartup.s NVIC Prioritybits

0x00000038 14 -2 PendSV_Handler NVIC_SYS_PRI3_R 23–21

0x0000003C 15 -1 SysTick_Handler NVIC_SYS_PRI3_R 31–29

0x00000040 16 0 GPIOPortA_Handler NVIC_PRI0_R 7–5

0x00000044 17 1 GPIOPortB_Handler NVIC_PRI0_R 15–13

0x00000048 18 2 GPIOPortC_Handler NVIC_PRI0_R 23–21

0x0000004C 19 3 GPIOPortD_Handler NVIC_PRI0_R 31–29

0x00000050 20 4 GPIOPortE_Handler NVIC_PRI1_R 7–5

0x00000054 21 5 UART0_Handler NVIC_PRI1_R 15–13

0x00000058 22 6 UART1_Handler NVIC_PRI1_R 23–21

0x0000005C 23 7 SSI0_Handler NVIC_PRI1_R 31–29

0x00000060 24 8 I2C0_Handler NVIC_PRI2_R 7–5

0x00000064 25 9 PWMFault_Handler NVIC_PRI2_R 15–13

0x00000068 26 10 PWM0_Handler NVIC_PRI2_R 23–21

0x0000006C 27 11 PWM1_Handler NVIC_PRI2_R 31–29

0x00000070 28 12 PWM2_Handler NVIC_PRI3_R 7–5

0x00000074 29 13 Quadrature0_Handler NVIC_PRI3_R 15–13

0x00000078 30 14 ADC0_Handler NVIC_PRI3_R 23–21

0x0000007C 31 15 ADC1_Handler NVIC_PRI3_R 31–29

0x00000080 32 16 ADC2_Handler NVIC_PRI4_R 7–5

0x00000084 33 17 ADC3_Handler NVIC_PRI4_R 15–13

0x00000088 34 18 WDT_Handler NVIC_PRI4_R 23–21

0x0000008C 35 19 Timer0A_Handler NVIC_PRI4_R 31–29

0x00000090 36 20 Timer0B_Handler NVIC_PRI5_R 7–5

0x00000094 37 21 Timer1A_Handler NVIC_PRI5_R 15–13

0x00000098 38 22 Timer1B_Handler NVIC_PRI5_R 23–21

0x0000009C 39 23 Timer2A_Handler NVIC_PRI5_R 31–29

0x000000A0 40 24 Timer2B_Handler NVIC_PRI6_R 7–5

0x000000A4 41 25 Comp0_Handler NVIC_PRI6_R 15–13

0x000000A8 42 26 Comp1_Handler NVIC_PRI6_R 23–21

0x000000AC 43 27 Comp2_Handler NVIC_PRI6_R 31–29

0x000000B0 44 28 SysCtl_Handler NVIC_PRI7_R 7–5

0x000000B4 45 29 FlashCtl_Handler NVIC_PRI7_R 15–13

0x000000B8 46 30 GPIOPortF_Handler NVIC_PRI7_R 23–21

0x000000BC 47 31 GPIOPortG_Handler NVIC_PRI7_R 31–29

0x000000C0 48 32 GPIOPortH_Handler NVIC_PRI8_R 7–5

0x000000C4 49 33 UART2_Handler NVIC_PRI8_R 15–13

0x000000C8 50 34 SSI1_Handler NVIC_PRI8_R 23–21

0x000000CC 51 35 Timer3A_Handler NVIC_PRI8_R 31–29

0x000000D0 52 36 Timer3B_Handler NVIC_PRI9_R 7–5

0x000000D4 53 37 I2C1_Handler NVIC_PRI9_R 15–13

0x000000D8 54 38 Quadrature1_Handler NVIC_PRI9_R 23–21

0x000000DC 55 39 CAN0_Handler NVIC_PRI9_R 31–29

0x000000E0 56 40 CAN1_Handler NVIC_PRI10_R 7–5

0x000000E4 57 41 CAN2_Handler NVIC_PRI10_R 15–13

0x000000E8 58 42 Ethernet_Handler NVIC_PRI10_R 23–21

0x000000EC 59 43 Hibernate_Handler NVIC_PRI10_R 31–29

0x000000F0 60 44 USB0_Handler NVIC_PRI11_R 7–5

0x000000F4 61 45 PWM3_Handler NVIC_PRI11_R 15–13

0x000000F8 62 46 uDMA_Handler NVIC_PRI11_R 23–21

0x000000FC 63 47 uDMA_Error NVIC_PRI11_R 31–29

Table5.1.SomeoftheinterruptvectorsfortheLM3S/TM4C(goestonumber154ontheM4).

InterruptsontheCortexTM-MprocessorarecontrolledbytheNestedVectoredInterruptController(NVIC).ToactivateaninterruptsourceweneedtosetitspriorityandenablethatsourceintheNVIC.Thisactivationisinadditiontothearmandenablesteps.Table5.1liststheinterruptsourcesavailableontheLM3S/LM4F/TM4Cfamilyofmicrocontrollers.Interruptnumbers0to15containthefaults,softwareinterruptandSysTick;theseinterruptswillbehandleddifferentlyfrominterrupts16to63.

Table5.2showsthetwelvepriorityregistersontheNVICoftheLM3S.Thereare35suchregistersontheTM4C.Eachregistercontainsan8-bitpriorityfieldforfourdevices.OntheLM3S/TM4Cmicrocontrollers,onlythetopthreebitsofthe8-bitfieldareused.Thisallowsustospecifytheinterruptprioritylevelforeachdevicefrom0to7,with0beingthehighestpriority.Theinterruptnumber(numbercolumninTable5.1)isloadedintotheIPSRregister.TheservicingofinterruptsdoesnotsettheIbitinthePRIMASK,soahigherpriorityinterruptcansuspendtheexecutionofalowerpriorityISR.IfarequestofequalorlowerpriorityisgeneratedwhileanISRisbeingexecuted,thatrequestispostponeduntiltheISRiscompleted.Inparticular,thosedevicesthatneedpromptserviceshouldbegivenhighpriority.

Address 31–29 23–21 15–13 7–5 Name

0xE000E400 GPIOPortD

GPIOPortC GPIOPortB

GPIOPortA

NVIC_PRI0_R

0xE000E404 SSI0,RxTx

UART1,RxTx

UART0,RxTx

GPIOPortE

NVIC_PRI1_R

0xE000E408 PWMGen1

PWMGen0 PWMFault

I2C0 NVIC_PRI2_R

0xE000E40C ADCSeq1 ADCSeq0 QuadEncoder

PWMGen2

NVIC_PRI3_R

0xE000E410 Timer0A Watchdog ADCSeq3

ADCSeq2

NVIC_PRI4_R

0xE000E414 Timer2A Timer1B Timer1A Timer0B NVIC_PRI5_R

0xE000E418 Comp2 Comp1 Comp0 Timer2B NVIC_PRI6_R

0xE000E41C GPIOPortG

GPIOPortF FlashControl

SystemControl

NVIC_PRI7_R

0xE000E420 Timer3A SSI1,RxTx UART2,RxTx

GPIOPortH

NVIC_PRI8_R

0xE000E424 CAN0 QuadEncoder1

I2C1 Timer3B NVIC_PRI9_R

0xE000E428 Hibernate Ethernet CAN2 CAN1 NVIC_PRI10_R

0xE000E42C uDMAError

uDMASoftTfr

PWMGen3

USB0 NVIC_PRI11_R

0xE000ED20 SysTick PendSV — Debug NVIC_SYS_PRI3_R

Table5.2.SomeoftheLM3S/TM4CNVICregisters.Bitsnotshownarezero.

TherearetwoenableregistersNVIC_EN0_R and NVIC_EN1_R ontheLM3S(fiveontheLM4F/TM4C).The32bitsin NVIC_EN0_R controltheIRQnumbers0to31(interruptnumbers16–47).InTable5.1weseeUART0isIRQ=5.ToenableUART0interruptswesetbit5in NVIC_EN0_R .Thebottom16bitsin NVIC_EN1_R controltheIRQnumbers32to47(interruptnumbers48–63).InTable5.1weseeTimer3AisIRQ=35.ToenableTimer3Ainterruptswesetbit3(35-32=3)in NVIC_EN1_R .NoteveryinterruptsourceisavailableoneveryLM3S/LM4F/TM4Cmicrocontroller,soyouwillneedtorefertothedatasheetforyourmicrocontrollerwhendesigningI/Ointerfaces.Writingzerostothe NVIC_EN0_R NVIC_EN1_R registershasnoeffect.Todisableinterruptswewriteonestothecorrespondingbitinthe NVIC_DIS0_Ror NVIC_DIS1_R register.

Figure5.10showsthecontextswitchfromexecutingintheforegroundtorunninganedge-triggeredISRfromPortC.AssumePortCinterruptsareconfiguredforaprioritylevelof5.TheIbitinthePRIMASKis0signifyinginterruptsareenabled.Theinterruptnumber(ISRNUM)intheIPSRregisteris0,meaningwearerunninginThreadmode(i.e.,themainprogram,andnotanISR).Handlermodeissignifiedbyanonzerovaluein

IPSR.WhenBASEPRIregisteriszero,allinterruptsareallowedandtheBASEPRIregisterisnotactive.

WhenaPortCinterruptistriggered,thecurrentinstructionisfinished.(a)EightregistersarepushedonthestackwithR0ontop.Theseregistersarepushedontothestackusingwhicheverstackpointerisactive:eithertheMSPorPSP.(b)ThevectoraddressisloadedintothePC(“Vectoraddress”columninTable5.2).(c)TheIPSRregisterissetto18(“Number”columninTable5.2)(d)Thetop24bitsofLRaresetto0xFFFFFF,signifyingtheprocessorisexecutinganISR.Bits[7:1]specifyhowtoreturnfrominterrupt.Bit0meansThumbmode.

0xE1ReturntoHandlermodeMSP(usingfloatingpointstateonLM4F/TM4C)

0xE9ReturntoThreadmodeMSP(usingfloatingpointstateonLM4F/TM4C)

0xEDReturntoThreadmodePSP(usingfloatingpointstateonLM4F/TM4C)

0xF1ReturntoHandlermodeMSP

0xF9ReturntoThreadmodeMSP

0xFDReturntoThreadmodePSP

Afterpushingtheregisters,theprocessoralwaysusesthemainstackpointer(MSP)duringtheexecutionoftheISR.Eventsb,c,anddcanoccursimultaneously

Figure5.10.Stackbeforeandafteraninterrupt.Toreturnfromaninterrupt,theISRexecutesthetypicalfunctionreturn BXLR .However,sincethetop24bitsofLRare0xFFFFFF,itknowstoreturnfrominterruptbypoppingtheeightregistersoffthestack.SincethebottomeightbitsofLRinthiscaseare0xF9,itreturnstothreadmodeusingtheMSPasitsstackpointer,inThumbmode.SincetheIPSRispartofthePSRthatispulled,soitisautomaticallyresetitspreviousstate.

AnestedinterruptoccurswhenahigherpriorityinterruptsuspendsanISR.ThelowerpriorityinterruptwillfinishafterthehigherpriorityISRcompletes.Whenoneinterruptpreemptsanother,theLRissetto0xFFFFFFF1,soitknowstoreturntohandlermode.TailchainingoccurswhenoneISRexecutesimmediatelyafteranother.Optimizationoccursbecausetheeightregistersneednotbepoppedonlytobepushedonceagain.Ifaninterruptistriggeredandisintheprocessofstackingregisterswhenahigherpriorityinterruptisrequested,thislatearrivalinterruptwillbeexecutedfirst.

OntheCortexTM-M4,ifaninterruptoccurswhileinfloatingpointstate,anadditional18wordsarepushedonthestack.These18wordswillsavethestateofthefloatingpointprocessor.Bits7-4oftheLRwillbe0b1110(0xE),signifyingitwasinterruptedduringafloatingpointstate.WhentheISRreturns,itknowstopullthese18wordsoffthestackandrestorethestateofthefloatingpointprocessor.

Prioritydeterminestheorderofservicewhentwoormorerequestsaremadesimultaneously.Priorityalsoallowsahigherpriorityrequesttosuspendalowerpriorityrequestcurrentlybeingprocessed.Usually,iftworequestshavethesamepriority,wedonotallowthemtointerrupteachother.NVICassignsapriorityleveltoeachinterrupttrigger.ThismechanismallowsahigherprioritytriggertointerrupttheISRofalowerpriorityrequest.Conversely,ifalowerpriorityrequestoccurswhilerunninganISRofahigherprioritytrigger,itwillbepostponeduntilthehigherpriorityserviceiscomplete.

5.5.Edge-triggeredInterruptsTable4.2listedtheregistersforPortA.Theotherportshavesimilarregisters.WewillbeginwithasimpleexamplethatcountsthenumberofrisingedgesonPortCbit4(Program5.6).Theinitializationrequiresmanysteps.(a)Theclockfortheportmustbeenabled.(b)Theglobalvariablesshouldbeinitialized.(c)Theappropriatepinsmustbeenabledasinputs.(d)Wemustspecifywhethertotriggerontherise,thefall,orbothedges.InthiscasewewilltriggerontheriseofPC4.(e)Itisgooddesigntoclearthetriggerflagduringinitializationsothatthefirstinterruptoccursduetothefirstrisingedgeaftertheinitializationhasbeenrun.Wedonotwishtotriggeronarisingedgethatmighthaveoccurredduringthepowerupphaseofthesystem.(f)Wearmtheedge-triggerbysettingthecorrespondingbitsintheIMregister.(g)WeestablishthepriorityofPortCbysettingbits23–21inthe NVIC_PRI0_R registeraslistedinTable5.2.WeactivatePortCinterruptsintheNVICbysettingbit2inthe NVIC_EN0_R register(“IRQnumber”inTable5.1).Thisinitializationisshowntoenableinterruptsinstep(i).However,inmostsystemswewouldnotenableinterruptsinthedeviceinitialization.Rather,itisgooddesigntoinitializealldevicesinthesystem,thenenableinterrupts.OntheLM4F/TM4CwealsoclearthecorrespondingbitsinAMSELandPCTL.

volatileuint32_tFallingEdges=0;

voidEdgeCounter_Init(void)

SYSCTL_RCGCGPIO_R|=0x04;//(a)activateclockforPortC

FallingEdges=0;//(b)initializecounter

GPIO_PORTC_DIR_R&=~0x10;//(c)makePC4in

GPIO_PORTC_DEN_R|=0x10;//enabledigitalI/OonPC4

GPIO_PORTC_IS_R&=~0x10;//(d)PC4isedge-sensitive

GPIO_PORTC_IBE_R&=~0x10;//PC4isnotbothedges

GPIO_PORTC_IEV_R&=~0x10;//PC4fallingedgeevent

GPIO_PORTC_ICR_R=0x10;//(e)clearflag4

GPIO_PORTC_IM_R|=0x10;//(f)arminterruptonPC4

NVIC_PRI0_R=(NVIC_PRI0_R&0xFF00FFFF)|0x00A00000;//(g)priority5

NVIC_EN0_R=4;//(h)enableinterrupt2inNVIC

EnableInterrupts();//(i)Program5.3

voidGPIOPortC_Handler(void)

GPIO_PORTC_ICR_R=0x10;//acknowledgeflag4

FallingEdges=FallingEdges+1;

Program5.6.Interrupt-drivenedge-triggeredinputthatcountsrisingedgesofPC4(EdgeInterrupt_xxx.zip).AllISRsmustacknowledgetheinterruptbyclearingthetriggerflagthatrequestedtheinterrupt.Foredge-triggeredPC4,thetriggerflagisbit4ofthe GPIO_PORTC_RIS_R register.Thisflagcanbeclearedbywritinga0x10to GPIO_PORTC_ICR_R .

Iftwoormoretriggerssharethesamevector,theserequestsarecalledpolledinterrupts,andtheISRmustdeterminewhichtriggergeneratedtheinterrupt.Iftherequestshaveseparatevectors,thentheserequestsarecalledvectoredinterruptsandtheISRknowswhichtriggercausedtheinterrupt.Example5.1illustratesthesedifferences.

Example5.1.Interfacetwoswitchesandsignalassociatedsemaphoreswheneachswitchispressed.

Solution:Wewillassumetheswitchesdonotbounce(interfacingswitchesthatbouncewillbecoveredlaterinthechapter).ThesemaphoreSW1willbesignaledwhenswitchSW1ispressed,andsimilarly,semaphoreSW2willbesignedwhenswitchSW2ispressed.Inthefirstsolution,wewillusevectoredinterruptsbyconnectingoneswitchtoPortCandtheotherswitchtoPortE.Sincethetwosourceshaveseparatevectors,theswitchonPortCwillautomaticallyactivate GPIOPortC_Handler andswitchonPortEwillautomaticallyactivate GPIOPortE_Handler .TheleftsideofFigures5.11and5.12showthesolutionwithvectoredinterrupts.

Figure5.11.Twosolutionsofswitch-triggeredinterrupts.

ThesoftwaresolutionusingvectoredinterruptsisinProgram5.7.WeinitializetwoI/Opinsasinputswithrisingedgeinterrupttriggers.Inthisway,wegetaninterruptrequestwhentheswitchistouched.I.e.,aninterruptoccursonthe0to1risingedgeeitherofPC4orPE4.Toacknowledgeaninterruptweclearthetriggerflag.Writinga0x10totheflagregister, GPIO_PORTn_ICR_R ,willclearbit4withoutaffectingtheotherbitsintheregister.Noticethattheacknowledgementusesan“ = ”insteadofan“ |= ”toavoidclearingallthebits.

volatileuint8_tSW1,SW2;//semaphores

voidVectorButtons_Init(void)

SYSCTL_RCGCGPIO_R|=0x14;//activateclockforPortsCandE

SW1=0;//clearsemaphores

SW2=0;

GPIO_PORTC_DIR_R&=~0x10;//makePC4in(PC4built-inbutton)

GPIO_PORTC_DEN_R|=0x10;//enabledigitalI/OonPC4

GPIO_PORTC_IS_R&=~0x10;//PC4isedge-sensitive(defaultsetting)

GPIO_PORTC_IBE_R&=~0x10;//PC4isnotbothedges(defaultsetting)

GPIO_PORTC_IEV_R|=0x10;//PC4risingedgeevent

GPIO_PORTC_ICR_R=0x10;//clearflag4

GPIO_PORTC_IM_R|=0x10;//arminterruptonPC4

NVIC_PRI0_R=(NVIC_PRI0_R&0xFF00FFFF)|0x00400000;//PortC=priority2

GPIO_PORTE_DIR_R&=~0x10;//makePE4in(PE4button)

GPIO_PORTE_DEN_R|=0x10;//enabledigitalI/OonPE4

GPIO_PORTE_IS_R&=~0x10;//PE4isedge-sensitive(defaultsetting)

GPIO_PORTE_IBE_R&=~0x10;//PE4isnotbothedges(defaultsetting)

GPIO_PORTE_IEV_R|=0x10;//PE4risingedgeevent

GPIO_PORTE_ICR_R=0x10;//clearflag4

GPIO_PORTE_IM_R|=0x10;//arminterruptonPE4

NVIC_PRI1_R=(NVIC_PRI1_R&0xFFFFFF00)|0x00000040;//PortE=priority2

NVIC_EN0_R=(NVIC_EN0_INT2+NVIC_EN0_INT4);//enableinterrupts2,4

EnableInterrupts();

voidGPIOPortC_Handler(void)

GPIO_PORTC_ICR_R=0x10;//acknowledgeflag4

SW1=1;//signalSW1occurred

voidGPIOPortE_Handler(void)

GPIO_PORTE_ICR_R=0x10;//acknowledgeflag4

SW2=1;//signalSW2occurred

Program5.7.Exampleofavectoredinterrupt(TwoButtonVector_xxx.zip).

Figure5.12.Flowchartsforavectoredandpolledinterrupt.

TherightsidesofFigures5.11and5.12showthesolutionwithpolledinterrupts.TouchingeitherswitchwillcauseaPortEinterrupt.TheISRmustpolltoseewhichoneorpossiblybothcausedtheinterrupt.Fortunately,eventhoughtheyshareavector,theacknowledgementsareseparate.Thecode GPIO_PORTE_ICR_R=0x10; willclearbit4inthestatusregisterwithoutaffectingbit5,andthecode GPIO_PORTE_ICR_R=0x20;willclearbit5inthestatusregisterwithoutaffectingbit4.Thismeansthetimingofoneswitchdoesnotaffectwhetherornotpushingtheotherswitchwillsignalitssemaphore.Ontheotherhand,whetherweareusingpolledorvectoredinterrupt,becausethereisonlyoneprocessor,thetimingofoneinterruptmaydelaytheservicingofanotherinterrupt.

ThepolledsolutionisProgram5.8.Ittakesthreeconditionstocauseaninterrupt.1)ThePE4andPE5arearmedintheinitialization;2)TheLM3S/LM4F/TM4Cisenabledforinterruptswiththe EnableInterrupts() function;3)Thetrigger GPIO_PORTE_RIS_RissetontherisingedgeofPE4orthetrigger GPIO_PORTE_RIS_R issetontherisingedgeofPE5.Becausethetwotriggershaveseparateacknowledgments,ifbothtriggersareset,bothwillgetserviced.Furthermore,thepollingsequencedoesnotmatter.

volatileuint8_tSW1,SW2;

voidPolledButtons_Init(void)

SYSCTL_RCGCGPIO_R|=0x10;//activateclockforPortE

SW1=0;SW2=0;//clearsemaphores

GPIO_PORTE_DIR_R&=~0x30;//makePE5-4in(PE5-4buttons)

GPIO_PORTE_DEN_R|=0x30;//enabledigitalI/OonPE5-4

GPIO_PORTE_IS_R&=~0x30;//PE5-4isedge-sensitive

GPIO_PORTE_IBE_R&=~0x30;//PE5-4isnotbothedges

GPIO_PORTE_IEV_R|=0x30;//PE5-4risingedgeevent

GPIO_PORTE_ICR_R=0x30;//clearflag5-4

GPIO_PORTE_IM_R|=0x30;//arminterruptsonPE5-4

NVIC_PRI1_R=(NVIC_PRI1_R&0xFFFFFF00)|0x00000040;//PortE=priority2

NVIC_EN0_R=NVIC_EN0_INT4;//enableinterrupt4inNVIC

EnableInterrupts();

voidGPIOPortE_Handler(void)

if(GPIO_PORTE_RIS_R&0x10)//pollPE4

GPIO_PORTE_ICR_R=0x10;//acknowledgeflag4

SW1=1;//signalSW1occurred

if(GPIO_PORTE_RIS_R&0x20)//pollPE5

GPIO_PORTE_ICR_R=0x20;//acknowledgeflag5

SW2=1;//signalSW2occurred

Program5.8.Exampleofapolledinterrupt(TwoButtonPoll_xxx.zip).

5.6.Interrupt-DrivenUARTFigure5.13showsadataflowgraphwithbufferedinputandbufferedoutput.FIFOsimplementedinthissectionarestaticallyallocatedglobalstructures.Becausetheyareglobalvariables,itmeanstheywillexistpermanentlyandcanbecarefullysharedbytheforegroundandbackgroundthreads.TheadvantageofusingaFIFOstructureforadataflowproblemisthatwecandecoupletheproducerandconsumerthreads.WithouttheFIFOwewouldhavetoproduceonepieceofdata,thenprocessit,produceanotherpieceofdata,thenprocessit.WiththeFIFO,theproducerthreadcancontinuetoproducedatawithouthavingtowaitfortheconsumertofinishprocessingthepreviousdata.Thisdecouplingcansignificantlyimprovesystemperformance.

Figure5.13.AdataflowgraphshowingtwoFIFOsthatbufferdatabetweenproducersandconsumers.Checkpoint5.6:WhatdoesitmeaniftheRxFifoinFigure5.13isempty?

Checkpoint5.7:WhatdoesitmeaniftheTxFifoinFigure5.13isempty?

ThesystemshowninFigure5.13hastwochannels,oneforinputandoneforoutput,andeachchannelemploysaseparateFIFOqueue.Program5.9showstheinterrupt-drivenUARTdevicedriver.TheflowchartforthisinterfacewasshownpreviouslyasFigure5.4.Duringinitialization,PortApins0and1areenabledasalternatefunctiondigitalsignals.ThetwosoftwareFIFOsofProgram3.10areinitialized.Thebaudrateissetat115200bits/sec,andthehardwareFIFOsareenabled.AtransmitinterruptwilloccurasthetransmitFIFOgoesfrom2elementsdownto1element.NotwaitinguntilthehardwareFIFOiscompletelyemptyallowsthesoftwaretorefillthehardwareFIFOandmaintainacontinuousoutputstream,achievingmaximumbandwidth.Therearetwoconditionsthatwillrequestareceiveinterrupt.First,ifthereceiveFIFOgoesfrom2to3elementsareceiveinterruptwillberequested.Atthistimethereisstill13freespacesinthereceiveFIFOsothelatencyrequirementforthisreal-timeinputwillbe130bittimes(about1ms).Theotherpotentialsourceofreceiverinterruptsisthereceivertimeout.ThistriggerwilloccurifthereceiverbecomesidleandtherearedatainthereceiverFIFO.Thistriggerwillallowtheinterfacetoreceiveinputdatawhenitcomesjustoneortwoframesatatime.IntheNVIC,thepriorityissetat2andUART0(IRQ=5)isactivated.Normally,onedoesnotenableinterruptsintheindividualinitializationfunctions.Rather,interruptsshouldbeenabledinthemainprogram,afterallinitializationfunctionshavecompleted.

Whenthemainthreadwishestooutputitcalls UART_OutChar ,whichwillputthedata

intothesoftwareFIFO.Next,itcopiesasmuchdatafromthesoftwareFIFOintothehardwareFIFOandarmsthetransmitter.ThetransmitterinterruptservicewillalsogetasmuchdatafromthesoftwareFIFOandputitintothehardwareFIFO.The copySoftwareToHardware functionhasacriticalsectionandiscalledbyboth UART_OutChar andtheISR.Toremovethecriticalsectionthetransmitteristemporarilydisarmedinthe UART_OutChar functionwhen copySoftwareToHardwareiscalled.Thishelperfunctionguaranteesdataistransmittedinthesameorderitwasproduced.

WheninputframesarereceivedtheyareplacedintothereceivehardwareFIFO.IfthisFIFOgoesfrom2to3elements,orifthereceiverbecomesidlewithdataintheFIFO,areceiveinterruptoccurs.Thehelperfunction copyHardwareToSoftware willgetfromthereceivehardwareFIFOandputintothereceivesoftwareFIFO.Whenthemainthreadwishedtoinputdataitcalls UART_InChar .ThisfunctionsimplygetsfromthesoftwareFIFO.IfthereceivesoftwareFIFOisempty,itwillspin.

#defineFIFOSIZE16//sizeoftheFIFOs(mustbepowerof2)

#defineFIFOSUCCESS1//returnvalueonsuccess

#defineFIFOFAIL0//returnvalueonfailure

AddIndexFifo(Rx,FIFOSIZE,char,FIFOSUCCESS,FIFOFAIL)

AddIndexFifo(Tx,FIFOSIZE,char,FIFOSUCCESS,FIFOFAIL)

//Assumesan80MHzbusclock,creates115200baudrate

voidUART_Init(void)//shouldbecalledonlyonce

SYSCTL_RCGCUART_R|=0x0001;//activateUART0

SYSCTL_RCGCGPIO_R|=0x0001;//activateportA

RxFifo_Init();//initializeemptyFIFOs

TxFifo_Init();

UART0_CTL_R&=~UART_CTL_UARTEN;//disableUART

UART0_IBRD_R=43;//IBRD=int(80000000/(16*115,200))=int(43.40278)

UART0_FBRD_R=26;//FBRD=round(0.40278*64)=26

UART0_LCRH_R=0x0070;//8-bitwordlength,enableFIFO

UART0_IFLS_R&=~0x3F;//clearTXandRXinterruptFIFOlevelfields

//configureinterruptforTXFIFO<=1/8full

//configureinterruptforRXFIFO>=1/8full

UART0_IFLS_R+=(UART_IFLS_TX1_8|UART_IFLS_RX1_8);

//enableTXandRXFIFOinterruptsandRXtime-outinterrupt

UART0_IM_R|=(UART_IM_RXIM|UART_IM_TXIM|UART_IM_RTIM);

UART0_CTL_R|=0x0301;//enableRXETXEUARTEN

GPIO_PORTA_PCTL_R=(GPIO_PORTA_PCTL_R&0xFFFFFF00)+0x00000011;//UART

GPIO_PORTA_AMSEL_R&=~0x03;//disableanalogonPA1-0

GPIO_PORTA_AFSEL_R|=0x03;//enablealtfunctonPA1-0

GPIO_PORTA_DEN_R|=0x03;//enabledigitalI/OonPA1-0

NVIC_PRI1_R=(NVIC_PRI1_R&0xFFFF00FF)|0x00004000;//UART0=priority2

NVIC_EN0_R=NVIC_EN0_INT5;//enableinterrupt5inNVIC

EnableInterrupts();

//copyfromhardwareRXFIFOtosoftwareRXFIFO

//stopwhenhardwareRXFIFOisemptyorsoftwareRXFIFOisfull

voidstaticcopyHardwareToSoftware(void)charletter;

while(((UART0_FR_R&UART_FR_RXFE)==0)&&(RxFifo_Size()<(FIFOSIZE-1)))

letter=UART0_DR_R;

RxFifo_Put(letter);

//copyfromsoftwareTXFIFOtohardwareTXFIFO

//stopwhensoftwareTXFIFOisemptyorhardwareTXFIFOisfull

voidstaticcopySoftwareToHardware(void)charletter;

while(((UART0_FR_R&UART_FR_TXFF)==0)&&(TxFifo_Size()>0))

TxFifo_Get(&letter);

UART0_DR_R=letter;

//inputASCIIcharacterfromUART

//spinifRxFifoisempty

charUART_InChar(void)

charletter;

while(RxFifo_Get(&letter)==FIFOFAIL);

return(letter);

//outputASCIIcharactertoSCI

//spinifTxFifoisfull

voidUART_OutChar(chardata)

while(TxFifo_Put(data)==FIFOFAIL);

UART0_IM_R&=~UART_IM_TXIM;//disableTXFIFOinterrupt

copySoftwareToHardware();

UART0_IM_R|=UART_IM_TXIM;//enableTXFIFOinterrupt

//atleastoneofthreethingshashappened:

//hardwareTXFIFOgoesfrom3to2orlessitems

//hardwareRXFIFOgoesfrom1to2ormoreitems

//UARTreceiverhastimedout

voidUART0_Handler(void)

if(UART0_RIS_R&UART_RIS_TXRIS)//hardwareTXFIFO<=2items

UART0_ICR_R=UART_ICR_TXIC;//acknowledgeTXFIFO

//copyfromsoftwareTXFIFOtohardwareTXFIFO

copySoftwareToHardware();

if(TxFifo_Size()==0)//softwareTXFIFOisempty

UART0_IM_R&=~UART_IM_TXIM;//disableTXFIFOinterrupt

if(UART0_RIS_R&UART_RIS_RXRIS)//hardwareRXFIFO>=2items

UART0_ICR_R=UART_ICR_RXIC;//acknowledgeRXFIFO

//copyfromhardwareRXFIFOtosoftwareRXFIFO

copyHardwareToSoftware();

if(UART0_RIS_R&UART_RIS_RTRIS)//receivertimedout

UART0_ICR_R=UART_ICR_RTIC;//acknowledgereceivertimeout

//copyfromhardwareRXFIFOtosoftwareRXFIFO

copyHardwareToSoftware();

Program5.9.Interrupt-drivendevicedriverfortheUARTusestwohardwareFIFOsandtwosoftwareFIFOstobufferdata(UART2_xxx.zip).

5.7.PeriodicInterruptsusingSysTickTheSysTicktimerisasimplewaytocreateperiodicinterrupts.Aperiodicinterruptisonethatisrequestedonafixedtimebasis.Thisinterfacingtechniqueisrequiredfordataacquisitionandcontrolsystems,becausesoftwareservicingmustbeperformedataccuratetimeintervals.Foradataacquisitionsystem,itisimportanttoestablishanaccuratesamplingrate.ThetimeinbetweenADCsamplesmustbeequal(andknown)inorderforthedigitalsignalprocessingtofunctionproperly.Similarlyformicrocontroller-basedcontrolsystems,itisimportanttomaintainboththeADCandDACtiming.LaterinChapter6,wewillseethegeneralpurposetimerscanalsocreateperiodicinterrupts.

Anotherapplicationofperiodicinterruptsiscalled“intermittentpolling”or“periodicpolling”.Figure5.14showsbusywaitsidebysidewithperiodicpolling.Inbusy-waitsynchronization,themainprogrampollstheI/Odevicescontinuously.Withperiodicpolling,theI/Odevicesarepolledonaregularbasis(establishedbytheperiodicinterrupt.)Ifnodeviceneedsservice,thentheinterruptsimplyreturns.Ifthepollingperiodis t,thenonaveragetheinterfacelatencywillbe½ t,andtheworstcaselatencywillbe t.Periodicpollingisappropriateforlowbandwidthdeviceswherereal-timeresponseisnotnecessary.ThismethodfreesthemainprogramfromtheI/Otasks.

Figure5.14.AnISRflowchartthatimplementsperiodicpolling.Weuseperiodicpollingifthefollowingtwoconditionsapply:

1.TheI/Ohardwarecannotgenerateinterruptsdirectly

2.WewishtoperformtheI/Ofunctionsinthebackground

Table5.3showstheSysTickregistersusedtocreateaperiodicinterrupt.SysTickhasa24-bitcounterthatdecrementsatthebusclockfrequency.LetfBUSbethefrequencyofthebusclock,andletnbethevalueoftheRELOADregister.ThefrequencyoftheperiodicinterruptwillbefBUS/(n+1).First,wecleartheENABLEbittoturnoffSysTickduringinitialization.Second,wesettheRELOADregister.Third,wewritetothe NVIC_ST_CURRENT_Rvaluetoclearthecounter.Lastly,wewritethedesiredmodetothecontrolregister,NVIC_ST_CTRL_R .WemustsetCLK_SRC=1,becauseCLK_SRC=0externalclockmodeisnotimplemented.

Address 31-24

23-17

16 15-3 2 1 0 Name

$E000E010 0 0 COUNT 0 CLK_SRC INTEN ENABLE NVIC_ST_CTRL_R

$E000E014 0 24-bitRELOADvalue NVIC_ST_RELOAD_R

$E000E018 0 24-bitCURRENTvalueofSysTickcounter NVIC_ST_CURRENT_R

Address 31-29 28-24

23-21 20-8

7-5 4-0 Name

$E000ED20 TICK 0 PENDSV 0 DEBUG 0 NVIC_SYS_PRI3_R

Table5.3.SysTickregisters.

WesetINTENtoenableinterrupts.WeestablishthepriorityoftheSysTickinterruptsusingtheTICKfieldinthe NVIC_SYS_PRI3_R register.WeneedtosettheENABLEbitsothecounterwillrun.WhentheCURRENTvaluecountsdownfrom1to0,theCOUNTflagisset.Onthenextclock,theCURRENTisloadedwiththeRELOADvalue.Inthisway,theSysTickcounter(CURRENT)iscontinuouslydecrementing.IftheRELOADvalueisn,thentheSysTickcounteroperatesatmodulon+1(…n,n-1,n-2…1,0,n,n-1,…).Inotherwords,itrollsovereveryn+1counts.Thus,theCOUNTflagwillbeconfiguredtotriggeraninterrupteveryn+1counts.

volatileuint32_tCounts;

#definePD0(*((volatileuint32_t*)0x40007004))

//periodhasunitsofthebusclock(e.g.,12.5nsor20ns)

voidSysTick_Init(uint32_tperiod)

SYSCTL_RCGCGPIO_R|=0x00000008;//activateportD

Counts=0;

GPIO_PORTD_DIR_R|=0x01;//makePD0out

GPIO_PORTD_PCTL_R&=~0x0000000F;//GPIO

GPIO_PORTD_AMSEL_R&=~0x01;//disableanalogonPD0

GPIO_PORTD_AFSEL_R|=0x03;//disablealtfunctonPD0

GPIO_PORTD_DEN_R|=0x01;//enabledigitalI/OonPD0

NVIC_ST_CTRL_R=0;//disableSysTickduringsetup

NVIC_ST_RELOAD_R=period-1;//reloadvalue

NVIC_ST_CURRENT_R=0;//anywritetocurrentclearsit

NVIC_SYS_PRI3_R=(NVIC_SYS_PRI3_R&0x00FFFFFF)|0x40000000;//priority2

NVIC_ST_CTRL_R=0x00000007;//enablewithcoreclockandinterrupts

EnableInterrupts();

voidSysTick_Handler(void)

PD0^=0x01;//togglePD0

Counts=Counts+1;

Program5.10.ImplementationofaperiodicinterruptusingSysTick(PeriodicSysTickInts_xxx.zip).Oneoftheproblemswithswitchesiscalledswitchbounce.Manyinexpensiveswitcheswillmechanicallyoscillateforuptoafewmillisecondswhentouchedorreleased.Itbehaveslikeanunderdampedoscillator.Thesemechanicaloscillationscauseelectricaloscillationssuchthataportpinwilloscillatehigh/lowduringthebounce.Insomecasesthisbounceshouldberemoved.

Example5.2.RedesignExample4.3ofamatrixkeyboardinterfaceusinginterruptsynchronization.TheISRwillputASCIIdataintotheFIFO.ThemainwillgetdatafromtheFIFO.

Solution:Figure4.40isredrawnhereasFigure5.15.Therearetwogoodsolutionstousinginterruptsynchronizationforthekeyboard.Theapproachimplementedhereusesperiodicpolling,becauseitaffordsasimplesolutiontobothbouncingandtwo-keyrollover.Thetimebetweeninterruptsisselectedtobelongerthanthemaximumbouncetime,butshorterthantheminimumtimebetweenkeystrikes.Ifyoutypetencharacterspersecond,theminimumtimebetweenrisingandfallingedgesisabout50ms.Sinceswitchbouncetimesarelessthan10ms,wewillpollthekeyboardevery25ms.Thismeanstheaveragelatencywillbe12.5ms,andthemaximumlatencywillbe25ms.

Figure5.15.Amatrixkeyboardinterfacedtothemicrocontroller(sameasFigure4.40).

TheinitializationcombinesProgram4.11withProgram5.10withtheadditionalinitializationoftheFIFO.ThePD0debuggingwillhavetoberemovedfromSysTickbecausePD0isusedtointerruptthekeyboard.Akeyisrecognizedifthescanningreturnsonekeyfound,andthiskeyisdifferentfromwhatitscanned25msago.

AddIndexFifo(Matrix,16,char,1,0)//createaFIFO

charstaticLastKey;

voidMatrix_Init(void)

LastKey=0;//nokeytyped

MatrixFifo_Init();

MatrixKeypad_Init();//Program4.11

SysTick_Init(1250000);//Program5.10,25mspolling

voidSysTick_Handler(void)charthisKey;int32_tn;

thisKey=MatrixKeypad_Scan(&n);//scan

if((thisKey!=LastKey)&&(n==1))

MatrixFifo_Put(thisKey);

LastKey=thisKey;

else

LastKey=0;//invalid

charMatrix_InChar(void)charletter;

while(MatrixFifo_Get(&letter)==FIFOFAIL);

return(letter);

Program5.11.Periodicpollinginterfaceofascannedkeyboard(MatrixKeypadPeriodic_xxx.zip).

OneoftheadvantagesofProgram5.11istwo-keyrollover.Whenpeopletypeveryfast,theysometimestypethenextkeybeforethereleasethefirstkey.Forexample,whentheoperatortypestheletters“BCD”slowlywithonefinger,thekeyboardstatusgoesinthissequence

<none>,<B>,<none>,<C>,<none>,<D>,<none>

Conversely,iftheoperatortypesquickly,therecanbetwo-keyrollover,whichcreatesthissequence

<none>,<B>,<BC>,<C>,<CD>,<D>,<none>

where<BC>meansbothkeys‘B’and‘C’aretouched.Two-keyrollovermeansthekeyboarddoesnotgothroughastatewherenokeysaretouchedbetweentypingthe‘B’andthe‘C’.Sinceeachofthekeysgoesthroughastatewhereexactlyonekeyispressedandisdifferentthanitwas25msago,Program5.11willhandletwo-keyrollover.

Asecondapproachistoarmthedeviceforinterruptsbydrivingallrowstozero.Inthismanner,wewillreceiveafallingedgeononeofthePortDinputswhenanykeyistouched.DuringtheISRwecouldscanthekeyboardandputthekeyintotheFIFO.Tosolvethebounceproblemthissolutionimplementsatimedelayfromkeytouchtowhenthesoftwarescansforkeys.Thisapproachusesacombinationofedge-triggeredinputsandoutputcompareinterruptstoperforminputinthebackground.Whenarmingforinterrupts,wesetallfourrowstooutputzero.Inthisway,afallingedgeinterruptwilloccuronanykeytouched.Whenanedge-triggeredinterruptoccurswewilldisarmthisinputandarmanoutputcomparetotriggerin10ms.ItisduringtheoutputcompareISRwescanthematrix.Ifthereisexactlyonekey,weenteritintotheFIFO.Aninterruptmayoccuronreleaseduetobounce.However,10msaftertherelease,whenwescanduringtheTimer0A_HandlertheMatrixKeypad_ScanfunctionwillreturnaNumofzero,andwewillignoreit.Thissolutionsolvesswitchbounce,butnottwo-keyrollover.YoucanfindthissolutiononthewebsiteasMatrixKeypadInt_xxx.zip.

Therearethreesolutionstodebounceanindividualswitchpostedontheexamplepage

1)Blindsynchronization:readswitch,andthenwait10msDebounceSysTick_xxx.zip

2)Periodicpolling:useaperiodicinterruptat10msDebounceSysTick_xxx.zip

3)Interrupt:edge-triggeredinterrupt,thentimedelayinterruptDebounceCombo_xxx.zip

5.8.Low-PowerDesignReducingtheamountofpowerusedbyanembeddedsystemwillsavemoneyandextendbatterylife.InCMOSdigitallogic,powerisrequiredtomakesignalsriseorfall.Mostmicrocontrollersallowyoutoadjustthefrequencyofthebusclock.MostmicrocontrollersallowyoutochangethebusclockusingaPLL.Selectingthebusclockfrequencyisatradeoffbetweenpowerandperformance.Tooptimizeforpower,wechoosetheslowestbusclockfrequencythatsatisfiestheminimumrequirementsofthesystem.Whenweimplementthesoftwaresystemwithinterrupts,itallowsustofocustheprocessoronexecutingtasks,whentheyneedtorun.Becausetherearefewerbackwardjumpswastingtime,interrupt-drivensystemstypicallywillbeabletoperformthesamefunctionsataslowerbusclock.

Capacitanceplaysamajorroleinhighspeeddigitalsystems.Thecurrent/voltagerelationshipacrossacapacitorisI=CdV/dt.Foragivendigitalcircuitthecapacitancesareapproximatelyfixed.AsweincreasethebusfrequencyusingthePLL,theslewratesinthesignalsmustincrease(largerdV/dt),requiringmorecurrenttooperate.Furthermore,CMOSlogicrequireschargetocreateatransition,soasthefrequencyincreasesthenumberoftransitionspersecondincreases,requiringmorecurrent(chargepersecondiscurrent).

Asecondfactorinlow-powerdesignfollowstheaxiom“turnthelightoffwhenyouleavetheroom.”Basically,weturnoffdeviceswhentheyarenotbeingused.MostI/Odevicesonthemicrocontrollerareinitializedasoff,sowehavetoturnthemontousethem.However,ratherthanturningitononceintheinitializationandleavingitoncontinuously,wecoulddynamicallyturnitonwhenneededthenturnitoffwhendone.InChapter9,wewilllearnwaystoturnoffexternalanalogcircuitswhentheyarenotneeded.

Ingeneral,aswereducethevoltage,thepowerisreduced.Somemicrocontrollerfamilieshaveversionsthatwillrunatdifferentvoltages.ForexampletheMC9S12C32canoperateateither3.3or5V.Ifadevicehasafixedresistancefromsupplyvoltagetoground,reducingfrom5to3.3Vwilldropthepowerbyafactorof(52-3.32)/52=56%.TheMSP430F2012canoperateonasupplyvoltagerangingfrom1.8to3.6V.ReducingthevoltageontheMSP430willdecreasethemaximumspeedatwhichwecanrunthebusclock.

Manymicrocontrollersallowustocontrolthecurrentoutputontheoutputpins.Reducingthecurrentonanoutputpinwillmakeitoperateslowerbutdrawlesscurrent.Anyunusedinputpinsshouldhaveinternalorexternalpull-up(orpull-down).Aninputpinnotconnectedtoanythingmayoscillateatafrequencyofanexternalfield,wastingpowerunnecessarily.Unusedpinscanalsobemadeoutputs.OnLM3S/LM4F/TM4CmicrocontrollerswecanturnoffindividualpinsthatwearenotusingbyclearingbitsintheDENregister(e.g., GPIO_PORTD_DEN_R ).Ifwehaveanentireportnotbeingused,wekeepitcompletelyoff.

Onewaytosavepoweristoperformalloperationsasbackgroundtasksandputtheprocessortosleepwhenintheforeground.The wfi instructionstopsthebusclock,andtheprocessorstopsexecutinginstructions.OnewaytouseitistomakeafunctionthatCprogramscancall.

WaitForInterrupt

WFI

BXLR

Aninterruptwillwakeupthesleepingprocessor.ALM3S811consumesabout100mAwhilerunningat50MHz.However,insleepmode,thesupplycurrentdropstoabout20mA,dependingonwhatelseisactive.TheLM3S/LM4F/TM4Cfamilyalsohasadeepsleepmode,whereitconsumesabout1mA.Toillustratethisapproach,considerthetwosystemsinProgram5.12.Bothsystemsexecute Stuff atafixedrate,buttheonewiththewaitforinterruptrequireslesspower.

intmain(void)

SysTick_Init(50000);

while(1)

WaitForInterrupt();

voidSysTick_Handler(void)

Stuff();

intmain(void)

SysTick_Init(50000);

while(1)

//runsatfullpower

voidSysTick_Handler(void)

Stuff();

Program5.12.Exampleshowinghowtosavepowerbyputtingtheprocessortosleep.MaintenanceTip:Wheneveryoursoftwareperformsabackwardjump(e.g.,waitingforanevent),itmaybepossibletoputtheprocessortosleep,thussavingpower.

5.9.DebuggingProfileOnewaytoseebothhowlongittakestoexecuteanISRandhowoftenitexecutesistotoggleanoutputpinthreetimes,asshowninProgram5.13.Figure5.16showsthetimebetweeninterruptsis1ms,andthetimetoexecuteoneISRis650ns.

Figure5.16.Profileshowingbothtimebetweeninterruptsandtimewithintheinterrupt(TM4C123,50MHz).#definePF2(*((volatileuint32_t*)0x40025010))

voidSysTick_Handler(void)

PF2^=0x04;//togglePF2

PF2^=0x04;//togglePF2

Stuff();

PF2^=0x04;//togglePF2

Program5.13.ProfilingboththeexecutiontimeforStuff()andthetimebetweenISRs.

5.10.Exercises5.1Syntactically,I/Oportsarepublicglobals.Inordertoseparatemechanismsfrompolicies(i.e.,improvethequalityofthesoftwaresystem),howshouldI/Obeactuallyused?

a)Localinallocation

b)Privateinscope

c)Globalinallocation

d)Likevolatilememory

e)Publicinscope

f)Likenonvolatilememory

5.2Whydoweaddthe volatile qualifierinallI/Oportdefinitions?

5.3Whathappensifaninterruptserviceroutinedoesnotacknowledgeordisarm?

a)Softwarecrashesbecausenomoreinterruptswillberequested

b)Thenextinterruptislost

c)Thisinterruptislost

d)Softwarecrashesbecauseinterruptsarerequestedoverandover.

5.4Themainprogramsynthesizesdataandaperiodicinterruptwilloutputthedataseparatedbyafixedtime.AFIFOqueueisusedtobufferdatabetweenamainprogram(e.g.,mainprogramcalls Fifo_Put ).Theinterruptserviceroutinecalls Fifo_Get andactuallyoutputs.ExperimentalobservationsshowthisFIFOisusuallyempty,andhasatmost3elements.Whatdoesitmean?Choosea-f.

a)ThesystemisCPUboundb)BandwidthcouldbeincreasedbyincreasingFIFOsize

c)ThesystemisI/Oboundd)TheFIFOcouldbereplacedbyaglobalvariable

e)Thelatencyissmallandboundedf)Interruptsarenotneededinthissystem

5.5Answerquestion5.4,undertheconditionthattheFIFOoftenbecomesfull.

5.6Anedge-triggeredinputisarmedsothatinterruptsoccurwhennewdataarrivesintothemicrocontroller.ConsiderthesituationinwhichaFIFOqueueisusedtobufferdatabetweentheISRandthemainprogram.TheISRinputsthedataandsavesthedatabycalling Fifo_Put .Whenthemainprogramwantsinputitcalls Fifo_Get .ExperimentalobservationsshowthisFIFOisusuallyempty,havingatmost3elements.Whatdoesitmean?Choosea-f.

a)ThesystemisCPUbound

b)BandwidthcouldbeincreasedbyincreasingFIFOsize

c)ThesystemisI/Obound

d)TheFIFOcouldbereplacedbyaglobalvariable

e)Thelatencyissmallandbounded

f)Interruptsarenotneededinthissystem

5.7Answerquestion5.6,undertheconditionthattheFIFOoftenbecomesfull.

5.8Considerthefollowinginterruptserviceroutine.Thegoalistomeasuretheelapsedtimefromoneinterruptcalltotheother.Whatqualifierdoyouplaceinthe XX positiontomakethismeasurementoperational?Choosefrom volatile static float const or public .

uint32_tElapsed;//timebetweeninterrupt

voidhandler(void)

XXuint16_tlast=0;

Elapsed=(NVIC_ST_CURRENT_R–last)&0x00FFFFFF;

last=NVIC_ST_CURRENT_R;

5.9WhatpurposemighttherebetousethePLLandslowdownthemicrocontroller?

a)ThesystemisCPUbound

b)Tomakethebatterieslastlongeronabattery-poweredsystem

c)Inordertoadjustthebaudratetoaconvenientvalue

d)Inordertobalancetheloadbetweenforegroundandbackgroundthreads

e)Toreducelatency

f)Noneoftheabove,becausethereisneverareasontorunslower

5.10Thisisafunctionaldebuggingquestion.However,thedebugginginstrumentstillneedstobeminimallyintrusive.Assumey= Function(x) isafunctionwith16-bitinput xand16-bitoutputyandiscalledfromanISRaspartofareal-timesystem.TheUART,PortFandPortGareunusedbythesystem,andPortFandPortGaredigitaloutputs.Thedebuggingcodewillbeplacedattheendjustbeforethereturn,unlessotherwisestated. UART_OutSDec outputsa16-bitsignedinteger. BufX and BufY are16-bitsignedglobalbuffersoflength100, n isaglobalvariableinitializedto0.Whichdebuggingcodewouldyouaddtoverifythecorrectnessofthisfunction?

a) GPIO_PORTF_DATA_R=x;GPIO_PORTG_DATA_R=y;

b) UART_OutSDec(x);UART_OutSDec(y);//busywait

c) if(n<100)BufX[n]=x;BufY[n]=y;n++;

d) GPIO_PORTF_DATA_R|=0x01;//atbeginning

GPIO_PORTF_DATA_R&=~0x01;//atend

e) if(n<100)BufX[n]=x;BufY[n]=TCNT;n++;

5.11Foureventsmustoccurforanedge-triggeredinterruptonPortEbit1tobegenerated:

1)Softwaresetsthearmbit(IMbit1)

2)Softwareenablesinterrupts(I=0)

3)Hardwaresetstheflagbit(RISbit1)

4)SoftwareconfigurestheNVICtoallowPortEinputinterrupts

Whichtimesequenceofthesefoureventscausetheinterrupttobegenerated?

a)Onlytheorder1,2,3,4

b)Onlytheorder1,2,4,3

c)Onlytheorder4,1,2,3

d)Anyorderwillgenerateaninterrupt

5.12Thefollowingmultithreadedsystemhasacriticalsection.Modifytheseprogramstoremovetheerror.Youmayassumetheinterruptsareenabledandarerunning.

uint32_tSec,Min;

voidmain(void)

while(1)

if((Min==10)&&(Sec==0))

UART_OutString(“done”);

voidInterruptHandler(void)

if(Sec==59)

Sec=0;Min++;

else

Sec++;

5.13ThefollowingmultithreadedsystemusesPortFasadebuggingprofile.Oneinterrupthashigherprioritythantheother.Stuff1 and Stuff2 areunrelated.Isthereacriticalsection?Ifyes,editthecodetoremovethecriticalsection.Ifno,justifyyouranswerin16wordsorless.

voidInterruptHandler1(void)

GPIO_PORTF_DATA_R^=0x02;

Stuff1();

voidInterruptHandler2(void)

GPIO_PORTF_DATA_R^=0x04;

Stuff2();

TheassemblycodefortheInterruptHandler1is

PUSHlr

LDRr0,[pc,#180];R0pointstoPortF

LDRr0,[r0,#0x00];R0isthevaluefromPortF

EORr0,r0,#0x02;togglebit1

LDRr1,[pc,#152];R1pointstoPortF

STRr0,[r1,#0x3FC];writenewvaluetoPortF

BL.WStuff0;callfunction

POPpc;returnfrominterrupt

5.14Whathappensiftwointerruptrequestsaremadeduringthesameinstruction?Isonelost?Ifbothareserviced,whichonegoesfirst?

5.15SpecifywhethereachstatementisTRUEorFALSE.

a)A signed char or int8_t variablecanstorevaluesfrom–128to+128.

b)Consideraninputdevice.Theinterfacelatencyisthetimefromwhenthesoftwareasksfornewdatauntilthetimenewdataareready.

c)Consideranoutputdevice.Theinterfacelatencyisthetimefromwhenthesoftwaresendsnewdatatotheoutputdeviceuntilthetimetheoutputoperationiscomplete.

d)The static qualifierisusedwithfunctionstospecifythefunctionispermanent,createdatcompiletimeandisneverdestroyed.E.g.,

staticint16_tAddTwo(int16_tin)returnin+2;

e)The static qualifierisusedwithavariabledefinedinsideafunctiontospecifythevariableispermanent,createdatcompiletime,initializedto0,andisneverdestroyed.E.g.,

voidfunction(int16_tin)staticint16_tmyData;

f)The const qualifierisusedwithaglobalvariabletospecifythevariableshouldbeallocatedinROM.E.g., constint16_tmyData=5;

g)Codethatisfriendlymeansitcanbeexecutedbymorethanonethreadwithoutcausingacrashorlossofdata.

h)The volatile qualifierisusedwithvariablestotellthecompilerthatcodethataccessesthisvariableshouldbeoptimizedasmuchaspossible.

i)Aread-modify-writeaccesstoasharedglobalvariablealwayscreatesacriticalsection.

j)ThecompilerautomaticallysetsI=1atthebeginningoftheinterruptserviceroutineandclearsI=0attheendsothatthecomputerrunswithinterruptsdisabledwhileservicingtheinterrupt.

5.16ModifyProgram5.9assumingthebusclockis8MHzandwanttoabaudrateof9600bits/sec.

5.17ModifyProgram5.9assumingthebusclockis6MHzandwanttoabaudrateof38400bits/sec.

5.18ModifyProgram5.9assumingthebusclockis25MHzandwanttoabaudrateof19200bits/sec.

5.19ModifyProgram5.9touseUART1,assumingthebusclockis50MHzandwanttoabaudrateof115200bits/sec.

5.20Whichofthefollowingdebugginginstrumentsisfaster?

GPIO_PORTD0^=0x01;

or

GPIO_PORTD0=0x01;GPIO_PORTD0=0x00;

5.21ModifyProgram5.10assumingthebusclockis8MHzandyouwanttointerruptevery1ms.

5.22ModifyProgram5.10assumingthebusclockis50MHzandyouwanttointerruptevery100 s.

5.23ModifyProgram5.10assumingthebusclockis6MHzandyouwanttointerruptevery1sec.

5.24TheseseveneventsalloccurduringeachSysTickinterrupt.Ordertheseeventsintoapropertimesequence.Morethanoneanswermaybecorrect.

1)TheCURRENTequals0andthehardwaresetstheSysTicktriggerflag

2)TheSysTickvectoraddressisloadedintothePC

3)TheSysTicktriggerflagisautomaticallyclearedbyhardware

4)ThesoftwareexecutestheSysTickISR

5)TheR0,R1,R2,R3,R12,LR,PCandPSRarepushedonthestack

6)TheR0,R1,R2,R3,R12,LR,PCandPSRarepulledfromthestack

7)Thesoftwareexecutes bxlr

D5.25DesignandimplementaFIFOthatcanholdupto4elements.Eachelementis10bytes.Therewillbethreefunctions: Init , Put oneelementintoFIFOand Get oneelementfromtheFIFO.ShowtheprivateRAM-basedvariables.WriteafunctionthatinitializestheFIFO.Writeafunctionthatputsone10-byteelementintotheFIFO.Writeafunctionthatgetsone10-byteelementfromtheFIFO.Documenthowparametersarepassedineachfunction.

D5.26DesignandimplementaFIFOthatusesthememorymanagertoallocatespace.Eachelementis32-bits.Callmallocandfreeonblocksof1024bytes(256words).IftheFIFOneedsmorespaceallocateitbycallingmalloc(1024).AfterallthedatafromoneblockisreturnedviaGet,thendeallocatethatblockbycallingFree.Youareallowedtopermanentlydefinepointersandcounters,butnopermanentbuffersareallowed.Therewillbethreefunctions: Fifo_Init , Fifo_Put oneelementintoFIFOand Fifo_Get oneelementfromtheFIFO,usingthesameprototypesasProgram3.7.

D5.27SolvetheFSMdescribedinExerciseD3.12usingSysTickinterrupts.Includethreecomponents:theFSMstructure,aninitializationfunction,andtheSysTickISR.Thereshouldbenobackwardsjumps.

D5.28SolvetheFSMdescribedinExerciseD3.13usingSysTickinterrupts.RunonepassthroughtheFSMevery10ms.Includethreecomponents:theFSMstructure,aninitializationfunction,andtheSysTickISR.Thereshouldbenobackwardsjumps.

D5.29SolvethetrafficlightFSMdescribedinExample3.1usingSysTickinterrupts.RuntheFSMintheISR.Includethreecomponents:theFSMstructure,aninitializationfunction,andtheSysTickISR.Thereshouldbenobackwardsjumps.

D5.30YouwilldesignandimplementtheExample3.2FSMusingtheedge-triggeredinterrupts.OneofthelimitationsoftheFSMimplementationsinChapter3isthattheyrequire100%oftheprocessortimeandrunintheforeground.Inthissystem,therearetwoinputsandtwooutputs.WewillimplementaFSMwherestatetransitionsonlyoccurontheedgesofoneofthetwoinputs.Theseedgesshouldcauseaninterrupt,andtheFSMcontrollerwillberunintheinterrupthandler.

5.11.LabAssignmentsLab5.1Theoverallobjectiveistocreateanalarmclock.Aperiodicinterruptestablishesthetimeofday.Input/outputofthesystemusesaninterruptingUARTport.Inputisusedtosetthetime,andsetthealarm.Designacommandinterpreterthatperformsthenecessaryoperations.Outputisusedtodisplaythecurrenttime.AflashingLEDorsoundbuzzercanbeusedtosignifythealarm.

Lab5.2Theoverallobjectiveistocreateasoftware-drivenvariable-frequencydigitalsquarewaveoutput.Aperiodicinterruptwillsetanoutputpinhigh/low.Input/outputofthesystemusesaninterruptingUARTport.Inputisusedtosetthefrequencyofthewave.Designacommandinterpreterthatperformsthenecessaryoperations.Connecttheoutputtoanoscilloscope.

Lab5.3Theoverallobjectiveistocreateaninterrupt-drivenLEDlightdisplay.Interface8to16coloredLEDstoindividualoutputpins.AperiodicinterruptwillchangetheLEDpattern.Connectoneortwoswitchestothesystem,andusethemtocontrolwhichLEDlightpatternisbeingdisplayed.Designalinkeddatastructurethatcontainsthelightpatterns.CreatedevicedriversfortheLEDoutputs,switchinputs,andperiodicinterrupt.ThemainprogramwillinitializetheLEDoutputs,switchinputs,andperiodicinterrupt.Alloftheinput/outputwillbeperformedintheISR.Youshouldcreateageneralpurposetimersystemthatacceptsafunctiontoexecuteandaperiod.E.g.,

voidmain(void)

LED_Init();//InitializeLEDoutputsystem

Switch_Init();//Initializeswitchinputsystem

SysTick_Init(&FSMcontroller,250);//RunFSMcontroller()every250ms

while(1)

Lab5.4Theoverallobjectiveistocreateaninterrupt-driventrafficlightcontroller.Thesystemhasthreedigitalinputsandsevendigitaloutputs.YoucansimulatethesystemwiththreeswitchesandsevenLEDs.TheinputsareNorth,East,andWalk.Theoutputsaresixforthetrafficlightandoneforawalksignal.Implementthecontrollerusingafinitestatemachine.ChooseaMooreorMealydatastructureasappropriate.AperiodicinterruptwillruntheFSM.ThemainprogramwillinitializetheLEDoutputs,switchinputs,andperiodicinterrupt.Alloftheinput/outputwillbeperformedintheISR.Youshouldcreateageneralpurposetimersystemthatacceptsafunctiontoexecuteandaperiod.E.g.,

voidmain(void)

Traffic_Init();//InitializeswitchesandLED

Timer_Init(&Traffic_Controller,100);//RunFSMevery100ms

while(1)

6.TimeInterfacingChapter6objectivesareto:•Useinputcapturetogenerateinterruptsandmeasureperiodorpulsewidth

•Useoutputcomparetocreateperiodicinterruptsandgeneratesquarewaves

•Usebothinputcaptureandoutputcomparetomeasurefrequency

•Interfacecoil-activateddeviceslikeaDCmotor,solenoid,andrelay

•Interfaceanopticaltachometer

•Generatewaveformsusingthepulse-widthmodulator

Thetimersystemsonstate-of-the-artmicrocontrollersareveryversatile.Overthelast30years,theevolutionofthesetimerfunctionshasparalleledthegrowthofnewapplicationspossiblewiththeseembeddedcomputers.Inotherwords,inexpensiveyetpowerfulembeddedsystemshavebeenmadepossiblebythecapabilitiesofthetimersystem.Inthischapterwewillintroducethesefunctions,thenusethemthroughouttheremainderofthebook.ToadjustpowertoaDCmotorwewilluseatime-basedmethodcalledpulse-widthmodulation.Tomeasuremotorspeedwewilluseatachometeranemploythetimertomeasureperiod.Inthisbook,wewillnotusethereal-timeclock(RTC).Forinformationaboutthesemodes,refertothedatasheetforyourspecificmicrocontroller.Ifwereviewtheapplicationsintroducedinthefirstchapter(seeSection1.1),wewillfindthatvirtuallyallofthemmakeextensiveuseofthetimerfunctionsdevelopedinthischapter.

6.1.InputCaptureorInputEdgeTimeMode

6.1.1.BasicPrinciplesTheTexasInstrumentsmicrocontrollershavetimersthatareseparateanddistinctfromSysTick,seeFigure6.1.Inputedgetimemode(orinputcapturemode)isusedtomaketimemeasurementsoninputsignals.Wecanuseinputcapturetomeasuretheperiodorpulsewidthofdigital-levelsignals.Theinputcapturesystemcanalsobeusedtotriggerinterruptsonrisingorfallingtransitionsofexternalsignals.AGeneralPurposeTimerModule(GPTM)hastwo16-bittimers,whichcanbeextendedto24bits.EachGPTMinputcapturemodulehas

Anexternalinputpin,e.g.,CCP0Atriggerflagbit,calledRawInterruptStatus,e.g.,CAERISTwoedgecontrolbits,Eventbits,e.g.,TAEVENTAnarmbit,calledinterruptmask,e.g.,CAEIMA16-bitor24-bitinputcaptureregister,e.g.,TARThevariousmembersoftheLM3S/TM4Cfamilyhavefromzerototwelveinputcapturemodules.Figure6.1showstheportpinsusedforinputcapturevaryfrommicrocontrollertomicrocontroller.TheinputcaptureandoutputcomparepinsarelabeledCCP0,CCP1,…AtimermodulehasassociatedI/Opins.TheevenCCPpinisattachedtoSubmoduleAandtheoddpintoSubmoduleB.SometimermodulesarenotattachedtoanyI/OpinsForexample,theTM4C1294haseighttimers,butTimer6andTimer7donothaveI/Opins.Timerswithoutpinscanbeusedtogenerateperiodicinterrupts,butnotforinputcapture.Tables2.7and2.8describehowtoattachI/OpinstothetimermodulesontheTM4C123andTM4C1294.

Figure6.1.InputcapturepinsontheLM3S811,theLM3S1968,theTM4C123,andtheTM4C1294.Inthisbookweusethetermarmtodescribethebitthatallows/deniesaspecificflagfromrequestinganinterrupt.TheTexasInstrumentsmanualsrefertothisbitasamask.I.e.,thedeviceisarmedwhenthemaskbitis1.Typically,thereisaseparatearmbitforeveryflagthatcanrequestaninterrupt.Anexternalinputsignalisconnectedtotheinputcapturepin.Duringinitializationwespecifywhethertherisingorfallingedgeoftheexternalsignalwilltriggeraninputcaptureevent.The16-bitcounterdecrementsattherateofthebusclock,whenithits0,itautomaticallyrollsoverto0xFFFFandcontinuestocountdown(Figure6.2).Twoorthreeactionsresultfromaninputcaptureevent:1)thecurrenttimervalueiscopiedintotheinputcaptureregister(TARorTBR),2)theinputcaptureflagisset(RIS)and3)aninterruptisrequestedifarmed(IM).Thismeansaninterruptcanberequestedonacaptureevent.WhenusingtheprescalerontheTM4C,thecountercanbeextendedto24bits(notonLM3Sparts).

31–3 2–0 Name

$4003.0000 GPTMCFG TIMER0_CFG_R

31–4 3 2 1-0

$4003.0004 TAAMS TACMR TAMR TIMER0_TAMR_R

31–4 3 2 1-0

$4003.0008 TBAMS TBCMR TBMR TIMER0_TBMR_R

14 13 11-10 8 6 5 3-2 0

$4003.000CTBPWMLTBOTE TBEVENTTBEN TAPWMLTAOTE TAEVENTTAEN TIMER0_CTL_R

31-11 10 9 8 7-4 2 1 0

$4003.0018 CBEIM CBMIM TBTOIM CAEIM CAMIM TATOIM TIMER0_IMR_R

31-11 10 9 8 7-4 2 1 0

$4003.001C CBERIS CBMRIS TBTORIS CAERIS CAMRIS TATORIS TIMER0_RIS_R

31-11 10 9 8 7-4 2 1 0

$4003.0020 CBEMIS CBMMIS TBTOMIS CAEMIS CAMMIS TATOMIS TIMER0_MIS_R

31-11 10 9 8 7-4 2 1 0

$4003.0020 CBECINT CBMCINTTBTOCINT CAECINTCAMCINT TATOCINTTIMER0_ICR_R

31–16 15–0

$4003.0028 TAILRH TAILRL TIMER0_TAILR_R

31–16 15–0

$4003.002C TBILRL TIMER0_TBILR_R

31–16 15–0

$4003.0030 TAMRH TAMRL _TAMATCHR_R

31–16 15–0

$4003.0034 TBMRL _TBMATCHR_R

31–8 7-0

$4003.0038 TAPSR TIMER0_TAPR_R

31–8 7-0

$4003.003C TBPSR TIMER0_TBPR_R

31–8 7-0

$4003.0040 TAPSMR TIMER0_TAPMR_R

31–8 7-0

$4003.0044 TBPSMR TIMER0_TBPMR_R

31–16 15-0

$4003.0048 TARH TARL TIMER0_TAR_R

31–16 15-0

$4003.004C TBRL TIMER0_TBR_R

Table6.1.Timer0registers.RTCbitsnotshown.Eachregisteris32bitswide.Shadedbitsarezero.Thebitsshowninboldwillbeusedinthissection.Timers1,2,…havethesameformats.

Theinputcapturemechanismhasmanyuses.Threeofcommonapplicationsare:

1.AnISRisexecutedontheactiveedgeoftheexternalsignal

2.Performtworisingedgeinputcapturesandsubtractthetwotogetperiod3.Performarisingedgeandthenafallingedgecaptureandsubtractthetwomeasurementstogetpulsewidth

Figure6.2.RisingorfallingedgeofCCP0causesthecountertobelatchedintoTAR,settingCAERIS.Checkpoint6.1:Whendoesaninputcaptureeventoccur?

Checkpoint6.2:Whathappensduringaninputcaptureevent?

Observation:Thetimerisveryaccuratebecauseofthestabilityofthecrystalclock.

Observation:Whenmeasuringperiodorpulse-width,themeasurementresolutionwillequalthebusclockperiod.

6.1.2.InputCaptureDetailsontheLM3S/TM4CNextwewilloverviewthespecificinputcapturefunctionsontheLM3S/TM4Cfamily.Thissectionisintendedtosupplementratherthanreplacethedatasheets.Whendesigningsystemswithinputcapture,pleaserefertothereferencemanualofyourspecificmicrocontroller.Table6.1showssomeoftheregistersforTimer0.Webegininitializationbyenablingtheclockforthetimerandforthedigitalportwewillbeusing.Weenablethedigitalpinandselectitsalternativefunction.WewilldisablethetimerduringinitializationbyclearingtheTAEN(orTBEN)bitinthe TIMER0_CTL_R register.Touse16-bitmode,wesetGPTMCFGfieldto4.WecleartheTAAMS(orTBAMS)bitforcapturemode.WesettheTACMR(orTBCMR)bitforinputedgetimemode.TheTAMR(orTBMR)fieldissetto3forcapturemode.Insummary,wewritea0x0007tothe TIMER0_TAMR_R registertoselectinputcapturemode.Table6.2liststheedgecapturemodesforTAEVENT(orTBEVENT.)

TAEVENT Activeedge

00 Captureonrising

01 Captureonfalling

10 Reserved

11 Captureonbothrisingandfalling

Table6.2.Twocontrolbitsdefinetheactiveedgeusedforinputcapture(TBEVENTisthesame).

Whenwearemeasuringtimewithprescaler,suchasperiodmeasurementandpulsewidthmeasurement,wesetthe24-bitreloadvalueto0xFFFFFF.Inthisway,the24-bitsubtractionoftwocaptureeventsyieldsthetimedifferencebetweenevents.Inparticular,wewillinitialize TIMER0_TAILR_R to0xFFFFand TIMER0_TAPR_R to0xFF.WearmtheinputcapturebysettingtheCAEIM(orCBEIM)bitinthe TIMER0_IMR_Rregister.Itisgoodpracticetoclearthetriggerflagintheinitializationsothatthefirstinterruptoccursdotoactionsoccurringaftertheinitialization,andnotduetoedgesthatmighthaveoccurredduringpowerup.Thetriggerflagsareinthe TIMER0_RIS_Rregister.Theseflagsareclearedbywriting1’sintocorrespondingbitsinthe TIMER0_ICR_R register.Afterallconfigurationbitsareset,theTimercanbeenabledbysettingtheTAEN(orTBEN)bitinthe TIMER0_CTL_R register.IfinterruptsarerequiredthentheNVICmustbeconfiguredbysettingthepriorityandenablingtheappropriateinterruptnumber.

Thereisan8-bitprescalerdefinedforeachsubmodulesAandB: TIMER0_TAPMR_R and TIMER0_TBPMR_R .TheseprescalersarenotactiveduringinputcapturemodeonLM3S,buttheprescalersontheTM4Careusedtoextendthe16-bittimerto24bits.

TheTAEVENTbitsof TIMER0_CTL_R registerspecifywhethertherisingorfallingedgeofCCP0willtriggeraninputcaptureeventonTimer0A.Twoorthreeactionsresultfromaninputcaptureevent:1)thecurrenttimervalueiscopiedintotheinputcaptureregister, TIMER0_TAR_R ,2)theinputcaptureflag(CAERIS)isset,and3)aninterruptisrequestedifthemaskbit(CAEIM)is1.TheCAERISandCBERISflagbitsinthe TIMER0_RIS_R registerdonotbehavelikearegularmemorylocation.Inparticular,theflagcannotbesetbysoftware.Rather,aninputcaptureoroutputcomparehardwareeventwillsettheflag.Theotherpeculiarbehavioroftheflagisthatthesoftwaremustwriteaonetothe TIMER0_ICR_R registerinordertocleartheflag.IfthesoftwarewritesazerototheTIMER0_ICR_R register,nochangewilloccur.FromTable6.1,weseetheCAERIStriggerflagisinbit2ofthe TIMER0_RIS_R register.Theproperwaytoclearthistriggerflagis

TIMER0_ICR_R=0x0004;

Writesthe TIMER0_RIS_R registerhavenoeffect.Noeffectoccursinthebitstowhichwewriteazerointhe TIMER0_ICR_R register.

Observation:Thephase-lock-loop(PLL)ontheARMwillaffectthetimerperiod.

Example6.1.Designameasurementsystemfortherobotthatcountsthenumberoftimesawheelturns.Thiscountwillbeameasureofthetotaldistancetravelled.Thedesiredresolutionis1/32ofaturn

Solution:Wheneveryoumeasuresomething,itisimportanttoconsidertheresolutionandrange.Thebasicideaistouseanopticalsensor(QRB1134)tovisualizethepositionofthewheel.Ablack/whitestripedpatternisattachedtothewheel,andanopticalreflectivesensorplacednearthestripes.ThesensorhasaninfraredLEDoutputandalightsensitivetransistor.Theresolutionwillbedeterminedbythenumberofstripesandtheabilityofthesensortodistinguishonestripefromanother.Therangewillbedeterminedbytheprecisionofthesoftwareglobalvariableusedtocountedges.

TheanodeandcathodeleadsofthesensorcontroltheamountofemittedIRlight.TheoperatingpointfortheLEDintheQRB1134is15mAat1.8V.ThecurrenttotheLEDiscontrolledbytheR1resistor.Inthiscircuit,theLEDcurrentwillbe(3.3-1.8V)/R1,whichwesetto15mAbymakingR1equalto100 .TheR2pull-upresistoronthetransistorcreatesanoutputswingatV1dependingonwhetherthesensorseesablackstripeorwhitestripe.Unfortunately,thesignalV1isnotdigital.Therail-to-railopamp,inopenloopmode,createsacleandigitalsignalatV2,whichhasthesamefrequencyasV1.ThenegativeterminalissettoavoltageapproximatelyinthecenterofV1,shownas+2VinFigure6.3.SlewrateisdefinedasdV/dt.Anuncertaintyinvoltage Vwilltranslatetoanuncertaintyintime,t= V/(dV/dt).Thustominimizetimeerror,wechooseaplacewithmaximumslewrate.Inotherwords,weshouldselectthethresholdattheplaceinthewavewheretheslopeisatmaximum.

WetheninterfaceV2toaninputcapturepin,andconfigurethesystemtotriggeraninterruptoneachrisingedge.SeveraloftheGPIOpinscouldhavebeenused,butweselectedPD4(CCP0)asshowninFigure6.3.ThismeansthatTimer0Awillbeused.

Becausethereare32stripesonthewheel,therewillbe32interruptseachtimethewheelrotatesonce.A32-bitglobalvariablewillbeusedtocountthenumberofrisingedges.Thiscountisabinaryfixed-pointnumberwitharesolutionof2-5revolutions.E.g.,ifthecountis100,thismeans100/32or3.125revolutions.Ifthecircumferenceofthewheelisfixed,andifthewheeldoesnotslip,thenthiscountisalsoameasureofdistancetraveled.WesolvedasimilarprobleminProgram5.6usingedge-triggeredinputs.Howeverinthissolution,wewilluseinputcapture.

Theinitializationsetsthedirectionregisterbit64to0,soPB6isaninput.Bit6in GPIO_PORTB_AFSEL_R isset,makingtimerchannel4aninputcapture.Bits2and3(TAEVENT)in TIMER0_CTL_R specifywewantTimer0AtocaptureontherisingedgeofPB6.Wearmtheinputcapturechannelbysettingbit2(CAEIM)in TIMER0_IMR_R .Itisgooddesignpracticetocleartriggerflagsintheinitialization,sothefirstinterruptisduetoarisingedgeontheinputoccurringaftertheinitializationandnotduetoeventsoccurringduringpoweruporbeforeinitialization.

Figure6.3.Anexternalsignalisconnectedtotheinputcapture.

volatileuint32_tCount;//incrementedoninterrupt

voidTimerCapture_Init(void)

SYSCTL_RCGCTIMER_R|=0x01;//activatetimer0

SYSCTL_RCGCGPIO_R|=0x00000002;//activateportB

Count=0;//allowtimetofinishactivating

GPIO_PORTB_DEN_R|=0x40;//enabledigitalI/OonPB6

GPIO_PORTB_AFSEL_R|=0x40;//enablealtfunctonPB6

GPIO_PORTB_PCTL_R=(GPIO_PORTB_PCTL_R&0xF0FFFFFF)+0x07000000;

TIMER0_CTL_R&=~0x00000001;//disabletimer0Aduringsetup

TIMER0_CFG_R=0x00000004;//configurefor16-bittimermode

TIMER0_TAMR_R=0x00000007;//configureforinputcapturemode

TIMER0_CTL_R&=~(0x000C);//TAEVENTisrisingedge

TIMER0_TAILR_R=0x0000FFFF;//startvalue

TIMER0_IMR_R|=0x00000004;//enablecapturematchinterrupt

TIMER0_ICR_R=0x00000004;//cleartimer0Acaptureflag

TIMER0_CTL_R|=0x00000001;//enabletimer0A

NVIC_PRI4_R=(NVIC_PRI4_R&0x00FFFFFF)|0x40000000;//Timer0A=priority2

NVIC_EN0_R=0x00080000;//enableinterrupt19inNVIC

EnableInterrupts();

voidTimer0A_Handler(void)

TIMER0_ICR_R=0x00000004;//acknowledgetimer0Acapturematch

Count=Count+1;

Program6.1.Countinginterruptusinginputcapture(InputCapture_xxx.zip).

AninputcaptureinterruptoccursoneachriseofCCP0.Thelatencyofthesystemisdefinedasthetimedelaybetweentheriseoftheinputcapturesignaltotheincrementof Count .Assumingtherearenootherinterrupts,andassumingthemainprogramdoesnotdisableinterrupts,thedelaywillbeontheorderof1ms.Thelatencymaybelargerifthereareothersectionsofcodethatexecutewiththeinterruptsdisabled,oriftherearehigherpriorityinterruptsactive.Theritual, TimerCapture_Init() ,setsinputcapturetointerruptontherise,andinitializestheglobal, Count .Theinterruptsoftwareperformsapoll,acknowledgestheinterruptandincrementstheglobalvariable.ActualmeasurementscollectedwiththisinterfaceareshowninFigure6.4.

Figure6.4.MeasuredV1andV2fromthecircuitinFigure6.3.

Checkpoint6.3:ExplainhowtochangeProgram6.1torunonaLM3S1968withtheinputconnectedtoCCP0/PB0.

Checkpoint6.4:ExplainhowtochangeProgram6.1tostillrunonaLM3S811butwiththeinputconnectedtoCCP2/PD5.

Checkpoint6.5:WritecodetoclearCBMRISinTimer1.

CommonError:Whentwosoftwaremodulesbothneedtosetthesameconfigurationregister,apoorlywritteninitializationbyonesoftwaremoduleundoestheinitializationperformedbytheother.

OntheLM4F120/TM4C123,theCCP0inputofTimer0canbeconnectedtoeitherPB6ortoPF0asdescribedinTables2.7and2.8.TousePB6,wesetPCTLbitsforPB6to7.TousePF0,wesetPCTLbitsforPF0to7.E.g.,

GPIO_PORTF_AFSEL_R|=0x01;

GPIO_PORTF_PCTL_R=(GPIO_PORTF_PCTL_R&0xFFFFFFF0)+0x00000007;

6.1.3.PeriodMeasurement

Beforeoneimplementsasystemthatmeasuresperiod,itisappropriatetoconsidertheissuesofresolution,precisionandrange.Theresolutionofaperiodmeasurementisdefinedasthesmallestchangeinperiodthatcanreliablybedetected.InExample6.2,thebusclockis80MHz.Thismeans,iftheperiodincreasesby12.5ns,thentherewillbeonemoreTimerclockbetweenthefirstrisingedgeandthesecondrisingedge.Inthissituation,the24-bitsubtractionwillincreaseby1,thereforetheperiodmeasurementresolutionis12.5ns.Theresolutionisthesmallestmeasurablechange.Resolutiondefinestheunitsofthemeasurement.Inthisfirstexample,ifthecalculationof Periodresultsin1000,thenitrepresentsaperiodof1000•12.5nsor12.5µs.Theprecisionoftheperiodmeasurementisdefinedasthenumberofseparateanddistinguishablemeasurements.Ifthe24-bitcounterisused,thereareabout16milliondifferentperiodsthatcanbemeasured.Wecanspecifytheprecisioninalternatives,e.g.,224,orinbits,e.g.,24bits.Thelastissuetoconsideristherangeoftheperiodmeasurement,whichisdefinedastheminimumandmaximumvaluesthatcanreliablybemeasured.Weareconcernedwhathappensiftheperiodistoosmallortoolarge.Agoodmeasurementsystemshouldbeabletodetectoverflowsandunderflows.Inaddition,wewouldnotlikethesystemtocrash,orhang-upiftheinputperiodisoutofrange.Similarly,itisdesirableifthesystemcandetectwhenthereisnoperiod.Foredgedetection,theinputmustbehighforatleasttwosystemclockperiodsandlowforatleasttwosystemclockperiods.

Example6.2.Designasystemthatmeasurestherotationalspeedofamotorshaftusingperiodmeasurementwithaprecisionof24bitsandaresolutionof12.5ns.

Solution:FordetailsofthesensorreferbacktoFigure6.3.Inthisexample,thedigitalinputsignalisconnectedtoaninputcapturepin,CCP0.IfthemotorshaftrotatesoncetherewillbeNrisingedgesonCCP0.Wewillmakethebusclockequal80MHz.EachrisingedgewillcauseTimer0Atogenerateaninputcaptureinterrupt(Figure6.5).

Figure6.5.Tomeasureperiodweconnecttheexternalsignalaninputcapture,PB6ontheTM4C123.

Theperiodiscalculatedasthedifferencein TIMER0_TAR_R latchvaluesfromonerisingedgetotheother.IfN=100,andthemotorisspinningat300RPM,thentheperiodwillbe[(60000ms/min)/(300RPM)/100edges/rotation)],whichwillbe2.00ms/edge,asshowninFigure6.6.

Forexample,iftheperiodis2000µs,theTimer0Ainterruptswillberequestedevery160,000cycles,andthe24-bitdifferencebetween TIMER0_TAR_R latchvalueswillbe160,000.ThissubtractionremainsvalidevenifthetimerreacheszeroandwrapsaroundinbetweenTimer0Ainterrupts.Ontheotherhand,thismethodwillnotoperateproperlyiftheperiodislargerthan224cycles,orabout209ms.

Figure6.6.Timingexampleshowingcounterrolloverduring24-bitperiodmeasurement.

Theresolutionis12.5nsbecausetheperiodmustincreasebyatleastthisamountbeforethedifferencebetweenTimer0Ameasurementswillreliablychange.Eventhougha24-bitcounterisused,theprecisionisalittlelessthan24bits,becausetheshortestperiodthatcanbehandledwiththisinterrupt-drivenapproachisabout1s.Ittakesabout1 stocompletethecontextswitch,executetheISRsoftware,andreturnfrominterrupt.Thisfactorisdeterminedbyexperimentalmeasurement.Inotherwords,astheperiodapproaches1s,ahigherandhigherpercentageofthecomputerexecutionisutilizedjustinthehandleritself.

Becausetheinputcaptureinterrupthasaseparatevectorthesoftwaredoesnotpoll.Aninterruptisrequestedoneachrisingedgeoftheinputsignal.Inthissituationwecountallthecyclesrequiredtoprocesstheinterrupt.TheperiodmeasurementsystemwrittenfortheTM4C123ispresentedinProgram6.2.The24-bitsubtractionisproducedbyandingthedifferencewith0x0FFFFFF,calculatingthenumberofbusclocksbetweenrisingedges.Thefirstperiodmeasurementwillbeincorrectandshouldbeneglected.

uint32_tPeriod;//24-bit,12.5nsunits

uint32_tstaticFirst;//Timer0Afirstedge,12.5nsunits

int32_tDone;//mailboxstatusseteachrising

voidPeriodMeasure_Init(void)

SYSCTL_RCGCTIMER_R|=0x01;//activatetimer0

SYSCTL_RCGCGPIO_R|=0x02;//activateportB

First=0;//firstwillbewrong

Done=0;//setonsubsequent

GPIO_PORTB_DIR_R&=~0x40;//makePB6input

GPIO_PORTB_AFSEL_R|=0x40;//enablealtfunctonPB6

GPIO_PORTB_DEN_R|=0x40;//configurePB6asT0CCP0

GPIO_PORTB_PCTL_R=(GPIO_PORTB_PCTL_R&0xF0FFFFFF)+0x07000000;

TIMER0_CTL_R&=~0x00000001;//disabletimer0Aduringsetup

TIMER0_CFG_R=0x00000004;//configurefor16-bitcapturemode

TIMER0_TAMR_R=0x00000007;//configureforrisingedgeevent

TIMER0_CTL_R&=~0x0000000C;//risingedge

TIMER0_TAILR_R=0x0000FFFF;//startvalue

TIMER0_TAPR_R=0xFF;//activateprescale,creating24-bit

TIMER0_IMR_R|=0x00000004;//enablecapturematchinterrupt

TIMER0_ICR_R=0x00000004;//cleartimer0Acapturematchflag

TIMER0_CTL_R|=0x00000001;//timer0A24-b,+edge,interrupts

NVIC_PRI4_R=(NVIC_PRI4_R&0x00FFFFFF)|0x40000000;//Timer0A=priority2

NVIC_EN0_R=1<<19;//enableinterrupt19inNVIC

EnableInterrupts();

voidTimer0A_Handler(void)

TIMER0_ICR_R=0x00000004;//acknowledgetimer0Acapture

Period=(First-TIMER0_TAR_R)&0x00FFFFFF;//12.5nsresolution

First=TIMER0_TAR_R;//setupfornext

Done=1;//setsemaphore

Program6.2.24-bitperiodmeasurement(PeriodMeasure_xxx.zip).

Example6.3.Designanohmmeterwitharangeof0to250k andaresolutionof10 .

Solution:Onewaytomeasureresistanceorcapacitanceisconverttheelectricalparametertotime.Inparticular,wewilluseaTLC555toconverttheinputresistance(Rin)toatimeperiod(P).Figure6.7showsthehardwarecircuit.The555iscalledanastablemultivibrator,createdasignalwavewithaperiodequaltoCT*(RA+2*RB)*ln(2).CTand

RAareconstants,butRBis10k plustheunknowninput,Rin.WecanthenusetheperiodmeasurementsoftwarefromProgram6.2.Theresistancemeasurementresolution( R)dependsontheperiodmeasurementresolution( t)andthecapacitorCT, t=CT* R*ln(2).Atabusfrequencyof50MHz,theperiodmeasurementresolutionis20ns.Ifwedesirea10 resistanceresolution,thecapacitorwillbeabout3nF.Inthiscircuit,a3.3nFcapacitorwasusedbecauseitisastandardvalue.Astheinputresistancevariesfrom0to250k ,theperiod(in20nsunits)variesfromabout3000to60000.Alinearfunctionisusedtoconvertthemeasuredperiod(20nsunits)toresistance(10 units).ThesameinitializationofProgram6.2canbeused.

Figure6.7.TheTLC555outputsasquarewavewithaperioddependingonRARBandCT.

Program6.3showstheISRofProgram6.2extendedtoincludetheperiodtoresistanceconversion.ThecoefficientsofthislinearfitweredeterminedbyempiricalcalibrationRin= 0.4817* Period -1466.5 .Thisequationisimplementedwithfixed-pointmath.

Althoughmathematically,itseemslikethesystemhasa10 resolutionandaprecisionof25000alternatives,theactualresolutionwillbelimitedbythestabilityoftheTLC555,thestabilityofcapacitor,andanyaddednoiseintothecircuit.

uint32_tPeriod;//16-bit,20nsunits

uint32_tResistance;//16-bit,10ohmunits

uint32_tstaticFirst;//Timer0Afirstedge

uint32_tDone;//mailboxstatusseteachrising

voidTimer0A_Handler(void)

TIMER0_ICR_R=0x00000004;//acknowledgetimer0Acapture

Period=(First-TIMER0_TAR_R)&0xFFFF;//20nsresolution

Resistance=(1973*Period-6006784)>>12;//conversion

First=TIMER0_TAR_R;//setupfornext

Done=0xFF;//setmailboxflag

Program6.3.16-bitperiodmeasurementusedtomeasureresistance(ResistanceMeasure_xxx.zip).

6.1.4.PulseWidthMeasurementThebasicideaofpulsewidthmeasurementistocauseaninputcaptureeventonboththerisingandfallingedgesofaninputsignal.Eachedgecapturesatimervalue.Thedifferencebetweenthesetwocapturedtimeswillbethepulsewidth.Justlikeperiodmeasurement,theresolutionisdeterminedbytherateatwhichthetimerisdecremented.Themaximumpulsewidthis216timestheresolution,andislimitedbythe16-bittimer.Whenconsideringmeasurementresolutionitisimportanttoconsidervoltagenoiseaswell.Forexample,inFigures6.3and6.5,anyvoltagenoiseonthesensorwillcauseatime-jitter,whichisanoiseinthetimemeasurement.IftheslewrateofthesensoratthethresholdvoltageisdV/dt,thenavoltagenoiseofVwillcauseatimeerrorof t,accordingto t= V/(dV/dt).

Example6.4.Designasystemtomeasureresistanceanduseittointerfacea10kΩjoystick.

Solution:Clearly,youcouldjustconnectthepotentiometeracrossthepowerrailsandmeasurethevoltagedropusingtheADC(whichwillbedescribedlater).However,itismuchcheaperandeasiertomeasuretimepreciselythantomeasurevoltageprecisely.Thismakessense,consideringthataninexpensiveclockcanrunformonthsbeforeitneedstobereset,butevenahighqualityvoltmetermeasurestoonlyafewdigitsofprecision.Therefore,wewillconverttheresistancetoapulsewidthusingexternalcircuitryandmeasurethepulseusinginputcapture.TheTM4C123microcontrollerrunsat80MHz,sothepulsewidthresolutionwillbe12.5ns.Therangewillbe1sto209ms.Withtheminimumdeterminedbythetimetoexecutesoftwareandthemaximumdeterminedbythe24-bitcounter(12.5ns*224.)

Theobjectiveistouseinputcapturepulsewidthmeasurementtomeasureresistance.Thisbasicapproachisemployedbymostjoystickinterfaces.Theresistancemeasurementrangeis0≤R≤10kΩ.Thedesiredresolutionis1Ω.Wewillusebusy-waitsynchronization.

Mostjoystickshavetwovariableresistances,butwewillshowthesolutionforjustoneofthepotentiometers.ThevariableresistanceRinFigure6.8isonechannelofthejoystick.Weuseamonostabletoconvertunknownresistance,R,totimedifferencet.Toperformhighqualitymeasurementswewillneedahighqualitycapacitor,becauseofthebasicconversionfollows t=R*C.PB5isadigitaloutputandPB6isaninputcaptureinput.A

risingedgeonPB5causesamonostablepositivelogicpulseonthe“Q”outputofthe74HC123.WechooseR1andC,sothattheresistanceresolutionmapsintoapulsewidthmeasurementresolutionof12.5ns,andtheresistancerange0≤R≤10kΩmapsinto125≤t≤250µs.Thefollowingequationdescribesthepulsewidthgeneratedbythe74HC123monostableasafunctionoftheresistancesandcapacitance.

t=0.45•(R+R1)•C

Foralinearsystem,withxasinputandyasoutput,wecanusecalculustorelatethemeasurementresolutionoftheinputandoutput.

Therefore,therelationshipbetweenthepulsewidthmeasurementresolution,∆t,andtheresultingresistancemeasurementresolutionisdeterminedbythevalueofthecapacitor.

t=0.45• R•C

Tomakea∆tof12.5nscorrespondtoa∆Rof1Ω,wechoose

C=∆t/(0.45∆R)=12.5ns/(0.45•1Ω)=27.8nF

WewilluseaC0Ghigh-stability27nFceramiccapacitor.Todesignfortheminimumpulsewidth,wesetR=0,t=0.45•R1•C.WechooseR1tomaketheminimumpulsewidth125 s,

R1=t/(0.45•C)=125µs/(0.45•27nF)=10.288kΩ

Wewillusea1%metalfilm10kΩresistorforR1.TochecktheminimumandmaximumpulsewidthswesetR=0andR=10kΩ,andcalculatet=0.45•(10kΩ)•27nF=121.5µs(closeto125),andt=0.45•(20kΩ)•27nF=243µs(closeto250).Theimportanceparametersforthe74HC123,R1andCarethelongtermstability.I.e.,theirperformanceshouldbeconstantovertime.Anydifferencesbetweenassumedvaluesandrealvaluesforthecapacitorandthe0.45constantcanbecompensatedforwithsoftwarecalibration.

Figure6.8.Tomeasureresistanceusingpulsewidthweconnecttheexternalsignalaninputcapture.

Themeasurementfunctionreturnstheresistance,R,inΩ.Forexample,iftheresistance,R,is1234Ω,thenreturnparameterwillbe1234.Wewillnotworryaboutresistances,R,greaterthan55535ΩorifRisdisconnected.ThesolutionisshownasProgram6.4.

#defineCALIBRATION0

voidResistanceMeasure_Init(void)

SYSCTL_RCGCTIMER_R|=0x01;//activatetimer0

SYSCTL_RCGCGPIO_R|=0x02;//activateportB

while((SYSCTL_PRGPIO_R&0x02)!=0x02);

GPIO_PORTB_DATA_R|=0x20;//setPB5high

GPIO_PORTB_DIR_R=(GPIO_PORTB_DIR_R&~0x40)|0x20;//PB5out,PB6in

GPIO_PORTB_DEN_R|=0x60;//enabledigitalPB5andPB6

GPIO_PORTB_AFSEL_R|=0x40;//enablealtfunctonPB6

GPIO_PORTB_PCTL_R=(GPIO_PORTB_PCTL_R&0xF0FFFFFF)+0x07000000;

TIMER0_CTL_R&=~0x00000001;//disabletimer0Aduringsetup

TIMER0_CFG_R=0x00000004;//configurefor16-bittimermode

TIMER0_TAMR_R=0x00000007;//configureforinputcapturemode

TIMER0_TAILR_R=0x0000FFFF;//startvalue

TIMER0_IMR_R&=~0x7;//disableallinterruptsfortimer0A

TIMER0_ICR_R=0x00000004;//cleartimer0Acapturematchflag

TIMER0_CTL_R|=0x00000001;//enabletimer0Aedge,nointerrupts

//returnresistanceinohms,rangeis0to10000ohm

uint16_tResistanceMeasure(void)uint16_trising;

GPIO_PORTB_DATA_R&=~0x20;//turnoffPB5

TIMER0_CTL_R&=~(0x000C);//risingedge

TIMER0_ICR_R=0x00000004;//cleartimer0Acaptureflag

GPIO_PORTB_DATA_R|=0x20;//turnonPB5,trigger74HC123

while((TIMER0_RIS_R&0x00000004)==0);//waitforrise

rising=TIMER0_TAR_R;//timerA0atrisingedge

TIMER0_ICR_R=0x00000004;//cleartimer0Aflag

TIMER0_CTL_R&=~(0x000C);

TIMER0_CTL_R+=0x00000004;//fallingedge

while((TIMER0_RIS_R&0x00000004)==0);//waitforfall

TIMER0_ICR_R=0x00000004;//cleartimer0Aflag

return(rising-TIMER0_TAR_R-CALIBRATION)&0xFFFF;

Program6.4.Measuringresistanceusingpulse-widthmeasurement.

Thedifficultywithpulsewidthmeasurementinthepreviousexamplewastheneedtoswitchfromrisingtofallingedgeduringeachmeasurement.Itwasnotaproblemwiththisproblembecausethesmallestpulsewidthwas1250 s.However,tohandleshorterpulseswewillneedtousetwoinputcapturepins.Onepinmeasuresthetimeoftheriseandtheotherpinmeasuresthetimeofthefall.Inorderforinputcapturetooperate,theinputmustbehighforatleasttwobusclocksandlowforatleasttwobusclocks.Otherwisetheminimumpulsewidthdoesnotdependonsoftwareexecutiontimeorinterruptlatency.Howevertheminimumperiodwilldependonsoftwarespeed.

Example6.5.Designasystemtomeasurepulsewidthusinginterrupts,withaprecisionof24bitsandaresolutionof12.5ns.

Solution:Inthisexample,thedigital-levelinputsignalisconnectedtotwoinputcapturepins,CCP0andCCP1(Figure6.9).Thebusclockisselectedtobe80MHzsothemeasurementresolutionwillbe12.5ns.TherisingedgetimewillbemeasuredbyTimer0BwithouttheneedofaninterruptandthefallingedgeinterruptswillbehandledbyTimer0A.Thepulsewidthiscalculatedasthedifferencein TIMER0_TBR_R-TIMER0_TAR_R latchvalues.InthisexampletheTimer0Ainterrupthandlersimplysetstheglobalvariable, PW ,atthetimeofthefallingedge.BecausenosoftwareisrequiredtoprocesstheTimer0Bmeasurement,thereisnosoftwarelimittotheminimumpulsewidth.Thereisthehardwarelimitrequiringatleasttwobusclockperiodswhilehighandtwobusclockperiodswhilelow.Ontheotherhand,softwareprocessingisrequiredtohandletheTimer0Asignal,sothereisaminimumperiod.E.g.,theremustbemorethan2µsfromonefallingedgetothenextfallingedge.ThistimedependsonsoftwareexecutionspeedintheISR,andthecontextswitch.Thisminimumperiodwillbelargerforsystemswithhigherpriorityinterrupts.Again,thefirstmeasurementmayormaynotbeaccurate.

Figure6.9.TherisingedgeismeasuredwithTimer0B,andfallingedgeismeasuredwithTimer0A.

Thepulsewidthmeasurementisperformedfromrisingedgetofallingedge.Theresolutionis12.5ns,determinedbythesystembusclock.Therangeisabout25nsto209mswithnooverflowchecking.Timer0Ainterruptsonlyoccuronthefallingedges.Theglobal, PW ,containsthemostrecentmeasurement. Done issetatthefallingedgebyTimer0Asignifyinganewmeasurementisavailable.Ifthefirstedgeafterthe PWMeasure2_Init(); isexecutedisafallingedge,thenthefirstmeasurementwillbeincorrect(because TIMER0_TBR_R isincorrect).Ifthefirstedgeafterthe PWMeasure2_Init(); isexecutedisarisingedge,thenthefirstmeasurementwillbecorrect.Noticehowlittlesoftwareoverheadisrequiredtoperformthesemeasurements(Program6.5).

uint32_tPW;//24bits,12.5nsunits

intDone;//seteachfalling

voidPWMeasure2_Init(void)//TM4C123code

SYSCTL_RCGCTIMER_R|=0x01;//activatetimer0

SYSCTL_RCGCGPIO_R|=0x02;//activateportB

Done=0;//allowtimetofinishactivating

GPIO_PORTB_DIR_R&=~0xC0;//makePB6,PB7inputs

GPIO_PORTB_DEN_R|=0xC0;//enabledigitalPB6,PB7

GPIO_PORTB_AFSEL_R|=0xC0;//enablealtfunctonPB6,PB7

GPIO_PORTB_PCTL_R=(GPIO_PORTB_PCTL_R&0x00FFFFFF)+0x77000000;

TIMER0_CTL_R&=~0x00000003;

TIMER0_CFG_R=0x00000004;//configurefor16-bittimermode

//****timer0Ainitialization****

TIMER0_TAMR_R=0x00000007;

TIMER0_CTL_R=(TIMER0_CTL_R&(~0x0C))+0x04;//fallingedge

TIMER0_TAILR_R=0x0000FFFF;//startvalue

TIMER0_TAPR_R=0xFF;//activateprescale,creating24-bit

TIMER0_IMR_R|=TIMER_IMR_CAEIM;//enablecapturematchinterrupt

TIMER0_ICR_R=TIMER_ICR_CAECINT;//cleartimer0Acapturematchflag

//****timer0Binitialization****

TIMER0_TBMR_R=0x00000007;

TIMER0_CTL_R=(TIMER0_CTL_R&(~0x0C00))+0x00;//risingedge

TIMER0_TBILR_R=0x0000FFFF;//startvalue

TIMER0_TBPR_R=0xFF;//activateprescale,creating24-bit

TIMER0_IMR_R&=~0x700;//disableallinterruptsfortimer0B

TIMER0_CTL_R|=0x00000003;//enabletimers

//****interruptinitialization****

NVIC_PRI4_R=(NVIC_PRI4_R&0x00FFFFFF)|0x40000000;//Timer0=priority2

NVIC_EN0_R=1<<19;//enableinterrupt19inNVIC

EnableInterrupts();

voidTimer0A_Handler(void)

TIMER0_ICR_R=0x00000004;//acknowledgetimer0Acaptureflag

PW=(TIMER0_TBR_R-TIMER0_TAR_R)&0x00FFFFFF;//fromrisetofall

Done=1;

Program6.5.Pulse-widthmeasurementusingtwoinputcaptures(HighPulseMeasureHW_xxx.zip).

6.2.OutputCompareorPeriodicTimerInoutputcompare(periodictimer)modethetimerisconfiguredasa16-bitdown-counterwithanoptional8-bitprescalerthateffectivelyextendsthecountingrangeofthetimerto24bits.Whenthetimercountsfrom1to0itsetsthetriggerflag.Onthenextcount,thetimerisreloadedwiththevalueinTIMER0_TAILR_R(or TIMER0_TBILR_R ).Weselectperiodictimermodebysettingthe2-bitTAMR(orTBMR)fieldofthe TIMER0_TAMR_R (or TIMER0_TBMR_R )to0x02.Ifwesetthisfieldto0x01,thetimerisinoneshotmode.Inperiodicmodethetimerrunscontinuously,andinoneshotmode,itrunsonceandstops.Theperiodicmodecanalsobeusedtocreatepulsewidthmodulatedoutputs.

Wewilluseoutputcomparetocreatetimedelays,triggeraperiodicinterrupts,andcontrolADCsampling.Wewillalsouseoutputcomparetogetherwithinputcapturetomeasurefrequency.Outputcompareandinputcapturecanalsobecombinedtomeasureperiodandfrequencyoverawiderangeofrangesandresolutions.Wemayruntheoutputcomparemodeswithorwithoutanexternaloutputpinattached.Eachperiodictimermodulehas

Anexternaloutputpin,e.g.,CCP0,

Aflagbit,e.g.,TATORIS

AcontrolbittoconnecttheoutputtotheADCasatrigger,e.g.,TAOTE,

Aninterruptarmbit,e.g.,TATOIM

A16-bitreloadregister,e.g., TIMER0_TAILR_RA8-bitprescaleregister,e.g., TIMER0_TAPR_RA8-bitprescalematchregister,e.g., TIMER0_TAPMR_R

ThemembersoftheLM3S/LM4F/TM4Cfamilyhavevaryingnumberoftimers.Whendesigningasystemusingthetimers,youwillneedtoconsultthedatasheetforyourparticularmicrocontroller.Inparticular,someofthechannelsdonothaveanassociatedoutputpin.Forexample,theTM4C1294haseighttimers,creatinguptoeightperiodicinterrupts,buthasonlysixI/Opins,asshowninFigure6.10.

Figure6.10.OutputcomparepinsontheLM3S811,theLM3S1968,theTM4C123,andtheTM4C1294.Toconnecttheoutputpintothetimer,wemustsetthealternativefunctionbitforthatpin.Tables2.7and2.8describehowtoattachI/OpinstothetimermoduleontheTM4C123andTM4C1294.Theoutputcomparepinisanoutputofthecomputer,hencecanbeusedfordebuggingortocontrolanexternaldevice.Anoutputcompareeventoccurs,changingthestateoftheoutputpin,whenthe16-bittimermatchesthe16-bit TIMER0_TAILR_Rregister.Thetimerwillbeusedwithouttheoutputpinifthecorrespondingalternativefunctionbitisclear.

Theoutputcompareeventoccurswhenatimercountsdowntozero.Thetimermodespecifieswhateffecttheoutputcompareeventwillhaveontheoutputpinortherestofthesystem.Ifthetimerisinone-shotorperiodictimermode,theTATORIS(orTBTORIS)bitoftheRawInterruptStatusregister( TIMER0_RIS_R )isset.IfthearmbitTATOIM(orTBTOIM)intheTIMER0_IMR_R registerisset,atimerinterruptisrequested.ThehardwarecanalsotriggeranADCconversionatthistime.Ifthetimerisinone-shotmode,itstopscountingafterthefirstoutputcompareevent.Inperiodictimermode,thetimercontinuescountingindefinitelyuntilexplicitlydisabledbyclearingtheTAEN(orTBEN)enablebitintheTIMER0_CTL_R register.Justliketheinputcapture,theoutputcompareflagisclearedbywritinga1toitscorrespondingbitintheInterruptClearRegister( TIMER0_ICR_R ).

Onesimpleapplicationofoutputcompareistocreateafixedtimedelay.Let delay bethenumberofbuscyclesyouwishtowait,upto65,535.Thestepstocreatethedelayare:

0)EnabletheGeneral-PurposeTimerModuleinofRCGCTIMER1)Ensurethatthetimerisdisabledbeforemakinganychanges(clearTAEN)

2)Putthetimermodulein16-bitmodebywriting0x4to TIMER0_CFG_R3)Write0x1toTAMR4)Loadthedesired delay into TIMER0_TAILR_R5)Writea1toCATOCINTof TIMER0_ICR_R toclearthetime-outflag

6)SettheTAENbittostartthetimerandbegincountingdownfrom delay7)PollTATORISof TIMER0_RIS_R ,waitisoverwhenthisbitisset

Asecondapplicationofoutputcompareistocreateaperiodicinterrupt.Let prescale bean8-bitnumberloadedinto TIMER0_TAPR_R .Thetimerfrequencywillbebusfrequencydividedbyprescale+1.Thedefaultprescaleis0,meaningthetimerfrequencyequalsthebusfrequency.Let period bethe16-bitvalueloadedinto TIMER0_TAILR_R .Thestepstocreatetheperiodicinterruptare:

0)EnabletheGeneral-PurposeTimerModuleinofRCGCTIMER1)Ensurethatthetimerisdisabledbeforemakinganychanges(clearTAEN)2)Putthetimermodulein32-bitmodebywriting0x00to TIMER0_CFG_R3)Write0x2toTAMRtoconfigureforperiodicmode4)Load period into TIMER0_TAILR_R5)Load prescale into TIMER0_TAPR_R6)Writea1toCATOCINTof TIMER0_ICR_R toclearthetime-outflag

7)SetTATOIENof TIMER0_IMR_R toarmthetime-outinterrupt

8)SetthepriorityinthecorrectNVICPriorityregister

9)EnablethecorrectinterruptinthecorrectNVICInterruptEnableregister

10)SettheTAENbittostartthetimerandbegincountingdownfrom period

Ifthebusperiodis t,thenthetimerinterruptperiodwillbe

t*( prescale +1)*( period +1).

AfewcyclesofinstructionsshouldseparateSteps0and1toensurethatthetimerisreceivingaclockbeforetheprogramattemptstouseit.MoveStep0earlierinyourprogramorinsertdummyinstructionsbetweenSteps0and1ifyouevergetaHardwareFault.Themaximum period canbe24bitsusingprescaleror32bitsusingTimerAandTimerBtogetherin32-bitmode.Resolutionis1buscycle.

Checkpoint6.6:Whendoesanoutputcompareeventoccurwheninone-shot/periodictimermode?

Checkpoint6.7:Whathappensduringanoutputcompareeventinone-shot/periodictimermode?

Example6.6.Designasystemtoexecuteausertaskataperiodicratewithunitsof12.5ns.

Solution:WewillgenerateaperiodicinterruptandcalltheusertaskfromtheISR.Assuminga80MHzbusclock,wedisabletheprescale,meaningthetimercountsevery12.5ns.Todefinetheusertaskwewillcreateaprivateglobalvariablecontainingapointertotheuser’sfunction.Wewillsetthevariableduringinitializationandcallthatfunctionatruntime.Anothernameforadynamicallysetfunctionpointerisahook.Themaximumpossiblevaluefor period is12.5ns*232,whichisabout53seconds.

void(*PeriodicTask)(void);//userfunction

Theinitializationsequencefollowsthe1–10outlinelistedabove(Program6.6).

voidTimer2A_Init(void(*task)(void),uint32_tperiod)

SYSCTL_RCGCTIMER_R|=0x04;//0)activatetimer2

PeriodicTask=task;//userfunction

TIMER2_CTL_R=0x00000000;//1)disabletimer2Aduringsetup

TIMER2_CFG_R=0x00000000;//2)configurefor32-bitmode

TIMER2_TAMR_R=0x00000002;//3)configureforperiodicmode

TIMER2_TAILR_R=period-1;//4)reloadvalue

TIMER2_TAPR_R=0;//5)busclockresolution

TIMER2_ICR_R=0x00000001;//6)cleartimer2Atimeoutflag

TIMER2_IMR_R=0x00000001;//7)armtimeoutinterrupt

NVIC_PRI5_R=(NVIC_PRI5_R&0x00FFFFFF)|0x80000000;//8)priority4

NVIC_EN0_R=1<<23;//9)enableIRQ23inNVIC

TIMER2_CTL_R=0x00000001;//10)enabletimer2A

EnableInterrupts();

voidTimer2A_Handler(void)

TIMER2_ICR_R=0x00000001;//acknowledgetimer2Atimeout

(*PeriodicTask)();//executeusertask

Program6.6.ImplementationofaperiodicinterruptusingTimer2A(PeriodicTimer0AInts_xxx.zip).

Example6.7.Designasystemthatgeneratesa50%dutycycle100HzsquarewaveonPB5.

Solution:Thisexamplegeneratesa50%dutycyclesquarewaveusingoutputcompare.Theoutputishighfor Period cyclesthenlowfor Period cycles.Program6.6willbeusedtorequestinterruptsataratetwiceasfastastheresultingsquarewavefrequency.Oneinterruptisrequiredfortherisingedgeandanotherforthefallingedge.Theoutputcompareinterrupthandlersimplyacknowledgestheinterruptandtogglestheoutputpin.

#definePB5(*((volatileuint32_t*)0x40005080))

voidTogglePB5(void)PB5^=0x20;

Therewillbesomesoftwarejitterduetothelatencyinprocessingtheinterrupt.Weaddtheselinestotheinitialization

SYSCTL_RCGCGPIO_R|=0x00000002;//activateclockforPortB

while((SYSCTL_PRGPIO_R&0x00000002)==0);//ready?

GPIO_PORTB_DEN_R|=0x20;//enabledigitalI/OonPB5

GPIO_PORTB_DIR_R|=0x20;//PB5isanoutput

TostartthesquarewavewecallinitializationinProgram6.6sointerruptsoccur200timesasecond(every5ms),creatingthe100Hzwave.

Timer2A_Init(&TogglePB5,400000);//400,000*12.5ns=5ms

6.3.PulseWidthModulationTheproblemwiththesolutiontoExample6.7isthattheminimumperiodisdeterminedbysoftwareexecutionspeed,thetimerunningwithinterruptsdisabled,andtheexistenceofhigherpriorityinterrupts.Generatingoutputwavesisanessentialtaskforreal-timesystems,sothemicrocontrollershavemultiplemethodstocreateoutputwaves.

6.3.1.PulseWidthModulationusingtheTimerModulePulsewidthmodulationisaneffectiveandthuspopularmechanismfortheembeddedmicrocontrollerstocontrolexternaldevices.ThetimercancreatePWMoutputsbysettingtheTAAMSbitandselectingperiodicmodeinTAMRfieldintheTIMER0_TAMR_Rregister.Theoutputisonefor High cyclesthenzerofor Low cycles.Thisexamplegeneratesavariabledutycyclesquarewaveusingoutputcompare.Outputcompareeventswillagainberequestedataratetwiceasfastastheresultingsquarewavefrequency.Oneeventisrequiredfortherisingedgeandanotherforthefallingedge.Intheexamplesbelow,wemakeHigh plus Low beaconstant.ByadjustingtheratioofHigh andLowthesoftwarecancontrolthedutycycle.

Thisimplementationoccursinhardwareanddoesnotrequireinterrupts.Therefore,itcangeneratewavescloseto0or100%dutycycle.IfwecleartheTAPWMLbitofthecontrolregisterweselectnormalPWMmode.Innormalmode,thecorrespondingoutputpinissetwhenthetimerisloadedwiththevalueinthe TIMER0_TAILR_R register.Whenitreachesthevaluestoredinthe TIMER0_TAMATCHR_R register,thepiniscleared.TheTAPWMLbitofthecontrolregisterinvertsthisbehavior.Inallmodes,thetimerisreloadedwiththevalueinthe TIMER0_TAILR_R registeronthecycleafteritreaches0x0000.InPWMoutputmode,thetimercontinuescountingindefinitelyuntilexplicitlydisabledbyclearingtheTAEN(orTBEN)bitinthe TIMER0_CTL_R register.Figure6.11showsthePWMoutputcanbeusedtointerfaceaDCmotortothemicrocontroller.

Figure6.11.ThePWMoutputofTimer0AcanadjustthepowertotheDCmotor.Program6.7configuresTimer0AforPWMoutput.Theusercalls PWM_Init oncetoturniton,andthencalls PWM_Duty toadjustthedutycycle.

//periodisnumberofbusclockcyclesinthePWMperiod

//highisnumberofbusclockcyclesthesignalishigh

voidPWM_Init(uint16_tperiod,uint16_thigh)

SYSCTL_RCGCTIMER_R|=0x01;//activatetimer0

SYSCTL_RCGCGPIO_R|=0x02;//activateportB

while((SYSCTL_PRGPIO_R&0x02)==0);

GPIO_PORTB_DEN_R|=0x01;//enabledigitalI/OonPB0

GPIO_PORTB_AFSEL_R|=0x01;//enablealtfunctonPB0

TIMER0_CTL_R&=~0x00000001;//disabletimer0Aduringsetup

TIMER0_CFG_R=0x00000004;//configurefor16-bittimermode

//configureforalternate(PWM)mode

TIMER0_TAMR_R=0x0000000A;//PWMandperiodicmode

TIMER0_TAILR_R=period-1;//timerstartvalue

TIMER0_TAMATCHR_R=period-high-1;//dutycycle=high/period

TIMER0_CTL_R|=0x00000001;//enabletimer0A16-b,PWM

voidPWM_Duty(uint16_thigh)//dutycycleishigh/period

TIMER0_TAMATCHR_R=TIMER0_TAILR_R-high;//dutycycle=high/period

Program6.7.SoftwaretogenerateaPWMoutputusingTimer0A(Timer0APWM_xxx.zip).Checkpoint6.8:WhendoesanoutputcompareeventoccurwheninPWMmode?

Checkpoint6.9:WhathappensduringanoutputcompareeventinPWMmode?

6.3.2.PulseWidthModuleusingthePWMModulePWMoutputsaresoimportant,themanymicrocontrollershavededicatedPWMmodules.ThenumberofPWMsandassociatedpinsvaryfromonemicrocontrollertothenext,seeFigure6.12.TheLM4F120hasnone,theTM4C123hassixteenandtheTM4C1294haseight.RefertoTables2.7and2.8.

Figure6.12.PWMpinsontheLM3S1968,theTM4C123,andtheTM4C1294.ThePWM0blockproducesthePWM0andPWM1outputs,thePWM1blockproducesthePWM2andPWM3outputs,thePWM2blockproducesthePWM4andPWM5outputs,andthePWM3ontheTM4C123blockproducesthePWM6andPWM7outputs.TheTM4C123hasasecondblockprovidingforanadditionaleightPWMoutputs.ThedesignofaPWMsystemconsidersthreefactors.ThefirstfactorisperiodofthePWMoutput.Mostapplicationschooseaperiod,initializethewaveformatthatperiod,andadjustthedutycycledynamically.Thesecondfactorisprecision,whichisthetotalnumberofdutycyclesthatcanbecreated.A16-bitchannelcanpotentiallycreateupto65536differentdutycycles.However,sincethedutycycleregistermustbelessthanorequaltotheperiodregister,theprecisionofthesystemisdeterminedbythevaluewrittentotheperiodregister.Thelastconsiderationisthenumberofchannels.DifferentmembersoftheLM3S/LM4F/TM4CfamilyhavefromzerotosixteenPWMoutputs.RefertothedatasheetforyourspecificmicrocontrollertodeterminehowmanyPWMoutputsithasandtowhichportsthePWMisconnected.

Program6.8showstheinitializationonaTM4C123forgeneratingaPWMonthePB6/PWM0pin.1)First,weactivatetheclockforthePWMmodule.2)Second,weactivatetheoutputpinasadigitalalternatefunction.3)Next,weselecttheclocktobeusedforthePWMinRCCregister.IfwedonotusethePWMdivider,thenitisclockedfromthebusclock.Withthedividerwecanchoose/2,/4,/8,/16,/32,or/64.AssumingtheLM3S1968isrunningat50MHz,thisprogramspecifiesthePWMclocktobe25MHz.4)WesetthePWMtocountdownmode.Wespecifyinthe PWM_0_GENA_Rregisterthatthecomparatoractionistosettoone,andtheloadactionissettozero.5)WespecifytheperiodinthePWM_0_LOAD_R register.6)WespecifythedutycycleinthePWM_0_CMPA_R register.7)Lastly,westartandenablethePWM.Wecall PWM0_Init oncetoturniton,andthencall PWM0_Duty toadjustthedutycycle.Assumethebusclockis6MHz,wecall PWM0_Init(30000,15000); tocreatea10msperiod50%dutycycleoutputonPWM0(PD0).

//periodis16-bitnumberofPWMclockcyclesinoneperiod(3<=period)

//dutyisnumberofPWMclockcyclesoutputishigh(2<=duty<=period-1)

//PWMclockrate=processorclockrate/SYSCTL_RCC_PWMDIV

//=BusClock/2(inthisexample)

voidPWM0_Init(uint16_tperiod,uint16_tduty)

SYSCTL_RCGCPWM_R|=0x01;//1)activatePWM0

SYSCTL_RCGCGPIO_R|=0x02;//2)activateportB

while((SYSCTL_PRGPIO_R&0x02)==0);

GPIO_PORTB_AFSEL_R|=0x40;//enablealtfunctonPB6

GPIO_PORTB_PCTL_R=(GPIO_PORTB_PCTL_R&0xF0FFFFFF)+0x04000000;//PWM0

GPIO_PORTB_AMSEL_R&=~0x40;//disableanalogfunonPB6

GPIO_PORTB_DEN_R|=0x40;//enabledigitalI/OonPB6

SYSCTL_RCC_R=0x00100000|//3)usePWMdivider

(SYSCTL_RCC_R&(~0x000E0000));//configurefor/2divider

PWM0_0_CTL_R=0;//4)re-loadingdown-countingmode

PWM0_0_GENA_R=0xC8;//lowonLOAD,highonCMPAdown

PWM0_0_LOAD_R=period-1;//5)cyclesneededtocountdownto0

PWM0_0_CMPA_R=duty-1;//6)countvaluewhenoutputrises

PWM0_0_CTL_R|=0x00000001;//7)startPWM0

PWM0_ENABLE_R|=0x00000001;//enablePB6/M0PWM0

voidPWM0_Duty(uint16_tduty)

PWM_0_CMPA_R=duty-1;//6)countvaluewhenoutputrises

Program6.8.Implementationofa16-bitPWMoutput.lowonLOAD,highonCMPA(PWM_xxx.zip).

6.4.FrequencyMeasurement

6.4.1.FrequencyMeasurementConceptsThedirectmeasurementoffrequencyinvolvescountinginputpulsesforafixedamountoftime.Thebasicideaistouseinputcapturetocountpulses,anduseoutputcomparetocreatethefixedtimeinterval, t.Forexample,wecouldinitializeinputcapturetodecrementacounterin TIMER0_TAR_R oneveryrisingedgeofourinputsignal.Atthebeginningofourfixedtimeinterval, TIMER0_TAR_R isinitializedto TIMER0_TAILR_R ,andattheendoftheinterval,wecancalculatefrequency:

f=( TIMER0_TAILR_R - TIMER0_TAR_R )/ t

Thefrequencyresolution,∆f,isdefinedtobethesmallestchangeinfrequencythatcanbereliablymeasuredbythesystem.Inorderforthesystemtodetectachange,thefrequencymustincrease(ordecrease)enoughsothatthereisonemore(oroneless)pulseduringthefixedtimeinterval.Therefore,thefrequencyresolutionis

∆f=1/ t

Thisfrequencyresolutionalsospecifiestheunitsofthemeasurement.

Example6.8.Designasystemthatmeasuresfrequencywitharesolutionof100Hz.

Solution:Ifwecountpulsesina10mstimeinterval,thenthenumberofpulsesrepresentsthesignalfrequencywithunits1/10msor100Hz.E.g.,ifthereare7pulsesduringthe10msintervalthenthefrequencyis700Hz.Forthissystem,themeasurementresolutionis100Hz,sothefrequencywouldhavetoincreaseto800Hz(ordecreaseto600Hz)forthechangetobedetected(Figure6.13.)

Figure6.13.Frequencymeasurementusingbothinputcaptureandoutputcompare.

ThehighestfrequencythatcanbemeasuredwillbedeterminedbyhowfasttheInputCapturehardwarecancountpulses.Sincetheremustbetwoclockperiodswhileitishighandtwoclockpulseswhileitislow,thefastestfrequencythatcanbemeasuredisthebusfrequencydividedbyfour.Theprecisionofthemeasurementisdeterminedbythenumberofbitsintheinputcaptureregister,whichinthiscaseis16bits.Inthisexample,thedigitallogicinputsignalisconnectedtoPC5(CCP1,attachedtoTimer0B).Therisingedgewilldecrementthecounterin TIMER0_TBR_R .

ThefrequencymeasurementsoftwareisgivenasProgram6.9.Thefrequencymeasurementcountsthenumberofrisingedgesina10msinterval.Themeasurementresolutionis100Hz(determinedbythe10msinterval).ATimer0Binputcaptureeventoccursoneachrisingedge,andaTimer0Aperiodicinterruptoccursevery10ms.Theforeground/backgroundthreadscommunicateviaamailbox.ThebackgroundthreadwillupdatethedatainFreq withanewmeasurementandsettheflagDone .When Done isset,theforegroundthreadwillreadtheglobal Freq andclear Done .

uint16_tFreq;/*Frequencywithunitsof100Hz*/

intDone;/*Seteachmeasurement,every10ms*/

Ifthebusclockis6MHz,thelargestfrequencywecanmeasurewillbe1.5MHz.Therefore,themaximumvaluesofthefrequencymeasurementwillbe15000,whichwillnotoverflowthe16-bitcounter.Afrequencyof0willresultinnoinputcaptureinterruptsandthesystemwillproperlyreportthefrequencyof0.Theprecisionofthissystemis15001alternativesorabout14bits.

voidFreqMeasure_Init(void)//****generalinitialization****

SYSCTL_RCGCTIMER_R|=0x01;//activatetimer0

SYSCTL_RCGCGPIO_R|=0x0C;//activateportsC,D

Freq=0;//allowtimetofinishactivating

Done=0;

GPIO_PORTC_DEN_R|=0x20;//enabledigitalI/OonPC5

GPIO_PORTC_AFSEL_R|=0x20;//enablealtfunctonPC5

GPIO_PORTD4=0x00;//turnoffPD4

GPIO_PORTD_DIR_R|=0x10;//makePD4out(PD4debugheartbeat)

GPIO_PORTD_DEN_R|=0x10;//enabledigitalI/OonPD4

TIMER0_CTL_R&=~(TIMER_CTL_TAEN|TIMER_CTL_TBEN);

TIMER0_CFG_R=TIMER_CFG_16_BIT;//configurefor16-bittimermode

//****timer0Ainitialization****

TIMER0_TAMR_R=TIMER_TAMR_TAMR_PERIOD;//periodicmode

TIMER0_TAILR_R=60000;//startvaluefor100Hzinterrupts

TIMER0_IMR_R|=TIMER_IMR_TATOIM;//enabletimeout(rollover)interrupt

TIMER0_ICR_R=TIMER_ICR_TATOCINT;//cleartimer0Atimeoutflag

//****timer0Binitialization****

TIMER0_TBMR_R=TIMER_TBMR_TBMR_CAP;//configureforcapturemode

TIMER0_CTL_R&=~TIMER_CTL_TBEVENT_M;

TIMER0_CTL_R+=TIMER_CTL_TBEVENT_POS;//risingedge

TIMER0_TBILR_R=0x0000FFFF;//maximumstartvalue

TIMER0_TBMATCHR_R=0;//countdowntozero

TIMER0_IMR_R&=~0x700;//disableallinterruptsfortimer0B

TIMER0_CTL_R|=(TIMER_CTL_TAEN|TIMER_CTL_TBEN);//enabletimers

//****interruptinitialization****

NVIC_PRI4_R=(NVIC_PRI4_R&0x00FFFFFF)|0x40000000;//priority2

NVIC_EN0_R=NVIC_EN0_INT19;//enableinterrupt19inNVIC

EnableInterrupts();

voidTimer0A_Handler(void)

GPIO_PORTD4=0x10;//heartbeat

TIMER0_ICR_R=TIMER_ICR_TATOCINT;//acknowledgetimer0Atimeout

Freq=(0xFFFF-TIMER0_TBR_R);//f=(pulses)/(fixedtime)

Done=-1;

//timer0Bneedstobere-initialized

TIMER0_CTL_R&=~TIMER_CTL_TBEN;//disabletimer0Bduringsetup

TIMER0_TBMR_R=TIMER_TBMR_TBMR_CAP;

TIMER0_CTL_R&=~TIMER_CTL_TBEVENT_M;

TIMER0_CTL_R+=TIMER_CTL_TBEVENT_POS;

TIMER0_TBILR_R=0x0000FFFF;//maximumstartvalue

TIMER0_TBMATCHR_R=0;//countdowntozero

TIMER0_IMR_R&=~0x700;//disableallinterruptsfortimer0B

TIMER0_CTL_R|=TIMER_CTL_TBEN;//re-enabletimer0B

GPIO_PORTD4=0x00;

Program6.9.Softwaretomeasurefrequencywitharesolutionof100Hz(FreqMeasure_xxx.zip).

6.4.2.UsingPeriodMeasurementtoCalculateFrequencyPeriodandfrequencyareobviouslyrelated,sowhenfacedwithaproblemthatrequiresfrequencyinformationwecouldmeasureperiod,andcalculatefrequencyfromtheperiod.

Ifwehaveabusclockof8MHz,theaperiodmeasurementsystemwillhavearesolutionof125ns.Assumepis16-bitperiodmeasurement.Witharesolutionof125ns,theperiodcanrangefromabout40to8192 s . Thiscorrespondstoafrequencyrangeof122Hzto25kHz.Wecancalculatefrequencyffromthisperiodmeasurement,f=8000000/p

Itiseasytoseehowthe40to8192 speriodrangemapsintothe122Hzto25kHzfrequencyrange,butmappingthe125nsperiodresolutionintoanequivalentfrequencyresolutionisalittletrickier.Ifthefrequencyisf,thenthefrequencymustchangetof+∆fsuchthattheperiodchangesbyatleast∆p=125ns.1/fistheinitialperiod,and1/(f+∆f)isthenewperiod.Thesetwoperiodsdifferby125ns.Inotherwords,

Wecanrearrangethisequationtorelate∆fasafunctionof∆pandf.

Thisverynonlinearrelationship,showninTable6.3,illustratesthatalthoughtheperiodresolutionisfixedat125ns,theequivalentfrequencyresolutionvariesfrom500Hzto0.0005Hz.Ifthesignalfrequencyisrestrictedtotherangefrom125to2828Hz,thenwecansaythefrequencyresolutionwillbebetterthan1Hz.

Frequency(Hz)

Period(µsec)

∆f(Hz)

25000 40 78.370

16000 63 32.064

8000 125 8.008

2000 500 0.500

1000 1000 0.125

500 2000 0.031

250 4000 0.008

125 8000 0.002

Table6.3.Relationshipbetweenfrequencyresolutionandfrequencywhencalculatedusingperiodmeasurement.

6.4.3.UsingFrequencyMeasurementtoCalculatePeriodSimilarly,whenfacedwithaproblemthatrequiresaperiodmeasurementwecouldmeasurefrequency,andcalculateperiodfromthefrequencymeasurement.Asimilarnonlinearrelationshipexistsbetweenthefrequencyresolutionandperiodresolution.Ingeneral,theperiodmeasurementapproachwillbefaster,butthefrequencymeasurementapproachwillbemorerobustinthefaceofmissededgesorextrapulses.

6.4.4.PeriodMeasurementwithFlexibleResolutionandRangeOneofthelimitationsofusinginputcapturealoneisthechoiceofresolutionandprecision.Inparticular,themeasurementresolutionofExample6.2istheperiodofthebusclock.Furthermoretheprecisionisfixedat16bits.Whenrunningat50MHz,thismeansthemaximumperiodthatcanbemeasuredisonly1.3ms.Inordertoextendtherangeofthemeasurementwewillusebothinputcaptureandoutputcompare.

Aperiodicinterruptwillincrementasoftwarecounter.Theperiodofthisinterruptspecifiesthemeasurementresolution.Theprecisionofthecountersetsthemeasurementprecision.ThesysteminExample6.9canmeasureperiodsofupto50days.Theinputcaptureinterruptwillbeusedtocollectthecounterinformationandtransmitthedatatoothersoftwareinthesystem.

Example6.9.Designasystemthatmeasuresperiodwitharesolutionof1ms.

Solution:Theobjectiveistomeasureperiodwitharesolutionof1ms.Thedigitallogicinputsignalisconnectedtoaninputcapture,PD4(CCP0).EachrisingedgewillcauseTimer0Atogenerateaninputcaptureinterrupt.Inaddition,Timer0Bisconfiguredforoutputcompareandisusedtoincrementasoftwarecounter, Cnt ,every1ms.Theperiodiscalculatedasthenumberof1msoutputcompareinterruptsbetweenonerisingedgeoftheinputcapturepintothenextrisingedgeoftheinputcapturepin.Forexample,iftheperiodis8192µs,therewillbe8outputcompareinterruptsbetweensuccessiveinputcaptureinterrupts.

ThesoftwaresolutionispresentedasProgram6.10.Theperiodmeasurementcountsthenumberof1msintervalsbetweensuccessiverisingedges.Theperiodmeasurementresolutionis1ms,becausetheperiodmustincreasebyatleast1msinorderfortheretobeadifferentnumberofcounts.Therangeis0to232-1ms,andtheprecisionof32bitsisdeterminedbythesizeofthe Cnt counter.ThereisaTimer0Ainterrupteachperiod,andtheexternalsignalisconnectedtoPD4(CCP0).Aperiodicinterruptata1000HzrateiscreatedwithTimer0B.Theforeground/backgroundthreadscommunicateviaamailboxcreatedwith3sharedglobals.Thebackgroundthreadwillupdate Period withanewmeasurementandset Done .When Done isset,theforegroundthreadwillreadtheglobal Period andclear Done . OverFlow issetbythebackgroundandreadbytheforegroundiftheperiodislargerthan4,294,967seconds(about50days).Thefirstmeasurementisincorrect. Cnt isaprivateglobal(onlyaccessedbytheperiodmeasurementmoduleandnotthemodulereadingthemailbox.)

uint32_tPeriod;//32-bitperiodinmsec

intOverFlow;//SetifPeriodistoobig

intDone;//SeteachrisingedgeofPD4

uint32_tstaticCnt;//numberofmsecinoneperiod

voidPeriodMeasure32_Init(void)

//****generalinitialization****

SYSCTL_RCGCTIMER_R|=0x01;//activatetimer0

SYSCTL_RCGCGPIO_R|=0x0C;//activateportsC,D

Period=0;//allowtimetofinishactivating

OverFlow=0;

Done=0;

Cnt=0;

GPIO_PORTC_DIR_R|=0x20;//makePC5out(PC5built-inLED)

GPIO_PORTC_DEN_R|=0x20;//enabledigitalI/OonPC5

GPIO_PORTD_AFSEL_R|=0x10;//enablealtfunctonPD4

TIMER0_CTL_R&=~(TIMER_CTL_TAEN|TIMER_CTL_TBEN);

TIMER0_CFG_R=TIMER_CFG_16_BIT;//configurefor16-bittimermode

//****timer0Ainitialization****

TIMER0_TAMR_R=(TIMER_TAMR_TACMR|TIMER_TAMR_TAMR_CAP);

TIMER0_CTL_R&=~TIMER_CTL_TAEVENT_M;

TIMER0_CTL_R+=TIMER_CTL_TAEVENT_POS;//risingedgeevent

TIMER0_TAILR_R=TIMER_TAILR_TAILRL_M;//maximumstartvalue

TIMER0_IMR_R|=TIMER_IMR_CAEIM;//enablecapturematchinterrupt

TIMER0_ICR_R=TIMER_ICR_CAECINT;//cleartimer0Acapturematchflag

//****timer0Binitialization****

TIMER0_TBMR_R=TIMER_TBMR_TBMR_PERIOD;

TIMER0_TBILR_R=6000;//startvaluefor1000Hz

TIMER0_IMR_R|=TIMER_IMR_TBTOIM;//enabletimeoutinterrupt

TIMER0_ICR_R=TIMER_ICR_TBTOCINT;//cleartimer0Btimeoutflag

TIMER0_CTL_R|=TIMER_CTL_TAEN;//enabletimer0A

//****interruptinitialization****

NVIC_PRI4_R=(NVIC_PRI4_R&0x00FFFFFF)|0x40000000;//priority2

NVIC_PRI5_R=(NVIC_PRI5_R&0xFFFFFF00)|0x00000040;//priority2

NVIC_EN0_R=NVIC_EN0_INT19+NVIC_EN0_INT20;

EnableInterrupts();

//InterruptonrisingedgeofPD4(CCP0)

voidTimer0A_Handler(void)

TIMER0_ICR_R=TIMER_ICR_CAECINT;//acknowledgetimer0Acapturematch

if(OverFlow)

Period=4294967295;//actualperiodmaybegreater

OverFlow=0;

else

Period=Cnt;

Cnt=0;

Done=-1;

//restartTimer0B

TIMER0_CTL_R&=~TIMER_CTL_TBEN;

TIMER0_TBILR_R=6000;//startvaluefor1000Hzinterrupts

TIMER0_ICR_R=TIMER_ICR_TBTOCINT;//cleartimer0Btimeoutflag

TIMER0_CTL_R|=TIMER_CTL_TBEN;//enabletimer0B

//Interruptevery1msafterrisingedgeofPD4(CCP0)

voidTimer0B_Handler(void)

GPIO_PORTC5=0x20;//heartbeat

TIMER0_ICR_R=TIMER_ICR_TBTOCINT;//acknowledgetimer0Btimeout

Cnt=Cnt+1;

if(Cnt==0)

OverFlow=-1;

GPIO_PORTC5=0x00;

Program6.10.Softwaretomeasureperiodwitharesolutionof1ms(LongPeriodMeasure_xxx.zip).

6.5.BinaryActuators

6.5.1.ElectricalInterfaceRelays,solenoids,andDCmotorsaregroupedtogetherbecausetheirelectricalinterfacesaresimilar.Wecanaddspeakerstothisgroupifthesoundisgeneratedwithasquarewave.Ineachcase,thereisacoil,andthecomputermustdrive(ornotdrive)currentthroughthecoil.Tointerfaceacoil,weconsidervoltage,currentandinductance.Weneedapowersupplyatthedesiredvoltagerequirementofthecoil.Iftheonlyavailablepowersupplyislargerthanthedesiredcoilvoltage,weuseavoltageregulator(ratherthanaresistordividertocreatethedesiredvoltage.)Weconnectthepowersupplytothepositiveterminalofthecoil,shownas+VinFigure6.14.Wewilluseatransistordevicetodrivethenegativesideofthecoiltoground.Thecomputercanturnthecurrentonandoffusingthistransistor.Thesecondconsiderationiscurrent.Inparticular,wemusthoweverselectthepowersupplyandaninterfacedevicethatcansupportthecoilcurrent.The7406isanopencollectordrivercapableofsinkingupto40mA.The2N2222isabipolarjunctiontransistor(BJT),NPNtype,withmoderatecurrentgain.TheTIP120isaDarlingtontransistor,alsoNPNtype,whichcanhandlelargercurrents.TheIRF540isaMOSFETtransistorthatcanhandleevenmorecurrent.BJTandDarlingtontransistorsarecurrent-controlled(meaningtheoutputisafunctionoftheinputcurrent),whiletheMOSFETisvoltage-controlled(outputisafunctionofinputvoltage).Wheninterfacingacoiltothemicrocontroller,weuseinformationlikeTable6.4toselectaninterfacedevicecapablethecurrentnecessarytoactivatethecoil.Itisagooddesignpracticetoselectadriverwithamaximumcurrentatleasttwicetherequiredcoilcurrent.WhenthedigitalPortoutputishigh,theinterfacetransistorisactiveandcurrentflowsthroughthecoil.WhenthedigitalPortoutputislow,thetransistorisnotactiveandnocurrentflowsthroughthecoil.

Thethirdconsiderationisinductanceinthecoil.The1N914diodeinFigure6.14providesprotectionfromthebackemfgeneratedwhentheswitchisturnedoff,andthelargedI/dtacrosstheinductorinducesalargevoltage(onthenegativeterminalofthecoil),accordingtoV=L∙dI/dt.Forexample,ifyouaredriving0.1Athrougha0.1mHcoil(Portoutput=1)usinga2N2222,thendisablethedriver(Portoutput=0),the2N2222willturnoffinabout20ns.ThiscreatesadI/dtofatleast5·106A/s,producingabackemfof500V!The1N914diodeshortsoutthisvoltage,protectingtheelectronicsfrompotentialdamage.The1N914iscalledasnubberdiode.

Device Type Maximumcurrent

LM3S/TM4C CMOS 8mA

7406 TTLlogic 40mA

PN2222 BJTNPN 150mA

2N2222 BJTNPN 500mA

TIP120 DarlingtonNPN 5A

IRF540 powerMOSFET 28A

Table6.4.Fourpossibledevicesthatcanbeusedtointerfaceacoilcomparedtothemicrocontroller.

Observation:Itisimportanttorealizethatmanydevicescannotbeconnecteddirectlyuptothemicrocontroller.Inthespecificcaseofmotors,weneedaninterfacethatcanhandlethevoltageandcurrentrequiredbythemotor.

Figure6.14.BinaryinterfacetoEMrelay,solenoid,DCmotororspeaker.Ifyouaresinking16mA(IOL)withthe7406,theoutputvoltage(VOL)willbe0.4V.However,whentheIOLofthe7406equals40mA,itsVOLwillbe0.7V.40mAisnotalotofcurrentwhenitcomestotypicalcoils.However,the7406interfaceisappropriatetocontrolsmallrelays.

Checkpoint6.10:Arelayisinterfacedwiththe7406circuitinFigure6.14.Thepositiveterminalofthecoilisconnectedto+5V,andthecoilrequires40mA.Whatwillbethevoltageacrossthecoilwhenactive?

Whendesigninganinterface,weneedtoknowthedesiredcoilvoltage(Vcoil)andcoilcurrent(Icoil).LetVbebethebase-emittervoltagethatactivatestheNPNtransistorandlethfebethecurrentgain.TherearethreestepswheninterfacinganN-channel(rightsideofFigure6.14.)

1)ChoosetheinterfacevoltageVequaltoVcoil(sinceVCEisclosetozero)

2)CalculatethedesiredbasecurrentIb=Icoil/hfe(sinceICequalsIcoil)

3)CalculatetheinterfaceresistorRb≤(VOH-Vbe)/Ib(choosearesistor2to5timessmaller)

WithanN-channelswitch,likeFigure6.14,currentisturnedonandoffbyconnecting/disconnectingonesideofthecoiltoground,whiletheothersideisfixedatthevoltagesupply.AsecondtypeofbinaryinterfaceusesP-channelswitchestoconnect/disconnectonesideofthecoiltothevoltagesupply,whiletheothersidefixedatground,asshowninFigure6.15.InothertoactivateaPNPtransistor(e.g.,PN2907orTIP125),theremustbeaVEBgreaterthan0.7V.InordertodeactivateaPNPtransistor,theVEBvoltagemustbe0.Becausethetransistorisacurrentamplifier,theremustbearesistorintothebaseinordertolimitthebasecurrent.

Figure6.15.PNPinterfacetoEMrelay,solenoid,DCmotororspeaker.TounderstandhowthePNPinterfaceontherightofFigure6.15operates,considerthebehaviorforthetwocases:thePortoutputishighandthePortoutputislow.IfthePortoutputishigh,itsoutputvoltagewillbebetween2.4and3.3V.ThiswillcausecurrenttoflowintothebaseofthePN2222,anditsVbewillsaturateto0.7V.ThebasecurrentintothePN2222couldbefrom(2.4-0.7)/1000to(3.3-0.7)/1000,or1.7to2.6mA.Themicrocontrollerwillbeabletosourcethiscurrent.ThiswillsaturatethePN2222anditsVCEwillbe0.3V.ThiswillcausecurrenttoflowoutofthebaseofthePN2907,anditsVEBwillsaturateto0.7V.IfthesupplyvoltageisV,thenthePN2907basecurrentis(V-0.7-0.3)/Rb.SincethePNPtransistorison,VECwillbesmallandcurrentwillflowfromthesupplytothecoil.Iftheportoutputislow,thevoltageoutputwillbebetween0and0.4V.ThisnothighenoughtoactivatethePN2222,sotheNPNtransistorwillbeoff.SincethereisnoICcurrentinthePN2222,the10kandRbresistorswillplace+VatthebaseofthePN2907.SincetheVEBofthePN2907is0,thistransistorwillbeoff,andnocurrentwillflowintothecoil.ForparametervaluesreferbacktoTable1.6.

MOSFETscanhandlesignificantlymorecurrentthanBJTorDarlingtontransistors.MOSFETsarevoltagecontrolledswitches.ThedifficultywithinterfacingMOSFETstoamicrocontrolleristhelargegatevoltageneededtoactivateit.TheleftsideofFigure6.16isanN-channelinterface.TheIRF540N-channelMOSFETcansinkupto28Awhenthegate-sourcevoltageisabove7V.Thiscircuitisnegativelogic.Whentheportpinishigh,the2N2222isactivemakingtheMOSFETgatevoltage0.3V(VCEofthePN2222).AVGSof0.3VturnsofftheMOSFET.Whentheportpinislow,the2N2222isoffmakingtheMOSFETgatevoltage+V(pulledupthroughthe10k resistor).TheVGSis+V,whichturnstheMOSFETon.

TherightsideofFigure6.16showsaP-channelMOSFETinterface.TheIRF9540P-channelMOSFETcansourceupto20Awhenthesource-gatevoltageisabove7V.TheFQP27P06P-channelMOSFETcansourceupto27Awhenthesource-gatevoltageisabove6V.Thiscircuitispositivelogic.Whentheportpinishigh,the2N2222isactivemakingtheMOSFETgatevoltage0.3V.ThismakesVSGequalto+V-0.3,whichturnsontheMOSFET.Whentheportpinislow,the2N2222isoff.Sincethe2N2222isoff,the10k pull-upresistormakestheMOSFETgatevoltage+V.InthiscaseVSGequals0,whichturnsofftheMOSFET.

Figure6.16.MOSFETinterfacestoEMrelay,solenoid,DCmotororspeaker.AnH-bridgeusesfourtransistors,allowingcurrenttoflowineitherdirection.Figures4.26and4.27showapplicationsoftheL293H-bridge,whileFigure6.17showsoneoftheH-bridgecircuitsinternaltotheL293.Eachoutputisatotem-poledrivecircuitwithaDarlingtontransistorsinkandapseudo-Darlingtonsource.If1Aishigh,Q1isonandQ2isoff.If1Aislow,Q1isoffandQ2ison.2AcontrolsQ3andQ4inasimilarfashion.If1Aishighand2Aislow,thenQ1Q4areonandcurrentflowslefttorightacrosscoilA.If1Aislowand2Aishigh,thenQ2Q3areonandcurrentflowsrighttoleftacrosscoilA.

Observation:WeusedtheL293tointerfaceunipolarandbipolarsteppermotorsbackinSection4.7.2.

Figure6.17.AnH-bridgecandrivecurrentineitherdirection.

6.5.2.ElectromagneticandSolidStateRelaysArelayisadevicethatrespondstoasmallcurrentorvoltagechangebyactivatingswitchesorotherdevicesinanelectriccircuit.Itisusedtoremotelyswitchsignalsorpower.Theinputcontrolisusuallyelectricallyisolatedfromtheoutputswitch.Theinputsignaldetermineswhethertheoutputswitchisopenorclosed.Relaysareclassifiedintothreecategoriesdependinguponwhethertheoutputswitchespower(i.e.,highcurrentsthroughtheswitch)orelectronicsignals(i.e.,lowcurrentsthroughtheswitch).Anotherdifferenceishowtherelayimplementstheswitch.Anelectromagnetic(EM)relayusesacoiltoapplyEMforcetoacontactswitchthatphysicallyopensandcloses.Thesolidstaterelay(SSR)usestransistorswitchesmadefromsolidstatecomponentstoelectronicallyalloworpreventcurrentflowacrosstheswitch).Thethreetypesare1.TheclassicgeneralpurposerelayhasanEMcoilandcanswitchACpower2.ThereedrelayhasanEMcoilandcanswitchlow-levelDCelectronicsignals3.Thesolidstaterelay(SSR)hasaninputtriggeredsemiconductorpowerswitchThreesolidstaterelaysareshowninFigure6.18.InterfacingaSSRissimilartointerfacinganLED.ASSRinterfacewasdevelopedasExample2.1andFigure2.33.SSRsallowthemicrocontrollertoswitchACloadsfrom1to30A.Theyareappropriateinsituationswherethepoweristurnedonandoffmanytimes.

TheinputcircuitofanEMrelayisacoilwithanironcore.Theoutputswitchincludestwosetsofsilverorsilver-alloycontacts(calledpoles.)Onesetisfixedtotherelayframe,andtheothersetislocatedattheendofleafspringpolesconnectedtothearmature.Thecontactsareheldinthe“normallyclosed”positionbythearmaturereturnspring.WhentheinputcircuitenergizestheEMcoil,a“pullin”forceisappliedtothearmatureandthe“normallyclosed”contactsarereleased(calledbreak)andthe“normallyopen”contactsareconnected(calledmake.)Thearmaturepullincaneitherenergizeorde-energizetheoutputcircuitdependingonhowitiswired.Relaysaremountedinspecialsockets,ordirectlysolderedontoaPCboard.

Thenumberofpoles(e.g.,singlepole,doublepole,3P,4Petc.)referstothenumberofswitchesthatarecontrolledbytheinput.Therelayshownbelowisadoublepolebecauseithastwoswitches.Single-throwmeanseachswitchhastwocontactsthatcanbeopenorclosed.Double-throwmeanseachswitchhasthreecontacts.Thecommoncontactwillbeconnectedtooneoftheothertwocontacts(butnotbothatthesametime.)TheparametersoftheoutputswitchincludemaximumAC(orDC)power,maximumcurrent,maximumvoltage,onresistance,andoffresistance.ADCsignalwillweldthecontactstogetheratalowercurrentvaluethananACsignal,thereforethemaximumratingsforDCareconsiderablesmallerthanforAC.Otherrelayparametersincludeturnontime,turnofftime,lifeexpectancy,andinput/outputisolation.Lifeexpectancyismeasuredin

numberofoperations.Figure6.19illustratesthevariousconfigurationsavailable.ThesequenceofoperationisdescribedinTable6.5.

Figure6.18.SolidstaterelayscanbeusedtocontrolpowertoanACappliance.

Figure6.19.Standardrelayconfigurations.

Form ActivationSequence

DeactivationSequence

A Make1 Break1

B Break1 Make1

C Break1,Make2 Break2,Make1

D Make1,Break2 Make2,Break1

E Break1,Make2,Break3

Table6.5.Standarddefinitionsforfiverelayconfigurations.

6.5.3.SolenoidsSolenoidsareusedindiscretemechanicalcontrolsituationssuchasdoorlocks,automaticdisk/tapeejectors,andliquid/gasflowcontrolvalves(on/offtype).MuchlikeanEMrelay,thereisaframethatremainsmotionless,andanarmaturethatmovesinadiscretefashion(on/off).Asolenoidhasanelectro-magnet.Whencurrentflowsthroughthecoil,a

magneticforceiscreatedcausingadiscretemotionofthearmature.EachofthesolenoidsshownFigure6.20hasacylindrically-shapedarmaturethemovesinthehorizontaldirectionrelativetothephotograph.Thesolenoidonthetopisusedinadoorlock,andthesecondfromtopisusedtoejectthetapefromavideocassetteplayer.Whenthecurrentisremoved,themagneticforcestops,andthearmatureisfreetomove.Themotionintheoppositedirectioncanbeproducedbyaspring,gravity,orbyasecondsolenoid.

Figure6.20.Photooffoursolenoids.

6.5.4.DCMotorInterfacewithPWMSimilartothesolenoidandEMrelay,theDCmotorhasaframethatremainsmotionless,andanarmaturethatmoves.Inthiscase,thearmaturemovesinacircularmanner(shaftrotation).

Inthepreviousinterfacesthemicrocontrollerwasabletocontrolelectricalpowertoadeviceinabinaryfashion:eitherallonoralloff.Sometimesitisdesirableforthemicrocontrollertobeabletovarythedeliveredpowerinavariablemanner.Oneeffectivewaytodothisistousepulsewidthmodulation(PWM).ThebasicideaofPWMistocreateadigitaloutputwaveoffixedfrequency,butallowthemicrocontrollertovaryitsdutycycle.ThesystemisdesignedinsuchawaythatHigh+Lowisconstant(meaningthefrequencyisfixed).Thedutycycleisdefinedasthefractionoftimethesignalishigh:

Hence,dutycyclevariesfrom0to1.Weinterfacethisdigitaloutputwavetoanexternalactuator(likeaDCmotor),suchthatpowerisappliedtothemotorwhenthesignalishigh,andnopowerisappliedwhenthesignalislow.WepurposelyselectafrequencyhighenoughsotheDCmotordoesnotstart/stopwitheachindividualpulse,butratherrespondstotheoverallaveragevalueofthewave.TheaveragevalueofaPWMsignalislinearlyrelatedtoitsdutycycleandisindependentofitsfrequency.LetP(P=V*I)bethepowertotheDCmotor,showninFigure6.21,whenthePP0signalishigh.NoticethecircuitinFigure6.21isoneoftheexamplespreviouslydescribedinFigure6.14.Underconditionsofconstantspeedandconstantload,thedeliveredpowertothemotoris

linearlyrelatedtodutycycle.

DeliveredPower=

Unfortunately,asspeedandtorquevary,thedevelopedemfwillaffectdeliveredpower.Nevertheless,PWMisaveryeffectivemechanism,allowingthemicrocontrollertoadjustdeliveredpower.

ADCmotorhasanelectro-magnetaswell.Whencurrentflowsthroughthecoil,amagneticforceiscreatedcausingarotationoftheshaft.Brushespositionedbetweentheframeandarmatureareusedtoalternatethecurrentdirectionthroughthecoil,sothataDCcurrentgeneratesacontinuousrotationoftheshaft.Whenthecurrentisremoved,themagneticforcestops,andtheshaftisfreetorotate.Theresistanceinthecoil(R)comesfromthelongwirethatgoesfromthe+terminaltothe–terminalofthemotor.Theinductanceinthecoil(L)arisesfromthefactthatthewireiswoundintocoilstocreatetheelectromagnetics.Thecoilitselfcangenerateitsownvoltage(emf)becauseoftheinteractionbetweentheelectricandmagneticfields.IfthecoilisaDCmotor,thentheemfisafunctionofboththespeedofthemotorandthedevelopedtorque(whichinturnisafunctionoftheappliedloadonthemotor.)Becauseoftheinternalemfofthecoil,thecurrentwilldependonthemechanicalload.Forexample,aDCmotorrunningwithnoloadmightdraw50mA,butunderload(friction)thecurrentmayjumpto500mA.

TherearesixconsiderationswhenselectingaDCmotor:speed,torque,voltage,current,size,andweight.Speedistherateinrotationsperminute(RPM)thatthemotorwillspin,andtorqueistheavailableforcetimesdistancethemotorcanprovideatthatspeed.Weselectthemotorvoltagetomatchtheavailablepowersupply.UnlikeLEDs,weMUSTnotusearesistorinserieswithamotortoreducethevoltage.Ingeneral,themotorvoltagematchesthepowersupplyvoltage.Wheninterfacingwewillneedtoknowmaximumcurrent.

Therearelotsofmotordriverchips,buttheyarefundamentallysimilartothecircuitsshowninFigure6.14.Forthe2N2222andTIP120NPNtransistors,iftheportoutputislow,nocurrentcanflowintothebase,sothetransistorisoff,andthecollectorcurrent,IC,willbezero.Iftheportoutputishigh,currentdoesflowintothebaseandVBEgoesaboveVBEsatturningonthetransistor.ThetransistorisinthelinearrangeifVBE≤VBEsatandIc=hfe·Ib.ThetransistorisinthesaturatedmodeifVBE≥VBEsat,VCE=0.3VandIc<hfe·Ib.WeselecttheresistorfortheNPNtransistorinterfacestooperaterightatthetransitionbetweenlinearandsaturatedmode.Westartwiththedesiredcoilcurrent,Icoil(thevoltageacrossthecoilwillbe+V-VCEwhichwillbeabout+V-0.3V).Next,wecalculatetheneededbasecurrent(Ib)giventhecurrentgainoftheNPN

Ib=Icoil/hfeknowingthecurrentgainoftheNPN(hfe),seeTable6.6.Finally,giventheoutputhighvoltageofthemicrocontroller(VOHisabout3.3V)andbase-emittervoltageoftheNPN(VBEsat)neededtoactivatethetransistor,wecancalculatethedesiredinterfaceresistor.

Rb≤(VOH-VBEsat)/Ib=hfe*(VOH-VBEsat)/Icoil

Theinequalitymeanswecanchooseasmallerresistor,creatingalargerIb.Becausetheofthetransistorscanvaryalot,itisagooddesignpracticetomaketheRbresistorabout½thevalueshownintheaboveequation.Sincethetransistorissaturated,theincreasedbasecurrentproducesthesameVCEandthusthesamecoilcurrent.

Parameter PN2222(IC=150mA)

2N2222(IC=500mA)

TIP120(IC=3A)

hfe 100 40 1000

hie 60 250to8000 70to7000

VBEsat 0.6 2 2.5V

VCEatsaturation

0.3 1 2V

Table6.6.Designparametersforthe2N2222andTIP120.

TheIRF540MOSFETisavoltage-controlleddevice,ifthePortoutputishigh,the2N2222ison,theMOSFETisoff,andthecoilcurrentwillbezero.IfthePortoutputislow,the2N2222isoff,thegatevoltageoftheMOSFETwillbe+V,theMOSFETison,andtheVDSwillbeverycloseto0.TheIRF540needsalargegatevoltage(>10V)tofullyturnsothedrainwillbeabletosinkupto28A.

Becauseoftheresistanceofthecoil,therewillnotbesignificantdI/dtwhenthedeviceisturnedon.ConsideraDCmotorasshowninFigure6.21withV=12V,R=50 andL=100 H.Assumeweareusinga2N2222withaVCEof1Vatsaturation.Initiallythemotorisoff(nocurrenttothemotor).Attimet=0,thedigitalportgoesfrom0to+3.3V,andtransistorturnson.Assumeforthissection,theemfiszero(motorhasnoexternaltorqueappliedtotheshaft)andthetransistorturnsoninstantaneously,wecanderiveanequationforthemotor(Ic)currentasafunctionoftime.ThevoltageacrossbothLCtogetheris12-VCE=11Vattime=0+.Attime=0+,theinductorisanopencircuit.Conversely,attime=∞,theinductorisashortcircuit.TheIcattime0-is0,andthecurrentwillnotchangeinstantaneouslybecauseoftheinductor.Thus,theIcis0attime=0+.TheIcis11V/50 =220mAattime=∞.

11V=Ic*R+L*dIc/dt

Generalsolutiontothisdifferentialequationis

Ic=I0+I1e-t/ dIc/dt=-(I1/ )e-t/

Weplugthegeneralsolutionintothedifferentialequationandboundaryconditions.

11V=(I0+I1e-t/ )*R-L*(I1/ )e-t/

Tosolvethedifferentialequation,thetimeconstantwillbe =L/R=2 sec.Usinginitialconditions,weget

Ic=220mA*(1-e-t/2 s)

Example6.10.Designaninterfacefortwo+12V1AgearedDCmotors.Thesetwomotorswillbeusedtopropelarobotwithtwoindependentdrivewheels.

Solution:WewillusetwocopiesoftheTIP120circuitinFigure6.21becausetheTIP120cansinkatleastthreetimesthecurrentneededforthismotor.Weselecta+12Vsupplyandconnectittothe+Vinthecircuit.Theneededbasecurrentis

Ib=Icoil/hfe=1A/1000=1mA

Thedesiredinterfaceresistor.

Rb≤(VOH-Vbe)/Ib=(5-2.5)/1mA=2.5k

Tocoverthevariabilityinhfe,wewillusea1k resistorinsteadofthe2.5k .Theactualvoltageonthemotorwhenactivewillbe+12-2=10V.

Thecoilsandtransistorscanvaryalot,soitisappropriatetoexperimentallyverifythedesignbymeasuringthevoltagesandcurrents.TwocopiesofProgram6.8areusedtocontroltherobot.TheperiodofthePWMoutputischosentobeabout10timesshorterthanthetimeconstantofthemotor.Theelectronicdriverwillturnonandoffatthisrate,butthemotoronlyrespondstotheaveragelevel.ThesoftwaresetsthedutycycleofthePWMtoadjustthedeliveredpower.Whenactive,theinterfacewilldrive+10Vacrossthemotor.Thecurrentwillbeafunctionofthefrictionappliedtotheshaft.

Figure6.21.DCmotorinterface.

SimilartothesolenoidandEMrelay,theDCmotorhasaframethatremainsmotionless(calledthestator),andanarmaturethatmoves(calledtherotor).AbrushedDCmotorhasanelectromagneticcoilaswell,locatedontherotor,andtherotorispositionedinsidethestator.InFigure6.22,NorthandSouthrefertoapermanentmagnet,generatingaconstantBfieldfromlefttoright.Inthiscase,therotormovesinacircularmanner.Whencurrentflowsthroughthecoil,amagneticforceiscreatedcausingarotationoftheshaft.AbrushedDCmotorusescommutatorstoflipthedirectionofthecurrentinthecoil.Inthisway,thecoilontherightalwayshasanupforce,andtheoneontheleftalwayshasadownforce.Hence,aconstantcurrentgeneratesacontinuousrotationoftheshaft.Whenthecurrentisremoved,themagneticforcestops,andtheshaftisfreetorotate.Inapulse-widthmodulatedDCmotor,thecomputeractivatesthecoilwithacurrentoffixedmagnitudebutvariesthedutycycleinordertoadjustthepowerdeliveredtothemotor.

Figure6.22.AbrushedDCmotorusesacommutatortoflipthecoilcurrent.AbrushlessDCmotor(BLDC),asthenameimplies,doesnothavemechanicalcommutatorsorbrushestoflipthecurrents.Itisasynchronouselectricmotorpoweredbydirectcurrentandhasanelectroniccommutationsystem,ratherthanamechanicalcommutatorandbrushes.InBLDCmotors,current-to-torqueandvoltage-to-rpmarelinearrelationships.ThecontrolleruseseitherthebackemfofthemotoritselforHall-effectsensorstoknowtherotationalangleoftheshaft.Thecontrollerusesthisangletosetthedirectionofthecurrentsintheelectromagnets,shownasthesixstepsequenceinFigure6.23.OtherdifferencesfromabrushedDCmotorarethattheBLDCpermanentmagnetsareintherotorandtheelectromagnetsareinthestator.Typically,therearethreeelectromagneticcoils,labeledPhaseA,PhaseB,andPhaseC,whicharearrangedinaWyeformation.Eachcoilcanbemodeledasaresistance,inductance,andemf,aspreviouslyshowninFigure6.14.TheHallsensorgoesthroughthesequence001,000,

100,110,111,011eachtimetheshaftrotatesonce.Itisasynchronousmotorbecausethecontrolleradjuststhephasecurrentaccordingtothesix-stepsequence.Forexample,iftheHallsensorreads001,thenthecontrollerplaces+VonPhaseAandgroundonPhaseC(step1).Inotherwords,thephasecurrentsaresynchronizedtotheshaftposition.Torotatethemotorintheotherdirection,wereversethecurrentsineachstep.Wewillseelaterforsteppermotorsthattheprocessisreversed.Forsteppermotors,thecontrollersetsthephasecurrentsandthemotormovestothatposition.ToadjustthepowertoaBLDCmotor,wechangethevoltage,V,orusePWMonthecontrolsignalsthemselves.ThePWMperiodshouldbeatleast10timesshorterthanthetimeforeachofthesixsteps.Inotherwords,thePWMfrequencyshouldbe60timesfasterthantheshaftrotationalfrequency.

BLDCmotorshavemanyadvantagesandfewdisadvantageswhencomparedtobrushedDCmotors.Becausetherearenobrushes,theyrequirelessmaintenanceandhencehavealongerlife.Therefore,theyareappropriateforapplicationswhereservicingisinconvenientorexpensive.BLDCmotorsproducemoreoutputtorqueperweightthanbrushedDCmotorsandhenceareusedforpilotlessairplanesandhelicopters.Becausetherotorismadeofpermanentmagnets,therotorinertiaisless,allowingittospinfasterandtochangequicker.Inotherwords,ithasfasteraccelerationanddeceleration.Removingthebrushesreducesfriction,whichalsocontributestotheimprovedspeedandacceleration.Ithasalinearspeed/torquerelationship.Becausethereisnobrushcontact,BLDCmotorsoperatemorequietlyandhavelessElectromagneticInterference(EMI).Theonlydisadvantagesarethecomplexcontrollerandincreasedcost.

Figure6.23.AbrushlessDCmotorusesanelectroniccommutator.

Example6.11.Interfacea24-V2-AbrushlessDCmotor.

Solution:AbrushlessDCmotorhasthreecoilsconnectedinaWyepattern.Eachofthephasescanbedrivenintooneofthreestates:24V,ground,orfloating.WewilluseMOSFETstosourceandsinkthecurrentrequiredbythemotor(Figure6.16).Remember,whenthemotorisunderload,thecurrentwillincrease.TheP-channelMOSFETwillconnectthe24Vtothephasewhenitsgatevoltageisbelow24V.TheN-channelMOSFETwilldrivethephasetogroundwhenitsgateisabovezero.ItwillbeimportanttopreventturningonbothMOSFETsatthesametime.Forsafetyreasons,wewillusedigitallogicintheinterfacesothedrivercanonlybeinthethreevalidstates.Table6.7showsthedesignspecificationforPhaseA.WhenEnAislow,bothMOSFETsareoffandthephasewillfloat(HiZ).WhenEnAishigh,theInAdetermineswhetherthephaseishighorlow.ThesixgatevoltagesarelabeledinFigure6.24asG1toG6.Thesegatevoltagesare24V,producedbythe10k pull-up,whenthecorresponding7406driveroutputisfloating.Alternatively,thesegatevoltagesare0.5Vwhenthe7406driveroutputislow.ItisgooddesigntouseintegrateddriversliketheULN2074,L293,TPIC0107,andMC3479ratherthanindividualtransistors.Inparticular,theentireinterfacecircuitinFigure6.24couldbereplacedwiththreeL6203fullbridgedrivers.

EnA InA G1 G2 P-chan N-chan PhaseA

1 1 Low Low On Off +24V

1 0 High High Off On Ground

0 X High Low Off` Off HiZ

Table6.7.ControlsignalsforonephaseofthebrushlessDCmotor.

Figure6.24.BrushlessDCmotorinterface.

TheInA,InB,andInCsignalsinFigure6.24areconnectedtoanyoutputports,whereastheEnA,EnB,andEnCsignalswillbeattachedtoPWMoutputs.ThePWMperiodwillbeselected60timesfasterthanthemotorspeedinrps.ThethreeHall-effectsensorsignalswillbeattachedtoinputcapturepins.Interruptswillbearmedforboththeriseandfallofthesethreesensors.Inthisway,anISRwillberunatthebeginningofeachofthesixsteps.TheBLDCmotorisasynchronousmotor,sothesixcontrolsignalsareafunctionoftheshaftposition.Inparticular,theISRwilllookuptheHallsensorsandoutputthepattern,asshowninTable6.8.

Step HS1 HS2 HS3 EnA InA EnB InB EnC InC A B C

1 0 0 1 PWM 1 0 X PWM 0 24V HiZ 0V

2 0 0 0 PWM 1 PWM 0 0 X 24V 0V HiZ

3 1 0 0 0 X PWM 0 PWM 1 HiZ 0V 24V

4 1 1 0 PWM 0 0 X PWM 1 0V HiZ 24V

5 1 1 1 PWM 0 PWM 1 0 X 0V 24V HiZ

6 0 1 1 0 X PWM 1 PWM 0 HiZ 24V 0V

Table6.8.Input-outputrelationshipsforthesynchronouscontroller.

Furthermore,whenanyoftheenablesignalsarescheduledtobehigh,theywillbepulsedusingpositivelogicPWM.ThesoftwarecanadjustthedeliveredpowertotheBLDCmotorbysettingthedutycycleofthePWM.ThesoftwareimplementationhasbeenleftasLab6.4.

6.6.IntegralControlofaDCMotorAcontrolsystemisacollectionofmechanicalandelectricaldevicesconnectedforthepurposeofcommanding,directing,orregulatingaphysicalplant.InthissectionthephysicalplantisaDCmotorasdescribedpreviouslyinSection6.5.4.Therealstatevariablesarethepropertiesofthephysicalplantthataretobecontrolled.Inthisexample,wewishtospinthemotorat1500RPM,or25rps.Thus,thestatevariableinthiscasewillbemotorspeed.Thesensorandstateestimatorcompriseadataacquisitionsystem.Thegoalofthisdataacquisitionsystemistoestimatethestatevariables.Wewillattachatachometertothemotorsothesystemcanmeasurespeed.Figures6.4and6.4illustrateonetypeoftachometerandhowthesignalisinterfacedtothemicrocontroller.Theestimatedstatevariables,X’(t),inthissystemwillbethemeasuredspeedin0.1rps.Theactuatorisatransducerthatconvertsthecontrolsystemcommands,U(t),intodrivingforces,V(t),thatareappliedthephysicalplant.InthisexamplewewillusethecircuitinFigure6.21,whichallowsthemicrocontrollertoadjustpowertothemotorusingthefunctionsinProgram6.8.Inthisexample,thebusclockis80MHz,sothePWMclockis40MHz.WewillfixthePWMperiod1ms,andinitializethePWMoutputat0.1%bycalling PWM0_Init(40000,40) .Wedefinetheactuatorcommand,U(t),astheparameter40to39960thatwepassinwhenwecall PWM0_Duty .Thereforewecanadjustthepowerfrom0.1%to99.9%.

Ingeneral,thegoalofthecontrolsystemistodrivetherealstatevariablestoequalthedesiredstatevariables.Inactualitythough,thecontrollerattemptstodrivetheestimatedstatevariablestoequalthedesiredstatevariables.Itisimportanttohaveanaccuratestateestimator,becauseanydifferencesbetweentheestimatedstatevariablesandtherealstatevariableswilltranslatedirectlyintocontrollererrors.Ifwedefinetheerrorasthedifferencebetweenthedesiredandestimatedstatevariables:

e(t)=X*(t)-X’(t)

Aclosed-loopcontrolsystemusestheoutputofthestateestimatorinafeedbacklooptodrivetheerrorstozero.ThecontrolsystemcomparesX’(t),tothedesiredstatevariables,X*(t),inordertodecideappropriateaction,U(t).SeeFigure6.25.

Figure6.25.Blockdiagramofamicrocomputer-basedclosed-loopcontrol

system.WecancombinetheperiodmeasurementfromSection6.1,thePWMoutputofSection6.3,andtheDCmotorinterfaceofSection6.5.4tobuildamotorcontroller.Oneeffectiveyetsimplecontrolalgorithmsisanintegralcontroller.Wespecifytheactuatoroutputastheintegraloftheaccumulatederrors.

whereKiisacontrollerconstant.Forthiscontroller,iftheerroriszerotheactuatorcommandremainsconstant.Ifthemotorisspinningtooslowly,thecontrollerwillincreasepower.Ifthemotorisspinningtooquickly,itwilldecreasepower.Foranintegralcontroller,theamountofincreaseordecreaseislinearlyrelatedtotheerror.Soiftheerrorislargeitadds(orsubtracts)alot,andiftheerrorissmallitadds(orsubtracts)alittle.Ifthetimeconstantofthemotoris100ms,thenwewillrunthecontroller10timesfaster.TheTimer2AISRinProgram6.11issettoexecuteevery10ms.ThesoftwareinProgram6.2willsettheglobalvariable Period withthemeasuredperiodin12.5nsunits.

uint32_tPeriod;//24-bit,12.5nsunits

uint32_tSpeed;//motorspeedin0.1rps

int32_tE;//speederrorin0.1rps

int32_tU;//dutycycle40to39960

voidTimer2A_Handler(void)

TIMER2_ICR_R=0x01;//acknowledgetimer2Atimeout

Speed=800000000/Period;//0.1rps

E=250-Speed;//0.1rps

U=U+(3*E)/64;//discreteintegral

if(U<40)U=40;//Constrainoutput

if(U>39960)U=39960;//40to39960

PWM0A_Duty(U);//output

Program6.11.ISRtoimplementanintegralcontroller

6.7.Exercises6.1ShowthechangesyouneedtomaketoProgram6.1torunonaLM3S1968withtheinputconnectedtoCCP2/PB1.

6.2ShowthechangesyouneedtomaketoProgram6.1torunonaLM3S1968withtheinputconnectedtoCCP3/PG4.

6.3IfTimer0AandTimer0Barebotharmedforinterrupt,arethetwosourcespolledorvectored?IfTimer0AandTimer1Aarebotharmedforinterrupt,arethetwosourcespolledorvectored?

6.4HowdoyouchangetheresolutionoftheperiodmeasurementresolutioninProgram6.2?

6.5Ifthebusclockwerechangedto50MHzwithoutanychangestoProgram6.2,whatwouldthenewperiodmeasurementrangebe?

6.6ShowthechangesyouneedtomaketoProgram6.6torunTimer1A.

6.7ShowthechangesyouneedtomaketoProgram6.6torunTimer2B.

6.8ShowthechangesyouneedtomaketoProgram6.7torunonaLM3S1968withtheoutputconnectedtoCCP2/PB1.

6.9ShowthechangesyouneedtomaketoProgram6.8torunonaLM3S1968withtheoutputconnectedtoPWM3/PH1.

D6.10Createtwosynchronized50%dutycyclesquarewavesusingPWMchannels.InterfacethetwoPWMoutputstoasteppermotorusingaL293.Ifyouusejust2PWMoutputsyouwillneedtoadddigitallogicinverters.WritesoftwarethatinitializedthePWMgiventhedesiredshaftspeedin1RPM,witharangeofspeedsfrom0to100RPM.Assumethereare200steps/revolution.Thestepperwillspinwithoutsoftwareoverheaduntilthefunctioniscalledagain.Ifthedesiredspeediszero,stopthemotoratoneofthevalidstates(5,6,10or9).

D6.11TheobjectiveofthisproblemistomeasurethefrequencyofasquarewaveconnectedtoCCP2.Thefrequencyrangeis0to2000Hzandtheresolutionis0.1Hz.Forexample,ifthefrequencyis567.83Hz,thenyoursoftwarewillsettheglobal Freq to5678.Don’tworryaboutfrequenciesabove2000Hz.

D6.12Theobjectiveofthisproblemistomeasurebodytemperatureusinginputcapture(periodmeasurement).Ashuntresistorisplacedinparallelwithathermistor.Thethermistor-shuntcombination,R,hasthefollowinglinearrelationshipfortemperaturesfrom90to110˚F.

R=100kΩ-(T-90˚F)•1kΩ/˚FwhereRistheresistanceofthethermistor-shunt

Inotherwords,theresistancevariesfrom100kΩto80kΩasthetemperaturevariesfrom90to110˚F.Therangeofyoursystemis90to110˚Fandtheresolutionshouldbebetterthan0.01˚F.YouwilluseaTLC555toconverttheresistancetoaperiod.TheperiodofaTLC555timeris0.693•CT•(RA+2RB).SetRA=R,andRB=50kΩ.

a)Thepulsewidthmeasurementresolutionwillbe125nsbecausethebusclockis8MHz.Choosethecapacitorvaluesothatthispulsewidthmeasurementresolutionmatchesthedesiredtemperatureresolutionof0.01˚F.

b)GiventhisvalueofC,whatisthepulsewidthat90˚F?Givetheanswerbothinµsandbusclockcycles.

c)GiventhisvalueofC,whatisthepulsewidthat110˚F?Givetheanswerbothinµsandbusclockcycles.

d)Writetheinitializationthatconfigurestheinputcaptureinterrupts.

e)Showtheinterrupthandlerthatperformsthetemperaturemeasurementtasksinthebackgroundandsetsaglobal, Temperature .Temperature willvaryfrom9000to11000astemperaturevariesfrom90to110˚F.

D6.13Designawinddirectionmeasurementinstrumentusingtheinputcapturetechnique.Again,youaregivenatransducerthathasaresistancethatislinearlyrelatedtothewinddirection.Asthewinddirectionvariesfrom0to360degrees,thetransducerresistancevariesfrom0to1000Ω.Thefrequenciesofinterestare0to0.5Hz,andthesamplingratewillbe1Hz.OnewaytointerfacethetransducertothecomputeristouseanastablemultivibratorliketheTLC555.TheperiodofaTLC555timeris0.693•CT•(RA+2RB).

a)Showthehardwareinterface

b)Writetheinitializationandinterruptserviceroutinethatmeasuresthewinddirectionandcreatesa16-bitunsignedresultwithunitsofdegrees.I.e.,thevaluevariesfrom0to359.Youdonothavetowritesoftwarethatsamplesat1Hz,simplyafunctionthatmeasureswinddirectioninthebackground.

D6.14Theobjectiveofthisproblemistodesignanunderwaterultrasonicrangingsystem.Thedistancetotheobject,d,canvaryfrom1to100m.Theultrasonictransducerwill

sendashort5µssoundpulseintothewaterinthedirectionofinterest.Thesoundwavewilltravelat1500m/secandreflectoffthefirstobjectitrunsinto.Thereflectedwavewillalsotravelat1500m/secbacktothetransducer.Thereflectedpulseissensedbythesametransducer.Yoursystemwilltriggertheelectronics(givea5µsdigitalpulse),measurethetimeofflight,thencalculatethedistancetotheobject.Usingperiodicinterrupts,thesoftwarewillissuea5µspulseoutonceasecond.Usinginterruptinginputcapture,thesoftwarewillmeasurethetimeofflight,∆t.Theinputcaptureinterrupthandlerwillcalculatedistance,d,asadecimalfixed-pointvaluewithunitsof0.01m,andenteritintoaFIFOqueue.Themainprogramwillcalltheritual,thengetdataoutoftheFIFOqueue.Themainprogramwillcall Alarm() ifthedistanceislessthan15m.Youdonothavetogivetheimplementationof Alarm() .YoumayuseanyoftheFIFOsinChapter3withoutshowingitsimplementation.Assumethebusclockis8MHz.

Figure6.25.InterfaceforQuestionD6.14.a)Deriveanequationthatrelatesthedistance,d,tothetimeofflight,∆t.

b)Usethisequationtocalculatetheminimumandmaximumpossibletimeofflight,∆t.

c)Givetheinitializationroutine.

d)GivetheISRsthatmeasuredistance.

6.8.LabAssignmentsLab6.1Theoverallobjectiveistointerfaceajoysticktothemicrocontroller.Thejoystickismadewithtwopotentiometers.Youcanusetwoastablemultivibrators(TLC555)toconvertthetworesistancesintotwoperiods.UsetwoperiodmeasurementchannelstoestimatetheX-Ypositionofthejoystick.Organizethesoftwareinterfaceintoadevicedriver,andwriteamainprogramtotesttheinterface.

Lab6.2Theoverallobjectiveistomeasuretemperature.Athermistorisatransducerwitharesistancethatisafunctionofitstemperature.Youcanuseanastablemultivibrator(TLC555)toconverttheresistanceintoaperiod.Useaperiodmeasurementchanneltoestimatetheresistanceofthethermistor.Useatablelookupwithlinearinterpolationtoconvertresistancetotemperature.Organizethesoftwareinterfaceintoadevicedriver,andwriteamainprogramthatoutputstemperaturetotheUARTchannel.

Lab6.3Theoverallobjectiveistomeasurelinearposition.Aslide-potisatransducerwitharesistancethatisafunctionofthelinearpositionoftheslide.Youcanuseanastablemultivibrator(TLC555)toconverttheresistanceintoaperiod.Useaperiodmeasurementchanneltoestimatetheresistanceofthepotentiometer.Useatablelookupwithlinearinterpolationtoconvertresistancetoposition.Organizethesoftwareinterfaceintoadevicedriver,andwriteamainprogramthatoutputspositiontotheUARTchannel.

Lab6.4Theobjectiveofthislabistocontrolaservomotor.InterfaceaservomotortothePWMoutputpinofthemicrocontroller.ThedesiredangleisinputfromtheUARTchannel(connectedtoPCrunningPuTTY.)Organizethesoftwareinterfaceintoadevicedriver,andwriteamainprogramthatinputsfromtheUARTchannelandmaintainsthePWMoutputtotheservo.Servosareapopularmechanismtoimplementsteeringinrobotics.Rangingfrommicroservoswith15oz-intorquetopowerfulheavy-dutysailboatservos,theyallshareseveralcommoncharacteristics.Aservoisessentiallyapositionablemotor.Theservo“knows”twothings:whereitis(theactualposition)andwhereitwantstobe(thedesiredposition).Whentheservoreceivesaposition,itattemptstomovetheservohorntothedesiredposition.Thetaskoftheservo,then,istomaketheactualpositionthedesiredposition.Thefirststeptounderstandinghowservosworkistounderstandhowtocontrolthem.Powerisusuallybetween4Vand6Vandshouldbeseparatefromsystempower(asservosareelectricallynoisy).Evensmallservoscandrawoveranampunderheavyloadsothepowersupplyshouldbeappropriatelyrated.Thoughnotrecommended,servosmaybedriventohighervoltagestoimprovetorqueandspeedcharacteristics.Servosarecommandedthrough“PulseWidthModulation,”orPWM,signalssentthroughthecommandwire.Essentially,thewidthofapulsedefinestheposition.Forexample,sendinga1.5mspulsetotheservo,tellstheservothatthedesiredpositionis90degrees.Inorderfortheservotoholdthisposition,thecommandmustbesentatabout50Hz,orevery20ms.Ifyouweretosendapulselongerthan2.5msorshorterthan0.5ms,theservowouldattempttooverdrive(andpossiblydamage)itself.Oncetheservohasreceivedthedesiredposition(viathePWMsignal)theservomustattempttomatchthedesiredandactualpositions.Itdoesthisbyturningasmall,gearedmotorleftorright.If,forexample,thedesiredpositionislessthantheactualposition,theservowillturntotheleft.Ontheotherhand,ifthedesiredpositionisgreaterthanthe

actualposition,theservowillturntotheright.Inthismanner,theservo“zeros-in”onthecorrectposition.Shouldaloadforcetheservohorntotherightorleft,theservowillattempttocompensate.Notethatthereisnocontrolmechanismforthespeedofmovementand,formostservos,thespeedisspecifiedindegrees/second.Formoreinformationrefertothedatasheetofyourservo.

Figure6.26.Exampleservomotor.

Lab6.5Theoverallobjectiveistomeasurecapacitance.Thesystemwillmeasurefrom100pFto1uFwitharesolutionof100pF.Youcanuseanastablemultivibrator(TLC555)toconvertthecapacitanceintoaperiod.Useaperiodmeasurementchanneltoestimatethecapacitance.Useatablelookupwithlinearinterpolationtoconvertperiodtocapacitance.Organizethesoftwareinterfaceintoadevicedriver,andwriteamainprogramthatoutputspositiontotheUARTchanneloronalocaldisplay.

Lab6.6.TheobjectiveofthislabistointerfaceabrushlessDCmotor.RatherthanusediscretetransistorslikeFigure6.21,useanintegratedmotorinterfacechipliketheL293orL6103.UsingthreepowerresistorsinaWyeshape,testthemotordrivecircuit.Writetwosoftwarefunctions:InitializationandPowerSet.Thesecondfunctioncanbeusedtoadjustthepowertothemotorfrom0to100%.AddperiodmeasurementstotheinputcaptureISRs,andusethemtomeasureshaftspeedinRPM.UsealogicanalyzertocollectdatasimilartoFigure8.62.Takemeasurementsofshaftspeedversusdutycycle.Taketransientmeasurementsofspeedversustime,asthedutycyclechangesfrom25to75%.

7.SerialInterfacingChapter7objectivesareto:•Presentthephysicaldriversandprotocolsforserialinterfacing

•DescribetheRS232standard

•Designinterfacesthatconvertbetween+5and+3.3Vdigitallogic

•PresentdifferentiallineprotocolssuchasRS422andUSB

•DescribeandemploytheSSIsynchronousserialprotocol

•DescribeandemploytheI2Cserialbus

•IntroducetheUSBprotocol

Inmanyapplications,asinglededicatedmicrocontrollerisinsufficienttoperformallthetasksoftheembeddedsystem.Onesolutionwouldbetousealargerandmorepowerfulmicrocontroller,andanotherapproachwouldbetodistributethetasksamongmultiplemicrocontrollers.Thissecondapproachhastheadvantagesofmodularityandexpandability.Toimplementadistributedsystem,weneedamechanismtosendandreceiveinformationbetweenthemicrocontrollers.Asecondscenariothatrequirescommunicationisacentralgeneral-purposecomputerlinkedtomultipleremoteembeddedsystemsforthepurposeofdistributeddatacollectionordistributedcontrol.Forsituationswheretherequiredbandwidthislessthanabout100,000bytes/sec,thebuilt-inserialportsofthemicrocontrollercanbeused.

InthischapterwewillinterfaceDACandADCdevicestothesynchronousserialport.InChapters8and10wecontinuebyshowinghowtoemploytheDACandADCforsignalgenerationanddataacquisition.WecanusetheI2Cbustointerfacesensorsandactuators.Inthischapterwewillfocusonserialchannelsthatemployadirectphysicalconnectionbetweenthemicrocontrollers,andlaterinChapter11weexpandthecommunicationsystemtoincludenetworks.

7.1.IntroductiontoSerialCommunicationSerialcommunicationinvolvesthetransmissionofonebitofinformationatatime.Onebitissent,atimedelayoccurs,thenthenextbitissent.Thissectionwillintroducetheuseofserialcommunicationasaninterfacingtechniqueforvariousmicrocontrollerperipherals.Sincemanyperipheraldevicessuchasprinters,keyboards,scanners,andmiceoperatehavetheirowncomputers,thecommunicationproblemcanbegeneralizedtooneoftransmittinginformationbetweentwocomputers.Theuniversalasynchronousreceiver/transmitter(UART)istheinterfacedevicethatimplementstheserialdatatransmission.Theserialchannelisthecollectionofsignals(orwires)thatimplementthecommunication,seeFigure7.1.Toimprovebandwidth,removenoiseandincreaserange,weplaceinterfacelogicbetweenthedigitallogicUARTdeviceandtheserialchannel.Wedefinethedataterminalequipment(DTE)asthecomputeroraterminalandthedatacommunicationequipment(DCE)asthemodemorprinter.

Figure7.1.Aserialchannelconnectstwodataterminalequipment(DTE)devices.Whentransmittinginaserialfashion,therearemanywaystoencodebinaryinformationontheline,asshowninFigure7.2.Thegoalistomaximizebandwidthandminimizeerrors.Non-return-to-zero(NRZ)encodingisabinarycodeinwhichthesignalisneveratzerovoltage.ThereismoreenergyinthewirecomparedtotypicalVOL/VOHdigitallogic,becausethereisalwaysavoltagecausingcurrenttoflowcontinuously.Energyisthefundamentalpropertyneededtocommunicateinformationoveradistance.Onasinglewire,voltagesaremeasuredrelativetoground(e.g.,RS232).Onatwistedpair,voltagesaredifferential(e.g.,RS422,USB).Thebinaryvaluesof1and0areencodedaspositiveornegativevoltagedifferences.Inpositivelogic,1isapositivevoltagedifferenceand0isanegativevoltagedifference.Innegativelogic,thevoltagerepresenting0ishigherthanthevoltagerepresenting1.

Non-return-to-zero-inverted(NRZI)isamethodofencodingbinarysignalastransitionsorchangesinthesignal.SimilartoNRZ,thesignalinaNRZIprotocolisneverzero.Thebinaryinformationisencodedasthepresenceorabsenceofatransitionataclockboundary,illustratedbythearrowsinFigure7.2.Boththetransmitterandreceiverwillsynchronizetheirclockssothereceiverknowswhentolookforthetransition.Atransitionisachangefrompositivetonegativeorfromnegativetopositive.Forexample,wecouldsenda1byplacingatransitiononthesignal,orsenda0bycausingnotransition.Also,NRZImighttaketheoppositeconvention,asinUniversalSerialBus(USB),whereatransitionmeans0andasteadylevelmeans1.

Manchesterencodingcodeencodesbinarybitsaseitheralow-to-hightransition,orahigh-to-lowtransition.Manchesterencodingisatypeofphaseencoding,whichmeanstheinformationtransmittedasphaseshifts.ItisusedwiththeIEEE802.3Ethernetprotocol.Thereisafixedtimeperiodinsidewhichonebitistransmitted.TheoriginalManchesterencodingschemedefined0asalow-to-high,and1asahigh-to-low.However,theIEEE802.3conventiondefinesthebitsintheoppositemanner.Thetransitionsthatsignify0or1occuratthemidpointofaperiod,shownasarrowsinFigure7.2.Theremaybeanadditionaltransitionsatthestartofaperiod;theseareextraanddonotsignifydata.Sinceeverybithasatleastonetransition,itiseasierforthereceivertoaligncorrectlyortosynchronizeitsclockwiththetransmitterclock.However,thecostofthiseaseofsynchronizationisadoublingofthebandwidthrequirementofthephysicalchannelascomparedtoNRZorNZRIencoding.Considerthecasewhereasystemiscommunicatingalongsequenceofzerosat1Mb/s.UsingtheNRZIUSBencoding,thelinewillbeasquarewaveat1MHz.However,usingIEEE802.3encoding,thelinewillbeasquarewaveat2MHz.

Figure7.2.Fourencodingsofthebinary10011011.Figure7.3showstheUARTpinsavailableonsomeLM3S/TM4Cmicrocontrollers.ThedescriptionsandsoftwaredriverswerepresentedearlierasProgram4.10and5.11.Tables2.7and2.8describehowtoattachI/OpinstotheUARTmodulesontheTM4C123/TM4C1294.

Figure7.3.SerialportpinsavailableonvariousLM3S/LM4F/TM4Cmicrocontrollers.

Theinterfacelogic(e.g.,MAX3232)convertsbetweendigitallogiclevelsandRS232logiclevels.Inthisprotocol,atypicalbidirectionalchannelrequires3wires(Rx,Tx,Ground.)WeuseRS422voltagelevelswhenwewantlongcablelengthsandhighbandwidths.ThebinarysignalisencodedontheRS422lineasavoltagedifference(e.g.,MAX485).Inthisprotocol,atypicalbidirectionalchannelrequires5wires(RxD+,RxD-,TxD+,TxD-,Ground.)TypicalvoltagelevelsareshowninTable7.1.Ifthetwocomputersareinthesamebox,wecanimplementtheserialchannelwithouttheinterfacedrivers.

+3.3Vlogic

+5Vlogic RS232level

RS422level

True Mark +3V +5V TxD=-5.5V

(TxD+-TxD-)=-3V

False Space +0.1V +0.1V TxD=+5.5V

(TxD+-TxD-)=+3V

Table7.1.Typicalvoltagelevelsforthedigitallogic,RS232andRS422protocols.

Aframeisacompleteandnondivisiblepacketofbits.Aframeincludesbothinformation(e.g.,data,characters)andoverhead(startbit,errorchecking,andstopbits.)Aframeisthesmallestpacketthatcanbetransmitted.TheRS232andRS422protocolshave1startbit,7/8databits,no/even/oddparity,and1/1.5/2stopbits.TheRS232idlelevelistrue(-5V).Thestartbitisfalse(+5V.)Atruedatabitis-5V,andafalsedatabitis+5V(Figure7.4).

Observation:RS232andRS422datachannelsareinnegativelogicbecausethetruevoltageislessthanthefalsevoltage.

Observation:TheRS232protocolalwayshasonestartbitandatleastonestopbit.

Checkpoint7.1:IftheRS232protocolhaseightdatabits,noparity,andonestop

bit,howmanytotalbitsareinaframe?

Figure7.4.ARS232frameshowing1start,8data,noparity,and1stopbit.

Paritycanbeusedtodetecterrors.Parityisgeneratedbythetransmitterandcheckedbythereceiver.Forevenparity,thenumberofonesinthedataplusparityisanevennumber.Foroddparity,thenumberofonesinthedataplusparityisanoddnumber.Iferrorsareunlikely,thenoperatingwithoutparityisfasterandsimpler.

Thebittimeisthebasicunitoftimeusedinserialcommunication.Itisthetimebetweeneachbit.Thetransmitteroutputsabit,waitsonebittime,andthenoutputsthenextbit.Thestartbitisusedtosynchronizethereceiverwiththetransmitter.Thereceiverwaitsontheidlelineuntilastartbitisfirstdetected.Afterthetruetofalsetransition,thereceiverwaitsahalfofabittime.Thehalfofabittimewaitplacestheinputsamplingtimeinthemiddleofeachdatabit,givingthebesttolerancetovariationsbetweenthetransmitterandreceiverclockrates.Inordertooperateproperlythedataavailableintervalmustoverlapthedatarequiredinterval(Section4.2).Next,thereceiverreadsonebiteverybittime.Thebaudrateisthetotalnumberofbits(information,overhead,andidle)pertimethataretransmittedintheserialcommunication.

baudrate=1/(bittime)

Wewilldefineinformationasthedatathatthe“user”intendstobetransmittedbythecommunicationsystem.Examplesofinformationinclude

•Characterstobeprintedonyourprinter

•Apicturefiletobetransmittedtoanothercomputer

•Adigitallyencodedvoicemessagecommunicatedtoyourfriend

•TheobjectcodefiletobedownloadedfromthePCtothemicrocontroller

Wewilldefineoverheadassignalsaddedtothecommunicationinordertoaffectreliabletransmission.Examplesofoverheadinclude

•Startbit(s)startbyte(s)orstartcode(s)

•Stopbit(s)stopbyte(s)orstopcode(s)

•Errorcheckingbitslikeparity,CRC(cyclicredundancycheck),andchecksum

•SynchronizationmessageslikeACK,NAK,XON,XOFF

Bandwidth,latency,andreliabilityarethefundamentalperformancemeasuresforacommunicationsystem.Although,inageneralsenseoverheadsignalscontain“information”,overheadsignalsarenotincludedwhencalculatingbandwidthorconsideringfullduplex,halfduplex,andsimplex.Insimilarway,ifwearesending2bitsofdata,butadd6bitsofzerostofillthebytefieldintheframe,weconsiderthatthereare2bitsofinformationperframe(not8bits.)Wewillusethethreetermsbandwidth,bitrateandthroughputinterchangeablytospecifythenumberofinformationbitspertimethataretransmitted.Thesetermsapplytoallformsofcommunication:

•Parallel

•Serial

•Mixedparallel/serial

Forserialcommunicationsystems,wecancalculate:

Latencyisthetimedelaybetweenwhenamessageissentandwhenitisreceived.Forthesimplesystemsinthischapter,atthephysicallayer,latencycanbecalculatedastheframesizeinbitsdividedbythebaudrateinbits/sec.ForexampleaRS232protocolwith10-bitframesrunningat9600-bpsbaudratewilltake1.04mstogofromtransmittertoreceiver.

Reliabilityisdefinedastheprobabilityofcorrupteddataorthemeantimebetweenfailures(MTBF).Oneoftheconfusingaspectsofbandwidthisthatitcouldmeantwothings.Thepeakbandwidthisthemaximumachievabledatatransferrateovershortperiodsduringtimeswhennothingelseiscompetingforresources.Whenwesaythebandwidthofaserialchannelwith10-bitframesandabaudrateof9600bpsis960bytes/s,wearedefiningpeakbandwidth.Atthecomponentlevel,itisappropriatetospecifypeakbandwidth.However,onacomplexsystem,therewillbedelayscausedbythetimeittakessoftwaretorun,andtherewillbetimeswhenthetransmissionwillbestalledduetoconditionslikefulloremptyFIFOs.Thesustainedbandwidthistheachievabledatatransferrateoverlongperiodsoftimeandundertypicalusageandconditions.Atthesystemlevel,itisappropriatetospecifysustainedbandwidth.

Thedesignparametersthataffectbandwidthareresistance,capacitanceandpower.Ittakesenergytoencodeeachbit,thereforethebandwidthinbitspersecondisrelatedtothepower,whichisenergypersecond.Capacitanceexistsbecauseofthephysicalproximityofthewiresinthecable.Thetimeconstant ofasimpleRCcircuitisR·C.Anincreaseincapacitancewilldecreasetheslewrateofthesignal(seeFigure1.14),limitingtherateatwhichsignalscanchange,therebyreducingthebandwidthofthedigitaltransmission.However,wecanincreasetheslewratebyusingmorepower.Wecanincreasetheenergyoverthesametimeperiodbyincreasingvoltage,increasingcurrent,ordecreasingresistance.

Afullduplexcommunicationsystemallowsinformation(data,characters)totransfersimultaneouslyinbothdirections.Afullduplexchannelallowsbits(information,errorchecking,synchronizationoroverhead)totransfersimultaneouslyinbothdirections,shownpreviouslyinFigure7.1.

Ahalfduplexcommunicationsystemallowsinformationtotransferinbothdirections,butinonlyonedirectionatatime.Halfduplexisatermusuallydefinedformodemcommunications,butinthisbookwewillexpanditsmeaningtoincludeanyserialprotocolthatallowscommunicationinbothdirections,butonlyonedirectionatatime.Afundamentalproblemwithhalfduplexisthedetectionandrecoveryfromacollision.Acollisionoccurswhenbothcomputerssimultaneouslytransmitdata.Fortunately,everytransmissionframeisechoedbackintoitsownreceiver.Thetransmitterprogramcanoutputaframe,waitfortheframetobetransmitted(whichwillbeechoedintoitsownreceiver)thenchecktheincomingparityandcomparethedatainordertodetectacollision.Ifacollisionoccurs,thenitwillprobablybedetectedbybothcomputers.Afteracollision,thetransmittercanwaitawhileandretransmittheframe.Thetwocomputersneedtodecidewhichonewilltransmitfirstafteracollisionsothatasecondcollisioncanbeavoided.

Observation:Mostpeoplecommunicateinhalfduplex.

Acommonhardwaremechanismforhalfduplexutilizesopendrainlogic.Themicrocontrolleropendrainmodehastwooutputstates:zeroandHiZ(Figure7.5).Thelogichighiscreatedwiththepassivepull-up.Withopendrain,thehalfduplexchannelisthelogicalANDofthetwoTxDoutputs.Inthissystem,thetransmittersimplytransmitsitsframewithoutneedingtoenableordisableadriver.Ifbothmicrocontrollerstransmitatthesametime,thezerosonthechannelwilldominatetheones,andthedatawillbecorrupted.

Figure7.5.Ahalfduplexserialchannelcanbeimplementedwithopendrainlogic.Checkpoint7.2:Whatisthedifferencebetweenfullduplexandhalfduplex?

Asimplexcommunicationsystemallowsinformationtotransferonlyinonedirection.TheXON/XOFFprotocolthatwewillcoverlaterisanexampleofacommunicationsystemthathasafullduplexchannel,butimplementssimplexcommunication.Thisissimplexbecauseinformationistransmittedfromthecomputertotheprinter,butonlyXON/XOFF(errorcheckingflags)aresentfromtheprinterbacktocomputer.Inthiscase,nodataissentfromprintertocomputer.

Inordertotransferinformationcorrectly,bothsidesofthechannelmustoperateatthesamebaudrate.Inanasynchronouscommunicationsystem,thetwodeviceshaveseparateanddistinctclocks.Becausethesetwoclocksaregeneratedseparately(oneoneachside),theywillnothaveexactlythesamefrequencyorbeinphase.Ifthetwobaudrateclockshavedifferentfrequencies,thephasebetweentheclockswillalsodriftovertime.Transmissionwilloccurproperlyaslongastheperiodsofthetwobaudrateclocksarecloseenough.The-5Vto+5Vedgeatthebeginningofthestartbitisusedtosynchronizethereceiverwiththetransmitter.IfthetwobaudrateclockperiodsinaRS232systemdifferbylessthan5%,thenafter10bitsthereceiverwillbeoffbylessthanahalfabittime(andnoerrorwilloccur.)Anylargerdifferencebetweenthetwoperiodsmaycauseanerror.

Inasynchronouscommunicationsystem,thetwodevicessharethesameclock(Figure7.6).Typicallyaseparatewireintheserialcablecarriestheclock.Inthisway,veryhighbaudratescanbeobtained.Anotheradvantageofsynchronouscommunicationisthatverylongframescanbetransmitted.Largerframesreducetheoperatingsystemoverheadforlongtransmissionsbecausefewerframesneedbeprocessedpermessage.Eventhoughinthischapter,wewilldesignvariouslowbandwidthsynchronoussystemsusingtheSSI,synchronouscommunicationisbestappliedtosystemsthatrequirebandwidthsabove1Mbps.Thecostofthisincreasedperformanceistheadditionalwireinthecable.Theclockmustbeinterfacedwithchanneldrivers(e.g.,RS232,RS422,optocouplers)similartothetransmitandreceivedatasignals.Ifthetwocomputersareinthesamebox,wecanimplementcommunicationwithouttheinterfacedrivers.

Checkpoint7.3:Whatisthedifferencebetweensynchronousandasynchronouscommunication?

Observation:Self-centeredpeopleemploysimplexcommunication.

Figure7.6.Nodesonasynchronouschanneloperateoffacommonclock.Checkpoint7.4:HowdoestheMAX3232inFigures7.1and7.6makeitbetter?

7.2.RS232InterfacingThebaudrateinaRS232systemcanbeashighas115,200bits/sec.Becauseatypicalcablehas50pF/foot,themaximumdistanceforRS232transmissionislimitedto50feet.Thereare21signalsdefinedforfullMODEM(MOdulate/DEModulate)communication(Figure7.7.)TheRS232standardusesaDB25connectorthathas25pins.TheEIA-574standardusesRS232voltagelevelsandaDB9connectorthathasonly9pins.TheEIA-561standardalsousesRS232voltagelevelsbutwithaRJ45connectorthathasonly8pins.

Figure7.7.RS232,EIA-574andEIA-561connectorsusedinmanyserialapplications.Table7.2showstheentiresetofRS232signals.ThemostcommonlyusedsignalsofthefullRS232standardareavailablewiththeEIA-561/EIA-574protocols.

Theframegroundisconnectedononesidetothegroundshieldofthecable.Theshieldwillprovideprotectionfromelectricfieldinterference.Thetwistedcablehasasmallareabetweenthewires.Thesmallerthearea,thelessthemagneticfieldpickup.Thereisonedisadvantagetoreducingtheareabetweentheconnectors.Thecapacitancetogroundisinverselyrelatedtotheseparationdistancebetweenthewires.Thusastheareadecreases,thecapacitancewillincrease.Thisincreasedcapacitiveloadwilllimitboththedistanceandthebaudrate.

DB25Pin

RS232Name

DB9Pin

EIA-574

Name

RJ45Pin

EIA-561

Name

Signal Description True DTE DCE

1 FG FrameGround/Shield

2 BA 3 103 6 103 TxD TransmitData -5V out in

3 BB 2 104 5 104 RxD ReceiveData -5V in out

4 CA 7 105/133 8 105/133 RTS RequesttoSend

+5V out in

5 CB 8 106 7 106 CTS CleartoSend +5V in out

6 CC 6 107 DSR DataSetReady +5V in out

7 AB 5 102 4 102 SG SignalGround

8 CF 1 109 2 109 DCD DataCarrierDetect

+5V in out

9 PositiveTestVoltage

10 NegativeTestVoltage

11 NotAssigned

12 sDCD secondaryDCD

+5V in out

13 sCTS secondaryCTS +5V in out

14 sTxD secondaryTxD -5V out in

15 DB TxC TransmitClk(DCE)

in out

16 sRxD secondaryRxD -5V in out

17 DD RxC ReceiveClock in out

18 LL LocalLoopback

19 sRTS secondaryRTS +5V out in

20 CD 4 108 3 108 DTR DataTerminalRdy

+5V out in

21 RL SQ SignalQuality +5V in out

22 CE 9 125 1 125 RI RingIndicator +5V in out

23 SEL SpeedSelectorDTE

in out

24 DA TCK SpeedSelectorDCE

out in

25 TM TM Testmode +5V in out

Table7.2.PinassignmentsfortheRS232EIA-574andEIA-561protocols.

Thesignalgroundisconnectedonbothsidestothesupplyreturn.Aseparatewireshouldbeusedforsignalground(donotusethegroundshieldtoconnectthetwosignalgrounds.)Thenoiseimmunitywillbedegradedifthegroundshieldisconnectedonbothsides.TherearemanyavailableRS232driverchips(e.g.,MaximMAX3232).Thesechipsemployachargepump(usingthe100nFcapacitors)tocreatethestandard+5and-5outputvoltageswithonlya+3.3Vsupply.ThefollowingoverviewstheRS232specificationsforanoutputsignal

MustbeabletowithstandashorttogroundorshorttoanotherwireTruevoltageisbetween-5and-15V(operatingrange)Falsevoltageisbetween+5and+15V(operatingrange)Shortcircuitcurrentlessthan0.5ASpendslessthan4%ofthetimeinthetransitionrangefrom-3to+3V

Similarly,thefollowingoverviewstheRS232specificationsforaninputsignal

Themaximumslewrateis30V/ sTrueinputis-3to-15VFalseinputis+3to+15VThetransitionrangeis-3to+3VTheinputimpedancebetween3000and7000Theinputcapacitanceincludingcablemustbelessthan2500pFTheinputopencircuitvoltagemustbelessthan2V

Onthedatalines,atrueormarkvoltageisnegative.Converselyforthecontrollines,atruesignalhasapositivevoltage.RequesttoSend(RTS)isasignalfromthecomputertotheMODEMrequestingtransmissionbeallowed.CleartoSend(CTS)istheacknowledgesignalbackfromtheMODEMsignifyingtransmissioncanproceed.DataSetReady(DSR)isaMODEMsignalspecifyingthatinput/outputcanoccur.DataCarrierDetect(DCD)isamodemsignalspecifyingthatthecarrierfrequencieshavebeenestablishedonitstelephoneline.RingIndicator(RI)isamodemsignalthatistruewhenthephonerings.TheReceiveClockandTransmitClockareusedtoestablishsynchronousserialcommunication.DataTerminalReady(DTR)isaprintersignalspecifyingthestatusoftheprinter.WhenDTRis+5V,theprinterisreadyandcanacceptmorecharacters.ConverselywhenDTRis-5V,theprinterisbusyandcannotacceptmorecharacters.Mostmicrocontrollersdonotsupportthesecontrollinesexplicitly.Ontheotherhand,itisstraightforwardtoimplementthesehardwarehandshakingsignalsusingsimpleinput/outputlines.Ifwewishtogenerateinterruptsonedgesoftheselines,thenwewoulduseedge-triggeredinputfeatures.

Example7.1.DesignasimplexprinterdriverwithDTRsynchronization.

Solution:Oneproblemwithprintersisthattheprinterbandwidth(theactualnumberofcharacterspersecondthatcanbeprinted)maybelessthanthemaximumbandwidthsupportedbytheserialchannel.Therearefiveconditionsthatmightleadtoasituationwherethecomputeroutputsserialdatatotheprinter,buttheprinterisn’treadytoacceptthedata.First,specialcharactersmayrequiremoretimetoprint(e.g.,carriagereturn,linefeed,tab,formed,andgraphics).Second,mostprintershaveinternalFIFOsthatcouldgetfull.IftheFIFOisnotfull,thenitcanacceptdataasfastasthechannelwillallow,butwhentheFIFObecomesfull,thecomputershouldstopsendingdata.Third,theprintercablemaybedisconnected.Fourth,theprintermaybedeselectedorattachedtodifferentcomputer.Fifth,theprinterpowermaybeoff.Theoutputinterfacesshownpreviouslyprovidenofeedbackfromtheprinterthatcouldbeusedtodetect/correctthesefiveproblems.Therearetwomechanisms,calledflowcontrol,tosynchronizethecomputerwithavariablerateoutputdevice.ThesetwoflowcontrolprotocolsarecalledDTRandXON/XOFF.

Thefirstmethodusesahardwaresignal,DTR(pin4ontheDB9connector),asfeedbackfromtheprintertothemicrocontroller,seeFigure7.8.DTRis–5Viftheprinterisbusyandisnotcurrentlyabletoaccepttransmission.DTRis+5Viftheprinterisreadyandabletoaccepttransmission.Thismechanismcanhandleallfiveoftheaboveconditionswheretheprinterisnotready.ThecomputerinputmechanismwillhandletheDTRprotocolusingadditionalsoftwarechecking.

WithastandardRS232interfacewhenDTRis–5V,PC4willbe+3.3V(whichwillstoptheoutputeveniftheUARTtransmitFIFOisnotfull).Thus,whenDTRis–5V,transmissionistemporarilysuspended.WhenDTRis+5V,PC4willbe0Vandtransmissioncanproceednormally.Inthisdesign,edge-triggeredinputonPC4willbeusedtodetectchangesintheDTRsignal.

TheDTRsignalfromtheprinterprovidesfeedbackinformationabouttheprinterstatus.WhenDTRis‑5V(PC4ishigh),theprinterisnotreadytoacceptmoredata.Inthiscase,ourcomputerwillpostponetransmittingmoreframes.WhenDTRis+5V(PC4islow),theprinterisreadytoacceptmoredata.Atthispoint,thecomputerwillresumetransmission.The Printer_OutChar(data) inProgram7.1iscalledbythemainprogramtoprint.

Figure7.8.HardwareinterfaceimplementingaRS232simplexchannelwithDTRhandshaking.

voidPrinter_Init2(void)

SYSCTL_RCGCUART_R|=0x01;//activateUART0

SYSCTL_RCGCGPIO_R|=0x05;//activateportsA,C

TxFifo_Init();//initializeemptyFIFOs

GPIO_PORTC_DIR_R&=~0x10;//makePC4in

GPIO_PORTC_DEN_R|=0x10;//enabledigitalI/OonPC4

GPIO_PORTC_IS_R&=~0x10;//PC4isedge-sensitive

GPIO_PORTC_IBE_R|=0x10;//PC4isbothedges

GPIO_PORTC_ICR_R=0x10;//clearflag4

GPIO_PORTC_IM_R|=0x10;//enableinterruptonPC4

UART0_CTL_R&=~UART_CTL_UARTEN;//disableUART

UART0_IBRD_R=43;//IBRD=int(80000000/(16*115,200))=int(43.40278)

UART0_FBRD_R=26;//FBRD=round(0.40278*64)=26

UART0_LCRH_R=0x0070;//8-bitwordlength,enableFIFO

UART0_IFLS_R&=~0x07;//clearTXinterruptFIFOlevelfield

UART0_IFLS_R+=UART_IFLS_TX1_8;//interruptforTXFIFO<=1/8full

UART0_IM_R|=UART_IM_TXIM;//enableTXFIFOinterrupt

UART0_CTL_R|=UART_CTL_UARTEN;//enableUART

GPIO_PORTA_PCTL_R=(GPIO_PORTA_PCTL_R&0xFFFFFF00)+0x00000011;//UART

GPIO_PORTA_AMSEL_R&=~0x03;//disableanalogonPA1-0

GPIO_PORTA_AFSEL_R|=0x03;//enablealtfunctonPA1-0

GPIO_PORTA_DEN_R|=0x03;//enabledigitalI/OonPA1-0

NVIC_PRI0_R=(NVIC_PRI0_R&0xFF00FFFF)|0x00400000;//priority2

NVIC_PRI1_R=(NVIC_PRI1_R&0xFFFF00FF)|0x00004000;//priority2

NVIC_EN0_R=(NVIC_EN0_INT2|NVIC_EN0_INT5);

EnableInterrupts();

//checkthestatusoftheprinteronPC4

//highmeansnotready

voidcheckStatus(void)

if(GPIO_PORTC_DATA_R&0x10)//PC4=1ifDTRis-5to-15

UART0_CTL_R&=~UART_CTL_TXE;//busy,sodisabletransmitter

else

UART0_CTL_R|=UART_CTL_TXE;//notbusy,soenabletransmitter

//copyfromsoftwareTXFIFOtohardwareTXFIFO

//stopwhensoftwareTXFIFOisemptyorhardwareTXFIFOisfull

voidcopySoftwareToHardware(void)

charletter;

while(((UART0_FR_R&UART_FR_TXFF)==0)&&(TxFifo_Size()>0))

TxFifo_Get(&letter);

UART0_DR_R=letter;

//outputASCIIcharactertoPrinter

//spinifTxFifoisfull

voidPrinter_OutChar(chardata)

while(TxFifo_Put(data)==FIFOFAIL);

checkStatus();

UART0_IM_R&=~UART_IM_TXIM;//disableTXFIFOinterrupt

copySoftwareToHardware();

UART0_IM_R|=UART_IM_TXIM;//enableTXFIFOinterrupt

//interruptwhenhardwareTXFIFOgoesfrom3to2orlessitems

voidUART0_Handler(void)

if(UART0_RIS_R&UART_RIS_TXRIS)//hardwareTXFIFO<=2items

UART0_ICR_R=UART_ICR_TXIC;//acknowledgeTXFIFO

//copyfromsoftwareTXFIFOtohardwareTXFIFO

copySoftwareToHardware();

if(TxFifo_Size()==0)//softwareTXFIFOisempty

UART0_IM_R&=~UART_IM_TXIM;//disableTXFIFOinterrupt

//interruptonanychangeofthestatusoftheprinteronPC4

voidGPIOPortC_Handler(void)

GPIO_PORTC_ICR_R=0x10;//acknowledgeflag4

checkStatus();

Program7.1.SoftwareimplementationofaprinterinterfacewithDTRsynchronization(SyncPrinter_xxx.zip).

7.3.RS422/USB/RS423/RS485BalancedDifferentialLinesInordertoincreasethebaudrateandmaximumdistance,thebalanceddifferentiallineprotocolswereintroduced.TheRS422signalisencodedinadifferentialsignal,A-B.TherearemanyRS422interfacechips,forexample,SP301orMAX13433.AfullduplexRS422channelisimplementedinFigure7.9withMAX13433drivers.Oftenterminatingresistorsareplacedattheendsoflongcables.E.g.,wecanplacea100 resistoratacrosstheA’B’inputs.

Figure7.9.RS422serialchannel.Becauseeachsignalrequires2wires,5wires(groundincluded)areneededtoimplementafullduplexchannel.WithRS232onetypicallyconnectsonereceivertoonetransmitter.ButwithRS422,upto10receiverscanbeconnectedtoonetransmitter.Table7.3summarizesfourcommonEIAstandards.

Specification RS232D RS423A RS422 RS485

Modeofoperation single-ended

single-ended

differential differential

Driversononeline 1 1 1 32

Receiversononeline

1 10 10 32

Maximumdistance(feet)

50 4,000 4,000 4,000

Maximumdatarate 20kb/s 100kb/s 10Mb/s 10Mb/s

Maximumdriveroutput

±25V ±6V -0.25to+6V

-7to+12V

±5V ±3.6V ±2V ±1.5V

DriverOutput(loaded)

DriverOutput(unloaded)

±15V ±6V ±5V ±5V

DriverLoadImpedance

3kΩto7kΩ 450Ωmin 100Ω 54Ω

Receiverinputvoltage

±15V ±12V ±7V -7to+12V

Receiverinputsensitivity

±3V ±200mV ±200mV ±200mV

Receiverinputresistance

3kΩto7kΩ 4kΩmin 4kΩmin 12kΩmin

Table7.3.SpecificationsfortheRS232RS423ARS422andRS485protocols.

Themaximumbaudrateat40feetis10Mbps.At4000feet,thebaudratecanonlybeashighas100Kbps.Table7.4showstwoimplementationsoftheRS422protocol.

DB25Pin

EIA-530Name

DB37Pin

RS449

Name

Signal Description DTE DCE

1 1 FG FrameGround/Shield

2 BA(A) 4 SD(A) TxD TransmitData out in

14 BA(B) 22 SD(B) TxD TransmitData out in

3 BB(A) 6 RD(A) RxD ReceiveData in out

16 BB(B) 24 RD(B) RxD ReceiveData in out

4 CA(A) 7 RS(A) RTS RequesttoSend out in

19 CA(B) 25 RS(B) RTS RequesttoSend out in

5 CB(A) 9 CS(A) CTS CleartoSend in out

13 CB(B) 27 CS(B) CTS CleartoSend in out

6 CC(A) 11 DM(A)

DSR DataSetReady in out

22 CC(B) 29 DM(B)

DSR DataSetReady in out

20 CD(A)

12 TR(A) DTR DataTerminalRdy

out in

23 CD(B) 30 TR(B) DTR DataTerminalRdy

out in

7 AB 19 SG SG SignalGround

8 CF(A) 13 RR(A) DCD DataCarrierDetect

in out

10 CF(B) 31 RR(B) DCD DataCarrierDetect

in out

15 DB(A)

5 ST(A) TxC TransmitClk(DCE)

in out

12 DB(B) 23 ST(B) TxC TransmitClk(DCE)

in out

17 DD(A)

8 RT(A) RxC ReceiveClock in out

9 DD(B)

26 RT(B) RxC ReceiveClock in out

18 LL 10 LL LocalLoopback out in

21 RL 14 RL RL RemoteLoopback

out in

24 DA(A)

17 TT(A) TCK SpeedSelectorDCE

out in

11 DA(B) 35 TT(B) TCK SpeedSelectorDCE

out in

25 TM 18 TM TM Testmode in out

Table7.4.PinassignmentsfortheEIA-530andRS449protocols.

InthissectionwewillintroducetheelectricalspecificationsoftheUniversalSerialBus(USB).Figure7.10showsthetwotypesofUSBconnectors.AsinglehostcomputercontrolstheUSB,andtherecanonlybeonehostperbus.Thehostcontrolstheschedulingofalltransactionsusingatoken-basedprotocol.TheUSBarchitectureisatieredstartopology,similarto10BaseTEthernet.Thehostisatthecenterofthestar,devicesareattachedtothehost.ThenumberofnodesonthebuscanbeextendedusingUSBhubs.Upto127devicescanbeconnectedtoanyoneUSBbusatanyonegiventime.USBplug’n’plugisimplementedwithdynamicallyloadableandunloadabledrivers.WhentheuserplugsthedeviceintotheUSBbus,thehostwilldetecttheconnection,interactwiththenewlyinserteddevice,andloadtheappropriatedriver.TheUSBdevicecanbeusedwithoutexplicitlyinstallingdriversorrebooting.Whenthedeviceisunplugged,thehostwillautomaticallyunloaditsdriver.

Figure7.10.USBconnectors.

USBusesfourshieldedwires,+5Vpower,GND,andatwistedpairdifferentialdatasignals,aslistedinTable7.5.ItusesaNRZI(NonReturntoZeroInvert)encodingschemetosenddatawithasyncfieldtosynchronizethehostandreceiverclocks.TheD+signalhasa15k pull-downresistortoground,andtheD-signalhasa1.5k pull-upresistorto+3.6V.Liketheotherprotocolsinthissection,thedataisencodedasadifferentialsignalbetweenD+andD-.Ingeneral,adifferential‘1’existswhenD+isgreaterthanD-.Morespecifically,adifferential‘1’istransmittedbypullingD+over2.8VandD-under0.3V.Thetransmittercreatesadifferential‘0’bymakingD-greaterthan2.8VandD+lessthan0.3V.Thereceiverrecognizesthedifferential‘1’whenD+is0.2VgreaterthanD-.Thereceiverwillconsidertheinputasadifferential‘0’whenD+0.2VlessthanD-.Thepolarityofthesignalisinverteddependingonthespeedofthebus.Thereforetheterms‘J’and‘K’statesareusedinsignifyingthelogiclevels.Atlowspeed,a‘J’stateisadifferential‘0’.Athighspeed,a‘J’stateisadifferential‘1’.USBinterfacesemploybothdifferentialandsingleendedoutputs.CertainbusstatesareindicatedbysingleendedsignalsonD+,D-orboth.Forexample,asingleendedzero(SE0)signifiesdeviceresetwhenheldformorethan10mS.Morespecifically,SE0isgeneratedbyholdingbothD-andD+low(<0.3V).USBcanoperateatthreespeeds.Thelowspeed/fullspeedbushasacharacteristicimpedanceof90 .HighSpeedmodeusesaconstantcurrentprotocoltoreducenoise.

Highspeeddataisclockedat480Mb/s

Fullspeeddataisclockedat12Mb/s

Lowspeeddataisclockedat1.5Mb/s

PinNumber

Color Function PinNumber

Function

1 Red VBUS(5V)

1 VBUS(5V)

2 White D- 2 D-

3 Green D+ 3 D+

4 Black Ground 4 ID

5 Ground

Table7.5.USBsignals.

7.3.1.RS422OutputSpecificationsTable7.6showstheoutputvoltagelevelsforRS422.

OutputVoltage

TrueorMark

-6≤A-B≤-2V

Transition -2≤A-B≤+2V

FalseorSpace

+2≤A-B≤+6V

Table7.6.OutputvoltagelevelsfortheRS422differentiallineprotocol.

AkeyRS422specificationisthattheoutputimpedanceoftheAandBoutputsshouldbebalanced.Iftheinput/outputimpedancesarebalancedthenaddednoiseinthecablecreatesacommonmodevoltage,andthecommonmoderejectionoftheinputwilleliminateit.

RAout=RBout≤100

Thetimeinthetransitionregionmustbelessthan

10%forbaudratesabove5Mbps,and

20nsforbaudratesbelow5Mbps

7.3.2.RS422InputSpecificationsLetA’andB’bethevoltagesattheinput.Table7.7definestheinputencoding.

InputVoltage

TrueorMark A’-B’≤-0.2V

Transition -0.2V≤A’-B’≤+0.2V

FalseorSpace +0.2V≤A’-B’

Table7.7.InputvoltagethresholdsfortheRS422differentiallineprotocol.

Asmentionedearlier,toprovidenoiseimmunitythecommonmodeinputimpedancesmustalsobebalanced

4kΩ≤RA’in=RB’in

Thebalancednatureoftheinterfaceproducesgoodnoiseimmunity.ThedifferentialinputimpedanceisspecifiedbyFigure7.11.Anypointwithintheshadedregionisallowed.

Figure7.11.RS422inputcurrentversusinputvoltagerelationship.EventhoughthegroundconnectionintheRS422cableisoptional,itisassumedthegroundsareconnectedsomewhere.Inparticular,theinterfacewilloperatewithacommonmodevoltageupto7volts.

|A’+B’|/2≤+7V

Checkpoint7.5:Whatistheeffectofcapacitanceonaserialchannel?

7.3.3.RS485HalfDuplexchannelRS485canbeeitherhalfduplexorfullduplex.TheRS485protocol,illustratedinFigure7.12,implementsahalfduplexchannelusingdifferentialvoltagesignals.TheSipexSP483orMaximMAX13431implementsthehalfduplexRS485channel.OneoftheadvantagesofRS485isthatupto32devicescanbeconnectedontoasingleserialbus.Whenmorethanonetransmittercandrivertheserialbus,theprotocolisalsocalled“multi-drop”.Normally,wemakebothDEandREactiveonalldevices.TotransmitthecomputersendstheserialframefromtheTxDoutputoftheUARTport.ThetransmittedframeisechoedintotheserialreceiveroftheSCIRxDline.ToreceiveaframethecomputeracceptsaserialframeontheRxDlineintheusualmanner.Whencarefulwhenselectingtheresistancesonahalfduplexnetworksothatthetotaldriverimpedanceisabout54Ω.

Figure7.12.AhalfduplexserialchannelisimplementedwithRS485logic.

RS422,RS485,Ethernet,andCANarehigh-speedcommunicationchannels.ThismeansthebandwidthandslewrateonthesignalsarehigherthanRS232.Thereisacorrespondencebetweenrisetime( )ofadigitalsignalandequivalentsinusoidalfrequency(f).ThederivativeofA∙sin(2 ft)is2 f∙A∙cos(2 ft).Themaximumslewrateofthissinusoidis2 f∙A.ApproximatingtheslewrateasA/ ,wegetacorrespondencebetweenfand

f=1/

Forexample,iftherisetimeis5ns,theequivalentfrequencyis200MHz.Noticethatthisequivalentfrequencyisindependentofbaudrate.Soevenat1000bits/sec,iftherisetimeis5ns,thenthesignalhasastrong200MHzfrequencycomponent!Todealwiththisissue,theRS232protocollimitstheslewratetoamaximumof30V/ s.Thismeansitwilltakeabout1 sforasignaltorisefrom-12to+12V.Consequently,RS232signals

havefrequencycomponentslessthan1MHz.However,totransmitfasterthanRS232,theprotocolmusthavefasterrisetimes.Electricalsignalstravelatabout0.6to0.9timesthespeedoflight.Thisvelocityfactor(VF)isapropertyofthecable.Forexample,VFforRG-6/Ucoaxcableis0.75,whereasVFisonly0.66forRG-58/Ucoaxcable.Usingtheslower0.66estimate,thespeedisv=2∙108m/s.Accordingtowavetheory,thewavelengthis =v/f.Estimatingthefrequencyfromrisetime,weget

λ=v*τ

Inourexample,arisetimeof5nsisequivalenttoawavelengthofabout1m.Asaruleofthumb,wewillconsiderthechannelasatransmissionlineifthelengthofthewireisgreaterthan /4.Anotherrequirementisforthediameterofthewiretobemuchsmallerthanthewavelength.Inatransmissionline,thesignalstraveldownthewiresaswavesaccordingtothewaveequation.Analysisofthewaveequationisoutsidethescopeofthisbook.However,youneedtoknowthatwhenawavemeetsachangeinimpedance,someoftheenergywilltransmit(agoodthing)andsomeoftheenergywillreflect(abadthing).Reflectionsareessentiallynoiseonthesignal,andiflargeenough,theywillcausebiterrorsintransmission.Wecanreducethechangeinimpedancebyplacingterminatingresistorsonbothendsofalonghigh-speedcable,asshowninFigure7.12.Theseresistorsreducereflections;hencetheyimprovesignaltonoiseratio.

7.4.LogicLevelConversionTherearemany3.3-Vdeviceswewishtointerfacetoa5-Vmicrocontroller,andtherearemany5-Vdeviceswewishtointerfacetoa3.3-Vmicrocontroller.Thissectionwillstudyvariousmethodstoconvertonelogicleveltoanother.Webeginwitha5-Voutputinterfacedtoa3.3-Vinput.Many3.3-Vinputsare5-Vtolerant,whichmeansnospecialinterfacecircuitsarerequired.Oneofthesimplestwaystoconvert5-Vlogicinto3.3-VlogicistousearesistordividerasshowninFigure7.13.ASchottkydiodecanalsobeusedtoconvert5Vinto3.3V,andconverta0.4Vintoa0.5V.TheSchottkydiodemustbefastandhavealowvoltagedrop.The7407isanotherwaytoconvertbetweenlogicfamilies.Whenthe7407inputis5V,itsoutputfloats,andthe3.3-Vpull-upmakesa3.3-Vsignal.Whenthe7407inputislow,itsoutputislow.

Many5Vinputsare3.3Vtolerant,whichmeansnospecialinterfacecircuitsarerequired.The7407canalsobeusedtointerface3.3-Vlogicinto5-Vlogic.TheVIHofthe7407is2V,sowhenthe7407inputis3.3V,itsoutputfloats,andthe5-Vpull-upmakesa5-Vsignal.Whenthe7407inputislow,itsoutputislow.AMOSFET,liketheBSS138,isapopularmethodtoconvertlogiclevelsbecauseitisfastandefficient.SparkFunmakesabreakoutboardwithresistor-dividerandBSS138circuits(www.sparkfun.comBOB-0874).

WecanproducethesameopencollectorbehaviorofanyI/Oportthathasadirectionregister.Weinitializetheportbywritingazerotothedataport.Onsubsequentaccessestotheopencollectorport,wewritethecomplementtothedirectionregister.I.e.,ifwewanttheI/Oportbittodrivelow,wesetthedirectionregisterbitto1,andifwewanttheI/Oportbittofloat(opencollector),wesetthedirectionregisterbitto0.

Itisgooddesignpracticetoreadtheerrataforthemicrocontrollersinyoursystem.Forexample,theLM3S811erratastates,“GPIObuffersarenot5-Vtolerantwhenusedinopen-drainmode.Pullinguptheopen-drainpinabove4Vresultsinhighcurrentdraw.”Furthermore,“ThepinsassociatedwithGPIOsignalsPB6,PC5,andPC6arenot5-Vtolerant.ApplyingavoltagetoanyofthesepinsthatisgreaterthanVDD(3.3V)willhaveundeterminedresults.”TheseparticularmistakesapplyspecificallytosiliconversionC0fortheLM3S811.However,ingeneralonemustreadtheerrataforthemicrocontrolleryouareusing.

Figure7.13.Circuitstointerfacebetween5-Vlogicand3.3-Vlogic.

7.5.SynchronousTransmissionandReceivingusingtheSSISSIallowsmicrocontrollerstocommunicatesynchronouslywithperipheraldevicesandothermicrocontrollers.TheSSIsystemcanoperateasamasterorasaslave.Thechannelcanhaveonemasterandoneslave,oritcanhaveonemasterandmultipleslaves.Withmultipleslaves,theconfigurationcanbeastar(centralizedmasterconnectedtoeachslave),oraring(eachnodehasonereceiverandonetransmitter,wherethenodesareconnectedinacircle.)Themasterinitiatesalldatacommunication.

StellarisandTivamicrocontrollershave0to4SynchronousSerialInterfaceorSSImodules.AnothernameforthisprotocolisSerialPeripheralInterfaceorSPI.ThefundamentaldifferencebetweenaUART,whichimplementsanasynchronousprotocol,andaSSI,whichimplementsasynchronousprotocol,isthemannerinwhichtheclockisimplemented.Twodevicescommunicatingwithasynchronousserialinterfaces(UART)operateatthesamefrequency(baudrate)buthavetwoseparateclocks.WithaUARTprotocol,theclocksignalisnotincludedintheinterfacecablebetweendevices.TwoUARTdevicescancommunicatewitheachotheraslongasthetwoclockshavefrequencieswithin±5%ofeachother.Twodevicescommunicatingwithsynchronousserialinterfaces(SSI)operatefromthesameclock(synchronized).WithaSSIprotocol,theclocksignalisincludedintheinterfacecablebetweendevices.Typically,themasterdevicecreatestheclock,andtheslavedevice(s)usestheclocktolatchthedatainandsenddataout.

TheSSIprotocolincludesfourI/Olines.TheslaveselectSSI0Fssisanoptionalnegativelogiccontrolsignalfrommastertoslavesignalsignifyingthechannelisactive.Thesecondline,SCK,isa50%dutycycleclockgeneratedbythemaster.TheSSI0Tx(masteroutslavein,MOSI)isadatalinedrivenbythemasterandreceivedbytheslave.TheSSI0Rx(masterinslaveout,MISO)isadatalinedrivenbytheslaveandreceivedbythemaster.Inordertoworkproperly,thetransmittingdeviceusesoneedgeoftheclocktochangeitsoutput,andthereceivingdeviceusestheotheredgetoacceptthedata.Figure7.14showstheI/Oportlocationsofthesynchronousserialportsforthethreemicrocontrollersdiscussedinthisbook.Tables2.7and2.8describehowtoattachI/OpinstotheSSImodulesontheTM4C123/TM4C1294.

Figure7.14.SynchronousserialportpinsonfourLM3S/TM4Cmicrocontrollers.OntheLM3S/TM4Ctheshiftregistercanbeconfiguredfrom4to16bits.Theshiftregisterinthemasterandtheshiftregisterintheslavearelinkedtoformadistributedregister.Figure7.15illustratescommunicationbetweenmasterandslave.Typically,themicrocontrollerandtheI/Odeviceslavearesophysicallyclosewedonotuseinterfacelogic.

TheSSIontheLM3S/TM4CemploystwohardwareFIFOs.BothFIFOsare8elementsdeepand4to16bitswide,dependingontheselecteddatawidth.WhenperformingI/OthesoftwareputsintothetransmitFIFObywritingtothe SSI0_DR_R registerandgetsfromthereceiveFIFObyreadingfromthe SSI0_DR_R register.

IfthereisdatainthetransmitFIFO,theSSImodulewilltransmitit.WithSSIittransmitsandreceivesbitsatthesametime.Whenadatatransferoperationisperformed,thisdistributed8-to32-bitregisterisseriallyshifted4to16bitpositionsbytheSCKclockfromthemastersothedataiseffectivelyexchangedbetweenthemasterandtheslave.Datainthemastershiftregisteraretransmittedtotheslave.Dataintheslaveshiftregisteraretransmittedtothemaster.Typically,themicrocontrollerismasterandtheI/Omoduleistheslave,butonecanoperatethemicrocontrollerinslavemode.WhendesigningwithSSI,youwillneedtoconsultthedatasheetsforyourspecificmicrocontroller.

Figure7.15.AsynchronousserialinterfacebetweenamicrocontrollerandanI/Odevice.Table7.8liststheSSI0registersontheLM3S/TM4C.TheLM3S/TM4Ccanoperateinslavemode,butwewillfocusonmastermode.ThePCTLbitsaredefinedinTables4.3and4.4.

Address 31-6 3 2 1 0 Name

$400F.E61C SSI3 SSI2 SSI1 SSI0 SYSCTL_RCGCSSI_R

31-16 15-8 7 6 5-4 3-0

$4000.8000 SCR SPH SPO FRF DSS SSI0_CR0_R

31-16 15-0

$4000.8008 Data SSI0_DR_R

7 6 5 4 3 2 1 0

$4000.8004 SOD MS SSE LBM SSI0_CR1_R

$4000.800C BSY RFF RNE TNF TFE SSI0_SR_R

$4000.8010 CPSDVSR SSI0_CPSR_R

$4000.8014 TXIM RXIM RTIM RORIM SSI0_IM_R

$4000.8018 TXRIS RXRIS RTRIS RORRIS SSI0_RIS_R

$4000.801C TXMIS RXMIS RTMIS RORMIS SSI0_MIS_R

$4000.8020 RTIC RORIC SSI0_ICR_R

$4005.8420 SEL SEL SEL SEL SEL SEL SEL SEL GPIO_PORTA_AFSEL_R

$4005.841C DEN DEN DEN DEN DEN DEN DEN DEN GPIO_PORTA_DEN_R

$4005.8400 DIR DIR DIR DIR DIR DIR DIR DIR GPIO_PORTA_DIR_R

$400F.E608 GPIOH GPIOG GPIOF GPIOE GPIOD GPIOC GPIOB GPIOA SYSCTL_RCGCGPIO_R

Table7.8.TheLM3S/TM4CSSI0registers.Eachregisteris32bitswide.Bits31–8arezero.

TheSSIclockfrequencyisestablishedbythe8-bitfieldSCRfieldinthe SSI0_CR0_Rregisterandthe8-bitfieldCPSDVSRfieldinthe SSI0_CPSR_R register.SCRcanbeany8‑bitvaluefrom0to255.CPSDVSRmustbeanevennumberfrom2to254.LetfBUSbethefrequencyofthebusclock.ThefrequencyoftheSSIis

fSSI=fBUS/(CPSDVSR*(1+SCR))

CommoncontrolfeaturesfortheSSImoduleinclude:

Baudratecontrolregister,usedtoselectthetransmissionrate

Datasize4to16bits,set4-bitDSSfieldequaltosize-1

Modebitsinthecontrolregistertoselect

Masterversusslave

Freescalemodewithclockpolarityandclockphase

TIsynchronousserialmode

Microwiremode

Interruptarmbit

Abilitytomaketheoutputsopendrain(opencollector)

CommonstatusbitsfortheSPImoduleinclude:

BSY,SSIiscurrentlytransmittingand/orreceivingaframe,

orthetransmitFIFOisnotempty

RFF,SSIreceiveFIFOisfull

RNE,SSIreceiveFIFOisnotempty

TNF,SSItransmitFIFOisnotfull

TFE,SSItransmitFIFOisempty

Thekeytopropertransmissionistoselectoneedgeoftheclock(shownas“T”inFigure7.16)tobeusedbythetransmittertochangetheoutput,andusetheotheredge(shownas“R”)tolatchthedatainthereceiver.Inthiswaydataislatchedduringthetimewhenitisstable.Dataavailableisthetimewhentheoutputdataisactuallyvalid,anddatarequiredisthetimewhentheinputdatamustbevalid.

Duringtransmission,theoutputdatawillbevalidfromS5maxaftertheclockedgeuntilS5minafterthenextclockedge.ThemaximumS5timeis1systembusperiod(e.g.,20ns)andtheminimumis0.Whenreceivingthesetuptime(S8)is1systembusperiodandtheholdtime(S9)is2systembusperiods.Inorderforthecommunicationtooccurwithouterror,thedataavailablefromthedevicethatisdrivingthedatalinemustoverlap(startbeforeandendafter)thedatarequiredbytheotherdevicethatisreceivingthedata.Itisthisoverlapthatwilldeterminethemaximumfrequencyatwhichsynchronousserialcommunicationcanoccur.TheconceptsofdataavailableanddatarequiredwerepresentedpreviouslyinSection4.2.

Figure7.16.Synchronousserialtimingshowingthedataavailableintervaloverlapsthedatarequiredinterval.Checkpoint7.6:Whatarethedefinitionsofsetuptimeandholdtime?

Observation:Becausetheclocksareshared,ifyouchangethebusclockfrequency,thetransferratewillchangeinbothmasterandslave.

TheFreescaleSPItimingisshowninFigure7.17(FRF=00).

Figure7.17.SynchronousserialFreescalesingletransfermode(nis4to16bits).TheSPItransmitsdataatthesametimeasitreceivesinput.IntheFreescalemodes,theSPIchangesitsoutputontheoppositeedgeoftheclockasitusestoshiftdatain.Therearethreemodecontrolbits(MS,SPO,SPH)thataffectthetransmissionprotocol.Ifthedeviceisamaster(MS=0)itgeneratestheSCLK,anddataisoutputontheSSI0Txpin,andinputontheSSI0Rxpin.TheSPOcontrolbitspecifiesthepolarityoftheSCLK.Inparticular,theSPObitspecifiesthelogicleveloftheclockwhendataisnotbeingtransferred.TheSPHbitaffectsthetimingofthefirstbittransferredandreceived.IfSPHis0,thenthedevicewillshiftdatainonthefirst(and3rd,5th,7th,…etc.)clockedge.IfSPHis1,thenthedevicewillshiftdatainonthesecond(and4th,6th,8th,…etc.)clockedge.ThedataistransmittedMSBfirst.

TheTIsynchronousserialtimingisshowninFigure7.18(FRF=01).ThereisathirdprotocolcalledMicrowire(FRF=10).Refertothedatasheetsfordetailsofthesemodes.

Figure7.18.SynchronousserialTIsingletransfermode.

Example7.2.InterfacetheMaximMAX5353digitaltoanalogconverter.

Solution:ThisfirstexampleshowsasynchronousserialinterfacebetweenthemicrocontrollerandaMaximMAX535312-bitdigitaltoanalogconverterasdrawninFigure7.19.Adigitaltoanalogconverter(DAC)acceptsadigitalinput(inourcaseanumberbetween0and4095)andcreatesananalogoutput(inourcaseavoltagebetween0andVREF*GAIN.)DetaileddiscussionofconverterswillbepresentedlaterinChapter8.Hereinthissectionwewillfocusonthedigitalhardwareandsoftwareaspectsoftheserialinterface.

Figure7.19.A12-bitDACinterfacedtotheSSIport.

Table7.9andFigure7.20describetheprotocol.Thefirst3bitssentwillbezero,thenthe12databitsthatspecifytheanalogoutput,andthenonemorezerowillbesent.TheSSI0Fsscontrolsignalwillbelowduringthe16-bittransmission.AswithanySPIinterface,therearebasicinterfacingissuestoconsider.

Wordsize.Inthiscaseweneedtotransmit16bitstotheDAC.TheMAX5353datasheetspecifiesthatthefirstthreebitsarecommandcodes,thenext12bitsaretheDACoutput(MSBtransmittedfirst),andthelastbitiszero.Bitorder.TheMAX5353requiresthemostsignificantbitsfirst.

Clockphase,clockpolarity.Therearetwoissuestoresolve.SincetheMAX5353samplesitsserialinputdataontherisingedgeoftheclock,theSSImustchangethedataonthefallingedge.SPO=SPH=0(Figure7.17)andSPO=SPH=1bothsatisfythisrequirement.Thesecondissueiswhichedgecomesfirsttheriseorthefall.Inthisinterfaceitprobablydoesn’tmatter.

Bandwidth.WelookatthetimingspecificationsoftheMAX5353.Theminimumclocklowwidthof40nsmeanstheshortestSSIperiodwecanuseis100ns.Thecommandsare:

C2 C1 C0 D11:D0

MSBLSB

S0 Description

X 0 0 12bitsofdata 0 Loadinputregister;DACregisterimmediatelyupdated.

X 0 1 12bitsofdata 0 Loadinputregister;DACregisterunchanged.

X 1 0 XXXXXXXXXXXX X UpdateDACregisterfrominputregister.

1 1 1 XXXXXXXXXXXX X Shutdown

0 1 1 XXXXXXXXXXXX X Nooperation

Table7.9.MAX5353protocols

Figure7.20.MAX5353DACserialtiming.

TheritualinitializestheFreescaleSPImastermode,16-bitdata,and8MHzbandwidth(Program7.2).TochangetheDACoutput,one16-bittransmissionissent( DAC_Out ).ThedatareturnedinthiscaseisnotsignificantbecausetheSSI0RxpininFigure7.19isleftnotconnected.Inthisexample,thebusclockis16MHz,andtheSSIclockwillbe8MHz.

voidDAC_Init(uint16_tdata)

SYSCTL_RCGCSSI_R|=0x01;//activateSSI0

SYSCTL_RCGCGPIO_R|=0x01;//activateportA

while((SYSCTL_PRGPIO_R&0x01)==0);//ready?

GPIO_PORTA_AFSEL_R|=0x2C;//enablealtfunctonPA2,3,5

GPIO_PORTA_DEN_R|=0x2C;//configurePA2,3,5asSSI

GPIO_PORTA_PCTL_R=(GPIO_PORTA_PCTL_R&0xFF0F00FF)+0x00202200;

GPIO_PORTA_AMSEL_R=0;//disableanalogfunctionalityonPA

SSI0_CR1_R=0x00000000;//disableSSI,mastermode

SSI0_CPSR_R=0x02;//8MHzSSIClk

SSI0_CR0_R&=~(0x0000FFF0);//SCR=0,SPH=0,SPO=0Freescale

SSI0_CR0_R|=0x0F;//DSS=16-bitdata

SSI0_DR_R=data;//load‘data’intotransmitFIFO

SSI0_CR1_R|=0x00000002;//enableSSI

voidDAC_Out(uint16_tcode)

while((SSI0_SR_R&0x00000002)==0);//SSITransmitFIFONotFull

SSI0_DR_R=code;//dataout,noreply

//sendthe16-bitcodetotheSSI,returnareply

//youwillneedtoenablePA4asSSI0Rxtousethisroutine

uint16_tDAC_Out2(uint16_tcode)uint16_treceive;

while((SSI0_SR_R&0x00000002)==0);//SSITransmitFIFONotFull

SSI0_DR_R=code;//dataout

while((SSI0_SR_R&0x00000004)==0);//SSIReceiveFIFONotEmpty

receive=SSI0_DR_R;//acknowledgeresponse

returnreceive;

Program7.2.FunctionstoinitializeandtosenddatatotheDACusingtheSSI(MAX5353_xxx.zip).

Example7.3.InterfacetheMaximMAX1246analogtodigitalconverter.

Solution:ThissecondSSIexampleshowsasynchronousserialinterfacebetweenthecomputerandaMaximMAX1246analogtodigitalconverter.Ananalogtodigitalconverteracceptsananaloginput(inourcaseavoltagebetween0and+2.5VononeofthefouranaloginputsCH3–CH0)andcreatesadigitaloutput(inourcaseanumberbetween0and4095.)DetaileddiscussionofconverterswillalsobepresentedlaterinChapter8.Hereinthissectionwewillfocusonthehardwareandsoftwareaspectsoftheserialinterface(Figure7.21).Again,thebasicinterfacingissuestoconsiderforthisinterfaceare:

Wordsize.Inthiscaseweneedtofirsttransmit8bitstotheADC,thenreceive12bitsbackfromtheADC.TheMAX1246datasheetsuggeststhatitwillembedthe12bitdataintotwo8-bittransmissions.

Bitorder.TheMAX1246requiresthemostsignificantbitsfirst.

Clockphase,clockpolarity.SincetheMAX1246samplesitsserialinputdataontherisingedgeoftheclock,theSPImustchangesthedataonthefallingedge.SPO=SPH=0andSPO=SPH=1bothsatisfythisrequirement.WewillusetheSPO=SPH=0modeassuggestedintheMaximdatasheet,referbacktoFigure7.17.

Bandwidth.WelookatthetimingspecificationsoftheMAX1246.ThemaximumSCLKfrequencyis2MHz,andtheminimumclocklow/highwidthsis200ns,sotheshortestSPIperiodwecanuseis500ns.

Figure7.21.Afourchannel12-bitADCinterfacedtotheSSIport.

Thefirst8bitssentwillspecifythechannelandADCmodeasshowninFigure7.22.Afterconversion,the12databitresultisreturned.Noticethatbit7ofthemodeselectisalwayshigh,andthe12-bitADCresultisembeddedintothemiddleofthetwo8-bittransmissions.Thesoftwarewillshiftthe16-bitdata3bitstotherightinordertoproducethe0to4095result.

Figure7.22.MAX1247ADCserialtiming.

BecausewewanttheCSsignaltoremainlowfortheentire24-bittransfer,wewillimplementitusingtheregularI/Opinfunctions.Theritualinitializesthedirectionregister,FreescaleSPImode,andbandwidth(Program7.3).Inthisexample,thebusclockis6MHz,andtheSSIclockwillbe1.5MHz.

RecallthatwhenthesoftwareoutputstotheSSIdataregister,the8-bitregisterintheSSIisexchangedwiththe8-bitregisterintheADC.ToreadtheADC,three8-bittransmissionsareexchanged.Onthefirstexchange,thesoftwarespecifiesthechannelandADCmode,thenthe12-bitADCdatareturnedduringthe2ndand3rdtransmission.AlloftheADCmodesinthe #define statementsimplementunipolarvoltagerange,singleended,andexternalclock.

#defineGPIO_PORTA3(*((volatileuint32_t*)0x40004020))

voidADC_Init2(void)

SYSCTL_RCGCSSI_R|=0x01;//activateSSI0

SYSCTL_RCGCGPIO_R|=0x01;//activateportA

while((SYSCTL_PRGPIO_R&0x01)==0);//ready?

GPIO_PORTA_PCTL_R=(GPIO_PORTA_PCTL_R&0xFF0F00FF)+0x00202200;

GPIO_PORTA_AMSEL_R=0;//disableanalogfunctionalityonPA

GPIO_PORTA_AFSEL_R|=0x34;//enablealtfunctonPA2,4,5

GPIO_PORTA_DIR_R|=0x08;//makePA3out(PA3!CSsignal)

GPIO_PORTA_DEN_R|=0x3C;//enabledigitalI/OonPA2,3,4,5

GPIO_PORTA3=0x08;//!CS=1

SSI0_CR1_R&=~SSI_CR1_SSE;//disableSSI

SSI0_CR1_R&=~SSI_CR1_MS;//mastermode(defaultsetting)

SSI0_CPSR_R=(SSI0_CPSR_R&~SSI_CPSR_CPSDVSR_M)+4;//1.5MHzSSIClk

SSI0_CR0_R&=~(SSI_CR0_SCR_M|//SCR=0

SSI_CR0_SPH|//SPH=0

SSI_CR0_SPO);//SPO=0

SSI0_CR0_R=(SSI0_CR0_R&~SSI_CR0_FRF_M)+SSI_CR0_FRF_MOTO;//Freescale

SSI0_CR0_R=(SSI0_CR0_R&~SSI_CR0_DSS_M)+SSI_CR0_DSS_8;//8-bitdata

SSI0_CR1_R|=SSI_CR1_SSE;//enableSSI

#defineSSI_SR_RNE0x00000004//SSIReceiveFIFONotEmpty

#defineSSI_SR_TFE0x00000001//SSITransmitFIFOEmpty

#defineCH00x9F//sampleonchannel0

#defineCH10xDF//sampleonchannel1

#defineCH20xAF//sampleonchannel2

#defineCH30xEF//sampleonchannel3

//sendthe8-bitcodetotheSSIafterwaitingforFIFOstoempty

//returnareply

uint8_tsendAfterWaiting(uint8_tcode)

while((SSI0_SR_R&SSI_SR_TFE)==0);//waituntilFIFOempty

SSI0_DR_R=code;//dataout

while((SSI0_SR_R&SSI_SR_RNE)==0);//waituntilresponse

returnSSI0_DR_R;//acknowledgeresponse

//takeoneADCmeasurementandreturntheresult

//NOTE:toensurecorrectoperation,useoneoftheCHndefinitions

uint16_tADC_In2(uint8_tcode)

uint16_tdata;

GPIO_PORTA3=0;//!CS=0

sendAfterWaiting(code);//sendchannel,mode

data=sendAfterWaiting(0)<<8;//msbyteofADC

data+=sendAfterWaiting(0);//lsbyteofADC

GPIO_PORTA3=0x08;//!CS=1

returndata>>3;//rightjustify

Program7.3.FunctionstoinitializeandtoreceivedatafromtheADCusingtheSSI(MAX1246_xxx.zip).

Example7.4.DesignanoutputportexpanderusingashiftregisterandSSI.

Solution:Sometimesweneedmoreoutputpinsthanavailableonourmicrocontroller.Ingeneral,theproperdesignapproachwouldbetoupgradetoamicrocontrollerwithmorepins.Howeverinsituationswherewedonothavethetimeormoneytochangemicrocontrollers,wecaninterfacea74HC595shiftregistertotheSSIportforaquicksolutionprovidingadditionaloutputpins.Basically,threepinsoftheSSI(SS,MOSI,andSCLK)willbeconvertedtoeightdigitaloutputsQonthe74HC595,asshowninFigure7.23.

Additionalshiftregisterscanbechainedtogether(connecttheQH’outputsofonetotheSERinputsofthenext)toprovideadditionaloutputswithoutrequiringmoreLM3S811pins.Thegateinput,G*,ofthe74HC595isgroundedsotheeightQoutputswillbecontinuouslydriven.TheSPIclockoutputisconnectedtothe74HC595clockinput(SCK)andtheSPIdataoutputisconnectedtothe74HC595datainput(SER).TheFreescaleSPImode(SPO=0,SPH=0)isselectedtotheLM3S811changestheoutputdataonthefalloftheclockandthe74HC595shiftsdatainontherise.Becausethe74HC595isfast(maximumclock25MHz,andsetuptimeof20ns),weruntheSSIasfastaspossible.Inthisexample,thebusclockis6MHz,andtheSSIclockwillbe3MHz.

AftereightbitsaretransferredfromtheLM3S811tothe74HC595,softwarewillcreatearisingedgeofRCK,causingthenewdatatobelatchedintothe74HC595.Ifthereisjustone74HC595likeFigure7.23,wecanusetheautomaticSSfeatureoftheSPItocreatearisingedgelatchonRCLK.Toenablethisfeature,wesetbit3AFSEL3ofGPIO_PORTA_AFSEL_R.Inthissolution,weperformoneSSItransmissiontochangeall8bitsoftheportoutput(Program7.4).TheSSpulse,seeFigure7.17,accordsautomaticallyanddoesnotrequiresoftwareoverheadtoproduce.However,ifwewerechainingmultipleshiftregisters,wewouldnotusetheautomaticSSfeature;ratherwewouldoutputallthedataandthenmanuallylatchitinwithexplicitoutputsonRCKbyusingPA3asaregularGPIOport.

Figure7.23.InterfacebetweentheLM3S811anda74HC595shiftregister.voidPort_Init(void)

SYSCTL_RCGCSSI_R|=0x01;//activateSSI0

SYSCTL_RCGCGPIO_R|=0x01;//activateportA

while((SYSCTL_PRGPIO_R&0x01)==0);//ready?

GPIO_PORTA_AFSEL_R|=0x2C;//enablealtfunctonPA2,3,5

GPIO_PORTA_PCTL_R=(GPIO_PORTA_PCTL_R&0xFF0F00FF)+0x00202200;

GPIO_PORTA_AMSEL_R=0;//disableanalogfunctionalityonPA

GPIO_PORTA_DEN_R|=0x2C;//enabledigitalI/OonPA2,3,5

SSI0_CR1_R&=~SSI_CR1_SSE;//disableSSI

SSI0_CR1_R&=~SSI_CR1_MS;//mastermode(defaultsetting)

SSI0_CPSR_R=(SSI0_CPSR_R&~SSI_CPSR_CPSDVSR_M)+2;//3MHzSSIClk

SSI0_CR0_R&=~(SSI_CR0_SCR_M|SSI_CR0_SPH|SSI_CR0_SPO);//SPO=SPH=0

SSI0_CR0_R=(SSI0_CR0_R&~SSI_CR0_FRF_M)+SSI_CR0_FRF_MOTO;//Freescale

SSI0_CR0_R=(SSI0_CR0_R&~SSI_CR0_DSS_M)+SSI_CR0_DSS_8;//8-bitdata

SSI0_CR1_R|=SSI_CR1_SSE;//enableSSI

uint8_tPort_Out(uint16_tcode)

while((SSI0_SR_R&SSI_SR_TNF)==0);//waituntilroominFIFO

SSI0_DR_R=code;//dataout

while((SSI0_SR_R&SSI_SR_RNE)==0);//waituntilresponse

returnSSI0_DR_R;//acknowledgeresponse

Program7.4.SoftwaretocontrolanoutputparallelportexpandedusingtheSSI(74HC595_xxx.zip).

Checkpoint7.7:HowwouldyouchangeProgram7.4torunat1MHz?

7.6.Inter-IntegratedCircuit(I2C)Interface

7.6.1.TheFundamentalsofI2CEversincemicrocontrollershavebeendeveloped,therehasbeenadesiretoshrinkthesizeofanembeddedsystem,reduceitspowerrequirements,andincreaseitsperformanceandfunctionality.TwomechanismstomakesystemssmalleraretointegratefunctionalityintothemicrocontrollerandtoreducethenumberofI/Opins.Theinter-integratedcircuitI2CinterfacewasproposedbyPhilipsinthelate1980sasameanstoconnectexternaldevicestothemicrocontrollerusingjusttwowires.TheSSIinterfacehasbeenverypopular,butittakes3wiresforsimplexand4wiresforfullduplexcommunication.In1998,theI2CVersion1protocolbecomeanindustrystandardandhasbeenimplementedintothousandsofdevices.TheI2Cbusisasimpletwo-wirebi-directionalserialcommunicationsystemthatisintendedforcommunicationbetweenmicrocontrollersandtheirperipheralsovershortdistances.Thisistypically,butnotexclusively,betweendevicesonthesameprintedcircuitboard,thelimitingfactorbeingthebuscapacitance.Italsoprovidesflexibility,allowingadditionaldevicestobeconnectedtothebusforfurtherexpansionandsystemdevelopment.Theinterfacewilloperateatbaudratesofupto100kbpswithmaximumcapacitivebusloading.Themodulecanoperateuptoabaudrateof400kbpsprovidedtheI2Cbusslewrateislessthan100ns.Themaximuminterconnectlengthandthenumberofdevicesthatcanbeconnectedtothebusarelimitedbyamaximumbuscapacitanceof400pFinallinstances.Theseparameterssupportthegeneraltrendthatcommunicationspeedcanbeincreasedbyreducingcapacitance.Version2.0supportsahighspeedmodewithabaudrateupto2.4MHz(supportedbyLM4F/TM4C).

Figure7.24showsablockdiagramofacommunicationsystembasedontheI2Cinterface.Themaster/slavenetworkmayconsistofmultiplemastersandmultipleslaves.TheSerialClockLine(SCL)andtheSerialDataline(SDA)arebothbidirectional.Eachlineisopendrain,meaningadevicemaydriveitloworletitfloat.Alogichighoccursifalldeviceslettheoutputfloat,andalogiclowoccurswhenatleastonedevicedrivesitlow.Thevalueofthepull-upresistordependsonthespeedofthebus.4.7k isrecommendedforbaudratesbelow100kbps,2.2k isrecommendedforstandardmode,and1k isrecommendedforfastmode.

Checkpoint7.8:Whyistherecommendedpull-upresistorrelatedtothebusspeed?

Checkpoint7.9:Whatdoesopendrainmean?

TheSCLclockisusedinasynchronousfashiontocommunicateonthebus.Eventhoughdatatransferisalwaysinitiatedbyamasterdevice,boththemasterandtheslaveshavecontroloverthedatarate.Themasterstartsatransmissionbydrivingtheclocklow,butifaslavewishestoslowdownthetransfer,ittoocandrivetheclocklow(calledclockstretching).Inthisway,devicesonthebuswillwaitforalldevicestofinish.Bothaddress(fromMastertoSlaves)andinformation(bidirectional)arecommunicatedinserialfashiononSDA.

Figure7.24.BlockdiagramofanI2Ccommunicationnetwork.ThebusisinitiallyidlewherebothSCLandSDAarebothhigh.ThismeansnodeviceispullingSCLorSDAlow.Thecommunicationonthebus,whichbeginswithaSTARTandendswithaSTOP,consistsoffivecomponents:

START(S)isusedbythemastertoinitiateatransferDATAissentin8-bitblocksandconsistsof7-bitaddressand1-bitdirectionfromthemaster

controlcodeformastertoslaves

informationfrommastertoslave

informationfromslavetomaster

ACK(A)isusedbyslavetorespondtothemasteraftereach8-bitdatatransfer

RESTART(R)isusedbythemastertoinitiateadditionaltransferswithoutreleasingthebus

STOP(P)isusedbythemastertosignalthetransferiscompleteandthebusisfree

ThebasictimingsforthesecomponentsaredrawninFigure7.25.Fornowwewilldiscussbasictiming,butwewilldealwithissueslikestretchingandarbitrationlater.Aslowslaveusesclockstretchingtogiveitmoretimetoreact,andmasterswillusearbitrationwhentwoormoremasterswantthebusatthesametime.AnidlebushasbothSCLandSDAhigh.AtransmissionbeginswhenthemasterpullsSDAlow,causingaSTART(S)component.ThetimingofaRESTARTisthesameasaSTART.AfteraSTARToraRESTART,thenext8bitswillbeanaddress(7-bitaddressplus1-bitdirection).Thereare128possible7-bitaddresses,however,32ofthemarereservedasspecialcommands.Theaddressisusedtoenableaparticularslave.Alldatatransfersare8bitslong,followedbya1-bitacknowledge.Duringadatatransfer,theSDAdatalinemustbestable(highorlow)whenevertheSCLclocklineishigh.ThereisoneclockpulseonSCLforeachdatabit,theMSBbeingtransferredfirst.Next,theselectedslavewillrespondwithapositiveacknowledge(Ack)oranegativeacknowledge(Nack).Ifthedirectionbitis0(write),thensubsequentdatatransmissionscontaininformationsentfrommastertoslave.

Forawritedatatransfer,themasterdrivestheRDAdatalinefor8bits,thentheslavedrivestheacknowledgeconditionduringthe9thclockpulse.Ifthedirectionbitis1(read),thensubsequentdatatransmissionscontaininformationsentfromslavetomaster.Forareaddatatransfer,theslavedrivestheRDAdatalinefor8bits,thenthemasterdrivestheacknowledgeconditionduringthe9thclockpulse.TheSTOPcomponentiscreatedbythemastertosignifytheendoftransfer.ASTOPbeginswithSCLandSDAbothlow,thenitmakestheSCLclockhigh,andendsbymakingSDAhigh.TherisingedgeofSDAwhileSCLishighsignifiestheSTOPcondition.

Figure7.25.TimingdiagramsofI2Ccomponents.

Checkpoint7.10:Whathappensifnodevicesendsanacknowledgement?

Figure7.26illustratesthecasewherethemastersends2bytesofdatatoaslave.Theshadedregionsdemarksignalsdrivenbythemaster,andthewhiteareasshowthosetimeswhenthesignalisdrivenbytheslave.Regardlessofformat,allcommunicationbeginswhenthemastercreatesaSTARTcomponentfollowedbythe7-bitaddressand1-bitdirection.Inthisexample,thedirectionislow,signifyingawriteformat.The1stthrough8thSCLpulsesareusedtoshifttheaddress/directionintoalltheslaves.Inordertoacknowledgethemaster,theslavethatmatchestheaddresswilldrivetheSDAdatalinelowduringthe9thSCLpulse.Duringthe10ththrough17thSCLpulsessendsthedatatotheselectedslave.TheselectedslavewillacknowledgebydrivingtheSDAdatalinelowduringthe18thSCLpulse.Aseconddatabyteistransferredfrommastertoslaveinthesamemanner.Inthisparticularexample,twodatabytesweresent,butthisformatcanbeusedtosendanynumberofbytes,becauseoncethemastercapturesthebusitcantransferasmanybytesasitwishes.Iftheslavereceiverdoesnotacknowledgethemaster,theSDAlinewillbelefthigh(Nack).ThemastercanthengenerateaSTOPsignaltoabortthedatatransferoraRESTARTsignaltocommenceanewtransmission.ThemastersignalstheendoftransmissionbysendingaSTOPcondition.

Figure7.26.I2CtransmissionoftwobytesfrommastertoslaveFigure7.27illustratesthecasewhereaslavesends2bytesofdatathemaster.Again,themasterbeginsbycreatingaSTARTcomponentfollowedbythe7-bitaddressand1-bitdirection.Inthisexample,thedirectionishigh,signifyingareadformat.Duringthe10ththrough17thSCLpulsestheselectedslavesendsthedatatothemaster.TheselectedslavecanonlychangethedatalinewhileSCLislowandmustbeheldstablewhileSCLishigh.ThemasterwillacknowledgebydrivingtheSDAdatalinelowduringthe18thSCLpulse.OnlytwodatabytesareshowninFigure7.27,butthisformatcanbeusedtoreceiveasanymanybytesthemasterwishes.Exceptforthelastbytealldataaretransferredfromslavetomasterinthesamemanner.Afterthelastdatabyte,themasterdoesnotacknowledgetheslave(Nack)signifying‘endofdata’totheslave,sotheslavereleasestheSDAlineforthemastertogenerateSTOPorRESTARTsignal.ThemastersignalstheendoftransmissionbysendingaSTOPcondition.

Figure7.27.I2Ctransmissionoftwobytesfromslavetomaster.

Figure7.28illustratesthecasewherethemasterusestheRESTARTcommandtocommunicatewithtwoslaves,readingonebytefromoneslaveandwritingonebytetotheother.Asalways,themasterbeginsbycreatingaSTARTcomponentfollowedbythe7-bitaddressand1-bitdirection.Duringthefirststart,theaddressselectsthefirstslaveandthedirectionisread.Duringthe10ththrough17thSCLpulsesthefirstslavesendsthedatatothemaster.Becausethisisthelastbytetobereadfromthefirstslave,themasterwillnotacknowledgelettingtheSDAdatafloathighduringthe18thSCLpulse,sothefirstslavereleasestheSDAline.RatherthanissuingaSTOPatthispoint,themasterissuesarepeatedstartorRESTART.The7-bitaddressand1-bitdirectiontransferredinthe20ththrough27thSCLpulseswillselectthesecondslaveforwriting.Inthisexample,thedirectionislow,signifyingawriteformat.The28thpulsewillbeusedbythesecondslavepullsSDAlowtoacknowledgeithasbeenselected.The29ththrough36thSCLpulsessendthedatatothesecondslave.Duringthe37thpulsethesecondslavepullsSDAlowtoacknowledgethedataitreceived.ThemastersignalstheendoftransmissionbysendingaSTOPcondition.

Figure7.28.I2Ctransmissionofonebytefromthefirstslaveandonebytetoasecondslave.Checkpoint7.11:IsthecommunicationinFigure7.28fullduplex,halfduplex,orsimplex?

Table7.10listssomeaddressesthathavespecialmeaning.Awritetoaddress0isageneralcalladdress,andisusedbythemastertosendcommandstoallslaves.The10-bitaddressmodegivestwoaddressbitsinthefirstframeand8moreaddressbitsinthesecondframe.Thedirectionbitfor10-bitaddressingisinthefirstframe.

Address R/W Description

0000000 0 Generalcalladdress

0000000 1 Startbyte

0000001 x CBUSaddress

0000010 x Reservedfordifferentbusformats

0000011 0 Reserved

00001xx x Highspeedmode

11110xx x 10-bitaddress

11111xx X Reserved

Table7.10.SpecialaddressesusedintheI2Cnetwork.

7.6.2.I2CSynchronizationTheI2Cbussupportsmultiplemasters.IftwoormoremasterstrytoissueaSTARTcommandonthebusatthesametime,bothclocksynchronizationandarbitrationwilloccur.Clocksynchronizationisprocedurethatwillmakethelowperiodequaltothelongestclocklowperiodandthehighisequaltotheshortestoneamongthemasters.Figure7.29illustratesclocksynchronization,wherethetopsetoftracesisgeneratedbythefirstmaster,andthesecondsetoftracesisgeneratedbythesecondmaster.Sincetheoutputsareopendrain,theactualsignalswillbethewired-ANDofthetwooutputs.Eachmasterrepeatsthesestepswhenitgeneratesaclockpulse.Itisduringstep3)thatthefasterdevicewillwaitfortheslowerdevice

1.DriveitsSCLclocklowforafixedamountoftime2.LetitsSCLclockfloat3.WaitfortheSCLtobehigh

4.Waitforafixedamountoftime,stopwaitingiftheclockgoeslow

Becausetheoutputsareopendrain,thesignalwillbepulledtoalogichighbythe2k resistoronlyifalldevicesreleasetheline(outputalogichigh).Conversely,thesignalwillbealogiclowifanydevicedrivesitlow.WhenmasterscreateaSTART,theyfirstdriveSDAlow,thendriveSCLlow.IfagroupofmastersareattemptingtocreateSTARTcommandsataboutthesametime,thenthewire-ANDoftheirSDAlineshasits1to0transitionbeforethewire-ANDoftheirSCLlineshasits1to0transition.Thus,avalidSTARTcommandwilloccurcausingalltheslavestolistentotheupcomingaddress.IntheexampleshowninFigure7.29,Master#2isthefirsttodriveitsclocklow.Ingeneral,theSCLclockwillbelowfromthetimethefirstmasterdrivesitlow(time1inthisexample),untilthetimethelastmasterreleasesitsclock(time2inthisexample.)Similarly,theSCLclockwillbehighfromthetimethelastmasterreleasesitsclock(time2inthisexample),untilthetimethefirstmasterdrivesitsclocklow(time3inthisexample.)

Figure7.29.I2Ctimingillustratingclocksynchronizationanddataarbitration.Therelativepriorityofthecontendingmastersisdeterminedbyadataarbitrationprocedure.Abusmasterlosesarbitrationifittransmitslogic“1”whileanothermastertransmitslogic“0”.ThelosingmastersimmediatelyswitchovertoslavereceivemodeandstopdrivingtheSCLandSDAoutputs.Inthiscase,thetransitionfrommastertoslavemodedoesnotgenerateaSTOPcondition.Meanwhile,astatusbitissetbyhardwaretoindicatelossofarbitration.IntheexampleshowninFigure7.29,master#1isgeneratinganaddresswithA7=1andA6=0,whilemaster#2isgeneratinganaddresswithA7=1andA6=1.Betweentimes2and3,bothmastersareattemptingtosendA7=1,andnoticetheactualSDAlineishigh.Attime4,master#2attemptstomaketheSDAhigh(A6=1),butnoticestheactualSDAlineislow.Ingeneral,themastersendingamessagetothelowestaddresswillwinarbitration.

Checkpoint7.12:IfMaster1sendsaddress0x30andMaster2sendsaddress#0x0F,whichonewinsarbitration?

Thethirdsynchronizationmechanismoccursbetweenmasterandslave.Iftheslaveisfastenoughtocapturedataatthemaximumrate,thetransferisasimplesynchronousserialmechanism.Inthiscasethetransferofeachbitfrommastertoslaveisillustratedbythefollowinginterlockedsequences,seeFigure7.25.

MastersequenceSlavesequence(nostretch)

1.DriveitsSCLclocklow

2.SettheSDAline

3.Waitforafixedamountoftime

4.LetitsSCLclockfloat

5.WaitfortheSCLtobehigh

6.Waitforafixedamountoftime6.CaptureSDAdataonlowtohighedgeofSCL

7.Stopwaitingiftheclockgoeslow

Iftheslaveisnotfastenoughtocapturedataatthemaximumrate,itcanperformanoperationcalledclockstretching.IftheslaveisnotreadyfortherisingedgeofSCL,itwillholdtheSCLclocklowitselfuntilitisready.Slavesarenotallowedtocauseany1to0transitionsontheSCLclock,butrathercanonlydelaythe0to1edge.Thetransferofeachbitfrommastertoslavewithclockstretchingisillustratedbythefollowingsequences

MastersequenceSlavesequence(clockstretching)

1.DriveitsSCLclocklow1.WaitfortheSCLclocktobelow

2.SettheSDAline2.DriveSCLclocklow

3.Waitforafixedamountoftime3.Waituntilit’sreadytocapture

4.LetitsSCLclockfloat4.LetitsSCLfloat

5.WaitfortheSCLclocktobehigh5.WaitfortheSCLclocktobehigh

6.Waitforafixedamountoftime6.CapturetheSDAdata

7.Stopwaitingiftheclockgoeslow

Clockstretchingcanalsobeusedwhentransferringabitfromslavetomaster

MastersequenceSlavesequence(clockstretching)

1.DriveitsSCLclocklow1.WaitfortheSCLclocktobelow

2.Waitforafixedamountoftime2.DriveSCLclocklow

3.Waituntilnextdatabitisready

4.LetitsSCLclockfloat4.LetitsSCLfloat

5.WaitfortheSCLclocktobehigh5.WaitfortheSCLclocktobehigh

6.CapturetheSDAinput

7.Waitforafixedamountoftime,

8.Stopwaitingiftheclockgoeslow

Observation:ClockstretchingallowsfastandslowdevicestoexistonthesameI2CbusFastdeviceswillcommunicatequicklywitheachother,butslowdownwhencommunicatingwithslowerdevices.

Checkpoint7.13:Arbitrationcontinuesuntilonemastersendsazerowhiletheothersendsaone.Whathappensiftwomastersattempttosenddatatothesameaddress?

7.6.3.LM3S/TM4CI2CDetailsLM3S/TM4CmicrocontrollershavezerototenI2Cmodules,seeFigure7.30.TheLM3Smicrocontrollersimplementjustasubsetofthestandard.Theysupportmasterandslavemodes,cangenerateinterruptsonstartandstopconditions,andallowI2Cnetworkswithmultiplemasters.Ontheotherhand,theLM3Smicrocontrollersdonotsupportgeneralcall,or10-bitaddressing.AsshowninFigure7.24,microcontrollerpinsSDAandSCLcanbeconnecteddirectlytoanI2Cnetwork.BecauseI2CnetworksareintendedtoconnectdevicesonthesamePCB,nospecialhardwareinterfaceelectronicsarerequired.Table7.11liststheI2CportsontheLM3S.TheLM3Scanoperateinslavemode,butwewillfocusonmastermode.Tables2.7and2.8describehowtoattachI/OpinstotheI2CmodulesontheTM4C123/TM4C1294.

7 6 5 4 3 2 1 0 Name

$4002.0000 SA R/S I2C0_MSA_R

$4002.0004 BUSBSY IDLE ARBLST DATACK ADRACK ERROR BUSY I2C0_MCS_R

$4002.0008 DATA DATA DATA DATA DATA DATA DATA DATA I2C0_MDR_R

$4002.000C TPR I2C0_MTPR_R

$4002.0010 IM I2C0_MIMR_R

$4002.0014 RIS I2C0_MRIS_R

$4002.0018 MIS I2C0_MMIS_R

$4002.001C IC I2C0_MICR_R

$4002.0020 SFE MFE LPBK I2C0_MCR_R

$4000.5420 SEL SEL SEL SEL SEL SEL SEL SEL GPIO_PORTB_AFSEL_R

$4000.551C DEN DEN DEN DEN DEN DEN DEN DEN GPIO_PORTB_DEN_R

$4000.550C SEL SEL SEL SEL SEL SEL SEL SEL GPIO_PORTB_ODR_R

$400F.E608 GPIOH GPIOG GPIOF GPIOE GPIOD GPIOC GPIOB GPIOA SYSCTL_RCGCGPIO_R

$400F.E620 I2C3 I2C2 I2C1 I2C0 SYSCTL_RCGCI2C_R

Table7.11.TheLM3SI2Cmasterregisters.Eachregisteris32bitswide.Bits31–8arezero.

Figure7.30.I/OportpinsforI2ConvariousLM3S/TM4Cmicrocontrollers.TherearesevenstepstoinitializeI2C0inmastermode.

Step1)weactivatetheI2C0clockbysettingbit0the SYSCTL_RCGCI2C_R register.

Step2)SinceI2C0usestwopinsonPortB,weneedtoactivatePortBclockinregister SYSCTL_RCGCGPIO_R .

Step3)WeconnectI2C0topinsPB3/PB2bysettingthealternativefunction.I.e.,wesetbits2and3of GPIO_PORTB_AFSEL_R register.

Step4)I2C0usesopendrainmode,sowesetbits2and3of GPIO_PORTB_ODR_Rregister.Theportwilldrivethelinelowwhenitwantstooutputazero.Conversely,tomakethelinehigh,opendrainmodecausestheoutputtofloatandthepull-upresistorinFigure7.24causesthelinetogohigh.

Step5)Weenablethedigitalcircuitsbysetting2and3of GPIO_PORTB_DEN_Rregister.

Step6)WeenablemastermodebysettingtheMFEbitinthe I2C0_MCR_R register.

Step7)Lastly,I2C0_MTPR_R istheI2CMasterTimerPeriodRegister,whichdeterminesthebaudratetransferredasamaster.Itisthemaster’sresponsibilitytogeneratetheI2Cclock.ThetimingoftheI2Cinterfaceisderivedfromthebusclock.WesetthebaudratebywritingtotheTPRfieldinthe I2C0_MTPR_R register.LetfBUSbethefrequencyofthebusclockandlett=1/fBUSbetheperiodofthebusclock.TheI2Cclockperiod(tbit)willbe20*(TPR+1)*t.Instandardmode,wesettheI2Cclockfrequencytobeabout100kHz(tbit=10 s).Infastmode,wesettheI2Cclockfrequencytobeabout400kHz.Forexample,tosettheI2Ctostandardspeed,assuming tisgiveninns,

tbit=20*(TPR+1)*t=10000ns

orTPR=500/t-1

TheR/Sbit(bit0)oftheMaster’sSlaveAddressRegister,I2C0_MSA_R ,specifieswhetherthenextdatatransferwillbeareceivefromslave(equals1)orawritetoslave(equals0).Bits7through1oftheMaster’sSlaveAddressRegistercontaintheunique7-bitaddressoftheslavethatthemasterisaddressing.Generally,notwodevicesshouldhavethesameslaveaddress.Manydevicescanpermanentlyacceptanewslaveaddressfromthemaster.I2C_MCS_ACK specifieswhetherornotthemasteracknowledgesdatareceivedfromtheslave.I2C_MCS_ACK isonlyusedwhentheI2CBusisareceiver,notatransmitter.

Whenreceivingdataasamaster,thisbitdeterminesifanacknowledgementwillbesentduringthe9thclockbit.1meansanacknowledgementwillbesent(Ack),and0meansnoacknowledgementwillbesent(Nack).Themastermaynotacknowledgethefinalbytesentbyaslave.ItmustissueaSTOPorrepeatedSTARTconditionafterthenegativeacknowledge.TheI2Cmodulewillalwaysacknowledgeaddressmatches,provideditisenabled.ASTARTorrepeatedstart(RESTART)willbesentifsoftwarewritesa1tothe I2C_MCS_START bit,providedthismicrocontrolleristhecurrentbusmaster.Attemptingarepeatedstartwhenthebusisownedbyanothermasterwillresultinlossofarbitration. I2C_MCS_START ispartoftheMaster’sControl/StatusRegister,whichisaspecialregister.Readsfromthisregisterreturninformationaboutthemaster’sstatus,andwritestothisregistercontrolthemaster’snexttransmission.Forexample,setthe I2C_MCS_ACK , I2C_MCS_STOP ,and/or I2C_MCS_START bitsandthe I2C_MCS_RUN bittobeginthenexttransmission.Afterwards,readthe I2C_MCS_ARBLST or I2C_MCS_ERROR bitstoseeifthemasterhaslostarbitrationofthebus.Manyapplicationsmayalsorequirereadingthe I2C_MCS_ADRACK and I2C_MCS_DATACK bitstoensurethattheslavehasacknowledgeditsaddressandanydata.Anunresponsivedevicemayrepresentaserioushardwareproblemtowhichthesystemmustreact.

Themastercanbeinthreemodes:idle,transmitandreceive.Table7.12summarizesthemodesandoperationofI2C.Combinationsnotlistedinthistableareeitherillegalornooperation.

State R/S ACK STOP START Run Action(andnewstate)

Idle 0 X 0 1 1 STARTconditionfollowedbySEND(goestoMasterTransmit)

0 X 1 1 1 STARTconditionfollowedbyaSENDandSTOPcondition(remainsinIdle).

1 0 0 1 1 STARTcondition,RECEIVEoperationwithnegativeACK(goestotheMasterReceive)

1 0 1 1 1 STARTconditionfollowedbyRECEIVEandSTOPcondition(remainsinIdlestate)

1 1 0 1 1 STARTconditionfollowedbyRECEIVE(goestotheMasterReceivestate)

MasterTransmit

X X 0 0 1 SENDoperation(remainsinMasterTransmit

state)

X X 1 0 0 STOPcondition(goestoIdlestate)

X X 1 0 1 SENDfollowedbySTOPcondition(goestoIdle

state)

0 X 0 1 1 RepeatedSTARTconditionfollowedbyaSEND(remainsinMasterTransmitstate)

0 X 1 1 1 RepeatedSTARTconditionfollowedbySENDandSTOPcondition(goestoIdlestate)

1 0 0 1 1

RepeatedSTART,thenRECEIVEoperationwithanegativeACK(goestoMasterReceivestate)

1 0 1 1 1 RepeatedSTARTconditionfollowedbyaSENDandSTOPcondition(goestoIdlestate)

1 1 0 1 1 RepeatedSTARTconditionfollowedbyRECEIVE(goestoMasterReceivestate)

Master

Receive

X 0 0 0 1 RECEIVEoperationwithnegativeACK(remains

inMasterReceivestate)

X X 1 0 0 STOPcondition(goestoIdlestate)

X 0 1 0 1 RECEIVEfollowedbySTOPcondition(goestoIdlestate)

X 1 0 0 1 RECEIVEoperation(remainsinMasterReceivestate)

1 0 0 1 1 RepeatedSTART,thenRECEIVEoperationwithanegativeACK(remainsinMasterReceivestate).

1 0 1 1 1 RepeatedSTART,thenRECEIVEandSTOPcondition(goestoIdlestate)

1 1 0 1 1 RepeatedSTARTconditionfollowedbyRECEIVE(remainsinMasterReceivestate)

0 X 0 1 1 RepeatedSTARTconditionfollowedbySEND(goestoMasterTransmitstate)

0 X 1 1 1 RepeatedSTARTconditionfollowedbySENDandSTOPcondition(goestoIdlestate)

Table7.12.I2Ccommands,actions,andstatechanges.

Figure7.31andTable7.13describethetimingofI2C.LettbitbetheI2CperiodasdefinedbythebusclockandtheTPRfieldinthe I2C0_MTPR_R register.BecauseSCLandSDAareopen-drain-typeoutputs,whichthecontrollercanonlyactivelydrivelow.Thesetuptime,tsetup,isthetimebeforetheriseofSCLthatthedataonSDAwillbevalid.ThedataremainsvalidthroughoutthetimeSCLishigh.Theholdtime,thold,isthetimeafterthefallofSCLthatthedataonSDAcontinuestobevalid.Therisetime,trise,ofSDAandSCLdependsonexternalsignalcapacitanceandpull-upresistorvalues(**).Thefalltime,tfall,isspecifieduptoamaximum50pFload.

Figure7.31.I2Ctimingintervals.

Parameter Minimum Typical Maximum

tstart 36t

thigh 24t

tlow 36t

18t

tsetup

thold 2t

trise ** ** **

tfall 9ns 10ns

tstop 24t

Table7.13.I2Ctimingintervals,wheretisthesystembusperiod(**dependsonexternalfactors).

7.6.4.I2CSingleMasterExampleTheobjectiveofthisexampleistopresentalow-leveldevicedriverforanI2Cnetworkwherethismicrocontrolleristheonlymaster,asshowninProgram7.5.Thissimpleexamplewillemploybusy-waitsynchronization. I2C_Init firstenablestheI2Cinterface,startingoutinmastermode.Sincethisistheonlymaster,itneedtoinitializeanyoftheslavemodesettings.

//Assumesa50MHzbusclock,20*(TPR+1)*20ns=10us,withTPR=24

#defineTPR(500/20–1)

voidI2C_Init(void)

SYSCTL_RCGCI2C_R|=0x0001;//activateI2C0

SYSCTL_RCGCGPIO_R|=0x0002;//activateportB

while((SYSCTL_PRGPIO_R&0x0002)==0);//ready?

GPIO_PORTB_AFSEL_R|=0x0C;//enablealtfunctonPB2,3

GPIO_PORTB_ODR_R|=0x0C;//enableopendrainonPB2,3

GPIO_PORTB_PCTL_R=(GPIO_PORTB_PCTL_R&0xFFFF00FF)+0x00003300;//I2C

GPIO_PORTB_DEN_R|=0x0C;//enabledigitalI/OonPB2,3

I2C0_MCR_R=0x00000010;//masterfunctionenable

I2C0_MTPR_R=TPR;//configurefor100kbpsclock

Program7.5.TM4C123I2Cinitializationinsinglemastermode(I2C_xxx.zip).

Program7.6containsthefunction I2C_Send2 thattransmitstwobytestoaslave,creatingatransmissionshowninFigure7.26.Inasystemwithmultiplemastersthisshouldchecktoseeifthebusisidlefirst.Becausethissystemhasjustonemaster,thebusshouldbeidle.However,thefirstlineofthefunctionmakescertainthattheI2Chardwareisfreeandthereisnotalreadyanothertransactionpending.Bysettingthe I2C_MCS_START bitoftheI2CMasterControl/StatusRegister, I2C0_MCS_R ,themicrocontrollerwillcreateaSTARTcondition.Inasystemwithmultiplemasters,itshouldchecktoseeifitlostbusarbitration( I2C_MCS_ARBLST ).Theslaveaddress(withbit0equalto0)willbesent.Thetwodatabytesaresent,thentheSTOPisissued.Ifthereisapossibilitytheslavedoesn’texist,datadoesn’tgetacknowledged,orbusarbitrationislostthenthisprogramcouldhavechecked I2C_MCS_ERROR aftereachtransfer.

uint32_tI2C_Send2(uint8_tslave,uint8_tdata1,uint8_tdata2)

while(I2C0_MCS_R&0x00000001);//waitforI2Cready

I2C0_MSA_R=(slave<<1)&0xFE;//MSA[7:1]isslaveaddress

I2C0_MSA_R&=~0x01;//MSA[0]is0forsend

I2C0_MDR_R=data1&0xFF;//preparefirstbyte

I2C0_MCS_R=(I2C_MCS_START//generatestart/restart

|I2C_MCS_RUN);//noack,nostop,masterenable

while(I2C0_MCS_R&0x00000001);//waitfortransmissiondone

//checkerrorbits

if((I2C0_MCS_R&

(I2C_MCS_DATACK|I2C_MCS_ADRACK|I2C_MCS_ERROR))!=0)

I2C0_MCS_R=I2C_MCS_STOP;//stop,noack,disable

return(I2C0_MCS_R&//returnerrorbitsifnonzero

(I2C_MCS_DATACK|I2C_MCS_ADRACK|I2C_MCS_ERROR));

I2C0_MDR_R=data2&0xFF;//preparesecondbyte

I2C0_MCS_R=(I2C_MCS_STOP//noack,stop,nostart

|I2C_MCS_RUN);//masterenable

while(I2C0_MCS_R&0x00000001);//waitfortransmissiondone

return(I2C0_MCS_R&//returnerrorbits

(I2C_MCS_DATACK|I2C_MCS_ADRACK|I2C_MCS_ERROR));

Program7.6.I2Ctransmissioninsinglemastermode(I2C_xxx.zip).Program7.7containsthefunction I2C_Recv2 thatreceivestwobytesfromaslave,

creatingatransmissionshowninFigure7.27.Bysettingthe I2C_MCS_START bit,themicrocontrollerwillcreateaSTARTcondition.Duringthefirsttransfer,theTx/Rxbitis1,sotheslaveaddress(withbit0equalto1)willbesent,andthemastergoesintoreceivemode.Duringthenexttwotransfers,themasterisinreceivemode,sodataflowsintothemicrocontroller.Totriggeranydatatransfer,thesoftwarewritesavalidvalueto I2C0_MCS_R withthe I2C_MCS_RUN bitset.Duringthefirstdatatransfer I2C_MCS_ACK is1,creatingapositiveacknowledgement.Converselyduringtheseconddatatransfer I2C_MCS_ACK is0,creatinganegativeacknowledgementandsignalingtotheslavethatthisisthelastdatatobetransferred.TheSTOPisrequestedwhenthe I2C_MCS_STOP bitofthe I2C0_MCS_R registerisset,anditautomaticallyisissuedafterthefinaldatatransfer.

Thisisasimpleconceptthatmayseemcomplicatedatfirst.Basically:

0)Initialize:turnonGPIOBandI2Cmodules,setupGPIOBpins,setupI2Cmaster

1)Putslaveaddressinto I2C0_MSA_R [7:1]

2)(receiveonly)Set I2C0_MSA_R [0]

2)(transmitonly)Clear I2C0_MSA_R [0]andputfirstbyteinto I2C0_MDR_R [7:0]

3)Writeavalidvalueto I2C0_MCS_R [3:0]tostarttransfer

4)Waitfortransfertofinish: while(I2C0_MCS_R&0x00000001);

5)(receiveonly)Readfirstbytefrom I2C0_MDR_R [7:0]

6)repeatSteps2-5untildone

Timingiscomplicatedandpotentiallyconfusing.Fortunately,itishandledbyhardwareyouarealreadygiven.Therearetwocomplicatedthingsforwhichyouareresponsible.ThefirstissueisunderstandingwhatcodestosendtotheI2Cslave.ThesecondcomplicatedissueisStep3.Thereare4bitstowriteto I2C0_MCS_R [3:0],andtheydependonthemaster’scurrentstateandnextstateasdescribedinTable7.12.

#defineI2C_MCS_ACK0x00000008//DataAcknowledgeEnable

#defineI2C_MCS_ADRACK0x00000004//AcknowledgeAddress

#defineI2C_MCS_STOP0x00000004//GenerateSTOP

#defineI2C_MCS_START0x00000002//GenerateSTART

#defineI2C_MCS_ERROR0x00000002//Error

#defineI2C_MCS_RUN0x00000001//I2CMasterEnable

#defineMAXRETRIES5//numberofreceiveattemptsbeforegivingup

uint16_tI2C_Recv2(uint8_tslave)uint8_tdata1,data2;

intretryCounter=1;

do

while(I2C0_MCS_R&0x00000001);//waitforI2Cready

I2C0_MSA_R=(slave<<1)&0xFE;//MSA[7:1]isslaveaddress

I2C0_MSA_R|=0x01;//MSA[0]is1forreceive

I2C0_MCS_R=(I2C_MCS_ACK//positivedataack

|I2C_MCS_START//nostop,yesstart/restart

|I2C_MCS_RUN);//masterenable

while(I2C0_MCS_R&0x00000001);//waitfortransmissiondone

data1=(I2C0_MDR_R&0xFF);//MSBdatasentfirst

I2C0_MCS_R=(I2C_MCS_STOP//generatestop,nostart

|I2C_MCS_RUN);//masterenable

while(I2C0_MCS_R&0x00000001);//waitfortransmissiondone

data2=(I2C0_MDR_R&0xFF);//LSBdatasentlast

retryCounter=retryCounter+1;//incrementretrycounter

//repeatiferror

while(((I2C0_MCS_R&(I2C_MCS_ADRACK|I2C_MCS_ERROR))!=0)

&&(retryCounter<=MAXRETRIES));

return(data1<<8)+data2;//usuallyreturns0xFFFFonerror

Program7.7.I2Creceptioninsinglemastermode(I2C_xxx.zip).Figure7.32showsalogicanalyzermeasurementtakenwithProgram7.7communicatingwithaTexasInstrumentsTMP102thermometer.Themainprogramcalls I2C_Recv2(0x48); Thefirsttransmissionsendsthe0x91(slaveaddress,read)command.Itthenreceivestwotransmissions,whichisthetemperatureencodedwith0.0625°Cresolution.

Figure7.32.LogicanalyzertransmissionofI2C_Recv2,withoneoutputandtwoinputs.

I2Ccanbeverydifficulttoconfigure.WesuggestyouobvervetheSCLandSDLonanoscilloscope.Firstchecktheclockrateandsecondverifytheoutputhighandoutputlowvoltagesarewithinspecification.LookupVIHandVILofboththemicrocontrollerandtheremotesensor.ThehighvoltagemeasuredbythescopeshouldbehigherthanVIHofbothdevices.ThelowvoltagemeasuredbythescopeshouldbelowerthanVILofbothdevices.

7.7.IntroductiontoUniversalSerialBus(USB)

7.7.1.BasicPrinciplesTheUniversalSerialBus(USB)isahost-controlled,token-basedhigh-speedserialnetworkthatallowscommunicationbetweenmanyofdevicesoperatingatdifferentspeeds.TheobjectiveofthissectionisnottoprovideallthedetailsrequiredtodesignaUSBinterface,butratheritservesasanintroductiontothenetwork.Thereis650-pagedocumentontheUSBstandard,whichyoucandownloadfromhttp://www.usb.org.Inaddition,therearequiteafewwebsitessetuptoassistUSBdesigners,suchastheonetitled“USBinaNutShell”athttp://www.beyondlogic.org/usbnutshell/.

Thestandardismuchmorecomplexthantheothernetworkspresentedinthischapter.Fortunately,however,thereareanumberofUSBproductsthatfacilitateincorporatingUSBintoanembeddedsystem.Inaddition,theUSBcontrollerhardwarehandlesthelow-levelprotocol.USBdevicesusuallyexistwithinthesameroom,andaretypicallylessthan4metersfromeachother.USB2.0supportsthreespeeds.

HighSpeed-480Mbits/s

FullSpeed-12Mbits/s

LowSpeed-1.5Mbits/sTheoriginalUSBversion1.1supportedjustfullspeedmodeandalowspeedmode.TheUniversalSerialBusishost-controlled,whichmeansthehostregulatescommunicationonthebus,andtherecanonlybeonehostperbus.Ontheotherhand,theOn-The-Gospecification,addedinversion2.0,includesaHostNegotiationProtocolthatallowstwodevicesnegotiatefortheroleofhost.TheUSBhostisresponsibleforundertakingalltransactionsandschedulingbandwidth.Datacanbesentbyvarioustransactionmethodsusingatoken-basedprotocol.USBusesatieredstartopology,usingahubtoconnectadditionaldevices.Ahubisatthecenterofeachstar.Eachwiresegmentisapoint-to-pointconnectionbetweenthehostandahuborfunction,orahubconnectedtoanotherhuborfunction,asshowninFigure7.33.Becausethehubprovidespower,itcanmonitorpowertoeachdeviceswitchingoffadevicedrawingtoomuchcurrentwithoutdisruptingotherdevices.Thehubcanfilterouthighspeedandfullspeedtransactionssolowerspeeddevicesdonotreceivethem.BecauseUSBusesa7-bitaddress,upto127devicescanbeconnected.

Figure7.33.USBnetworktopology.

TheelectricalspecificationforUSBwasintroducedinFigure7.10andTable7.5,usingfourshieldedwires(+5Vpower,D+,D-andground).TheD+andD-aretwistedpairdifferentialdatasignals.ItusesNonReturntoZeroInvert(NRZI)encodingtosenddatawithasyncfieldtosynchronizethehostandreceiverclocks.

USBdriverswilldynamicallyloadandunload.Whenadevicepluggedintothebus,thehostwilldetectthisaddition,interrogatethedeviceandloadtheappropriatedriver.Similarly,whenthedeviceisunplugged,thehostwilldetectitsabsenceandautomaticallyunloadthedriver.TheUSBarchitecturecomprehendsfourbasictypesofdatatransfers:

•ControlTransfers:Usedtoconfigureadeviceatattachtimeandcanbeusedforotherdevice-specificpurposes,includingcontrolofotherpipesonthedevice.

•BulkDataTransfers:Generatedorconsumedinrelativelylargequantitiesandhavewidedynamiclatitudeintransmissionconstraints.

•InterruptDataTransfers:Usedfortimelybutreliabledeliveryofdata,forexample,charactersorcoordinateswithhuman-perceptibleechoorfeedbackresponsecharacteristics.

•IsochronousDataTransfers:OccupyaprenegotiatedamountofUSBbandwidthwithaprenegotiateddeliverylatency.(Alsocalledstreamingreal-timetransfers).

Isochronoustransferallowsadevicetoreserveadefinedaboutofbandwidthwithguaranteedlatency.Thisisappropriateforreal-timeapplicationslikeinaudioorvideoapplications.Anisochronouspipeisastreampipeandis,therefore,alwaysunidirectional.Anendpointdescriptionidentifieswhetheragivenisochronouspipe’scommunicationflowisintooroutofthehost.Ifadevicerequiresbidirectionalisochronouscommunicationflow,twoisochronouspipesmustbeused,oneineachdirection.

AUSBdeviceindicatesitsspeedbypullingeithertheD+orD-lineto3.3V,asshowninFigure7.34.Apull-upresistorattachedtoD+specifiesfullspeed,andapull-upresistorattachedtoD-meanslowspeed.Thesedevice-sideresistorsarealsousedbythehostorhubtodetectthepresenceofadeviceconnectedtoitsport.Withoutapull-upresistor,thehostorhubassumesthereisnothingconnected.Highspeeddevicesbeginasafullspeeddevice(1.5kto3.3V).Onceithasbeenattached,itwilldoahighspeedchirpduringresetandestablishahighspeedconnectionifthehubsupportsit.Ifthedeviceoperatesinhighspeedmode,thenthepull-upresistorisremovedtobalancetheline.

Figure7.34.Pull-upresistorsonUSBdevicessignalspecifythespeed.Likemostcommunicationsystems,USBismadeupofseverallayersofprotocols.LiketheCANnetworkpresentedearlier,theUSBcontrollerswillberesponsibleforestablishingthelow-levelcommunication.EachUSBtransactionconsistsofthreepackets

TokenPacket(header),OptionalDataPacket,(information)andStatusPacket(acknowledge)

Thehostinitiatesallcommunication,beginningwiththeTokenPacket,whichdescribesthetypeoftransaction,thedirection,thedeviceaddressanddesignatedendpoint.Thenextpacketisgenerallyadatapacketcarryingtheinformationandisfollowedbyahandshakingpacket,reportingifthedataortokenwasreceivedsuccessfully,oriftheendpointisstalledornotavailabletoacceptdata.Dataistransmittedleastsignificantbitfirst.SomeUSBpacketsareshowninFigure7.35.Allpacketsmuststartwithasyncfield.Thesyncfieldis8bitslongatlowandfullspeedor32bitslongforhighspeedandisusedtosynchronizetheclockofthereceiverwiththatofthetransmitter.PID(PacketID)isusedtoidentifythetypeofpacketthatisbeingsent,asshowninTable7.14.

Theaddressfieldspecifieswhichdevicethepacketisdesignatedfor.Being7bitsinlengthallowsfor127devicestobesupported.Address0isnotvalid,asanydevicewhichisnotyetassignedanaddressmustrespondtopacketssenttoaddresszero.Theendpointfieldismadeupof4bits,allowing16possibleendpoints.Lowspeeddevices,howevercanonlyhave2additionalendpointsontopofthedefaultpipe.CyclicRedundancyChecksareperformedonthedatawithinthepacketpayload.Alltokenpacketshavea5-bitCRCwhiledatapacketshavea16-bitCRC.EOPstandsforEndofpacket.StartofFramePackets(SOF)consistofan11-bitframenumberissentbythehostevery1ms±500nsonafullspeedbusorevery125µs±0.0625µsonahighspeedbus.

Figure7.35.USBpackettypes.

Group PIDValue

PacketIdentifier

Token

0001 OUTToken,Address+endpoint

1001 INToken,Address+endpoint

0101 SOFToken,Start-of-Framemarkerandframenumber

1101 SETUPToken,Address+endpoint

Data

0011 DATA0

1011 DATA1

0111 DATA2(highspeed)

1111 MDATA(highspeed)

Handshake

0010 ACKHandshake,Receiveracceptserror-freedatapacket

1010 NAKHandshake,devicecannotacceptdataorcannotsenddata

1110 STALLHandshake,Endpointishaltedorpiperequestnotsupported

0110 NYET(NoResponseYetfromreceiver)

Special

1100 PREamble,Enablesdownstreambustraffictolow-speeddevices.

1100 ERR,SplitTransactionErrorHandshake

1000 Split,High-speedSplitTransactionToken

0100 Ping,High-speedflowcontrolprobeforabulk/controlendpoint

Table7.14.USBPIDnumbers.

USBfunctionsareUSBdevicesthatprovideacapabilityorfunctionsuchasaPrinter,ZipDrive,Scanner,Modemorotherperipheral.Mostfunctionswillhaveaseriesofbuffers,typically8byteslong.Endpointscanbedescribedassourcesorsinksofdata,shownas EP0In , EP0Out etc.inFigure7.36.Asthebusishostcentric,endpointsoccurattheendofthecommunicationschannelattheUSBfunction.Thehostsoftwaremaysendapackettoanendpointbufferinaperipheraldevice.Ifthedevicewishestosenddatatothehost,thedevicecannotsimplywritetothebusasthebusiscontrolledbythehost.Therefore,itwritesdatatoendpointbufferspecifiedforinput,andthedatasitsinthebufferuntilsuchtimewhenthehostsendsaINpackettothatendpointrequestingthedata.Endpointscanalsobeseenastheinterfacebetweenthehardwareofthefunctiondeviceandthefirmwarerunningonthefunctiondevice.

Figure7.36.USBdataflowmodel.Whilethedevicesendsandreceivesdataonaseriesofendpoints,theclientsoftwaretransfersdatathroughpipes.Apipeisalogicalconnectionbetweenthehostandendpoint(s).Pipeswillalsohaveasetofparametersassociatedwiththemsuchashowmuchbandwidthisallocatedtoit,whattransfertype(Control,Bulk,IsoorInterrupt)ituses,adirectionofdataflowandmaximumpacket/buffersizes.StreamPipescanbeusedsendunformatteddata.Dataflowssequentiallyandhasapre-defineddirection,eitherinorout.Streampipeswillsupportbulk,isochronousandinterrupttransfertypes.Streampipescaneitherbecontrolledbythehostordevice.MessagePipeshaveadefinedUSBformat.Theyarehost-controlled,whichareinitiatedbyarequestsentfromthehost.Dataisthentransferredinthedesireddirection,dictatedbytherequest.Thereforemessagepipesallowdatatoflowinbothdirectionsbutwillonlysupportcontroltransfers.

7.7.2.ModularUSBInterfaceTherearetwoapproachestoimplementingaUSBinterfaceforanembeddedsystem.In

themodularapproach,wewillemployaUSB-to-parallel,orUSB-to-serialconverter.ThemodularapproachisappropriateforaddingUSBfunctionalitytoanexistingsystem.Forabout$30,wecanbuyaconvertercablewithaUSBinterfacetoconnecttothepersonalcomputer(PC)andaserialinterfacetoconnecttotheembeddedsystem,asshowninFigure7.37.TheembeddedsystemhardwareandsoftwareisstandardRS232serial.ThesesystemscomewithPCdevicedriverssothattheUSB-serial-embeddedsystemlookslikeastandardserialport(COM)tothePCsoftware.TheadvantageofthisapproachisthatsoftwaredevelopmentonthePCandembeddedsystemissimple.ThedisadvantageofthisapproachisnoneofthepowerandflexibilityofUSBisutilized.Inparticular,thebandwidthislimitedbytheRS232line,andthedatastreamisunformatted.SimilarproductsareavailablethatconvertUSBtotheparallelport.Companiesthatmaketheseconvertersinclude

IOGearInc.http://www.iogear.com

WyseTechnologyhttp://www.wyse.com

D-LinkCorporationhttp://www.dlink.com

ComputerPeripheralSys,Inc.http://www.cpscom.com

Jo-DanInternational,Inc.http://www.jditech.com

Figure7.37.ModularapproachtoUSBinterfacing.ThesecondmodularapproachistopurchaseaUSBparallelinterfacemodule.Thesedevicesallowyoutosendandreceivedatausingparallelhandshakeprotocolssimilartotheinput/outputexamplesinChapter3.TheytypicallyincludeaUSB-enabledmicrocontrollerandreceiver/transmitFIFObuffers.Thisapproachismoreflexiblethantheserialcablemethod,becauseboththemicrocontrollermoduleandtheUSBdriverscanbetailoredpersonalized.Inparticular,somemodulesallowyoutoburnPIDandVIDnumbersintoEEPROM.Theadvantages/disadvantagesofthisapproacharesimilartotheserialcable,inthatthedataisunformattedandyouwillnotbeabletoimplementhigh

bandwidthbulktransfersornegotiateforreal-timebandwidthavailablewithisochronousdatatransfers.Companiesthatmakethesemodulesinclude

FutureTech.DevicesInter.Ltd.http://www.ftdichip.com/

ActiveWire,Inc.http://www.activewireinc.com

DLPDesign,Inc.http://www.dlpdesign.com

ElexolPtyLtd.http://www.elexol.com

7.7.3.IntegratedUSBInterfaceThesecondapproachtoimplementingaUSBinterfaceforanembeddedsystemistointegratetheUSBcapabilityintothemicrocontrolleritself.Thismethodaffordsthegreatestflexibilityandperformance,butrequirescarefulsoftwaredesignonboththemicrocontrollerandthehost.Overthelast15yearsUSBhasbeenreplacingRS232serialcommunicationasthepreferredmethodforconnectingembeddedsystemstothepersonalcomputer.ManufacturersofmicrocontrollershaveintroducedversionsoftheproductwithUSBcapability.EverycompanythatproducesmicrocontrollershasmembersofthefamilywithUSBfunctionality.ExamplesincludetheMicrochipPIC18F2455,AtmelAT89C5131A,FTDIFT245BM,FreescaleMCF51Jx,STMicrosystemsSTM32F102,TexasInstrumentsMSP430F5xx,andTexasInstrumentsLM4F120/TM4C123.Figure7.38showstheUSBconfigurationontheEK-TM4C123GXLLaunchPadEvaluationKit,whichiscapableofoperatingasadevice,ahostoron-the-go(OTG).TouseUSBpopulateR25andR29.TheLM4F120canbeonlyconfiguredasaUSBdevice.TheTivaWare ® softwarelibraryhas14exampleprojectsforthisevaluationboard,includingserialporttranslator,securedigitalcard,mouse,andkeyboardinterface.

Figure7.38.TheTM4C123LaunchPadkitsupportsUSBhost,device,andOTGmodes.TooperateaUSBinterfaceatfullspeedDMAsynchronizationwillberequired,sothatdataistransferreddirectlyfrommemorytoUSBoutput,orfromUSBinputtomemory.

7.8.Exercises7.1Foreachtermgiveadefinitionin32wordsorless.

a)Asynchronousb)Baudratec)Bandwidth

d)Breake)DCEf)DTE

g)Evenparityh)Fullduplexi)Frame

j)Framingerrork)Halfduplexl)Mark

m)Negativelogicn)NRZo)NRZI

p)Opendrainq)Overrunr)Positivelogic

s)Simplext)Spaceu)Synchronous

7.2In32wordsorless,describethesimilaritiesanddifferencesbetweenthesepairsofterms

a)Baudrateversusbandwidthb)Positivelogicversusnegativelogic

c)XONversusXOFFd)Fullduplexversushalfduplex

e)DS275versusMAX232f)SCIversusSPI

g)NRZversusNRZIh)DTEversusDCE

7.3Whatfundamentalelectricalpropertyisusedtotransferdigitaldataacrossadistance?

A)VoltageD)Frequency

B)CurrentE)Phase

C)EnergyF)Wavelength

7.4LookupintheLM3S/LM4F/TM4CdatasheetwhattheBUSYbitintheUARToperates.Inparticular,whenisitset?Whenisitclear?Canthesoftwareclearit?Canittriggeraninterrupt?

7.5In16wordsorless,explainwhytheUARTreceiverISRinProgram5.9interruptsonbothRXRISandRTRIS.InparticularwhendoesRXRIStriggerandwhendoesRTRIStrigger?

7.6DrawaplotsimilartoFigure7.2forthebinarydata00110111.

7.7DrawaplotsimilartoFigure7.2forthebinarydata10011100.

7.8Consideraserialportoperatingwithabaudrateof10,000bitspersecond.DrawthewaveformoccurringattheU0Txoutput(voltagelevelsare+3.3and0)whentheASCII‘a’(0x61)istransmittedonUART0.Theprotocolis1start,8dataand1stopbit.UART0isinitiallyidle,andthesoftwarewritesthe0x61to UART0_DR_R attime=0.ShowtheU0Txlinebeforeandaftertheframe,assumingthechannelisidlebeforeandaftertheframe.

7.9Consideraserialportoperatingwithabaudrateof1000bitspersecond.DrawthewaveformoccurringattheU1Txoutput(voltagelevelsare+3.3and0)whentheASCII‘B’(0x42)istransmittedonUART1.Theprotocolis1start,8dataand1stopbit.UART1isinitiallyidle,andthesoftwarewritesthe0x42to UART1_DR_R attime=0.ShowtheU1Txlinebeforeandaftertheframe,assumingthechannelisidlebeforeandaftertheframe.

7.10AssumetheUARTbaudrateis1000bits/secandtheprotocolis1start,8data,noparityand1stopbit.Whatisthechannelbandwidth?Ifweused2stopbitsinsteadof1withoutchangingthebaudrate,wouldthebandwidthbehigher,lowerorthesame?

7.11ThedatainFigure7.39wasmeasuredonaU0Rxserialinput,whichwethinkisoneframe,butitmightbetwoframes.Theserialformatis1-start,8-bit,and1-stopbit.

a)WhatisthebaudrateinCaseA?

b)WhatdataisbeingtransferredinCaseA?Givethenumber(s)inhexadecimal.

c)WhatisthebaudrateinCaseB?

d)WhatdataisbeingtransferredinCaseB?Givethenumber(s)inhexadecimal.

Figure7.39.Serialtransmissionforexercise7.11.7.12RXRISandRTRISinterruptsarearmedsothatinterruptsoccurwhennewdataarrivesintotheLM3S/LM4F/TM4C.ConsiderthesituationlikeProgram5.9inwhichasoftwareFIFOqueueisusedtobufferdatabetweentheISRandthemainprogram.TheISRreadsdatafromtheUARTandsavesthedatabycalling Fifo_Put .Whenthemainprogramwantsinputitcalls UART_InChar ,whichinturncalls Fifo_Get .ExperimentalobservationsshowthisFIFOisusuallyempty,andhasatmost3elements.Whatdoesitmean?ChooseA-F.

A)ThesystemisCPUbound

B)BandwidthcouldbeincreasedbyincreasingFIFOsize

C)ThesystemisI/Obound

D)TheFIFOcouldbereplacedbyaglobalvariable

E)Thelatencyissmallandbounded

F)Interruptsarenotneededinthissystem

7.13AslavedevicewillbeinterfacedtothemasterLM3S/LM4F/TM4CusingSSI.ThetimingisshowninFigure7.40.TherearethreesignalsthatwillbeoutputsoftheLM3S/LM4F/TM4Candinputstothedevice(Enable,Clock,andData).Thetimingoftheexternaldeviceisshownbelow.AssumingaFreescaleSPIprotocol,whatSPH,SPOmodeshouldyouuse?

Figure7.40.Serialtransmissionforexercise7.13.D7.14.WritesoftwarethatcreatestheSSIprotocolonaregulardigitalport.Morespecifically,interfacetheDACinExample7.2toportpinsPD3,PD2,PD1,andPD0.(PD0isnotused).

D7.15.WritesoftwarethatcreatestheSSIprotocolonaregulardigitalport.Morespecifically,interfacetheADCinExample7.3toportpinsPB3,PB2,PB1,andPB0.

D7.16.WritesoftwarethatcreatestheSSIprotocolonaregulardigitalport.Morespecifically,interfacethe74HC595inExample7.4toportpinsPD3,PD2,PD1,andPD0.(PD0isnotused).

D.7.17Thisproblemaddressestheissueofcapacitiveloadingonahigh-speedserialtransmissionlinelikeSSI.TheSSIportsoftwomicrocontrollersareconnectedwithaVERYlongcable.Wewillmodelthiscableasasingleresistorinserieswithacapacitor,asshowninFigure7.41.Forthisquestion,assumeanidealtransmitter(outputimpedanceof0)andanidealreceiver(inputimpedanceofinfinity).LettheresistanceRbe1 ,andthecapacitanceCbe10nF.Considera3.3-V100-nspulse(T=100ns)ontheoutputofthetransmitter(labeledasVin)asmightoccurwitha5-MbpsSSItransmission.DeriveanequationforVoutasafunctionoftimeforthefirst100ns.Showyourwork.CalculatevaluesforVoutattimeequals0andtimeequals100ns.CreateaVinversustimesketchsimilartotherightofFigure7.41,andaddasketchofVoutonthissameplot.Showboth0totimeTandtimeTtotime2T.

Figure7.41.CircuitmodelforexercisesD7.17andD7.18.D.7.18SolveexerciseD7.17withR=10 ,C=100nFandT=10ns.Justifywhetherornotthissystemwillwork?

D7.19Theoutputofoneshiftregisterisconnectedtotheinputofasecondshiftregister,asshowninFigure7.42.Thetworegistersarecontrolledbythesame50%dutycycleClock.Theperiodoftheclockist1.ThedataisshiftedoutonthefallingedgeofClock.Thetimet2=[10nsmin,200nsmax]isthedelayfromthefallingedgeofClockuntilwhentheoutputisvalid.ThedataisshiftedintothesecondregisterontherisingedgeofClock.Thetimet3=50nsisthetimebeforetherisingedgeofClockthatthedatamustbevalid.Thetimet4=20nsisthetimeafterthatsamerisingedgeofClockthatthedatamustcontinuetobevalid.Whatisthesmallestt1clockperiodthatreliablytransversesdatafromoneshiftregistertotheother?

Figure7.42.CircuitandtimingforexerciseD7.19.

7.9.LabAssignmentsLab7.1TheoverallobjectiveistoredesigntheLab6.2or6.3dataacquisitionsystemtoemploytwomicrocontrollers.Theslavemicrocontrollerwillperformthedataacquisition(positionortemperature),andthemastermicrocontrollerwillinterfacetothePC.ThetwomicrocontrollerswillbelinkedusingtheirSSIports.Themastermicrocontrollerwillfetchdatafromtheslave,andtransmitittothePC,usingUARTinterruptsynchronization.TheuserwillinteractwithaPCrunningPuTTY.

Lab7.2TheoverallgoalistodevelopaninterruptingUARTdevicedriverthatimplementsfixed-pointinput/output.Thefixed-pointconstantis0.001.Thefull-scalerangeisfrom0to65.534.The Fix_InDec functionshouldprovideforflexibleoperation.Forexample“50.5”isreturnedas50500,and“1.4595”isreturnedas1460.Theoperatorcreatestheinputbytypingonthekeyboard,andtheoutputisanumberpassedbackasthereturnparameterofthefunction.Noticethat Fix_InDecroundstheinputtotheclosestfixed-pointresult(e.g.,1.4595roundsto1.460and1.4604alsoroundsto1.460).Somenumberslike1.2345678mightbeconsideredillegalbecausetheycauseoverflowofintermediateresults.Inthecommentsofyoursoftware,pleasediscusswhyyouchoseyourparticularimplementationmethodovertheotheravailablechoices.Pleasehandlethebackspacecharacter,allowingtheoperatortoerasecharacters.Inparticular,youarefreetouseiterativeorrecursivealgorithms.Youarefreetomodifytheprototypesaswellashandleillegalinputsinanywayyoufeelisappropriate.Youmustdetectillegalinput,butyouhaveachoiceastohowyoursystemrespondstotheillegalinput.Onepossibilityforhandlinganillegalnumberwouldbetoreturn65535,whichyoucoulddefineasanillegalnumber.Asecondpossibilityforwhenanillegalnumberistypedistooutputanerrormessage,andrequiretheoperatortoenterthenumberagain.Forexample,50isoutputas“0.050”and12345isoutputas“12.345”.

Lab7.3.TheoverallgoalistointerfaceaDS1620temperaturecontrollertothemicrocontroller.Createthefunctionstoinitialize,sethightemperature,setlowtemperatureandreadcurrenttemperature.Whenreadingthetemperatureyouwillhavetostartaconversionandwaitfortheconversiontobecomplete.Performexperimentstodeterminetheaccuracyofthetemperaturemeasurement.InterfacethreeLEDstoTHTLTCOMoutputs,andperformexperimentstoverifythetemperaturecontrollerintheDS1620isoperatingproperly.

Lab7.4.Theoverallobjectiveofthislabistodesign,implementandtestanoutputportexpander.YouwillusethreeI/Opinsofthemicrocontroller,andfour74HC595shiftregisters.Youwilldesignhardwareandsoftwarethatsupportsfour8-bitoutputports.Theoutputportsdonotneedtobereadable.Measurehowlongittakesforthemicrocontrollertoperformoutputstoall32bits.

Lab7.5Theoverallobjectiveofthislabistodesign,implementandtestaninputportexpander.YouwillusetheeI/Opinsofthemicrocontroller,andfour74HC165shiftregisters.Youwilldesignhardwareandsoftwarethatsupportsfour8-bitinputports.Theinputportsdonotneedtobelatchedbyanexternalsignal.Measurehowlongittakesforthemicrocontrollertoperforminputsfromall32bits.

Lab7.6TheobjectiveofthislabistodesignadigitalclockusingaDS1307externalclockchip.ThefirststepistointerfacetheclockchiptothemicrocontrollerusinganI2Cnetwork.Thesecondstepistodesignlow-leveldriverstoallowthemicrocontrollertosendandreceivedatafromtheDS1307.Thenextsoftwarelayerincludesfunctionslike SetTime FormatTime and ReadTime .ThehighestlevelisamainprogramthatimplementsadigitalclockusinganLEDorLCDdisplay.2,3,or4momentaryswitcheswillbeusedtocontroltheoperationofthedigitalclock.

Lab7.7TheobjectiveofthislabistodesignadigitalthermometerusingaDS1631Aexternalthermometerchip.ThefirststepistointerfacethethermometerchiptothemicrocontrollerusinganI2Cnetwork.Thesecondstepistodesignlow-leveldriverstoallowthemicrocontrollertosendandreceivedatafromtheDS1631A.Thenextsoftwarelayerincludesfunctionslike SetMode and ReadTemperature .ThehighestlevelisamainprogramthatimplementsadigitalthermometerusinganLEDorLCDdisplay.2or3momentaryswitcheswillbeusedtocontroltheoperationofthedigitalthermometer.

8.AnalogInterfacingChapter8objectivesareto:• Designanalogamplifiersandfilters

•StudybuildingblocksfordataacquisitionincludingDACandADC

•DiscussthefunctionalityoftheADContheLM3SandTM4C

Mostembeddedsystemsincludecomponentsthatmeasureand/orcontrolrealworldparameters.Theserealworldparameters,likeposition,speed,temperature,andvoltage,usuallyexistinacontinuousoranalogform.Therefore,thedesignofanembeddedsysteminvolvingtheseparametersrarelyusesonlybinaryordigitallogic.Rather,wewilloftenneedtoamplify,filterandeventuallyconverttodigitalformthesesignals.Inthischapterwewilldeveloptheanalogcircuitbuildingblocksusedinthedesignofdataacquisitionsystemsandcontrolsystems. Acomputerengineerinterestedinthefieldofembeddedsystemswillfindmorejobopportunitiesifheorshecandevelopmicrocontrollerskillsalongwithanalogcircuitdesignskills.

8.1.ResistorsandCapacitors

8.1.1.ResistorsAsengineers,weuseresistorsandcapacitorsformanypurposes.Theresistororcapacitortypeisdefinedbythemanufacturingprocess,thematerialsused,andthetestingperformed.Theperformanceandcostofthesedevicesvarysignificantly.Forexample,a5%1/4wattcarbonresistorcostslessthan1¢,whilea0.01%thinfilmorwirewoundresistormaycost$6.Itisimportanttounderstandbothourcircuitrequirementsandtheresistorparameterssothatwematchthecorrectresistortypetoeachapplicationyieldinganacceptablecost-performancebalance.Wemustspecifyinourtechnicaldrawingsthedevicetypeandtolerance(e.g.,1%metalfilm),sothatyourprototypecanbeeffectivelymanufactured.ThecharacteristicsofvariousresistortypesareshowninTable8.1.

Type Range Tolerance Temperaturecoef

MaxPower

Carboncomposition

1Ωto22MΩ 5to10% 200to700ppm/˚C

1W

Carbonfilm 1Ωto22MΩ 1to10% 200to1500ppm/˚C

2W

Metalfilm 0.01Ωto68MΩ

>0.05% 2to300ppm/˚C 1W

Thickandthinfilm

0.001Ωto100GΩ

0.1to20%

5to1000ppm/˚C

100W

Wirewound 0.005Ωto167kΩ

>0.0005%

10to900ppm/˚C

30W

Table8.1.Generalspecificationofvarioustypesofresistorcomponents.

Carboncompositionresistorsaremanufacturedwithhot-pressedcarbongranules.Variousamountsoffillerareaddedtoachieveawiderangeofresistancevalues.Filmresistorsaremadebydepositingpuremetals,suchasnickel,oranoxidefilm,suchastin-oxide,ontoaninsulatingceramicrodorsubstrate.Awirewoundresistor,ismadebywindingathinmetalalloywiresuchasNichromeontoaninsulatingceramicintheformofaspiralhelix.

Theleastexpensivetypeofathrough-holeresistoriscarbonfilm.Weaddthemtodigitalcircuitsas+3.3-Vpull-ups.Inordertoimprovetheaccuracyandstabilityofourprecisionanalogcircuits,wewilluseresistorswithalowertoleranceandsmallertemperaturecoefficient.Formostapplications1%thickfilmormetalfilmresistorswillbesufficienttobuildouranalogamplifiercircuits.Forsurfacemountconstructionweuseeitherthickfilmorthinfilmresistors,whichcomeinawiderangeofsizesandtolerances.Wirewoundresistorsaremanufacturedbytwistingaverylongverythinwirelikeaspring.Thewireiscoiledupanddownashaftinsuchawaytotryandcanceltheinductance.Sincesomeinductanceremains,wirewoundresistorsshouldnotbeusedforhighfrequency(above1MHz)applications.

Observation:Allresistorsproducewhite(thermal)noise.

Observation:Wirewoundresistorsdonotgenerate1/fnoise,whereastheothertypesdo.

8.1.2.CapacitorsSimilarly,capacitorscomeinawidevarietyofsizesandtolerances.Polarizedcapacitorsoperatebestwhenonlypositivevoltagesareapplied.Nonpolarizedorbipolarcapacitorsoperateforbothpositiveandnegativevoltages.Weselectacapacitorbasedonthefollowingparameters:capacitancevalue,polarized/nonpolarized,maximumvoltagelevel,tolerance,leakagecurrent(resistance),temperaturecoefficient,usefulfrequencyresponse,andtemperaturerange.Themaximumvoltageratingisimportantforhighvoltagecircuits,butisoflesserimportanceforembeddedmicrocontrollersystems.Table8.2comparesvariouscapacitortypeswecoulduseinourcircuit.

Type Range Tolerance Tempcoef

Leakage Frequencies

Polystyrene 10pFto2.7µF

±0.5% Excellent 10GΩ 0to1010Hz

Polypropylene 100pFto50µF

Excellent Good Excellent

Teflon 1000pFto2µF

Excellent Best Best

Mica 1pFto0.1µF ±1to±20% 1000MΩ 103to1010Hz

Ceramic 1pFto0.01µF

±5to±20% Poor 1000MΩ 103to1010Hz

100MΩ

Paper(oil-soaked)

1000pFto50µF

±10to±20%

100to108Hz

Mylar(polyester)

5000pFto10µF

±20% Poor 10GΩ 103to1010Hz

Tantalum 0.1µFto220µF

±10% Poor

Electrolytic 0.47µFto0.01F

±20% Ghastly 1MΩ 10to104Hz

Table8.2.Generalspecificationofvarioustypesofcapacitorcomponents(WolfandSmith,StudentReferenceManual,PrenticeHall,pg.302,1990andHorowitzandHill,TheArtofElectronics,CambridgeUniversityPress,pg.22,1989).

Wewillusecapacitorsfortwopurposesinourmicrocontroller-basedembeddedsystems.First,wewillplacethemontheDCpowerlinestofilterthesupplyvoltagetoourcircuits.Avoltagesupplytypicallywillincluderipple,whichisaddednoiseontopoftheDCvoltagelevel.Therearetwophysicallocationstoplacethesupplyfilters.Thefirstlocationisattheentrypointofthesupplyvoltageontothecircuitboard.Therearetwoapproachestothisboard-levelfilter.Ifthesupplynoiseismostlyvoltageripple,thentwocapacitorsinparallelcanbeused.Thelargeamplitudepolarizedcapacitor(e.g.,1to47µFelectrolyticortantalum)willremovelowfrequencylargeamplitudevoltagenoise,andthenonpolarizedcapacitor(e.g.,0.01to0.47µFceramic)willremovehighfrequencyvoltagenoise.Twodifferenttypesofcapacitorsareusedbecausetheyareeffective(i.e.,behavelikeacapacitor)atdifferentfrequencies.The∏filter(CLC)isveryeffectiveinremovingcurrentspikenoise.TheCLCparametersdependontheamplitudeofthecurrentripple.TheinductorinFigure8.1canbeaferritebead.AtDCthebeadisessentiallyashortcurrent.Theferritebeadincreasesbothitsrealandreactiveimpedanceathighfrequencies.Thebeadshouldbeselectedtohavelargeimpedanceatthedigitalclockfrequency.Panasonicmakesaseriesofferritebeads.ThePanasonicEXC-ELDR25ChasaDCresistanceof0.08 ,canconduct7ADC,buthasan80- impedanceat24MHz.

Figure8.1.DCsupplyfilters.

Inadditiontotheboard-levelsupplyfilter,wewilladdbypasscapacitorsatthesupplypinsofeachchip.ItwillbeimportanttoplacethesecapacitorsasclosetothepinasphysicallypossibleasillustratedinFigure8.2.Anonpolarizedcapacitor(e.g.,0.01to0.1µFceramic)willsmooththesupplyvoltageasseenbythechip.Placingthecapacitorclosetothechippreventscurrentsurgesfromonechipfromaffectingthevoltagesupplyofanother.

Figure8.2.PCBlayoutpositioningthebypasscapacitorclosetothechip.Thesecondapplicationofcapacitorsinourembeddedsystemswillbeinthelinearanalogcircuitsofthelowpassfilter,thehighpassfilter,thederivativecircuit,andtheintegratorcircuit.Fortheseapplicationswewillselectanonpolarizedcapacitorevenifthesignalamplitudeisalwayspositive.Inaddition,weusuallywantalow-tolerance,low-leakagecapacitortoimprovetheaccuracyofthelinearanalogcircuit.Ceramiccapacitorsarealow-costmedium-qualitychoiceforanalogcircuitdesign.Theycomeinthreetolerances,seeTable8.2.ThebestceramicisC0G,whichhasa1to10%toleranceandatemperaturecoefficientof30ppm/oCor±0.3%over-55to125oC.ThemiddlegradeisX7Rceramic,whichhasa5to20%toleranceandatemperaturecoefficientof±15%over-55to125oC.ThelowestcostceramicisZ5U,whichhasa20%toleranceandatemperaturecoefficientof22to-56%over10to86oC.WecanuseZ5Uforbypasscapacitorsonpowerlines,butweshoulduseeitherC0GorX7Rforanalogfilters.

Partnumber Type Tolerance Cost

06035A102FAT2A C0G ±1% $0.846

06035C102JAT2A X7R ±5% $0.11

06035C102KAT2A X7R ±10% $0.022

Table8.3.CostofAVXceramiccapacitors:1000pF,50V,surfacemount0603package(2013pricesonwww.digikey.comforquantity10).

Observation:Themaximumvoltageratingofacapacitorislimitedtoitssize.

Commonerror:Polarizedcapacitorsoftenhaveanonlinearcapacitanceversusfrequencyresponse,thereforeusingtheminananalogfilterwillcausedistortion.

Commonerror:Ifyoudesignanelectroniccircuitandneglecttoexplicitlyspecifytheresistorandcapacitortypes,thenanincorrectsubstitutionmayoccurinthelayout/manufacturingstageoftheproject.

8.2.OpAmps

Whilethedesignofanalogelectronicsisnotanexplicitobjectiveofthisbook,wewillincludeabriefdiscussionofanalogcircuitdesignissuesoftenrelatedtoembeddedsystems.Forexample,lowcost,smallsize,singlevoltagesupply,andlowpowerarefourcharacteristicstypicalofembeddedsystems.Otherfactorstoconsiderarereliability,noise,frequencyresponse,availability,andtemperaturerange.

Overadozenmanufacturersproducethousandsofanalogintegratedcircuits.Manufacturersinclude:AnalogDevices,AvagoTechnologies,CirrusLogic,Fairchild,Honeywell,Intersil,LinearTechnology,MaximIC,Microchip,NationalSemiconductor,NXPSemiconductors,ONSemiconductor,SiliconLaboratories,TexasInstruments,andToshiba.Thefactthattherearesomanyopampsavailablemakesthechoiceconfusing.Themanufacturersdopublishaselectionguideoftheirproductstoassistinfindinganappropriatepart.

Table8.4listsperformanceparametersoffouropamps,andthechoiceoftheseparticulardevicestypifythekindsofopampsusedinanembeddedsystem,buttheselectionoftheseparticularfourisnotmeantasarecommendation.Manyopampscomeinavarietyofpackagesizesandareavailablein1,2,or4opampsperpackage.Wewilluserail-to-railopamps,liketheonesinTable8.4todesignanalogcircuitsthatrunonasingle+3.3Vsupply.TheOPA330andMAX494arelow-powerdevices.

Theopamprailsareitstwopowersupplies,-Vsand+Vs.Atypicalopamppoweredwith±12‑Vrailsoperatesproperlywhenitsinputsandoutputsarebetween-10to+10V.Arail-to-railopampoperatesinalinearfashionforoutputvoltagesallthewayfromtheminusrailtotheplusrail.Someopampsoperate“rail-to-rail”forboththeinputsandoutput.

Singleopamp

Doubleopamp

Quadopamp

OPA330

OPA2330

OPA4330

OPA350

OPA2350

OPA4350

TLC2272

TLC2274

MAX492

MAX494

Description Lowpower

Highspeed Rail-to-Rail

Rail-to-Rail

K,Openloopgain

100dB 122dB 104dB 108dB

Rcm,Inputimpedance

4pF 1013Ω||6.5pF

1012Ω||8pF

2pF 2MΩ

Rdiff,Inputimpedance

1013Ω||2.5pF

1012Ω||8pF

Vos,Offsetvoltage

50V 0.5mV 3mV 0.5mV

Ios,Offsetcurrent 500pA 10pA 100pA 6nA

Ib,Biascurrent 1nA 10pA 100pA 60nA

en,Noisedensity 55nV/√Hz 5nV/√Hz 50nV/√Hz

25nV/√Hz

f1,Gain*bandwidthproduct

350kHz 38MHz 2.18MHz 500kHz

dV/dt,Slewrate 0.16V/ s 22V/ s 3.6V/ s 0.2V/ s

+Vs,Voltagesupply

1.5to5.5V

2.7to5.5V

0to5or±5V

2.7to6V

Is,Supplycurrent 35A 7.5mA 3mA 170 A

Cost $2 $4 $2 $6

Table8.4.Parametersforvariousrail-to-railCMOSopampsusedinthischapter(with–Vsgrounded).

Checkpoint8.1:Whatistherelationshipbetweenbandwidthandsupplycurrent?

Observation:Theopampshouldbemuchfasterthanthesignalyouaretryingtoprocess.

8.2.1.IdealOpAmpModelWewillbeginourdiscussionofopampswiththeidealopamp.Forsimpleanalogcircuitsusinghighqualitydevices,theidealmodelwillbesufficient.Withanidealopamp,theoutputvoltage,Vout,islinearlyrelatedtothedifferencebetweentheinputvoltages,

Vout=K•(Vy–Vx)

wherethegain,K,isaverylargenumber,asshowninFigure8.3.

Voltagerangesoftheinputsandoutputsareboundedbythesupplyvoltages,+Vsand-Vs.Opampcircuitsfoundinmanytraditionalanalogdesigntextbooksarepoweredwith±12-Vsupplies.Ontheotherhand,itwillreducesystemcosttopowerourembeddedsystemswithasinglevoltage.Morespecifically,ifthemicrocontrolleroperateswith+3.3-Vsupply,wewillruntheanalogcircuitsalsoon+3.3V.Inparticular,wewilluserail-to-railopampsandset+Vsto+3.3Vand-Vstoground.

Nomatterhowwepowertheanalogcircuits,weassumetheinputandoutputvoltageswillexistbetween-Vsand+Vs.Theinputcurrentsintotheopampareverysmall.Becausetheinputimpedanceoftheopampislarge,theidealmodelassumesIxandIyarezero.

Figure8.3.Regularopampandasinglesupplyrail-to-railopamp.Ifafeedbackresistorisplacedbetweentheoutputandthenegativeterminaloftheopamp,thenthisnegativefeedbackwillselectanoutputsuchthatVxisveryclosetoVy.Intheidealmodel,weletVx=Vy.OnewaytojustifythisbehavioristorecallthatVout=K•(Vy-Vx).SinceKisverylarge,theonlywayforVouttobebetween-Vsand+VsisforVxtobeveryclosetoVy.

Wecandesignathresholddetectorusingpositiveornofeedback.Athresholddetectorhasabinaryoutput(trueorfalse)dependingonwhetherornotaninputsignalisgreaterthanathresholdvalue.PositivefeedbackornofeedbackdrivesVouttoequal-Vsor+VsIfafeedbackresistorisplacedbetweentheoutputandthepositiveterminaloftheopamp,thenthisfeedbackwillsaturatetheoutputtoeitherthepositiveornegativesupply.Withnofeedback,theoutputwillalsosaturate.InbothcasestheoutputwillsaturatetothepositivesupplyifVy>Vx,andtothenegativesupplyifVx>Vy.Wewillseelater,thatpositivefeedbackcanbeusedtocreatehysteresis.

8.2.2.RealisticOpAmpModelAlthoughtheinputimpedanceofanopampislarge,itisnotinfiniteandsomecurrententerstheinputterminals.Figure8.4illustratesthedefinitionofinputimpedance.Wedefinethecommon-modeinputimpedance,Rcm,asthecommonmodevoltagedividedbythecommonmodecurrent.Wedefinethedifferentialinputimpedance,Rdiffasthedifferentialvoltagedividedbythedifferentialcurrent.Theseparametersvaryconsiderablyfromopamptoopamp.TheCMOSandFETdeviceshaveverylargeinputimpedances.

Figure8.4.Definitionofopampinputimpedance.Thenextrealisticparameterwewilldefineisopen-loopoutputimpedance.Whentheopampisusedwithoutfeedback,theopampoutputimpedanceisdefinedastheopencircuitvoltagedividedbytheshortcircuitcurrent.Theoutputimpedanceisameasureofhowmuchcurrenttheopampcansourceorsink.TheopenloopoutputimpedanceoftheTLC2274is140Ω.

Checkpoint8.2:TheMAX494hasanoutputshort-circuitcurrentof30mA.Assuminganoutputof5V,whatisitsoutputimpedance?

Observation:Theinputandoutputimpedance’softheopamparenotnecessarilythesameastheinputandoutputimpedance’softheentireanalogcircuit.

AsillustratedinFigure8.5,theopampoffsetvoltageVosisdefinedasthevoltagedifferencebetweenVyandVxwhichyieldsanoutputofzero.TheoffsetcurrentIosisdefinedasthecurrentdifferencebetweeninputs.Therewillbeanoutputerrorequaltotheoffsetvoltagetimesthegainofamplifier.Similarly,theoffsetcurrentcreatesanoffsetvoltagethroughtheresistorsofthecircuit.Someopampsprovidethecapabilitytoaddanexternalpotentiometertonullifythetwooffseterrors.Theuseofanulloffsetpotincreasesmanufacturingcostsandincursalaborcosttoadjustitperiodically.Therefore,theoverallsystemcostmaybereducedbyusingmoreexpensiveopampsthatdonotrequireanulloffsetpot.Alternatively,ifthegainandoffsetaresmallenoughnottosaturatetheoutput,thentheoffseterrorcanbecorrectedinsoftwarebyadding/subtractinganappropriatecalibrationconstant.

TheopampbiascurrentIbisdefinedasthecommoncurrentcomingoutofbothVyandVx.Wecanreducetheeffectofbiascurrentbyselectingresistorsinourcircuitinordertoequalizetheeffectiveimpedancetogroundfromthetwoinputterminals.

Figure8.5.Definitionofopampoffsetvoltage,offsetcurrent,andbiascurrent.Checkpoint8.3:ConsiderthesituationwhereaMAX494isusedtocreateananalogamplifierwithgain100.Whatwillbetheoutputerrorduetooffsetvoltage?

Checkpoint8.4:Whycan’taTLC2274isbeusedtocreateanamplifierwithgain1000?

Theinputvoltagenoise,Vn,arisesfromthethermalnoisegeneratedintheresistivecomponentswithintheopamp.Duetothewhite-noiseprocess,themagnitudeofthenoiseisafunctionofthebandwidth(BW)oftheanalogcircuit.Thisparametervariesquiteabitfromopamptoopamp.TocalculatetheRMSamplitudeoftheinputvoltagenoise,weneedtocalculate,Vn=en•√(BW).Toreducetheeffectofnoisewecanlimitthebandwidthoftheanalogsystemusingananaloglowpassfilterorbuyabetteropamp.Theoutputvoltagenoisewillbetheinputvoltagenoisemultipliedbythegainofthecircuit.

Therearetwoapproachestodefiningthetransientresponseofouranalogcircuits.Inthefrequencydomainwecanspecifythefrequencyandphaseresponse.Inthetimedomain,wecanspecifythestepresponse.Formostsimpleanalogcircuitsdesignedwithopamps,thefrequencyresponsedependsontheopampperformanceandtheanalogcircuitgain.Iftheunity-gainopampfrequencyresponseisf1,thenthefrequencyresponseatgain,G,willbef1/G.Thebandwidth,BW,isdefinedasthefrequencyatwhichthegain(Vout/Vin)dropsto0.707oftheoriginal.ThevoltagegainindecibelsGdbisrelatedtothevoltagegaininV/V.WhenVout/Vinequals0.707(√½),theGdb=-3dB.

Theoutputslewrateisthemaximumslopethattheoutputcangenerate.Slewrateisimportantifthecircuitmustresponsequicklytochangesininput(e.g.,asensordetectingdiscreteevents).Alternatively,bandwidthisimportantifthecircuitisrespondingtoacontinuouslychanginginput(e.g.,audioandvideo).

Checkpoint8.5:ConsiderthesituationwhereaMAX494isusedtocreateananalogamplifierwithagainof100.Whatwillbethebandwidthofthiscircuit?Giventhisbandwidth,whatwillbetheRMSoutputvoltagenoise?

Whenweconsidertheperformanceofalinearamplifiernormallywespecifythevoltagegain,inputimpedanceandoutputimpedance.Thesethreeparameterscanbelumpedintoasingleparameter,Adb,calledthepowergain.LetVinRinbetheinputsandVoutRoutbetheoutputsofouramplifier.TheinputandoutputpowersarePin=V2in/RinandPout=V2out/Routrespectively.Thenthepowergainindecibelshasvoltagegainandimpedancecomponents.

8.2.3.OpAmpCircuitDesignTheopenlooporsaturatedmodeperformanceofaOPA2350opampcanbestudiedbylookingatthesimplecircuitinFigure8.6.Weusesaturatedmodetocreateathresholddetector.Whentheinput,V1,isabovethereference,Vt,thentheoutput,V2,saturatesto+3.3V.Similarly,whenV1isbelowVtthenV2saturatesto0V.

Figure8.6.ApositivelogicvoltagecomparatorusingaOPA2350opamp.TheOPA2350willoperateonasupplyrangeof2.7to5.5V.Noticethatitwillcreateadigitaloutputwhen+Vsis3.3Vand-Vsisground.TheinputvoltagesoninputpinsoftheOPA2350mustbebetween-0.3and+3.6V.TheshortcircuitoutputcurrentoftheOPA2350is80mA.TheOPA2350slewrateis22V/µsec,withasettlingtimeof500ns.

Awiderangeofanalogcircuitscanbedesignedbyfollowingthesesimpledesignrules.1.Choosequalitycomponents.

Itisimportanttouseopampswithgoodenoughparameters.Similarlyweshoulduselowtoleranceresistorsandcapacitors.Ontheotherhand,oncethepreliminaryprototypehasbeenbuiltandtested,thenwecouldcreatealternativedesignswithlessexpensivecomponents.Becauseaworkingprototypeexists,wecanexplorethecost/performancetradeoff.

2.Negativefeedbackisrequiredtocreatealinearmodecircuit.

Asmentionedearlier,thenegativefeedbackwillproducealinearinput/outputresponse.Inparticular,weplacearesistorbetweenthenegativeinputterminalandtheoutput(Figure8.7).Acorollarytothisiswealmostneverplacearesistorbetweenthepositiveinputterminalandtheoutput.

Figure8.7.Negativefeedbackiscreatedbyplacingaresistorbetweenthe-inputandoutput.3.Assumenocurrentflowsintotheopampinputs.

Sincetheinputimpedanceoftheopampislargecomparedtotheotherresistancesinthecircuit,wecanassumethatIx=Iy=0.

4.Assumenegativefeedbackequalizestheopampinputvoltages.

Iftheanalogcircuitisinlinearmodewithnegativefeedback,thenwecanassumeVx=Vy.

5.Chooseresistorvaluesinthe1kΩto1MΩrange.

Inordertohavetheresistorsinthecircuitbemuchlargerthantheoutputimpedanceoftheopampandmuchsmallerthantheinputimpedanceoftheopamp,wechooseresistorsinthe1kΩto1MΩrange.Ifwecan,itisbettertorestrictvaluestothe10kΩto100kΩrange.Ifwechooseresistorsbelow1kΩ,thencurrentswillincrease.Ifthecurrentsgettoolargethebatterieswilldrainfasterandtheopampmaynotbeabletosourceorsinkenoughcurrent.Astheresistorsgoabove1MΩ,thewhitenoiseincreases,thecurrenterrors(IosIbIn)becomemoresignificant.Inaddition,lowtoleranceresistorsareexpensiveinsizesabove2MΩ.

6.Theanalogcircuitbandwidthdependsonthegainandtheopampperformance.

Lettheunity-gainopampfrequencyresponsebef1,andlettheanalogcircuitgainbeG.Thefrequencyresponseorbandwidthoftheanalogcircuit,BW,willbef1/G.Designthecircuitbandwidth10timesfasterthanthesignalyouaretryingtoprocess.

7.Equalizetheeffectiveresistancetogroundatthetwoopampinputterminals.

Tostudythebiascurrents,considerallothervoltagesourcesasshortstoground,andallothercurrentsourcesasopencircuits.Adjusttheresistancevaluesinthecircuitsothattheimpedancefromthe+terminaltogroundisthesameastheimpedancefromthe-terminaltoground.Inthisway,thebiascurrentswillcreateacommonmodevoltage,whichwillnotappearattheopampoutputbecauseofthecommonmoderejectionoftheopamp.

8.Theimpedanceisthevoltagedividedbythecurrent.

Iftheanalogcircuithasasingleinputvoltage,thentheinputimpedance,Zin,issimplytheinputvoltagedividedbytheinputcurrentasshowninFigure8.8.Theoutputimpedance,Zout,istheopencircuitoutputvoltagedividedbytheshortcircuitoutputcurrent.

Figure8.8.Definitionofinputandoutputimpedanceforananalogcircuitwithasingleinput.Iftheinputstageoftheanalogcircuitisadifferentialamplifierwithtwoinputvoltages,thenwecanspecifythecommonmodeinputimpedance,Zcm,andthedifferentialmodeinputimpedance,Zdiff.SeeFigure8.9.

Figure8.9.Definitionofinputimpedanceforananalogcircuitwithtwoinputs.Observation:Inmostcases,thedifferentialmodeinputimpedanceoftheanalogcircuitwillbethedifferentialmodeinputimpedanceoftheopamp.

9.MatchinputimpedancestoimproveCMRR.

Iftheinputstageoftheanalogcircuitisadifferentialamplifierwithtwoinputvoltages,averyimportantperformanceparameteriscalledthecommonmoderejectionratio(CMRR).Itisassumedthatthesignalofinterestisthedifferentialvoltage,whereascommonmodevoltagesareconsiderednoise.I.e.,VdiffwillbethedesiredsignalandVcmistheaddednoise.TheCMRRisdefinedtobetheratioofthedifferentialgaindividedbythecommonmodegain.SeeFigure8.10.ThereforeadifferentialamplifierwithalargeCMRRwillpassthesignalandrejectthenoise.Indecibels,itiscalculatedas

CMRR=20•log10(Gdiff/Gcm)

Figure8.10.Definitionofcommonmoderejectionratio(CMRR).Remember,VdiffisthesignalandVcmisnoise.TherearetwosetsofimpedanceswemustmatchtoachieveagoodCMRR.Eachamplifierinputhasaseparateinputimpedancetoground,shownasZin1andZin2inFigure8.11.ToimproveCMRR,wemakeZin1equaltoZin2.Similarly,thesignalsource(Vdiff)hasaseparateoutputimpedance,Zout1andZout2.Again,wetrytomakeZout1equaltoZout2.Unfortunately,ifZin1doesnotequalZin2orifZout1

doesnotequalZout2thenacommonmodesignal(e.g.,addednoiseinthecable)willappearasadifferentialsignaltotheanalogcircuitandthusbepresentintheoutput.

Figure8.11.Circuitmodelforimprovingcommonmoderejectionratio(CMRR).10.Rail-to-railconsiderations.

Whendesigningwithrail-to-railopamps,wemustguaranteethatthevoltagesatallinputandoutputpinsoftheopampsnevergooutsidetherangeof-Vsto+Vs.

Wewillbeginwithaninvertingamplifier(Figure8.12),whichisasimplelinearmodeanalogcircuit.ThegainistheR2/R1ratio.NoticethatthegainresponseisindependentofR3.Thus,wecanchooseR3tobetheparallelcombinationofR1||R2sothattheeffectofthebiascurrentsisreduced.BecauseIyiszero,Vyisalsozero.Becauseofnegativefeedback,VxequalsVy.Thus,Vxequalszerotoo.BecauseVxiszero,IinisVin/R1andI2is-Vout/R2.BecauseIxiszero,IinequalsI2.SettingIinequaltoI2yields

Vout=-(R2/R1)•Vin

Theinputimpedance(Zin)ofthiscircuit(definedasVin/Iin)isR1.IfthecircuitwerebuiltwithanOPA227,whichhasagainbandwidthproductof8MHz,thebandwidthofthiscircuitwillbe8MHzdividedbythegain,R2/R1.

Figure8.12.Invertingamplifier,builtwithOP227,poweredwith+12and-12V.Commonerror:ThislowinputimpedanceofZinmaycauseloadingonthepreviousanalogstage.

Observation:Theinvertingamplifierinputimpedanceisindependentoftheopampinputimpedance.

Thenegativefeedbackwillreducetheoutputimpedanceoftheamplifier,Zout,toavaluemuchlessthantheoutputimpedanceoftheopampitself,Rout.TocalculateZout,wefirstdeterminetheopencircuitvoltage

Vopen=-(R2/R1)•Vin

Wenextdeterminetheshortcircuitcurrent,Ishort.Thismeansweconsiderwhatwouldhappeniftheoutputwereshortedtoground.Iftheoutputisshorted,thecircuitisnolongerinfeedbackmode,andVxwillnotequalVy.Infact,VxwillbeasimplevoltagedividerfromVinthroughR1andR2toground,

Vx=Vin•R2/(R1+R2)

Becauseofthelargeopen-loopgain,theidealoutputwillattempttobecome

Vo=K•(Vy-Vx)=-K•Vin•R2/(R1+R2)

Theshortcircuitcurrentwillbeafunctionoftheidealoutputvoltage,andtheoutputresistanceoftheopamp,

Ishort=Vo/Rout

Theoutputimpedanceofthecircuitisdefinedtobetheopencircuitvoltagedividedbytheshortcircuitcurrent,whichforthisinvertingamplifieris

Zout=Vopen/Ishort=Rout•(R2+R1)/(K•R1)

Observation:Theoutputimpedanceofanalogcircuitsusingopampswithnegativefeedbackistypicallyinthem s.

Amixed-signaldesignincludesbothanaloganddigitalcomponents.Theclassicapproachtocombininganaloganddigitalcircuitsistopowertheanalogsystemwithalownoise±12Vpowersupply,maintainseparateanaloganddigitalgrounds,andconnecttheanaloggroundtothedigitalgroundonlyattheADC.OneofthelimitationsoftheADCbuiltintoamicrocontrolleristhatanalogsignalsextendingbeyondthe0to+3.3Vrangewillpermanentlydamagethemicrocontroller.Oneapproachtoallowingsignedanalogvoltages,whilestillusingasinglevoltagesupplyandprotectingthemicrocontrolleristocreateananaloggroundthatisatadifferentpotentialfromthedigitalground.Forourmicontrollersystems,whicharepoweredwitha+3.3Vsupply,wewillcreateananaloggroundthatisat1.5Vrelativetothedigitalground.Thus,analogsignalsrangingfrom-1.5to+1.5Vareactually0to+3Vrelativetothemicrocontrollerdigitalground.Thefirststeptoimplementingthisapproachistouseananalogreferencechip,liketheonesshowninTable8.5,tocreatealow-noise+1.50Vsignal(theanalogground).Thesecondstepistoconnectpowertotheanalogcircuitswith-Vssettodigitalground,andthe+Vssettothe+3.3Vsupply.Wewilluserail-to-railopampsthatoperateon3.3Vpower.Thelaststepistoreplaceallconnectionstoanaloggroundwiththelow-noise+1.50Vreferencevoltage.Figure8.13showstheinvertingamplifier,redesignedfromFigure8.12

tooperateonasingle+3.3Vsupply.ThesignalsVinandVoutareallowedtovaryfrom0to+3Vrelativetodigitalground,butrelativetotheanalogground,thesesignalswillvaryfrom-1.5to+1.5V.AnadjustableshuntvoltagereferencesliketheLM4041alsocanbeusedtocreateconstantanalogvoltages.

Part Voltage(V)

±Accuracy(mV)

AD1580,AD589,REF1004,MAX6120,LT1034,LM385

1.2 1to15

MAX6101,REF3312,ADR1581 1.25 2

MAX6108 1.6 3

ADR420,ADR520,REF191,MAX6191,LT1790,LM4120

2.048 1to10

AD580,REF03,REF1004,MAX6192,MAX6225,LT1389,LM336

2.5 1to75

AD1583,ADR530,ADR423,REF193,MAX6163,LT1461,LM4120

3 1.5to10

ADR366,REF196,MAX6331,LT1461,LM3411,LM4120

3.3 4to10

AD1584,ADR540,ADR292,REF198,MAX6241,LT1790,LM4040

4.096 2to8

Table8.5.Parametersofvariousprecisionreferencevoltagechips.

CommonError:Precisionreferencechipsdonotprovidemuchoutputcurrentandshouldnotbeusedtopowerotherchips.

WecananalyzetheresponseofthecircuitinFigure8.13byassuminganidealopamp.Sincetherearenocurrentsintotheinputs,thevoltageatthepositiveinputwillbe1.50V.Becauseofnegativefeedback,thevoltageatthenegativeinputwillalsobe1.50V.ThecurrentthroughR1willbe(Vin-1.5)/R1.ThecurrentthroughR2willbe(1.5-Vout)/R2.Sincetherearenocurrentsintotheinputsoftheopamp,thesetwocurrentsareequal,

(Vin-1.5)/R1=(1.5-Vout)/R2orVout=1.5-(Vin-1.5)R2/R1Definetheanalogsignalsrelativeto1.5V,i.e.,V’in(Vin-1.5)andV’out(Vout-1.5).ThecircuitinFigure8.13implementsanegativegaininverter.

V’out=-(R2/R1)V’inObservation:Itisimportantinthisschemetoseparatethedigitalandanaloggroundsavoidingdirectconnectionsbetweenthetwogrounds.

Figure8.13.Invertingamplifierwithaneffective-1.5Vto+1.5Vanalogsignalrange.Thesecondlinearmodecircuitwewillstudyisthenoninvertingamplifier,asshowninFigure8.14.Thegainis1+R2/R1.Thenoninvertingamplifiercannothaveagainlessthan1.Justliketheinvertingamp,thegainresponseisindependentofR3.So,wechooseR3tobetheparallelcombinationR1||R2sothattheeffectofthebiascurrentsisreduced.

Figure8.14.Noninvertingamplifier.BecauseIyiszero,VyequalsVin.BecauseofnegativefeedbackVxequalsVy.Thus,VxequalsVintoo.Calculatingcurrentsweget,I1isVin/R1andI2is(Vout-Vin)/R2.BecauseIxiszero,I1equalsI2.SettingI1equaltoI2yields

Vout=(1+R2/R1)•Vin

UsingthesimpleopamprulesIyiszero,sotheinputimpedance(Zin)ofthiscircuit(definedasVin/Iin)wouldbeinfinite.Inthissituationwespecifytheamplifierinputimpedancetobetheopampinputimpedance.IfthecircuitwerebuiltwithanOPA350,whichhasagainbandwidthproductof38MHz,thebandwidthofthiscircuitwillbe38MHzdividedbythegain.

Checkpoint8.6:IfthenoninvertingamplifierinFigure8.14werebuiltwithanOPA350,whatwouldbetheinputimpedanceoftheamplifier?

Thecalculationoftheoutputimpedanceofthisampfollowsthesameapproachastheinvertingamp,

Zout=Rout•(R2+R1)/(K•R1)

Thefollowingdesignprocesscanbeusedtobuildanyanalogcircuitintheformof

Vout=A1V1+A2V2+…+AnVn+B

whereA1A2…AnBareconstantsandV1V2…Vnareinputvoltages.ThecircuitwillbedesignedwithoneopampbeginningwiththeboilerplateshowninFigure8.15.

Figure8.15.Boilerplatecircuitmodelforlinearcircuitdesign.Thefirststepistochooseareferencevoltagefromavailablereferencevoltagechips,likeonesshownintheTable8.5.SomeofthemanufacturersthatproducevoltagereferencesareAnalogDevices,TexasInstruments,LinearTechnology,Maxim,andNationalSemiconductor.Theparameterstoconsiderwhenchoosingavoltagereferencearevoltage,packageconfiguration,accuracy,temperaturecoefficient,andpower.Inparticular,letVrefbethisreferencevoltage.

CommonError:Ifyouuseresistordividerfromthepowersupplytocreateavoltageconstant,thenthepowersupplyripplewillbeaddeddirectlytoyouranalogsignal.

Thesecondstepistorewritethedesignequationintermsofthereferencevoltage,Vref.Inparticular,wemakeAref=B/Vref.

Vout=A1V1+A2V2+…+AnVn+ArefVref

whereA1A2…AnArefareconstantsandV1V2…Vnareinputvoltages.Thethirdstepistoaddagroundinputtotheequation.Groundiszerovolts(Vg=0),butitisnecessarytoaddthisgroundsothatthesumofallthegainsisequaltoone.

Vout=A1V1+A2V2+…+AnVn+ArefVref+AgVg

ChooseAgsuchthatA1+A2+…+An+Aref+Ag=1

Inotherwords,letAg=1–(A1+A2+…+An+Aref)

Thefourthstepistochooseafeedbackresistor,Rf,intherangeof10k to1M .Thelargerthegains,thelargerthevalueofRfmustbe.Thencalculateinputresistorstocreatethedesiredgains.Inparticular,

|A1|=Rf/R1soR1=Rf/|A1|

|A2|=Rf/R2soR2=Rf/|A2|

|An|=Rf/RnsoRn=Rf/|An|

|Aref|=Rf/RrefsoRref=Rf/|Aref|

|Ag|=Rf/RgsoRg=Rf/|Ag|

Observation:Wewillgetalow-costsolutionifwechoosestandardresistorvalues,asshowninTables9.1,9.2,9.3,or9.4.

Thelaststepistobuildthecircuit.Ifthegainispositive,thentheinputresistorisconnectedtothepositiveterminaloftheopamp.Conversely,ifthegainisnegative,thentheinputresistorisconnectedtothenegativeterminaloftheopamp.Thefeedbackresistor,Rf,willalwaysbeconnectedfromthenegativeinputtotheoutput.

Example8.1.Designananalogcircuitwithtwoinputsandoneoutput.Thefirstinputvariesfrom1to1.5Vandthesecondinputvariesfrom0.75to1V.Theoutputwillbeequallysensitivetothetwoinputs,sothegainofthesecondinputwillbetwicethegainofthefirstinput.Theoutputshouldrangefrom0to3V.Inparticular,buildacircuitwithatransferfunctionofVout=3V1+6V2–7.5.

Solution:Thefirststepistochooseareferencevoltage.TheREF3312+1.25-Vvoltagereferencewillbeused.Thesecondstepistorewritethedesignequationintermsofthereferencevoltage.NoticethatAref=-7.5/1.25=-6.

Vout=3V1+6V2–6Vref

Thethirdstepistoaddagroundinputtotheequationsothatthesumofallthegainsisequaltoone.Notice,Ag=1–(3+6–6)=-2

Vout=3V1+6V2–6Vref–2Vg

Thefourthstepistochooseafeedbackresistor,Rf=60k .Thisvalueisacommonmultipleofthegains:6,3,2.Thencalculateinputresistorstocreatethedesiredgains.

R1=Rf/2=20k

R2=Rf/6=10k

Rref=Rf/6=10k

Rg=Rf/2=30k

Thelaststepistobuildthecircuit,asshowninFigure8.16.Thepositivegaininputsareconnectedtotheplusinputoftheopampandthenegativegaininputsareconnectedtotheminusinputoftheopampinput.

Figure8.16.Alinearopampcircuit.

Tables9.1,9.2,9.3and9.4liststandardresistancevalues.Wedesigninganalogcircuitsweneedtoselectfromthesespecificvaluestoreducecostandsimplifymanufacturing.

Theinstrumentationampwillamplifyadifferentialvoltage,Vout=G•(V2-V1).Weuseinstrumentationampsinapplicationsthatrequirealargegain(above100),ahighinputimpedance,andagoodcommonmoderejectionratio.AninstrumentationampcanbebuiltusingthreehighqualityopampsasshowninFigure8.17.

Figure8.17.Instrumentationamplifiermadewiththreeopamps.

Observation:Inordertoachievequalityperformancewitha3-op-ampinstrumentationampcircuit,wemustuseprecisionresistorsandqualityopamps.

CommonError:Ifyouuseapotentiometerinplaceofonethegainresistorsintheabovecircuit,thenfluctuationsinthepotentiometerresistancethatcanoccurwithtemperature,vibration,andtimewillhaveastrongeffectontheamplifiergain.

Becauseofthewiderangeofapplicationsthatrequireinstrumentationamplifiers,chipmanufacturershavedevelopedintegratedsolutions.Inmanycaseswecanachievehigherperformanceatreducedcostbyutilizinganintegratedcircuit.Thegainisselectedbyexternaljumpersorexternalresistors.TheMAX4460,AD627,andINA122aresingle-supplyrail-to-railinstrumentationamps.TheAD627andINA122havethesamepinsandthesameformulaforcalculatinggain,asshowninFigure8.18.

Figure8.18.IntegratedinstrumentationamplifiermadewithINA122orAD627.CommonError:IfyouuseapotentiometerastheRGgainresistorsintheabovecircuit,thenfluctuationsinthepotentiometerresistancethatcanoccurwithtemperature,vibration,andtimewillhaveastrongeffectontheamplifiergain.

Wecanuseavoltagecomparatortodetecteventsinananalogwaveform.Theinputvoltagerangeisdeterminedbytheanalogsupplyvoltagesofthecomparator.Theoutputistakesontwovalues,shownanVhandVlinFigure8.19.Acomparatorwithhysteresishastwothresholds,Vt+andVt-.Inboththepositiveandnegativelogiccasesthethreshold(Vt+orVt-)dependsonthepresentvalueoftheoutput.

Figure8.19.Input/outputresponseofvoltageconverterswithhysteresis.Hysteresispreventssmallnoisespikesfromcreatingafalsetrigger.

PerformanceTip:Inordertoeliminatefalsetriggering,weselectahysteresislevel(Vt+–Vt-)greaterthanthenoiselevelinthesignal.

InFigure8.20,arail-to-railopampisusedtodesignavoltagecomparator.Sincetheoutputswingsfrom0to3.3V,itcanbeconnecteddirectlytoaninputpinofthemicrocontroller.Ontheotherhand,since+3.3and0areusedtopowertheopamp,theanaloginputmustremaininthe0to+3.3Vrange.ThehysteresislevelisdeterminedbytheamplitudeoftheoutputandtheR1/(R1+R2)ratio.Iftheoutputisat0V,thevoltageatthe+terminalisVin*R2/(R1+R2).Theoutputswitcheswhenthevoltageatthe+terminalgoesabove1.65.BysolvingforVin*200k/(10k+200k)=1.65,weseeVinmustgoabove+1.73fortheoutputtoswitch.Similarly,iftheoutputisat+3.3V,thevoltageatthe+terminalcanbecalculatedasVin+(3.3-Vin)*R1/(R1+R2).Theoutputswitchesbackwhenthevoltageatthe+terminalgoesbelow1.65.BysolvingforVin+(3.3-Vin)*R1/(R1+R2)=1.65,weseeVingobelow+1.57beforethe+terminaloftheopampfallsbelow1.65V.Inlinearmodecircuitsweshouldnotusethesupplyvoltagetocreatevoltagereferences,butinasaturatedmodecircuit,powersupplyripplewillhavelittleeffectontheresponse.

Figure8.20.Avoltagecomparatorwithhysteresisusingarailtorailopamp.Insomemedicalandindustrialapplicationsweneedtodesignanaloginstrumentationthatisisolatedfromearthground.Inanindustrialsetting,isolationisonewaytoreducenoisepickupfromlargeEMfieldsproducedbyheavymachinery.Inmedicalapplicationsweneedtoprotectthepatientfrompotentiallydangerousmicroshocks,Thus,themedicalinstrumentmustbeisolated.Therearethreeapproachestoisolation,asshowninFigure8.21.Inthefirstapproach,shownatthetopofFigure8.21,ananalogisolationbarrieriscreatedbetweenthepreampandamp.Thiswastheoriginalapproachusedinanaloginstrumentsbeforetheadventofmixedanalog-digitalsystems.Itisexpensive,bulkyandintroducesaverylargetransfererror.Itisnotappropriateforembeddedapplicationsthatuseamicrocontroller.Inthesecondapproach,weusedigitalisolation.The6N139opticalisolatorisaneffectivelow-costmechanismtoimplementdigitalisolation.Thisisthemostcommonapproachusedfornewdesignswhenahardconnectionbetweenthedataacquisitionsystemandbuildingisrequired.Itisfast,small,cheap,andwillnotintroduceerrors.Thethirdapproachrunstheentiresystemwithbatteries.Thisisaveryattractiveapproachduetotheavailabilityofhigh-qualitylow-powerLED/LCDdisplaysandwirelessnetworks,suchasBluetooth,ZigBee,and802.11b.

Figure8.21.Analogisolation,digitalisolation,andbattery-poweredallprovideprotectionfrommicroshocks.

8.3.AnalogFilters

8.3.1.SimpleAnalogFiltersWewillusealow-passfiltertoremoveunwantedhighfrequencysignals.Wecanaddacapacitorinparallelwiththefeedbackresistorintheinvertingamplifiertocreateasimpleonepolelow-passfilter.Theimpedanceofaresistor,R2inparallelthecapacitor,Cisafunctionoffrequency,seeFigure8.22.

Figure8.22.Aparallelcombinationofaresistorandcapacitormakeafrequency-dependentimpedance.InFigure8.23,thefeedbackpathhasbotharesistorandcapacitor.Thereforethegainofthecircuitis-Z/R1,whichexhibitslow-passbehavior.Thecutofffrequencyisdefinedtobethefrequencyatwhichthegaindropsto0.707ofitsoriginalvalue.Inthissimplelow-passfilter,thecutofffrequency,fc,is1/(2πR2C).Thegaindropsoffathighfrequencies.

Vout/Vin=(-R2/R1)*(1/(1+j R2C))

Figure8.23.Onepolelow-passanalogfilter.Weclassifythislow-passfilterasonepole,becausethetransferfunctionhasonlyonepairofpolesinthes-plane.Onepolelow-passfiltershaveagainversusfrequencyresponseof

Wewilluseahigh-passfilter(HPF)toremoveunwantedlowfrequencysignals.Soundandbiopotentialsareexamplesofsignalswherethelowfrequencycomponentsarenotwanted.Ifweplaceacapacitorinseriesitwillblocklowfrequencies.Oneofthedifficultiesinhigh-passfiltersforembeddedsystemsisthefactthatweoftenpowertheanalogelectronicswithasinglesupply.Becausewecannotprocessnegativevoltages,wemustuseananaloggrounddifferentfromthedigitalgroundwhenbuildinghigh-passfilters,asillustratedinFigure8.24.Thefilterontheleftisasimpleapproach.Themiddleoneislowcostandnotappropriateforhighfidelitysystems.Theoneontherightisappropriatefordifferentialsignals.Eachofthesefiltershasaone-poleHPFshape,with2 f= ,and1/fc=2 RC.

or

Figure8.24.Onepolehigh-passanalogfiltersoperatingonasinglesupplyvoltage.

8.3.2.ButterworthFiltersHigherorderanalogfilterscanbedesignedusingmultiplecapacitors.Oneoftheadvantagesofthetwo-poleButterworthanalogfilteristhataslongasthecapacitorsmaintainthe2/1ratio,theanalogcircuitwillbeaButterworthfilter.Figure8.25isatemplateforaSalen-Keyformofthefilter.Thedesignstepsforthetwo-poleButterworthlow-passfilterareasfollows:

1)Selectthecutofffrequency,fc2)Dividethetwocapacitorsby2πfc(letC1A,C2Abethenewcapacitorvalues)

C1A=141.4µF/2πfcC2A=70.7µF/2πfc3)Locatetwostandardvaluecapacitors(withthe2/1ratio)withthesameorderofmagnitudeasthedesiredvalues.Wecancreatecapacitorswitha2/1ratiousingthreecapacitorsofthesamevalue.LetC1B,C2Bbethesestandardvaluecapacitors,letxbethisconveniencefactor

C1B=C1A/x

C2B=C2A/x

4)Adjusttheresistorstomaintainthecutofffrequency

R=10kΩ•x

5)Vrefcanbesettoanalogground,e.g.,1.50Vor0V.

Figure8.25.Two-poleButterworthlow-passanalogfilter(seethefileLPF.xls).Theanalogfiltersinthissectionallrequirelowleakage,highaccuracyandlowtemperaturecoefficientcapacitorslikeC0Gceramic.

PerformanceTip:Ifyouchoosestandardvalueresistorsnearthedesiredvalues,youwillsavemoneyandthecircuitwillstillbeaButterworthfilter.Theonlydifferenceisthatthecutofffrequencywillbeslightlyofffromtheoriginalspecification.

Wecanuseasimilarapproachtodesignatwo-poleButterworthhigh-passfilter.Figure8.26isatemplateforaSalen-KeyformoftheHPF.Thedesignstepsforthetwo-poleButterworthhigh-passfilterareasfollows:

1)Selectthecutofffrequency,fc2)Dividethetwocapacitorsby2πfc(letCAbethenewcapacitorvalues)

CA=10µF/2πfc3)Locateastandardvaluecapacitorwiththesameorderofmagnitudeasthedesiredvalue.LetCB,bethisstandardvalue,letxbethisconveniencefactor

CB=CA/x

4)Adjustthetworesistorstomaintainthecutofffrequency

R1=70.7kΩ•xandR2=141.4kΩ•x

5)Vrefshouldbesettoanalogground,e.g.,1.50V.

Figure8.26.Two-poleButterworthhigh-passanalogfilter.ManyanalogICmanufacturersprovidedesigntools.FilterProisafreedesigntoolfromTexasInstrumentsyoucanusetodesignanalogfilters(www.ti.com).Usingthesedesigntoolsyouwillbeabletocreatefiltersmuchbetterthantheonespresentedinthisbook.

8.4.DigitaltoAnalogConvertersAnanalogsignalisonethatiscontinuousinbothamplitudeandtime.Neglectingquantumphysics,mostsignalsintheworldexistascontinuousfunctionsoftimeinananalogfashion(e.g.,voltage,current,position,angle,speed,force,pressure,temperature,andflowetc.)Inotherwords,thesignalhasanamplitudethatcanvaryovertime,butthevaluecannotinstantaneouslychange.Torepresentasignalinthedigitaldomainwemustapproximateitintwoways:amplitudequantizingandtimequantizing.Fromanamplitudeperspective,wewillfirstplacelimitsonthesignalrestrictingittoexistbetweenaminimumandmaximumvalue(e.g.,0to+3V),andsecond,wewilldividethisamplituderangeintoafinitesetofdiscretevalues.Therangeofthesystemisthemaximumminustheminimumvalue.Theprecisionofthesystemdefinesthenumberofvaluesfromwhichtheamplitudeofthedigitalsignalisselected.Precisioncanbegiveninnumberofalternatives,binarybits,ordecimaldigits.Theresolutionisthesmallestchangeinvaluethatissignificant.Figure8.27showsatemperaturewaveform(solidline),withacorrespondingdigitalrepresentationsampledat1Hzandstoredasa5-bitintegernumberwitharangeof0to31oC.Becauseitisdigitizedinbothamplitudeandtime,thedigitalsamples(individualdots)inFigure8.27mustexistatanintersectionofgreylines.Becauseitisatime-varyingsignal(mathematically,thisiscalledafunction),wehaveoneamplitudeforeachtime,butitispossiblefortheretobe0,1,ormoretimesforeachamplitude.

Thesecondapproximationoccursinthetimedomain.Timequantizingiscausedbythefinitesamplinginterval.Forexample,thedataaresampledevery1secondinFigure8.27.Inpracticewewilluseaperiodictimertotriggerananalogtodigitalconverter(ADC)todigitizeinformation,convertingfromtheanalogtothedigitaldomain.Similarly,ifweareconvertingfromthedigitaltotheanalogdomain,weusetheperiodictimertooutputnewdatatoadigitaltoanalogconverter(DAC).TheNyquistTheoremstatesthatifthesignalissampledwithafrequencyoffs,thenthedigitalsamplesonlycontainfrequencycomponentsfrom0to½fs.Conversely,iftheanalogsignaldoescontainfrequencycomponentslargerthan½fs,thentherewillbeanaliasingerrorduringthesamplingprocess.Aliasingiswhenthedigitalsignalappearstohaveadifferentfrequencythantheoriginalanalogsignal.

Figure8.27.Ananalogsignalisrepresentedinthedigitaldomainas5-bit

discretesamples.

8.4.1.DACOperationandPerformanceMeasuresADACconvertsdigitalsignalsintoanalogformasillustratedinFigure8.28.AlthoughonecaninterfaceaDACtoaregularoutputport,mostDACsareinterfacedusinghigh-speedsynchronousprotocols.TheDACoutputcanbecurrentorvoltage.Additionalanalogprocessingmayberequiredtofilter,amplifyormodulatethesignal.WecanalsouseDACstodesignvariablegainorvariableoffsetanalogcircuits.

TheDACprecisionisthenumberofdistinguishableDACoutputs(e.g.,4096alternatives,12bits).TheDACrangeisthemaximumandminimumDACoutput.TheDACresolutionisthesmallestdistinguishablechangeinoutput.Theresolutionisthechangeinoutputthatoccurswhenthedigitalinputchangesby1.Theunitsofrangeandresolutionareinvoltsorampsdependingonwhethertheoutputisvoltageorcurrent.

Range(volts)=Precision(alternatives)•Resolution(volts)

Figure8.28.A12-bitDACprovidesanalogoutput.A12-bitADCprovidesanaloginput.TheDACaccuracyis(Actual-Ideal)/IdealwhereIdealisreferredtotheNationalInstituteofStandardsandTechnology(NIST).OnecanchoosethefullscalerangeoftheDACtosimplifytheuseoffixed-pointmath.Forexample,ifan8-bitDAChadafullscalerangeof0to2.55volts,thentheresolutionwouldbeexactly10mV.ThismeansthatiftheDACdigitalinputwere123(decimal),thentheDACoutputvoltagewouldbe1.23volts.

ADACgainerrorisashiftintheslopeoftheVoutversusdigitalinputstaticresponse.ADACoffseterrorisashiftintheVoutversusdigitalinputstaticresponse.TheDACtransientresponsehasthreecomponents:delayphase,slewingphase,ringingphase.Duringthedelayphase,theinputhaschangedbuttheoutputhasnotyetbeguntochange.Duringtheslewingphase,theoutputchangesrapidly.Duringtheringingphase,theoutputoscillateswhileitstabilizes.Forpurposesoflinearity,letm,nbedigitalinputs,andletf(n)betheanalogoutputoftheDAC,seeFigure8.29.Onequantitativemeasureoflinearityisthecorrelationcoefficientofalinearregressionfitofthef(n)responses.If∆istheDACresolution,itislinearif

f(n+1)-f(n)=f(m+1)-f(m)=∆foralln,m

TheDACismonotonicif

sign(f(n+1)-f(n))=sign(f(m+1)-f(m))foralln,m

Conversely,theDACisnonlinearif

f(n+1)-f(n)≠f(m+1)-f(m)forsomen,m

PracticallyspeakingallDACsarenonlinear,buttheworstnonlinearityisnonmonotonicity.TheDACisnonmonotonicif

sign(f(n+1)-f(n))≠sign(f(m+1)-f(m))forsomen,m

Figure8.29.NonlinearandnonmonotonicDACs.

Example8.2.Designa2-bitbinary-weightedDACwitharangeof0to+3.3Vusingresistors.

Solution:Webeginbyspecifyingthedesiredinput/outputrelationshipofthe2-bitDAC.Therearetwopossiblesolutionsdependinguponwhetherwewantaresolutionof0.825Vor1.1V,asshownasV1andV2inTable8.6.BothsolutionsarepresentedinFigure8.30.

N Q1Q0 V1(V) V2(V)

0 00 0.000 0.0

1 03.3 0.825 1.1

2 3.30 1.650 2.2

3 3.33.3 2.475 3.3

Table8.6.Specificationsofthe2-bitDAC.

Assumetheoutputhighvoltage(VOH)ofthemicrocontrolleris3.3V,anditsoutputlowvoltage(VOL)is0.Withabinary-weightedDAC,wechoosetheresistorratiotobe2/1soQ1bitistwiceassignificantastheQ0bit,asshowninFigure8.30.Consideringthecircuitontheright,ifbothQ1andQ0are0,theoutputV2iszero.IfQ1is0andQ0is+3.3V,theoutputV2isdeterminebytheresistordividernetwork

whichis1.1V.IfQ1is+3.3VandQ0is0,theoutputV2isdeterminebythenetwork

whichis2.2V.IfbothQ1andQ0are+3.3V,theoutputV2is+3.3V.TheoutputimpedanceofthisDACisapproximately20k ,whichmeansitcannotsourceorsinkmuchcurrent.

Figure8.30.Twosolutionsfora2-bitDAC.

Example8.3.Designa6-bitDACwitharangeof0to+3.3VusingE961%standardresistors.

Solution:1%errorisofcourse1partoutof100,while6-bitprecisionwillbeonly1partoutof64.Soweexpectitwillbepossibletobuilda6-bitDACwith1%parts.Webeginthedesignbyspecifyingthedesiredinput/outputrelationshipofthe6-bitDAC.AllzeroswillmaptoVout=0,andalloneswillmaptoVout=3.3V.Theexponentialweightingofthebitscorrespondstothebasiselementsina6-bitnumber.Letb5,b4,b3,b2,b1,b0bethe6-bitDACinput,thedesiredperformanceis

Vout=3.3V*(32*b5+16*b4+8*b3+4*b2+2*b1+b0)/63

OnesetofE96standardresistorswithvaluesapproximatingthedesiredexponentialweightingisshownontheleftofFigure8.31.TheperformanceoftheDACisshownontheleft.ThisDACremainsmonotoniceveniftheresistorsarevariedby±1%.

Figure8.31.A6-bitbinary-weightedDAC.

ItisnotfeasibletoconstructaDACwithmorethan8bitsusingthebinaryweightedtechniqueforthreereasons.First,ifonechoosestheresistorvaluesfromthepractical10kΩto1MΩrange,thenthemaximumprecisionwouldbe1MΩ/10kΩ,whichequals100orabout7bits.Thesecondproblemisthatitwouldbedifficulttoavoidnonmonotonicitybecauseasmallpercentagechangeinthesmallresistor(e.g.,theonecausingthelargestgain)wouldoverwhelmtheeffectsofthelargeresistor(e.g.,theonecausingthesmallestgain.)Forexample,ifyoutriedtoadda7thbittotheDACinFigure8.31usinga523k 1%resistor,theDACcouldbecomenonmonotonic.Third,thesummingDACincludestheerrors,uncertaintyandnoiseinthedigitalpowersupply.Toaddressallthreeoftheselimitations,theR-2Rladderisused.Itispracticaltobuildresistornetworkssuchthatalltheresistancesareequal.Tocreatea2Rcomponent,weusetworesistorsinseries.Resistanceerrorswillchangeallresistorsequally.Thistypeoferroraffectstheslope,Vfs,butnotthelinearityorthemonotonicity.InFigure8.32eachofthethreedigitalinputs(bit2,bit1,bit0)controlsacurrentswitch.Whenthedigitalsignalistrue,thereferencevoltageisappliedtotheladder.Whenthedigitalsignalisfalse,thatpositionontheladderisavirtualground(0V).UsingareferencevoltageinsteadoftheVOHofthemicrocontrollergreatlyreducesthenoiseintheoutput.

Figure8.32.3-bitunsignedR-2RDAC.Toanalyzethiscircuitwewillconsiderthethreebasiselements(1,2,and4).Ifthesethreecasesaredemonstrated,thenthelawofsuperpositionguaranteestheotherfivewillwork.WhenoneofthedigitalinputsistruethenVrefisconnectedtotheR-2Rladder,andwhenthedigitalinputisfalse,thentheconnectionisgrounded.SeeFigure8.33.

Figure8.33.Analysisofthethreebasiselements100,010,001ofthe3-bitunsignedR-2RDAC.Ineachofthethreetestcases,thecurrentacrosstheactiveswitchisI0=Vref/(3R).Thiscurrentisdividedby2ateachbranchpoint.I.e.,I1=I0/2,andI2=I1/2.Currentinjectedfromthelowerbitswillbedividedmoretimes.Sinceeachstagedividesbytwo,theexponentialbehaviorisproduced.AnactualDACisimplementedwithacurrentswitchratherthanavoltageswitch.Nevertheless,thissimplycircuitillustratestheoperationoftheR-2Rladderfunction.Whentheinputis001,Vrefispresentedtotheleft.Theeffectiveimpedancetogroundis3R,sothecurrentinjectedintotheR-2RladderisI0=Vref/(3R).Thecurrentisdividedinhalfthreetimes,andI001=Vref/(24R).

Whentheinputis010,Vrefispresentedinthemiddle.Theeffectiveimpedancetogroundisstill3R,sothecurrentinjectedintotheR-2RladderisI0=Vref/(3R).Thecurrentisdividedinhalftwice,andI010=Vref/(12R).

Whentheinputis100,Vrefispresentedontheright.Theeffectiveimpedancetogroundisonceagain3R,sothecurrentinjectedintotheR-2RladderisI0=Vref/(3R).Thecurrentisdividedinhalfonce,andI100=Vref/(6R).

UsingtheLawofSuperposition,theoutputvoltageisalinearcombinationofthethreedigitalinputs,Iout=(4b2+2b1+b0)Vref/(3R).Acurrenttovoltagecircuitisusedtocreateavoltageoutput.ToincreasetheprecisiononesimplyaddsmorestagestotheR-2Rladder.

Manymanufacturers,likeAnalogDevices,TexasInstruments,SipexandMaximproduceDACs.TheseDACshaveawiderangeofperformanceparametersandcomeinmanyconfigurations.ThefollowingparagraphsdiscussthevariousissuestoconsiderwhenselectingaDAC.AlthoughweassumetheDACisusedtogenerateananalogwaveform,theseconsiderationswillgenerallyapplytomostDACapplications.

Precision/range/resolution.Thesethreeparametersaffectthequalityofthesignalthatcanbegeneratedbythesystem.ThemorebitsintheDACthefinerthecontrolthesystemhasoverthewaveformitcreates.Asimportantasthisparameteris,itisoneofthemoredifficultspecificationstoestablishapriori.Asimpleexperimentalproceduretoaddressthisquestionistodesignaprototypesystemwithaveryhighprecision(e.g.,12,14,16,or20bits.)Thesoftwarecanbemodifiedtouseonlysomeoftheavailableprecision.Forexample,the12-bitMax5353hardwaredevelopedinExample7.2,canbereducedto4,8,or10bitsusingthefollowingfunctions.Thebottombitsaresettozero,insteadofshiftingsotherestofthesystemwilloperatewithoutchange.

voidDAC_Out4(uint16_tcode)

DAC_Out(code&0xFF00);//ignorebottom8bits

voidDAC_Out8(uint16_tcode)

DAC_Out(code&0xFFF0);//ignorebottom4bits

voidDAC_Out10(unsignedintcode)

DAC_Out(code&0xFFFC);//ignorebottom2bits

Program8.1.Softwareusedtotesthowmanybitsarereallyneeded.Multipleversionsofthesoftware(e.g.,4-bit,8-bit,10-bit,and12-bitDAC)areusedtoseeexperimentallytheeffectofDACprecisionontheoverallsystemperformance.Figure8.34illustrateshowDACprecisionaffectsthequalityofthegeneratedwaveform.

Figure8.34.Thewaveformontheleftusesa4-bitDAC,whileononeontherightusesa12-bitDAC.Channels.EventhoughmultiplechannelscouldbeimplementedusingmultipleDACchips,itisusuallymoreefficienttodesignamultiplechannelsystemusingamultiplechannelDAC.SomeadvantagesofusingaDACwithmorechannelsthanoriginallyconceivedarefutureexpansion,automatedcalibration,andautomatedtesting.AmultiplechannelDACallowsyoutoupdateallchannelsatthesametime.

Configuration.DACscanhavevoltageorcurrentoutputs.CurrentoutputDACscanbeusedinawidespectrumofapplications(e.g.,addinggainandfiltering),butdorequireexternalcomponents.DACscanhaveinternalorexternalreferences.AninternalreferenceDACiseasiertouseforstandarddigitalinput/analogoutputapplications,buttheexternalreferenceDACcanoftenbeusedinvariablegainapplications(multiplyingDAC).SometimestheDACgeneratesaunipolaroutput,whileothertimestheDACproducesbipolaroutputs.

Speed.ThereareacoupleofparametersmanufacturersusetospecifythedynamicbehavioroftheDAC.Themostcommonissettlingtime,anotherismaximumoutputrate.WhenoperatingtheDACinvariablegainmode,wearealsointerestedinthegain/bandwidthproductoftheanalogamplifier.Whencomparingspecificationsreportedbydifferentmanufacturersitisimportanttoconsidertheexactsituationusedtocollecttheparameter.Inotherwords,onemanufacturermaydefinesettlingtimeasthetimetoreach0.1%ofthefinaloutputafterafullscalechangeininputgivenacertainloadontheoutput,whileanothermanufacturermaydefinesettlingtimeasthetimetoreach1%ofthefinaloutputaftera1voltchangeininputunderadifferentload.ThespeedoftheDACtogetherwiththespeedofthecomputer/softwarewilldeterminetheeffectivefrequencycomponentsinthegeneratedwaveforms.Boththesoftware(rateatwhichthesoftwareoutputsnewvaluestotheDAC)andtheDACspeedmustbefastenoughforthegivenapplication.Inotherwords,ifthesoftwareoutputsnewvaluestotheDACataratefasterthantheDACcanrespond,thenerrorswilloccur.Figure8.35illustratestheeffectofDACoutputrateonthequalityofthegeneratedwaveform.AccordingtotheNyquistTheoremstatesthedigitaldataratemustbegreaterthantwicethemaximumfrequencycomponentofthedesiredanalogwaveform.However,bothwaveformsinFigure8.35satisfytheNyquistTheorem,butincreasingtheoutputratebyeightimprovesthesignaltonoiseratiobyeight.31dBisaratioofabout35to1,and49dBisaratioofabout281to1.Ifthegoalistocreateasinewaveatafixedfrequency,wecouldimprovetheSNRgreatlybyusingananaloglowpassfilter.

Experimentaldataofa32-output523Hzsine-waveExperimentaldataofa256-output523Hzsine-wave

Signal/noiseratiois31dB(3dB–-28dB)Signal/noiseratiois49dB(3dB–-46dB)

Figure8.35.Thewaveformontherightwascreatedbya12-bitDACwitheighttimestheoutputratethantheleft.VoltageversustimedataontopandtheFourierTransform(frequencyspectrumdBversuskHz)ofthedataonthebottom.Thereisapointinthespectrumat0,whichistheDC

component.However,thesignalisthe523Hzbumpwithamagnitudeof3dB,representingthesinewave.Thenoisesarealltheotherpointsnotat0or523Hz.Thelargestnoiseontheleftis-28dB.Thelargestnoiseontherightis-46dB.Power.Therearethreepowerissuestoconsider.Thefirstconsiderationisthetypeofpowerrequired.Olderdevicesrequirethreepowervoltages(e.g.,+5and-5V),whilemostdeviceswilloperateonasinglevoltagesupply(e.g.,+2.7,+3.3,or+5V.)Ifasinglesupplycanbeusedtopowerallthedigitalandanalogcomponentsthentheoverallsystemcostswillbereduced.Thesecondconsiderationistheamountofpowerrequired.Somedevicescanoperateonlessthan0.1mWandareappropriateforbattery-operatedsystemsorforsystemswhereexcessheatisaproblem.Thelastconsiderationistheneedforalow-powersleepmode.SomebatteryoperatedsystemsneedtheDAConlyintermittently.Intheseapplications,wewishtogiveashutdowncommandtotheDAC,sothatitdrawslesscurrentwhennotneeded.

Interface.ThreeapproachesexistforinterfacingtheDACtothecomputer.Inadigitallogicorparallelinterface,theindividualdatabitsareconnectedtoadedicatedcomputeroutputport.Forexample,a12-bitDACrequiresa12-bitoutputportbitstointerface.Thesoftwaresimplywritestotheparallelport(s)tochangetheDACoutput.ThesecondapproachiscalledµP-busormicroprocessor-compatible.Thesedevicesareintendedtobeinterfacedontotheaddress/databusofanexpandedmodemicrocontroller.Thethirdapproachisahigh-speedserialinterfacelikeI2CorSPI.TheSSI/MAX5353interfaceisanexampleofahigh-speedserialinterface.ThisapproachrequiresthefewestnumberofI/Opins.EvenifthemicrocontrollerdoesnotsupporttheSPIinterfacedirectly,thesedevicescanbeinterfacedtoregularI/Opinsviathebit-bangingsoftwareapproach.

Package.DIPpackagesareconvenientforcreatingandtestinganoriginalprototype.Ontheotherhandsurfacemountpackagesrequirelessboardspace.BecausesurfacemountpackagesdonotrequireholesinthePCboard,circuitswiththesedevicesareeasier/cheapertoproduce.

Cost.Costisalwaysafactorinengineeringdesign.BesidethedirectcostsoftheindividualcomponentsintheDACinterface,otherconsiderationsthataffectcostinclude:1)powersupplyrequirements;2)manufacturingcosts;3)thelaborinvolvedinindividualcalibrationifrequired;and4)softwaredevelopmentcosts.

8.4.2.DACWaveformGeneration

OneapplicationthatrequiresaDACiswaveformgeneration.Inthissection,wewilldiscussvarioussoftwaremethodsforcreatinganalogwaveformswithaDAC.Ineachcase,wewillbeusingtheMAX5353hardware/softwareinterfaceintroducedinExample7.2.Inaddition,wewilluseanoutputcaptureinterruptforthetiming,sothatthewaveformgenerationoccursinthebackground.TheritualsforinitializingtheperiodicinterruptareshowninChapter6.Inordertogetafaircomparisonbetweenthevariousmethods,eachimplementationwillgenerate32interruptsperwaveform.Inthefirstapproach,weassumethereexistsatimetovoltagefunction,called Wave() ,whichwecancalltodeterminethenextDACvaluetooutput.ThewaveformontheleftofFigure8.36couldbegeneratedbyPrograms8.2and8.3.

constfloatA=2048.0,B=1000.0,C=2*pi*31.25,D=-500.0,E=2*pi*125.0;

uint16_tWave(floattime)floatresult;

result=A+B*cos(C*time)+D*sin(E*time);

return(uint16_t)result;

Program8.2.Waveformisdefinedbyamathematicalfunction.Thesimplestsolutiongeneratesanoutputcompareinterruptataregularrate.Theadvantageofthisapproachisthatcomplexwaveformscanbeencodedwithasmallamountofdata.Inthisparticularexample,theentirewaveformcanbestoredas5datapoints( 2048.01000.031.25-500.0125.0 ).Thedisadvantageofthistechniqueisthatnotallwaveformshaveasimplefunction,andthissoftwarewillrunslowerascomparedtotheothertechniques.Ifyouweretoimplementthisapproach,performancewouldbeimprovedbyreplacingthefloating-pointmathwithfixed-point.

floatTime;//incrementedevery1ms

voidTimer0A_Handler(void)

TIMER0_ICR_R=TIMER_ICR_CAECINT;//acknowledge

Time=Time+0.001;

DACout(Wave(Time));

Program8.3.Periodicinterruptusedtocreatetheanalogoutputwaveform.Inthesecondapproach,weputthewaveforminformationinalargestaticallyallocatedglobalarray,seeProgram8.4.EveryinterruptwefetchanewvalueoutofthedatastructureandoutputittotheDAC.Inthiscasetheoutputcompareinterruptalsooccursataregularrate.Assumetheritualinitializes Time=0 .Runningat50MHzbusclock,theISRinProgram8.4executesinlessthan1 s.

uint16_tTime;//incrementedevery1ms,0-31

constuint16_tWave[32]=3048,2675,2472,2526,2755,2957,

2931,2597,2048,1499,1165,1139,1341,1570,1624,1421,1048,714,624,863,

1341,1846,2165,2206,2048,1890,1931,2250,2755,3233,3472,3382;

uint16_tTime;//every1ms

voidTimer0A_Handler(void)

TIMER0_ICR_R=TIMER_ICR_CAECINT;//acknowledge

Time=(Time+1)&0x1F;

DACout(Wave[Time]));

Program8.4.Periodicinterruptusedtocreatetheanalogoutputwaveform.SincetheoutputrateisequalandfixedthesefirsttwomethodshavethesameperformanceasillustratedintheleftofFigure8.36.Thesolidlineisthedesiredwaveformandthedottedlineistheactualgeneratedcurve.

Figure8.36.Generatedwaveforms.Leftuseseitherthefunctionorthetablelookuptechnique.Centerusesasmalltableandinterpolation.Rightusesavariableoutputrate.Ifthesizeofthetablegetslargeitispossibletostoreasmallertableinmemoryanduselinearinterpolationtorecoverthedatapointsinbetweenthestoredsamples.ThecenterofFigure8.36showsthegeneratedwaveformderivedfromonly9oftheoriginal32datapoints.Tosimplifythesoftware,thefirstdatapointisrepeatedasthelastdatapoint.ForeachpointwewillneedtosaveboththeDACvalueandtimelengthofthecurrentlinesegment.Forthe9saveddatapointswesimplyoutputthedata,butfortheotherpoints,wemustperformalinearinterpolationtogetthevaluetooutputtotheDAC,seeProgram8.5.Assumetheritualinitializes I=J=0 .Signed16-bitnumbersareusedsothesubtractionsoperateproperly.Inotherwords,someoftheintermediatecalculationscanbenegative.

int16_tI;//incrementedevery1ms

int16_tJ;//indexintothesetwotables

constint16_tTime[10]=0,2,6,10,14,18,22,25,30,32;//timeinmsec

constint16_tWave[10]=3048,2472,2931,1165,1624,624,2165,1890,3472,3048;//last=first

uint16_tTime;//every1ms

voidTimer0A_Handler(void)

TIMER0_ICR_R=TIMER_ICR_CAECINT;//acknowledge

if((++I)==32)I=0;J=0;

if(I==Time[J])

DACout(Wave[J]);

elseif(I==Time[J+1])

J++;

DACout(Wave[J]);

else

DACout(Wave[J]+((Wave[J+1]-Wave[J])*(I-t[J]))/(Time[J+1]-Time[J]));

Program8.5.Periodicinterruptusedtocreatetheanalogoutputwaveform.ThesoftwareintheprevioustechniqueschangestheDACatafixedrate.Whilemostofthetimethisisadequate,therearesomewaveformsforwhichuneventimesbetweenoutputsseemappropriate.Inourtestsignal,thereareplacesinthewavewherethesignalvariesslowlyinadditiontoplacesinthewavewithrapidlychangingvalues.Noticethedatapointsinthisfigureareplacedatuneventimeintervalstomatchthevariousphasesofthissignal.Thisgeneratedwaveformisstillcreatedwith32points,butplacingthepointsclosertogetherduringphaseswithlargeslopesimprovestheoverallaccuracy,seeProgram8.6.Thetabledatastructurewillencodeboththevoltage(asaDACvalue)andthetime.Thetimeparameterisstoredasa∆tinTimer0cyclestosimplifyservicingtheoutputcompareinterrupt.

uint16_tTime;//incrementedeverysample,0to31

constuint16_tWave[32]=

3048,2675,2472,2526,2817,2981,2800,2337,1901,1499,1165,1341,1570,1597,

1337,952,662,654,863,1210,1605,1950,2202,2141,1955,1876,2057,

2366,2755,3129,3442,3382;

constuint16_tdt[32]=//timeincrementinTimer0cycles

2000,2000,2000,2500,2500,2000,2000,1500,1500,2000,4000,2000,2500,

2000,2000,2000,2000,1500,1500,1500,1500,2000,2500,2000,2000,2000,

1500,1500,1500,2000,2500,2000;

voidTimer0A_Handler(void)

Time=(Time+1)&0x1F;

DACout(Wave[Time]);//thisoutputamplitude

TIMER0_TAILR_R=dt[Time];//thistimeduration

TIMER0_ICR_R=TIMER_ICR_CAECINT;//acknowledge

Program8.6.Periodicinterruptatunequalratesusedtocreatetheanalogoutputwaveform.

8.4.3.PWMDACWecanusethePWMmoduletocreateaDAC.ThePWMdigitalsignalisfeedthroughananaloglow-passfiltertocreateaDCvoltagelinearlyrelatedtothedutycycleofthePWMwave.TheprecisionoftheDACwillbetheprecisionofthecounterusedtocreatethePWM.Inparticular,theprecisionwillbethenumberweputintothe PWM_0_LOAD_RregisteroftheLM3S/TM4C.Forexample,ifwecallProgram6.8with PWM0_Init(1024,512) ,thentheDACprecisionwillbe1024alternativesor10bits.ThefrequencyoftheDACistherateatwhichtheDACoutputcanbechangedanddependsonthefrequencyofthePWMoutput.Inparticular,wecanchangethePWMdutycycleonlyoncepercycle.Ifthebusclockis8MHz,andweinitializewith PWM0_Init(4000,2000) ,thePWMperiodwillbe1ms.Thismeanswecanupdatethe12-bitDAC1000timesasecond.However,ifthebusclockis50MHz,andweinitializewith PWM0_Init(250,125) ,thePWMperiodwillbe10 s.Thismeanswecanupdatethe8-bitDAC100,000timesasecond.ItisusuallybestforthePWMsignalfrequencytobe10to100timeshigherthanthedesiredbandwidthofanalogsignalstobeproduced.Generally,thehigherthePWMfrequency,thelowertheorderoffilterrequired,andtheeasieritistobuildasuitablefilter.Letfmaxbethehighestfrequencycomponentwewishtocreateintheanalogoutput,andletfPWMbethePWMfrequency.WewanttheanalogLPFtorejectfPWMandpassfmax.

Example8.4.Designasystemthatoutputsa1-kHzsinewaveusingaPWMDAC.

Solution:ThissolutionwillusetheperiodictimershowninProgram6.6andthePWMdrivershowninProgram6.8.WewillrunthePWMoutputwaveat100kHz,whichismuchfasterthanthedesiredsinewave.Wecancreatean8-bitPWMDACusingabusclockof50MHzandinitializingwith PWM0_Init(250,125) .50MHz/100kHzis250alternatives,whichabout8bits.TochangetheamplitudeoftheDACweadjustthedutycycleofthePWMbycalling PWM0_SetDuty() .Wewilloutput32pointstothe8-bitPWMDACforeachcycleinthesinewave,seeProgram8.7.TheLPFmustpass1kHzandreject100kHz.WechoosetheLPFcutoffat3kHz,whichisbetweenfPWMandfmax.ApassiveLPFcanbemadewitharesistorandcapacitor,fc=1/(2 RC).AnRequalto22k andCequalto2.2nFwillcreateaLPFat3.3kHz,asshowninFigure8.37.Ifweneedlowoutputimpedance,wecouldreplacethepassivefilterwithanactivefilterlikeFigure8.25.

Figure8.37.PWMDACusedtocreatea1kHzsinewave.constuint16_tWave[32]=

125,143,159,175,189,200,208,213,215,213,208,

200,189,175,159,143,125,107,91,75,61,50,

42,37,35,37,42,50,61,75,91,107

;

voidOutputSineWave(void)

staticuint8_tindex=0;//countingindexofoutputsequence

PWM0_Duty(Wave[index]);//outputnextvalueinsequence

index=(index+1)&0x1F;//incrementcounter

intmain(void)//PWMcodeinProgram6.8

SysCtlClockSet(SYSCTL_SYSDIV_4|SYSCTL_USE_PLL|SYSCTL_OSC_MAIN|

SYSCTL_XTAL_8MHZ);

PWM0_Init(250,125);//initializePWM0,100kHz

Timer0A_Init(&OutputSineWave,31);//Program6.6,31us,32kHz

while(1)

WaitForInterrupt();

Program8.7.SoftwareforPWMDACcreatinga1kHzsinewave(PWMSine_xxx.zip).

8.5.AnalogtoDigitalConverters

8.5.1.ADCParametersAnADCconvertsananalogsignalintodigitalform.Theinputsignalisusuallyananalogvoltage(Vin),andtheoutputisabinarynumber.TheADCprecisionisthenumberofdistinguishableADCinputs(e.g.,4096alternatives,12bits).TheADCrangeisthemaximumandminimumADCinput(volts,amps).TheADCresolutionisthesmallestdistinguishablechangeininput(volts,amps).Theresolutionisthechangeininputthatcausesthedigitaloutputtochangeby1.

Range(volts)=Precision(alternatives)•Resolution(volts)

Normallywedon’tspecifyaccuracyforjusttheADC,butratherwegivetheaccuracyoftheentiresystem(includingtransducer,analogcircuit,ADCandsoftware).Therefore,accuracywillbedescribedlaterinChapter10aspartofthesystemsapproachtodataacquisitionsystems.AnADCismonotonicifithasnomissingcodes.Thismeansiftheanalogsignalisaslowrisingvoltage,thenthedigitaloutputwillhitallvaluessequentially.TheADCislineariftheresolutionisconstantthroughtherange.Letf(x)betheinput/outputADCtransferfunction.Onequantitativemeasureoflinearityisthecorrelationcoefficientofalinearregressionfitofthef(x)responses.TheADCspeedisthetimetoconvert,calledtc.TheADCcostisafunctionofthenumberandpriceofinternalcomponents.TherearefourcommonencodingschemesforanADC.Table8.7showstwoencodingschemesfora12-bitunipolarADC,andTable8.8showstwoencodingschemesfora12-bitbipolarADC.

UnipolarCodes

StraightBinary ComplementaryBinary

+Vmax 1111,1111,1111 0000,0000,0000

+Vmax/2 1000,0000,0000 0001,1111,1111

+Vmax/1024 0000,0000,0001 1111,1111,1110

+0.00 0000,0000,0000 1111,1111,1111

Table8.7.Unipolarcodesfora12-bitADCwitharangeof0to+Vmax.

BipolarCodes

OffsetBinary Two’sComplementBinary

+Vmax 1111,1111,1111 0111,1111,1111

+Vmax/2 1000,0000,0000 0100,0000,0000

+Vmax/512 1000,0000,0001 0000,0000,0001

+0.00 1000,0000,0000 0000,0000,0000

-Vmax/512 0111,1111,1111 1111,1111,1111

-Vmax/2 0100,0000,0000 1100,0000,0000

-Vmax 0000,0000,0000 1000,0000,0000

Table8.8.Bipolarcodesfora12-bitADCwitharangeof-Vmaxto+Vmax.

TheTM4Cusesstraightbinary,hasaprecisionof12bits,andhasarangeof0to3.3V.TheLM3Shasa10-bitADCwitharangeof0to3V.Toconvertbetweenstraightbinaryandcomplementarybinarywesimplycomplement(change0to1,change1to0)allthebits.Toconvertbetweenoffsetbinaryand2’scomplement,wecomplementjustthemostsignificantbit.Theexclusive-oroperationcanbeusedtocomplementbits.

JustliketheDAC,onecanchoosethefullscalerangetosimplifytheuseoffixed-pointmath.Forexample,ifa12-bitADChadafullscalerangeof0to4.095volts,thentheresolutionwouldbeexactly1mV.ThismeansthatiftheADCinputvoltagewere1.234volts,thentheresultwouldbe1234(decimal).

Thetotalharmonicdistortion(THD)ofasignalisameasureoftheharmonicdistortionpresentandisdefinedastheratioofthesumofthepowersofallharmoniccomponentstothepowerofthefundamentalfrequency.Basically,itisameasureofallthenoiseprocessesinanADCandusuallyisgivenindBfullscale.Asimilarparameterissignal-to-noiseanddistortionratio(SINAD),whichismeasuredbyplacingapuresinewaveattheinputoftheADC(signal)andmeasuringtheADCoutput(signalplusnoise).Wecancompareprecisioninbitstosignal-to-noiseratioindBusingtherelationdB=20log10(2n).Forexample,the12-bitMAX1247ADChasaSINADof73dB.Noticethat20log10(212)is72dB.

Dynamicrange,expressedindB,isdefinedastherangebetweenthenoisefloorofadeviceanditsspecifiedmaximumoutputlevel.ThedynamicrangeistherangeofsignalamplitudeswhichtheADCcanresolve.IfanADCcanresolvesignalsfrom1mVto1V,ithasadynamicrangeof20*log(1V/1mV)=60dB.Dynamicrangeisimportantincommunicationapplications,wheresignalstrengthsvarydramatically.Ifthesignalistoolarge,itsaturatestheADCinput.Ifthesignalistoosmall,itgetslostinthequantizationnoise.

Theeffectivenumberofbits(ENOB)specifiesthedynamicperformanceofanADCataspecificinputfrequencyandsamplingrate.Inanidealsituation,ADCerrorconsistsonlyofquantizationnoise(resolution=range/precision).Astheinputfrequencyincreases,the

overallnoise(particularlyinthedistortioncomponents)alsoincreases,therebyreducingtheENOBandSINAD.

8.5.2.ADCConversionTechniquesThemostpervasivemethodforADCconversionisthesuccessiveapproximationtechnique,asillustratedinFigure8.38.A12-bitsuccessiveapproximationADCisclocked12times.Ateachclockanotherbitisdetermined,startingwiththemostsignificantbit.Foreachclock,thesuccessiveapproximationhardwareissuesanew“guess”onVdacbysettingthebitundertesttoa“1”.IfVdacisnowhigherthantheunknowninput,Vin,thenthebitundertestiscleared.IfVdacislessthanVin,thenthebitundertestisremains1.Inthisdescription,bitisanunsignedintegerthatspecifiesthebitundertest.Fora12-bitADC,bitgoes2048,1024,512,256,128,64,…,1.DoutistheADCdigitaloutput,andZisthebinaryinputthatistrueifVdacisgreaterthanVin.

Figure8.38.A12-bitsuccessiveapproximationADC.Observation:ThespeedofasuccessiveapproximationADCrelateslinearlywithitsprecisioninbits.

Thesigmadeltaanalogtodigitalconverterisusedinmanyaudioapplications.Itisacosteffectiveapproachto16-bit44kHzsampling(CDqualityaudio).SigmadeltaconvertershaveaDAC,acomparatoranddigitalprocessingsimilartothesuccessiveapproximationtechnique.WhilesuccessiveapproximationconvertershaveDACswiththesameprecisionastheADC,sigmadeltaconvertersuseDACswithamuchsmallerprecisionthantheADC.A1-bitDACissimplyadigitalsignalitself.ThedigitalsignalprocessingwillrunataclockfrequencyfasterthantheoverallADCsystem,calledoversampling.ItusescomplexsignalprocessingtodrivetheoutputvoltageV0toequaltheunknowninputVininatime-averagedsense.The“delta”partofthesigmadeltaconverteristhesubtractor,whereV1=V0–Vin.Nextcomesthe“sigma”partthatimplementsananalogintegration.IfV0toequaltheunknowninputVininatime-averagedsense,thenV2willbezero.ThecomparatorteststheV2signal.IfV2ispositivethenV0ismadesmaller.IfV2isnegativethenV0ismadelarger.ThisDAC-subtractor-integrator-comparator-digitalloopisexecutedataratemuchfasterthantheeventualdigitaloutputrate.

Averysimplealgorithm,showninFigure8.39isruncontinuously.ForeverytimethroughtheouterwhileloopthereisoneADCoutput.Thisalgorithmismuchtoosimpletobeappropriateinanactualconverter,butitdoesillustratethesigmadeltaapproach.Fora10-bitconversion,theDACoutputrateis1024timestheADCconversionrate.Weassumetheinputvoltage,Vin,isbetween0and+3V.DACisanoutputofthesigma-delta

processingthatsetsthe1-bitDAC.Zisthecomparatoroutput,whichisaninputtothesignalprocessing.

Figure8.39.Blockdiagramofthesigma-deltaADCconversiontechnique.Inthisverysimplesolution,theDACissetto1(V0=+3)ifZis0(V2<0).Conversely,theDACissetto0(V0=0)ifZis1(V2>0).EachtimetheDACissetto1,sumisincremented.Attheendof1024passes,thevaluesumisrecordedastheADCsample.Sincethereare1024passesthroughtheloop,sumwillvaryfrom0to1023.Forexample,iftheVinis1.5V,thenhalfoftheDACoutputswillbe1andtheotherhalf0.ThiswillmakeV0oscillatebetween0and3V,witha50%dutycycle,V1willoscillatebetween-1.5and+1.5witha50%dutycycle,andthetime-averagedV2willbezero.

AsecondexampleisillustratedinFigure8.40.TheinputVinis2.25V,sotheoutputshouldbe2.25/3*1024or768.ThesigmadeltawilladjusttheDACoutputsothatV1=V0–Vin.isequalto0inatime-averagedsense.V1is2.25Vfor25%ofthetimeand‑0.75Vfor75%ofthetime.ThreeoutofeveryfourDACoutputsarehigh,sothreeoutofeveryfourtime,V2willbeabove0.Thereforeafter1024timesthroughtheloop,768ofthemwillincrementsum,yieldingthecorrectADCresult.IftheVininputrises,ahigherpercentageofDACoutputswillbehigh,increasingsum.IftheVininputfalls,alowerpercentageofDACoutputswillbehigh,decreasingsum.

Figure8.40.Exampleoperationofasigma-deltaconversion.Inarealsigmadeltatheoverclockrateistypically8to1or16to1.Multiplebitsareobtainedeachtimethroughtheoutput-inputcycleusingDSPalgorithms.

AnotherADCtechniqueiscalledflash.Flashconvertersareveryexpensiveandveryfast.Figure8.41showsa3-bitflash.TheMAX104isa±5V,1Gsps,8-BitADCwithon-chip2.2GHzsample/hold.

Figure8.41.Blockdiagramofa3-bitflashADC.

8.5.3.Sample/HoldAsampleandhold(S/H)isananaloglatch,illustratedinFigure8.42.Analternativenameforthisanalogcomponentistrackandhold.ThepurposeoftheS/HistoholdtheADCanaloginputconstantduringtheconversion.ThereisS/HattheinputofmostADC,includingtheonesontheStellaridandTivamicrocontrollers.Thefirstphaseofmostanalogtodigitalconversionsisthesamplingphase,wheretheinputvoltageVin,isrecordedasachargeonthecapacitorC.

Figure8.42.Thesampleandholdhasadigitalinput,ananaloginputandananalogoutput.Thedigitalinput,Control,determinestheS/Hmode.TheS/Hisinsamplemode,whereVoutequalsVin,withtheswitchisclosed.TheS/Hisinholdmode,Voutisfixedbecausetheswitchisopen.Theacquisitiontimeisthetimefortheoutputtoequaltheinputafterthecontrolisswitchedfromholdtosample.ThisisthetimetochargethecapacitorC.Theaperturetimeisthetimefortheoutputtostabilizeafterthecontrolisswitchedfromsampletohold.Thisisthetimetoopentheswitch,whichisusuallyquitefast.Thedrooprateistheoutputvoltageslope(dVout/dt)whenControlequalshold.Normallythegain,K,shouldbeoneandtheoffset,Voff,shouldbezero.ThegainandoffseterrorspecifyhowcloseistheVouttothedesiredVinwhenControlequalssample,

Vout=KVin+VoffTochoosethecapacitor,C:

1)Oneshoulduseahighqualitycapacitorwithhigh

insulationresistanceandlowdielectricabsorption.

2)AlargervalueofCwilldecrease(improve)thedrooprate.

IfthedroopcurrentisIDR,thenthedroopratewillbedVout/dt=IDR/C

3)AsmallerCwilldecrease(improve)theacquisitiontime.

ThesystemwillrequireasampleandholdiftheinputsignalcouldchangeduringanADCconversion.TherewillbeatimeduringwhichtheADCsamplestheinputvoltage,tsamp.LetthemaximumslewrateoftheinputsignalbedVin/dt.IftheslewratetimesthesamplingtimeislargerthantheADCresolution,weshouldaddasampleandholdmoduletokeeptheanaloginputstableduringconversion.TheLM3S/LM4F/TM4CADCmoduleshaveabuilt-inS/H.

8.5.4.InternalADCTable8.9showstheADCregisterbitsrequiredtoperformperiodicsampling.Wewillshowtimertriggeringononeandontwochannels.Formorecomplexconfigurationsrefertothespecificdatasheet.ThevalueinADC0_PC_Rspecifiesthemaximumsamplingrate,seeTable8.10.TheLM3S1968,TM4C123,andTM4C1294cansampleupto1millionsamplespersecond,butthemaximumsamplingrateonmanyLM3Smicrocontrollersisonly500K.Refertothedatasheetofyourspecificmicrocontrollerformaximumpossiblesamplingrate,thenumberofchannels,andthenumberofbits.TheADChasfoursequencers,butwewillpresentcodeusingsequencers2and3.WesettheADC0_SSPRI_Rregisterto0x3210tomakesequencer0thehighestandsequencer3thelowestpriority.Weneedtomakesureeachsequencerhasauniquepriority.Wesetbits15–12(EM3)intheADC0_EMUX_Rregistertospecifyhowsequencer3willbetriggered.Similarly,wesetbits11–8(EM2)tospecifyhowsequencer2willbetriggered.Table8.11showsthevariouswaystotriggeranADCconversion.Inthissection,wewillusetimertriggering(EM3=0x5).

Address 31-2 1 0 Name

$400F.E638 ADC1 ADC0 SYSCTL_RCGCADC_R

31-14 13-12 11-10 9-8 7-6 5-4 3-2 1-0

$4003.8020 SS3 SS2 SS1 SS0 ADC0_SSPRI_R

31-16 15-12 11-8 7-4 3-0

$4003.8014 EM3 EM2 EM1 EM0 ADC0_EMUX_R

31-4 3 2 1 0

$4003.8000 ASEN3 ASEN2 ASEN1 ASEN0 ADC0_ACTSS_R

$4003.8028 SS3 SS2 SS1 SS0 ADC0_PSSI_R

$4003.8004 INR3 INR2 INR1 INR0 ADC0_RIS_R

$4003.8008 MASK3MASK2 MASK1MASK0 ADC0_IM_R

$4003.8FC4 Speed ADC0_PC_R

$4003.800C IN3 IN2 IN1 IN0 ADC0_ISC_R

31-28 27-24 23-20 19-16 15-12 11-8 7-4 3-0

$4003.8040 MUX7 MUX6 MUX5 MUX4 MUX3 MUX2 MUX1 MUX0 ADC0_SSMUX0_R

31-16 15-12 11-8 7-4 3-0

$4003.8060 MUX3 MUX2 MUX1 MUX0 ADC0_SSMUX1_R

$4003.8080 MUX3 MUX2 MUX1 MUX0 ADC0_SSMUX2_R

$4003.80A0 MUX0 ADC0_SSMUX3_R

31 30 29 28 27 26 … 8 7 6 5 4 3 2 1 0

$4003.8044 TS7IE7END7D7TS6IE6… D2TS1IE1 END1D1TS0IE0 END0D0ADC0_SSCTL0_R

15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

$4003.8064 TS3IE3END3D3TS2IE2END2D2TS1IE1 END1D1TS0IE0 END0D0ADC0_SSCTL1_R

$4003.8084 TS3IE3END3D3TS2IE2END2D2TS1IE1 END1D1TS0IE0 END0D0ADC0_SSCTL2_R

$4003.80A4 TS0IE0 END0D0ADC0_SSCTL3_R

31-10 11-0

$4003.8048 DATA ADC0_SSFIFO0_R

$4003.8068 DATA ADC0_SSFIFO1_R

$4003.8088 DATA ADC0_SSFIFO2_R

$4003.80A8 DATA ADC0_SSFIFO3_R

Table8.9.SomeoftheADCregisters.Eachregisteris32bitswide.LM3Shas10-bitdata

Value Description

0x7 1Msamples/second

0x5 500Ksamples/second

0x3 250Ksamples/second

0x1 125Ksamples/second

Table8.10.TheSpeedbitsintheADC0_PC_Rregister.

Whenusingsequencer0,wecanspecifyuptoeightchannelstoconvertonasinglecommand.Sequencers1and2allowuptofourchannelstobeconverted.Thecontrolbitsforsequencer2areshowninTable8.9.TheADC0_SSMUX2_RregisterspecifieswhichchannelstoconvertandtheADC0_SSCTL2_Rregisterspecifieshowmanysamplestotakeandif/whentointerrupt.Forexample,ifwewishtoconvertchannels2,3,and6,wesetMUX0,MUX1,andMUX2fieldsto2,3,6respectivelyinADC0_SSMUX2_R.TheADC0_SSCTL2_RbitEND2issettospecifythreeconversions,assumingEND0=0andEND1=0.TheIE2bitcanbesettorequestaninterruptafterthethirdconversion.10-bitresultsfromsequencer2arereadfromtheADC0_SSFIFO2register.AllotherbitsintheADC0_SSCTL2_Rregisterwillbeclearforthisexample(notemperatureordifferentialmeasurements).

TheADC0_RIS_Rregisterhasflagsthataresetwhentheconversioniscomplete,assumingtheIE0bitisset.TheADC_IM_Rregisterhasinterruptarmbits.TheADC0_ISC_Rregisterhasinterrupttriggerbits.TheIN3bitissetwhenbothINR3andMASK3areset.WecleartheINR3andIN3bitsbywritingan8totheADC0_ISC_Rregister.TheinterruptvectorforADCsequencer3isat0x00000084.

Value Event

0x0 Softwarestart

0x1 AnalogComparator0

0x2 AnalogComparator1

0x3 AnalogComparator2

0x4 External(GPIOPB4)

0x5 Timer

0x6 PWM0

0x7 PWM1

0x8 PWM2

0xF Always(continuouslysample)

Table8.11.TheADCEM3,EM2,EM1,andEM0bitsintheADC0_EMUX_Rregister.

Thereare13stepstoconfiguretheADCtosampleasinglechannelataperiodicrate.Themostaccuratesamplingmethodistimer-triggeredsampling(EM3=0x5).OntheTM4C123,theMUXfieldsare4bitswide,allowingustospecifychannels0to11.OntheTM4C1294,thechannelrangesfrom0to19(seeTable3.2).

Step1.WeenabletheADCclockintheSYSCTL_RCGCADC_Rregister.

Step2.Bits3–0oftheADC0_PC_RregisterspecifythemaximumsamplingrateoftheADC.Inthisexample,wewillsampleslowerthan125kHz,sothemaximumsamplingrateissetat125kHz.ThiswillrequirelesspowerandproducealongersamplingtimeasdescribedtheS/Hsection,creatingamoreaccurateconversion.

Step3.Wewillsetthepriorityofeachofthefoursequencers.Inthiscase,weareusingjustonesequencer,sotheprioritiesareirrelevant,exceptforthefactthatnotwosequencersshouldhavethesamepriority.ThedefaultconfigurationhasSampleSequencer0withthehighestpriority,andSampleSequencer3asthelowestpriority.

Step4.Next,weneedtoconfigurethetimertorunatthedesiredsamplingfrequency.WeenabletheTimer0clockbysettingbit0oftheSYSCTL_RCGCTIMER_Rregister.ThisinitializationissimilartoProgram6.6withtwochanges.Firstwesetbit5oftheTIMER0_CTL_RregistertoactivateTAOTE,whichistheTimerAoutputtriggerenable.Secondly,wedonotarmanyTimer0interrupts.Therateatwhichthetimerrollsoverdeterminesthesamplingfrequency.LetprescalebethevalueloadedintoTIMER0_TAPR_R,andletperiodbethevalueloadedintoTIMER0_TAILR_R.Iftheperiodofthebusclockfrequencyis t,thentheADCsamplingperiodwillbe

t*(prescale+1)*(period+1)

ThefastestsamplingrateisdeterminedbythespeedoftheprocessorhandlingtheADCinterruptsandbythespeedofthemainprogramconsumingthedatafromtheFIFO.Ifthebusclockis80MHz,theslowestpossiblesamplingrateforthisexampleis80MHz/232,whichisabout0.018Hz,whichisevery53seconds.

Step5.Beforeconfiguringthesequencer,weneedtodisableit.Todisablesequencer3,wewritea0tobit3(ASEN3)intheADC0_ACTSS_Rregister.Disablingthesequencerduringprogrammingpreventserroneousexecutionifatriggereventweretooccurduringtheconfigurationprocess.

Step6.WeconfigurethetriggereventforthesamplesequencerintheADC0_EMUX_Rregister.Forthisexample,wewritea0101tobits15–12(EM3)specifyingtimertriggermodeforsequencer3.

Step7.Foreachsampleinthesamplesequence,configurethecorrespondinginputsourceintheADC0_SSMUXnregister.Inthisexample,wewritethechannelnumber(0,1,2,or3)tobits3–0intheADC0_SSMUX3_Rregister.

Step8.Foreachsampleinthesamplesequence,weconfigurethesamplecontrolbitsinthecorrespondingnibbleintheADC0_SSCTLnregister.Whenprogrammingthelastnibble,ensurethattheENDbitisset.FailuretosettheENDbitcausesunpredictablebehavior.Sequencer3hasonlyonesample,sowewritea0110totheADC0_SSCTL3_Rregister.Bit3istheTS0bit,whichweclearbecausewearenotmeasuringtemperature.Bit2istheIE0bit,whichwesetbecausewewanttorequestaninterruptwhenthesampleiscomplete.Bit1istheEND0bit,whichissetbecausethisisthelast(andonly)sampleinthesequence.Bit0istheD0bit,whichweclearbecausewedonotwishtousedifferentialmode.

Step9.Ifinterruptsaretobeused,writea1tothecorrespondingmaskbitintheADC0_IM_Rregister.Wewantaninterrupttooccurwhentheconversioniscomplete(setbit3,MASK3).

Step10.Weenablethesamplesequencerlogicbywritinga1tothecorrespondingASENn.Toenablesequencer3,wewritea1tobit3(ASEN3)intheADC0_ACTSS_Rregister.

Step11.ThepriorityoftheADC0sequencer3interruptsareinbits13–15oftheNVIC_PRI4_Rregister.

Step12.Sincewearerequestinginterrupts,weneedtoenableinterruptsintheNVIC.ADCsequencer3interruptsareenabledbysettingbit17intheNVIC_EN0_Rregister.

Step13.Lastly,wemustenableinterruptsinthePRIMASKregister.

Thetimerstartstheconversionataregularrate.Bit3(INR3)intheADC0_RIS_Rregisterwillbesetwhentheconversionisdone.Thisbitisarmedandenabledforinterrupting,soconversioncompletewilltriggeraninterrupt.TheIN3bitintheADC0_ISC_Rregistertriggerstheinterrupt.TheISRacknowledgestheinterruptbywritinga1tobit3(IN3).The12-bitresultisreadfromtheADC0_SSFIFO3_Rregister.Thebookwebsiteforhasexamplecode.Inordertoreducelatencyofotherinterruptrequestsinthesystem,thisISRsimplystoresthe12-bitconversioninaFIFO,tobeprocessedlaterinthemainprogram.Program8.8showstheinitializationandinterrupt

serviceroutinetoaffecttheperiodicsampling.Fortheportpin,wedisableitsDEN,clearitsDIR,setitsAFSELandenableitsAMSELbit.

voidADC0_InitTimer0ATriggerSeq3PD3(uint32_tperiod)

volatileuint32_tdelay;

SYSCTL_RCGCADC_R|=0x01;//1)activateADC0

SYSCTL_RCGCGPIO_R|=0x08;//PortDclock

delay=SYSCTL_RCGCGPIO_R;//allowtimeforclocktostabilize

GPIO_PORTD_DIR_R&=~0x08;//makePD3input

GPIO_PORTD_AFSEL_R|=0x08;//enablealternatefunctiononPD3

GPIO_PORTD_DEN_R&=~0x08;//disabledigitalI/OonPD3

GPIO_PORTD_AMSEL_R|=0x08;//enableanalogfunctionalityonPD3

ADC0_PC_R=0x01;//2)configurefor125Ksamples/sec

ADC0_SSPRI_R=0x3210;//3)seq0ishighest,seq3islowest

SYSCTL_RCGCTIMER_R|=0x01;//4)activatetimer0

delay=SYSCTL_RCGCGPIO_R;

TIMER0_CTL_R=0x00000000;//disabletimer0Aduringsetup

TIMER0_CTL_R|=0x00000020;//enabletimer0AtriggertoADC

TIMER0_CFG_R=0;//configurefor32-bittimermode

TIMER0_TAMR_R=0x00000002;//configureforperiodicmode

TIMER0_TAPR_R=0;//prescalevaluefortrigger

TIMER0_TAILR_R=period-1;//startvaluefortrigger

TIMER0_IMR_R=0x00000000;//disableallinterrupts

TIMER0_CTL_R|=0x00000001;//enabletimer0A32-b,periodic

ADC0_ACTSS_R&=~0x08;//5)disablesamplesequencer3

ADC0_EMUX_R=(ADC0_EMUX_R&0xFFFF0FFF)+0x5000;//6)timertrigger

ADC0_SSMUX3_R=4;//7)PD3isanalogchannel4

ADC0_SSCTL3_R=0x06;//8)setflagandendafterfirstsample

ADC0_IM_R|=0x08;//9)enableSS3interrupts

ADC0_ACTSS_R|=0x08;//10)enablesamplesequencer3

NVIC_PRI4_R=(NVIC_PRI4_R&0xFFFF00FF)|0x00004000;//11)priority2

NVIC_EN0_R=1<<17;//12)enableinterrupt17inNVIC

EnableInterrupts();//13)enableinterrupts

voidADC0Seq3_Handler(void)

ADC0_ISC_R=0x08;//acknowledgeADCsequence3completion

Fifo_Put(ADC0_SSFIFO3_R);//passtoforeground

Program8.8.SoftwaretosampledatausingtheADC(ADCT0ATrigger_xxx.zip).Checkpoint8.7:Iftheinputvoltageis1.0V,whatvalue,in10-bitunsignedright-justifiedmode,willtheLM3SADCreturn?WhatwillaTM4Cwitha12-bitADCreturn?

Sequencer3canonlysampleoneanaloginput.Whenwewishtosamplemultiplechannelswithonetrigger,weneedtousesequencers0,1,or2.Thenextexampleshowshowtosampletwochannelsusingsequencer2.TheTM4C123haspinsthatcouldbe12analoginputandtheTM4C1294has20possibleanalogpins.BothofthesemicrocontrollershavetwoADCmodulesandeachmodulehas4sequencers.

Example8.5.WritesoftwaretosampleADCchannels4and5at1kHz.Channel4ontheTM4C123isPD3andchannel5isPD2.

Solution:Thissolutionwillusetheperiodictimertoestablishthe1000Hzsamplingrate,similartoProgram8.8.WespecifythechannelstosampleintheADC0_SSMUX2_Rregister.0x0054meansfirstsamplechannel4thensamplechannel5.TheADC0_SSCTL2_RbitEND1issettospecifytwoconversions,assumingEND0=0.TheIE1bitissettorequestaninterruptafterthesecondconversion.Wereadthetwo12-bitresultsfromtheADC0_SSFIFO2register.AllotherbitsintheADC0_SSCTL2_Rregisterwillbeclearforthisexample(notemperatureordifferentialmeasurements).

voidADC_Init(void)//assumesa80MHzbusclock

SYSCTL_RCGCADC_R|=0x01;//1)activateADC0

SYSCTL_RCGCGPIO_R|=0x08;//PortDclock

SYSCTL_RCGCTIMER_R|=0x01;//4)activatetimer0

Ch1Fifo_Init();//initializeFIFOs

Ch3Fifo_Init();//waitforclockstostabilize

GPIO_PORTD_DIR_R&=~0x0C;//makePD3-2input

GPIO_PORTD_AFSEL_R|=0x0C;//enablealternatefunctiononPD3-2

GPIO_PORTD_DEN_R&=~0x0C;//disabledigitalI/OonPD3-2

GPIO_PORTD_AMSEL_R|=0x0C;//enableanalogfunctionalityonPD3-2

ADC0_PC_R=0x01;//2)configurefor125Ksamples/sec

ADC0_SSPRI_R=0x3210;//3)Priorityofsequencers

TIMER0_CTL_R=0x00000000;//disabletimer0Aduringsetup

TIMER0_CTL_R|=0x00000020;//enabletimer0AtriggertoADC

TIMER0_CFG_R=0;//configurefor32-bittimermode

TIMER0_TAMR_R=0x00000002;//configureforperiodicmode

TIMER0_TAPR_R=0;//prescalevaluefortrigger

TIMER0_TAILR_R=79999;//80000cyclesis1ms

TIMER0_IMR_R=0x00000000;//disableallinterrupts

TIMER0_CTL_R|=0x00000001;//enabletimer0A32-b,periodic

ADC0_ACTSS_R&=~0x04;//5)disablesamplesequencer2

ADC0_EMUX_R=(ADC0_EMUX_R&0xFFFFF0FF)+0x0500;//6)timertrigger

ADC0_SSMUX2_R=0x0054;//7)PD3-2arechannel4,5

ADC0_SSCTL2_R=0x0060;//8)setflagandendaftersecond

ADC0_IM_R|=0x04;//9)enableSS2interrupts

ADC0_ACTSS_R|=0x04;//10)enablesamplesequencer2

NVIC_PRI4_R=(NVIC_PRI4_R&0xFFFFFF00)|0x00000040;//ADC2priority2

NVIC_EN0_R=1<<16;//12)enableinterrupt16

EnableInterrupts();//13)enableallinterrupts

voidADC0Seq2_Handler(void)

ADC0_ISC_R=0x04;//acknowledgeADCsequence2completion

Ch1Fifo_Put(ADC0_SSFIFO2_R);//PD3,Channel4first

Ch3Fifo_Put(ADC0_SSFIFO2_R);//PD3,Channel5second

Program8.9.Softwaretosamplechannels4and5at1kHz(ADC_TwoChan_xxx.zip).

8.5.5.MultipleAccessCircularQueueAmultipleaccesscircularqueue(MACQ)isusedfordataacquisitionandcontrolsystems.AMACQisafixedlengthorderpreservingdatastructure,seeFigure8.43.Thesourceprocess(ADCsamplingsoftware)placesinformationintotheMACQ.Onceinitialized,theMACQisalwaysfull.TheoldestdataisdiscardedwhenthenewestdataisPutintoaMACQ.ThesinkprocesscanreadanyofthedatafromtheMACQ.TheReadfunctionisnon-destructive.ThismeansthattheMACQisnotchangedbytheReadoperation.

Figure8.43.Whendataisputintoamultipleaccesscircularqueue,theoldestdataislost.ForexampleconsidertheproblemofweatherforecastingasshowninFigure8.44.

Figure8.44.Applicationofthemultipleaccesscircularqueue.Theweathermanmeasuresthetemperatureeverydayat12noon,andputsthetemperatureintotheMACQ.Topredicttomorrow’stemperature,shelooksatthetrendoverthelast3days.LetT(0)betoday’stemperature,T(1)beyesterday’stemperature,etc.Wecouldpredicttomorrow’sbyexecutingthisdigitalequation

U=(170•T(0)+60•T(1)+36•T(2))/256

TheMACQisusefulforimplementingdigitalfiltersandlinearcontrolsystems.OnecommonapplicationoftheMACQisthereal-timecalculationofderivative.AlsoassumetheADCsamplingistriggeredevery1ms.x(n)willrefertothecurrentsample,andx(n-1)willbethesample1msago.Thereareacoupleofwaystoimplementthediscretetimederivative.Thesimpleapproachis

d(n)=(x(n)–x(n-1))/Δt

Inpractice,thisfirstorderequationisquitesusceptibletonoise.Anapproachgeneratinglessnoisecalculatesthederivativeusingahigherorderequationlike

d(n)=(x(n)+3x(n-1)–3x(n-2)–x(n-3))/Δt

TheCimplementationofthisdiscretederivativeusesaMACQ.Since∆tis1ms,wesimplyconsiderthederivativetohaveunitsmV/msandnotactuallyexecutethedivideby∆toperation.Signedarithemeticisusedbecausetheslopemaybenegative.

int32_tx[4];//MACQ(mV)

int32_td;//derivative(V/s)

voidADC3_Handler(void)

ADC0_ISC_R=0x08;//acknowledgeADCsequence3completion

x[3]=x[2];//shiftdata

x[2]=x[1];//unitsofmV

x[1]=x[0];

x[0]=(3300*ADC_SSFIFO3_R)/4096;//inmV

d=x[0]+3*x[1]-3*x[2]-x[3];//inV/s

Fifo_Put(d);//passtoforeground

Program8.9.Softwareimplementationoffirstderivativeusingamultipleaccesscircularqueue.WhentheMACQholdsmanydatapointsitcanbeimplementedusingapointerorindextothenewestdata.Inthisway,thedataneednotbeshiftedeachtimeanewsampleisadded.ThedisadvantageofthisapproachisthataddresscalculationisrequiredduringtheReadaccess.Forexample,wecouldimplementa16-elementaveragingfilter.Morespecifically,wewillcalculatetheaverageofthelast16samples.

uint32_tData[32];//twocopies

uint32_t*Pt;//pointertocurrent

uint32_tSum;//sumofthelast16samples

voidLPF_Init(void)

Pt=&Data[0];Sum=0;

//calculateonefilteroutput

//calledatsamplingrate

//Input:newADCdata

//Output:filteroutput,DACdata

uint32_tLPF_Calc(uint32_tnewdata)

Sum=Sum-*(Pt+16);//subtracttheone16samplesago

if(Pt==&Data[0])

Pt=&Data[16];//wrap

else

Pt—;//makeroomfordata

*Pt=*(Pt+16)=newdata;//twocopiesofthenewdata

returnSum/16;

Program8.10.Digitallowpassfilterimplementedbyaveragingtheprevious16samples(cutoff=fs/32).

8.6.Exercises8.1Foreachtermgiveadefinitionin32wordsorless.

a)Thermalnoiseb)CLCor filter

c)Voltagecomparatord)Differentialamplifier

e)Lowpassfilterf)Railtorail

g)Commonmoderejectionratioh)Outputimpedance

i)Shuntreferencej)Hysteresis

k)Analogisolationl)Complementarybinary

m)Analogmultiplexorn)Sampleandhold

8.2Foreachopampparametergiveadefinitionin32wordsorless.

a)Openloopgainb)Gainbandwidthproduct

c)Offsetvoltaged)Offsetcurrent

e)Biascurrentf)Commonmodeinputimpedance

g)Differentialmodeinputimpedanceh)Slewrate

i)Noisedensityj)Supplycurrent

8.3ForeachDACparametergiveadefinitionin32wordsorless.

a)Precisionb)Range

c)Resolutiond)Monotonic

e)Linearityf)Settlingtime

g)Supplycurrenth)Slewrate

8.4ForeachADCparametergiveadefinitionin32wordsorless.

a)Precisionb)Range

c)Resolutiond)Totalharmonicdistortion

e)Nomissingcodesf)Conversiontime

g)Supplycurrenth)Bandwidth

8.5Foreachpairoftermsexplainthesimilaritiesanddifferencesin32wordsorless

a)Carbonversusmetalfilmresistorb)Ceramicversustantalumcapacitor

c)Linearversusbuck-boostregulatord)Regularversusrail-to-railopamp

e)Opampversusinstrumentationampf)One-poleversustwo-polefilter

g)Li-ionversusNiMHbatteryh)Negativefeedbackversuspositivefeedback

8.6DescribeeachADCtypein32wordsorless.

a)Flashb)Sigmadelta

c)Successiveapproximationd)Pipeline

8.7Amicrocontrollerwithan8-bitDACisusedtocreateasoftware-controlledanalogoutput.The8-bitsignedDACusesoffsetbinaryformatisconnectedtoaparallelportofthemicrocontrollerandhasa-2Vto+2Vrange.Nistheportoutput(DACinput)andVistheDACoutput.

a)DerivethelinearrelationshipbetweenNandV.ShowboththeequationthatexpressesVintermsofN,andtheequationthatexpressesNintermsofV.

b)WhatistheDACprecision?Statetheunits.

c)WhatistheDACresolution?Statetheunits.

8.8Amicrocontrollerwitha10-bitDACisusedtocreateasoftware-controlledanalogoutput.The10-bitsignedDACusestwo’scomplementformatisconnectedtoaparallelportofthemicrocontrollerandhasa-1Vto+1Vrange.Nistheportoutput(DACinput)andVistheDACoutput.

a)DerivethelinearrelationshipbetweenNandV.ShowboththeequationthatexpressesVintermsofN,andtheequationthatexpressesNintermsofV.

b)WhatistheDACprecision?Statetheunits.

c)WhatistheDACresolution?Statetheunits.

8.9Ifa10-bitADChasarangeof0to+10volts,whatisitsresolution?

8.10Givethreeequivalentwaystospecifytheprecisionofa12-bitADC.

8.11Ifan8-bitADChasarangeof0to3volts,whatwillbeoutputforaninputvoltageof1voltassumingstraightbinaryencoding?

8.12Ifan8-bitADCtakesinputsrangingfrom-2.5Vto+2.5V,whatwilltheoutputcorrespondingto+1Vassumingoffsetbinaryencoding?

D8.13.Design2-bitanalogtodigitalconverter.Therangeis0to3.2V.Forinputvoltagesbetween0and0.8Vmaketheoutput00,forinputsbetween0.8and1.6Vmaketheoutput01,forinputsbetween1.6and2.4Vmaketheoutput10,andforinputsbetween2.4and3.2Vmaketheoutput11.Userail-to-railopampspoweredat3.3V.Specifyresistorvaluesandcapacitorvalues.

D8.14.Designavariablegainanalogamplifier.TheanaloginputisVin,the3-bitdigitalinputs(connectedtoamicrocontrolleroutputdigitaloutput)areB2,B1,B0andtheanalogoutputisVout.Useananalogswitch,suchasaMAX4783.Exactlyoneofthedigitalinputswillbeone,andthegainshouldbe

B2,B1,B0 Gain(Vout/Vin)

001 1

010 10

100 100

D8.15.Designananalogcircuitwiththefollowingspecifications

twosingle-endedinputs(notdifferential)

anyinputimpedanceisOK

transferfunctionVout=5•V1-3•V2+5

YouarelimitedtooneOPA227opampandonereferencechip(youchooseit).Givechipnumbersbutnotpinnumbers.Specifyallresistorvalues.Youwilluse+12and–12Vanalogsupplyvoltages.

D8.16.Designaninstrumentationamp,usinganAD627,withthefollowingtransferfunction.

Vout=500*(V2-V1)

D8.17.DesignananalogcircuitwiththefollowingtransferfunctionVout=3-2*Vin.Theinputisasinglevoltage(notdifferential).Theinputrangeis0to1.5Vandtheoutputrangeis0to3V.Useananalogreferenceandonerail-to-railopamp.Showyourworkandlabelallchipnumbersandresistorvalues.Youdonothavetoshowpinnumbers.

D8.18Theinput,Vin,isdifferential,notsingle-ended.DesignananalogcircuitwithatransferfunctionofVout=30*Vin+1.5poweredbyasingle+3.3Vsupply.Youmayuseanyoftheanalogchipsinthischapter.Theinputrangeis-0.05Vto+0.05V,andtheoutputrangeis0to+3V.Labelallchips,resistorsandcapacitorsasneeded.

D8.19.Theinput,Vin,issingle-ended,notdifferential.DesignananalogcircuitwithatransferfunctionofVout=30*Vin-3usingonerail-to-railopamppoweredbyasingle+3.3Vsupply.Theinputrangeis0.1Vto0.2V,andtheoutputrangeis0to+3V.

D8.20.Designatwo-poleButterworthanaloghighpassfilterwithacutoffof10Hz.

D8.21.Designatwo-poleButterworthanaloglowpassfilterwithacutoffof250Hz.

D8.22.Designatwo-poleButterworthanaloglowpassfilterwithacutoffof10000Hz.

D8.23.Designananalogbandpassfilterwithacutoffsof1and1000Hz.First,designatwo-poleButterworthanaloghighpassfilter,andthenfollowitwithatwo-poleButterworthanaloglowpassfilter.

8.7.LabAssignmentsLab8.1.Theoverallobjectiveofthislabistodesignavariablegainamplifier,wherethegainiscontrolledbyadigitaloutputofthemicrocontroller.Thegainsettingsare10,20,50,and100.Aspreparation,designtwoseparatecircuitsusingdifferentapproaches.Evaluatethebandwidth,noise,errorduetooffsetvoltage,andcost.Buildandexperimentallymeasurebandwidth,noise,andoffseterror.

Lab8.2.Mostdigitalmusicdevicesrelyonhigh-speedDACstocreatetheanalogwaveformsrequiredtoproducehigh-qualitysound.InthislabyouwillcreateaverysimplesoundgenerationsystemthatillustratesthisapplicationoftheDAC.Yourgoalistoplayyourfavoritesong.Forthefirststep,youwillbuildaDAC.YouarefreetodesignyourDACwithaprecisionanywherefrom5to8bits.Youwillconvertthebinarybits(digital)toananalogoutputcurrentusingasimpleresistornetwork.ThethirdstepistoconverttheDACanalogoutputtospeakercurrentusinganaudioamplifier.Thefourthstepistodesignalow-leveldevicedriverfortheDAC.Thefifthstepistodesignadatastructuretostorethesoundwaveform.Thesixthstepistoorganizethemusicsoftwareintoadevicedriver.Thelaststepistowriteamainprogramthatinputsfromthreebinaryswitchesandperformsthethreepublicfunctions.

Lab8.3.TheoverallobjectiveofthislabisdesignthemusicplayerdescribedinLab8.2withatwochannelDACchip,anddesigntwoaudioamplifiers.Thesystemwillimplementtwo-channelstereosound.

9.System-LevelDesignChapter9objectivesareto:•Discussissuesassociatedwithmanufacturabilityandtestability

•Describepowersources

•Designmethodstochargebatteries

•Presentapproachesforlow-powerdesign

•IntroducePCBdesign

Chapters1to8ofthisbookhavepresentedembeddedsystemsfromaninterfacingorcomponentlevel.Theremainingchapterswillfocusonsystemsleveldesign.Thechapterbeginswithadiscussionofselectingresistorandcapacitorcomponents.Next,itwilldescribepowersources,includingbatteriesandbatterychargers.Oneoftheimportantaspectsofembeddedsystemsislow-poweroperation.ThechapterwillconcludewithanintroductiontoPCBdesign.

9.1.DesignforManufacturabilityUsingstandardvaluesforresistorsandcapacitorsmakesfindingpartsquicker.Standardvaluesfor1%resistorsrangefrom10 to2.2M .WecanmultiplyanumberinTables9.1,9.2,9.3,and9.4bypowersof10toselectastandardvalueresistor.Forexample,ifweneeda5k 1%resistor,theclosestnumberis49.9*100,or4.99k .

Sometimesweneedapairofresistorswithaspecificratio.Thereare19pairsofresistorswitha2to1ratio(e.g.,20/10).Thereisonlyonepairwitha3to1ratio,102/34.Similarly,thereisonlyonepairwitha4to1ratio,102/25.5.Thereare19pairsofresistorswitha5to1ratio(e.g.,100/20).Thereare5pairsofresistorswitha7to1ratio(e.g.,93.1/13.3,105/15,140/20,147/21,196/28).Therearenopairswithratiosof6,8,or9.

Usingstandardvaluescangreatlyreducemanufacturingcostsbecausepartsarelessexpensive,andpartsforoneprojectcanbeusedinotherprojects.CeramiccapacitorscanbereadilypurchasedasE6,E12,orE24standards.Filtersscaleoverafairlywiderange.Ifaresistorisincreasedbyafactorofxandthecapacitorisreducedbybyafactorofx,thefilterresponsewillremainunchanged.Forexample,theresponseofafilterthatuses100k and0.1 Fwillbethesameasafilterwith20k and0.5F.Resistorsthataretoolowwillincreasepowerconsumptioninthecircuit,andresistorvaluesthataretoohighwillincreasenoise.1%resistorsbelow100 andabove10M arehardtoobtain.Precisioncapacitorsbelow10pFandabove1 Farehardtoobtain.High-speedapplicationsuselowervaluesofresistorsinthe100 to1k range,precisionequipmentoperatesbestwithresistorsinthe100k to1M range,whileportableequipmentuseshighervaluesinthe100k to10M range.

E12standardvaluesfor10%resistorsrangefrom10 to22M .WecanmultiplyanumberinTable9.1bypowersof10toselectastandardvalue10%resistororcapacitor.TheE6seriesiseveryothervalueandtypicallyavailablein20%tolerances.

10 12 15 18 22 27 33 39 47 56 68 82

Table9.1.E12Standardresistorandcapacitorvaluesfor10%tolerance.

E24standardvaluesfor5%resistorsrangefrom10 to22M .WecanmultiplyanumberinTable9.2bypowersof10toselectastandardvalue5%resistor.Forexample,ifweneeda25k 5%resistor,theclosestnumberis24*1000,or24k .Capacitorsrangefrom10pFto10 F,althoughceramiccapacitorsabove1 Fcanbequitelarge.Thephysicaldimensionsofacapacitoralsodependontheratedvoltage.Youcanalsoget1%resistorsand1%capacitorsintheE24series.Forexample,ifyouneeda0.05 Fcapacitor,youcanchoosean0.047 FE12,ora0.051 FE24capacitor.

10 11 12 13 15 16 18 20 22 24 27 30

33 36 39 43 47 51 56 62 68 75 82 91

Table9.2.E24Standardresistorandcapacitorvaluesfor5%tolerance.

Table9.3showstheE96standardresistancevaluesfor1%resistors.Table9.4showsE192standardresistancevaluesfor0.5,0.25,0.1%tolerances.Tables9.1and9.2refertobothresistorsandcapacitors,buttheE96andE192standardsreferonlytoresistors.

10.0 10.2 10.5 10.7 11.0 11.3 11.5 11.8 12.1 12.4 12.7 13.0

13.3 13.7 14.0 14.3 14.7 15.0 15.4 15.8 16.2 16.5 16.9 17.4

17.8 18.2 18.7 19.1 19.6 20.0 20.5 21.0 21.5 22.1 22.6 23.2

23.7 24.3 24.9 25.5 26.1 26.7 27.4 28.0 28.7 29.4 30.1 30.9

31.6 32.4 33.2 34.0 34.8 35.7 36.5 37.4 38.3 39.2 40.2 41.2

42.2 43.2 44.2 45.3 46.4 47.5 48.7 49.9 51.1 52.3 53.6 54.9

56.2 57.6 59.0 60.4 61.9 63.4 64.9 66.5 68.1 69.8 71.5 73.2

75.0 76.8 78.7 80.6 82.5 84.5 86.6 88.7 90.9 93.1 95.3 97.6

Table9.3.E96Standardresistorvaluesfor1%tolerance(resistorsonly,notforcapacitors).

Checkpoint9.1:LetR=100k .FindanE24capacitorsuchthat1/(2 RC)isascloseto1000Hzaspossible.

Checkpoint9.2:RatherthanusinganE96resistor,findtwoE24resistorssuchthattheseriescombinationisascloseto127k aspossible.

10.0 10.1 10.2 10.4 10.5 10.6 10.7 10.9 11.0 11.1 11.3 11.4

11.5 11.7 11.8 12.0 12.1 12.3 12.4 12.6 12.7 12.9 13.0 13.2

13.3 13.5 13.7 13.8 14.0 14.2 14.3 14.5 14.7 14.9 15.0 15.2

15.4 15.6 15.8 16.0 16.2 16.4 16.5 16.7 16.9 17.2 17.4 17.6

17.8 18.0 18.2 18.4 18.7 18.9 19.1 19.3 19.6 19.8 20.0 20.3

20.5 20.8 21.0 21.3 21.5 21.8 22.1 22.3 22.6 22.9 23.2 23.4

23.7 24.0 24.3 24.6 24.9 25.2 25.5 25.8 26.1 26.4 26.7 27.1

27.4 27.7 28.0 28.4 28.7 29.1 29.4 29.8 30.1 30.5 30.9 31.2

31.6 32.0 32.4 32.8 33.2 33.6 34.0 34.4 34.8 35.2 35.7 36.1

36.5 37.0 37.4 37.9 38.3 38.8 39.2 39.7 40.2 40.7 41.2 41.7

42.2 42.7 43.2 43.7 44.2 44.8 45.3 45.9 46.4 47.0 47.5 48.1

48.7 49.3 49.9 50.5 51.1 51.7 52.3 53.0 53.6 54.2 54.9 55.6

56.2 56.9 57.6 58.3 59.0 59.7 60.4 61.2 61.9 62.6 63.4 64.2

64.9 65.7 66.5 67.3 68.1 69.0 69.8 70.6 71.5 72.3 73.2 74.1

75.0 75.9 76.8 77.7 78.7 79.6 80.6 81.6 82.5 83.5 84.5 85.6

86.6 87.6 88.7 89.8 90.9 92.0 93.1 94.2 95.3 96.5 97.6 98.8

Table9.4.E192Standardresistorvaluesfortolerancesbetterthan1%(resistorsonly,notforcapacitors).

9.2.Power

9.2.1.RegulatorsOneoftheimportantaspectsofasystemispower.ManyembeddedsystemsarepoweredwithanACadapter.Othernamesforthisadapterthattakes120VACinandoutputsanunregulatedDCvoltageincludepoweradapter,powerblock,wallwart,wallcube,andpowerbrick.A9-V,500-mAunregulatedACadaptermeansthevoltageisabove9Vforallcurrentslessthan500mA.However,theactualvoltagecanvaryconsiderably.Forexample,thevoltagemightrangefrom13Vatnocurrentdownto9.5Vat500mA.Therefore,aregulatorwillbeusedtoprovideaconstantvoltagetopowerthesystem.Therearemanyconsiderationswhenchoosingaregulator.Linearregulators,likethe78x05seriesandtheLP2981,usetransistors,areferencediode,andfeedbacktocreateaconstantoutputvoltage(Figure9.1).Linearregulatorsneedaninputvoltage,Vin,largerthantheoutputvoltage,Vout.IfthecurrentisIout,thenpowerislostintheregulatoranddissipatedasheatintheamountof(Vin‑Vout)*Iout.Linearregulatorshaveavoltagedropout,whichspecifieshowmuchhigherVinneedstobeaboveVoutinorderforittowork.Thedropoutvoltageforthe5-V78L05regulatoris2V,meaningVinmustbelargerthan+7Vfortheoutputtobe+5V.Conversely,thedropoutvoltagefortheLP2981isonly0.2V.Forexample,a3.7-VLi‑ionbatterywithaLP2981-3.3regulatorcouldbeusedtopowera3.3Vsystem.

Abuck-boostregulatorusesaninductorandaswitchingcircuittocreatetheconstantoutputvoltage.TheTPS63002buck-boostregulatorwillcreatea+5Voutputaslongastheinputisbetween1.8and5.5V,makingitsuitableforbattery-poweredapplications.Buck-boostregulatorsareratedfortheirefficiencyandhaveasizeable100-kHzswitchingnoiseinthepowerline.Sincelinearregulatorshavelowernoise,theyaremoresuitableforprecisionanalogelectronics.

Regulatorscanbefixedoutput(e.g.,3.3or5V)oradjustable.Adjustableregulatorsusetworesistorstosettheoutputvoltagelevel.ManyregulatorfamilieshavebothfixedoutputandadjustableversionssuchasLM1086,LM1117,LT1761,andTPS6300x.Adjustableregulatorsareconvenientifthesystemneedstohaveauser-controlledswitchtodecidewhetherthesystemispoweredat3.3or5V.Wechoosearegulatorthathasamaximumcurrentlargerthanthecurrentrequirementsofthesystem.Forexample,themaximumcurrentsforthe78L05,78M05,7805,and78S05are100mA,500mA,1A,and2A,respectively.Thelineregulationparameterspecifieshowtheoutputvoltagewillvaryasafunctionofinputvoltage.Theloadregulationparameterspecifieshowtheoutputvoltagewillvaryasafunctionofthecurrenttothesystem.

Figure9.1.Regulatorscanbeusedtocreateaconstantvoltagetopowerthesystem.TheTPS6300xdevicesarebuck-boost,whiletheothersarelinearregulators.Observation:Regulatorsareverydifferentfromeachother,soitisessentialtoreadthedatasheetandfollowallthedesignrecommendationsconcerningexternalcomponents,heatsinks,andlayout.

Checkpoint9.3:WhichregulatorinFigure9.1cancreatea5Vsupplyfroma3.7Vbattery?

9.2.2.Low-PowerDesignTosaveenergy,ourparentstaughtusto“turnoffthelightwhenyouleavetheroom.”Wecanusethissameapproachtoconserveenergyinourembeddedsystem.Therearemanywaystoplaceanalogcircuitsinlow-powermode.Someanalogcircuitshavealow-powermodethatthesoftwarecanselect.Forexample,theMAX535312-bitDACrequires280 Afornormaloperation,butthesoftwarecanplaceitintoshut-downmode,reducingthesupplycurrentto2A.Someanalogcircuitshaveadigitalinputthatthemicrocontrollercancontrolplacingthecircuitinactiveorlow-powermode.Forexample,theMC34119audioamplifierhasaCDpin,seeFigure9.2.Whenthispinislow,theamplifieroperatesnormallywithasupplycurrentof3mA.However,whenCDpinisabove2V,thesupplycurrentdropsdownto65 A.So,whenthesoftwarewishestooutputsound,itsendsacommandtotheMAX5353toturnonandmakesPB0equalto0.Conversely,whenthesoftwarewishestosavepower,itsendsashutdowncommandtotheMAX5353andmakesPB0high.

Figure9.2.Audioamplifierthatcanbeplacedintolow-powermode.Themosteffectivewaytoplaceanalogcircuitinalow-powerstateistoactuallyshutoffitspower.Someregulatorshaveadigitalsignalthemicrocontrollercancontroltoapplyorremovepowertotheanalogcircuit.Forexample,whentheOFFpinoftheMAX604regulatorishigh,thevoltageoutputisregulatedto+3.3V,asshowninFigure9.3.Conversely,whentheOFFpinislow,theregulatorgoesintoshut-downmode,andnocurrentissuppliedtotheanalogcircuit.Whenthesoftwarewishestoturnoffpowertotheanalogcircuit,itmakesPB0equalto0.Conversely,whenthesoftwarewishesenabletheanalogcircuit,itmakesPB0high.Themicrocontrolleritselfalwayswillbepowered.However,mostmicrocontrollerscanputthemselvesintoalow-powerstate.

Wecansavepowerbydesigningwithlow-powercomponents.Manyanalogcircuitsrequireasmallamountofcurrent,evenwhenactive.TheMAX494requiresonly200 Aperamplifier.Iftherearetenopampsinthecircuit,thetotalsupplycurrentwillbe2mA.Forcurrentslessthan8mA,wecanusetheoutputportitselftopowertheanalogcircuit,asshowninFigure9.4.Toactivatetheanalogcircuit,themicrocontrollermakesthePB0high.Toturnthepowertotheanalogcircuitoff,themicrocontrollermakesPB0low.

Figure9.3.Powertoanalogcircuitscanbecontrolledbyswitchingon/offtheregulator.

Figure9.4.Powertoanalogcircuitscanbedeliveredfromaportoutputpin.AswementionedinChapter1,considerablepowercanbesavedbyreducingthesupplyvoltage.Amicrocontrolleroperatingat3.3Vrequireslessthanhalfthepowerforanequivalent+5Vsystem.Powercanbesavedbyturningoffmodules(likethetimer,ADC,UART,andSSI)whennotinuse.Wheneverpossible,slowingdownthebusclockwiththePLLwillsavepower.Manymicrocontrollerscanputthemselvesintoalow-powersleepmodetobeawakenedbyatimerorexternalevent.

Thereareanumberoffactorsthataffectthesupplycurrenttoamicrocontroller.Thefirstisbusfrequency;powerincreaseslinearlywithbusfrequency.Asecondfactoristurningon/offI/Oports.Athirdfactorisactivatingasleepmode.TheLM3S/LM4F/TM4Chasasleepmode,adeepsleepmode,andahibernatemode.ThedeepsleepmodeonaLM3S811requiresabout1mAofsupplycurrent.HibernatemodeontheTM4C123requires5 A.Conversely,thedeepsleepmodeonaMSP430F2012microcontrollerislessthan1 A.

9.2.3.BatteryPowerAbatteryisasourceofenergythatcanbeusedinanembeddedsystemtomakethesystemportable.Anotherapplicationofbatteriesistosupplypowertoamission-criticalsystemwhentheregularACpowerislosttemporarily.Typically,abatteryhasthreeparts.Theanodeisthenegativeterminalofthebattery,thecathodeisthepositiveterminal,andtheelectrolyteisaliquidsolutionthatacceptsstoresandreleasesenergy.Thesethreecomponentscanbeconstructedfrommanydifferentmaterialsandconfiguredinanalmostendlessarrayofsizesandshapes.Thetype,size,andshapeofthematerialsplayamajorroleindeterminingthebatteryperformance.Aprimarybatteryisusedonceanddiscarded,andasecondarybatterycanberechargedandreused.

Therearemanyparameterstoconsiderwhenselectingabattery.Nominalvoltageisthetypicalvoltageofthebatterywhenfullycharged.Somebatteriesmaintainafairlyconstantvoltageoutputwhileenergyisbeingdischarged.However,otherbatterieswilldropthevoltagesteadilyduringusage.Physicalparametersofthebattery(suchasvolume,weight,andshape)oftenplayasignificantroleintheoverallappealofanembeddedsystem.Peakcurrentisthemaximumcurrentthebatterycandeliver.Shelf-life,operatingtemperature,andstoragetemperatureareotherparameterstoconsiderwhenchoosingabattery.Memoryeffectisanobservableconditioninsomerechargeablebatteriesthatcausesthemtoholdlesschargeovertime.Theenergystorageofabatteryistypicallydefinedinamp-hours,becausethevoltageisassumedconstant.Thestandardunitsofenergyarewatt-hours(1W-hris3600J).Onecanestimatetheoperationtimeofabattery-poweredembeddedsystembydividingtheenergystoragebytheaveragecurrentrequiredtorunthesystem.Thepowerbudgetembodiesthisconcept.LetEbethebatteryspecificationinamp-hoursandtlifebethedesiredlifetimeoftheproduct;thenwecanestimatetheaveragecurrentoursystemisallowedtodraw:

AverageCurrent≤E / tlifeCheckpoint9.4:Amedicaldeviceimplantedinsidethebodyhasa500mA-hourbatterythatmustlast5years.Whatisyourpowerbudget?

Heavy-dutybatteries,werefirstmadewithzinc-carboninthemid-1800s,butnowaremadewithzincchloride.Theyarealow-cost,low-performancebatterybutarenotappropriateformostembeddedapplications.Analkalinebatteryismadewithalkalinemanganese.Alkalinebatteriesareappropriateforsituationsthatrequirelongshelflife,butsizeandweightarenotimportant.Therearetwokindsofleadacidbatteries.Floodedlead-acidventinflammablegassesandrequireadditionalwatertomaintaintheproperspecificgravityoftheacid.Valve-regulatedlead-acid(VRLAalsocalledsealedleadbattery)haveaboutatwo-to-oneadvantageoverthefloodedtypebatteryinspecificenergyandenergydensity.IntheVRLAcell,theventforthegasspaceincorporatesapressurereliefvalvetominimizethegaslossandtopreventdirectcontactbetweentheheadspaceandoutsideair.Leadacidbatteriescanbeusedforbackinguppoweronsystemsthatrequirelargecurrents.Leadacidbatterieshaveamaximumstoragetimeofsixmonthsattemperaturesbetween20and30°C,afterwhichtheyrequireafresheningcharge.Zincchloride,alkaline,andlead-acidbatteriesallhavevoltagesthatdropasenergyisdrainedfromthem.Inthesesystems,thevoltagecanbemonitoredasameasureoftheenergyleftinthebattery.However,embeddedsystemsthatusethesetypesofbatterywillrequireavoltageregulatortomaintainaconstantvoltagefortheelectronics.Forexample,theMSP430microcontrollerwilloperatewithapowersupplyvoltageanywherefrom1.8to3.6V.TheLM3S/TM4Cmicrocontrollerswillrunwithasupplyvoltagefrom3.0to3.6V.

Nickel-cadmium(NiCad)andnickel-metalhydride(NiMH)arelow-costrechargeablebatteriesthatusedtobepopularforembeddedsystems.NiMHbatterieshaveabouttwicethestoragecapacityasNiCad.CertainNiCadbatteriesgraduallylosetheirmaximumenergycapacityiftheyarerepeatedlyrechargedafterbeingonlypartiallydischarged.MostNiMHbatteriesdonotsufferfromamemory-effect.TheNiMHbatteriesoperatebetween10to55°C,andhaveaprojectedlifeofsevenandahalfyearsat30°C.You

shouldcyclenewNiMHbatteriesthreetofivetimestoachievepeakperformance.CyclingorconditioningaNiMHbatteryisperformedbycompletelydischargingitthencompletelyrechargingit.Atroomtemperature,NiMHbatterieswillself-dischargein30to60dayswithoutusage,dependingonenvironmentalconditions.Ingeneral,youcanexpectNiMHbatteriestolastupto500recharges.

Thesearchforalighterbatterythatusesmetalliclithiumasitsanodewasdrivenbythefactthatlithiumisthelightestandthemostelectropositiveofmetals.Thespecificenergyoflithiummetal(1727Ah/lb)isgreaterthanlead(118Ah/lb)andcadmium(218Ah/lb).ThereareawholerangeofbatteriesbasedonLithium,bothsingleuse(usedincameras)andrechargeable.ThemostcommonrechargeabletypeiscalledLithium-ion(Li-ion).Whenenergyisbeingdischarged,thelithiumionmovesfromtheanodetothecathode.Duringcharging,thelithiumionmovesfromthecathodetotheanode.Becauseoftheirexcellentenergy-to-weightandenergy-to-sizeratios,Lithium-ionrechargeablebatteriesarecommonlyemployedinportableembeddedsystems.Table9.5showsenergystoragefortypicalAA-sizedbatteries(50mmtallby14mmdiameter).Table9.6listssomeLi-Ionrechargeablebatteries.

Battery Voltage(V)

Energy(mAh)

Type

Alkaline 1.5 2000 Primary

Lithium 1.5 3000 Primary

NiCad 1.2 1200 Secondary

NiMH 1.2 1800 Secondary

Li-ion 3.6 1900 Secondary

Table9.5.EnergystoragefordifferentAA-sizedbatterytypes.

Battery Shape Energy(mAh)

Size(mm) Weight(g)

GMBpowerGM041124

ThinPack 60 4.0×11×24

1.2

GMBpowerGM041429

ThinPack 120 4.0×14×29

2.8

GMBpowerGM041842

ThinPack 250 4.0×18×42

5.5

TrustFire10440 AAA 600 10.25×46.25

9.5

UltraFire14500 AA 900 14×50 31

Tenergy30027 Cylinder 2200 69×19 54

Tenergy31000 Pack 4400 65×37×18

170

Tenergy31002 Pack 6600 69×54×18

255

Table9.6.Example3.7VLi-Ionbatteries(http://www.powerstream.com/http://www.gmbattery.com/http://www.batteryjunction.com/).

Checkpoint9.5:Amedicaldeviceimplantedinsidethebodyhasanaveragecurrent20 Aandmustlastfor1year.WhichbatteryinTable9.6wouldyouchoose?

Tomakethesystemmoreconvenient,battery-poweredembeddedsystemscouldincludeabuilt-inrecharger.Thesystemcanrunandchargewhilepluggedin.However,itwillalsorunoffthebatterywhenACpowerisnotavailable.Therearemanybatterychargingcircuitsavailable.Figure9.5showsachargingcircuitaNiMHbattery.R1isdependsonthesmallestVinfromthewallcube.IfVinislargerthan10V,thesetR1=(10-5V)/5mA=1k .R3andR4arepartofathermalshut-downsafetycircuit,whichisnotimplementedinthissimpleversion.TheRsenseresistorsetsthefastprogrammingcurrent:Ifast=2.5V/Rsense,whichis1Aforthiscircuit.ThePGM0,PGM1,PGM2,andPGM3pinsspecifythechargingprotocolandthenumberofNiMHcells,whichis7cellsor8.4Vforthiscircuit.Figure9.6showsachargingcircuitforaLi-ionbattery.Youshouldrefertothespecificdatasheetstoworkthroughtheresistorandcapacitorvaluesdependingonthevoltageandcurrentofthebattery.

Figure9.5.Chargingcircuitforan8.4-VNiMHbatterywitha1-Achargingcurrent.

Figure9.6.Chargingcircuitfora7.2-VLi-ionwitha1.2-Achargingcurrent.

9.3ToleranceThefirststepineffectivecommunicationisestablishingaclearagreementonthedefinitionofterms.Broadlyput,tolerancemeansputtingupwitherror.However,thedictionarydefinitionsoftoleranceanderrorarenotadequateforengineeringdesignaslistedinTable9.7.Fromanengineeringperspective,toleranceisthequantitativedifferencebetweenthedesiredparameterandtheactualvalue.Forexample,a±1%1000 resistormayhaveanactualvaluefrom990 to1010 .Sometimesaparameterislistedastheminimumandmaximumvalues.Forexample,theoffsetvoltageofanOPA2350opampwillbebetween-500and+500 V.Specifyingtolerancewillhaveaprofoundimpactonbothpriceandperformance.Ifweoverspecify(use1%resistorswhen5%wouldhavebeenOK),thenthesystemcostsincrease.However,ifweunderspecify(use5%resistorsforasystemneeding1%),thensomeofthedeviceswemanufacturewillnotoperateproperly.

Tolerance

1)afair,objective,andpermissiveattitudetowardthosewhoseopinions,practices,race,religion,nationality,etc.,differfromone’sown;freedomfrombigotry.

2)afair,objective,andpermissiveattitudetowardopinionsandpracticesthatdifferfromone’sown.

3)interestinandconcernforideas,opinions,practices,etc.,foreigntoone’sown;aliberal,undogmaticviewpoint.

4)theactorcapacityofenduring;endurance:Mytoleranceofnoiseislimited.

Error

1)adeviationfromaccuracyorcorrectness;amistake,asinactionorspeech:Hisspeechcontainedseveralfactualerrors.

2)beliefinsomethinguntrue;theholdingofmistakenopinions.

3)theconditionofbelievingwhatisnottrue:inerroraboutthedate.

4)amoraloffense;wrongdoing;sin.

5)Baseball.amisplaythatenablesabaserunnertoreachbasesafelyoradvanceabase,orabattertohaveaturnatbatprolonged,asthedroppingofaballbattedintheair,thefumblingofabattedorthrownball,orthethrowingofawildball,butnotincludingapassedballorwildpitch.

5)Medicine/Medical,Immunology.a)thepowerofenduringorresistingtheactionofadrug,poison,etc.:atolerancetoantibiotics.b)thelackoforlowlevelsofimmuneresponsetotransplantedtissueorotherforeignsubstancethatisnormallyimmunogenic.

Table9.7.Dictionarydefinitionsoftoleranceanderrorhttp://dictionary.reference.com

Anotherfactorrelatedtotoleranceistemperaturecoefficient.Propertiesofmostdeviceswillchangewithtemperature.Forexample,theTemperatureCoefficientofResistance(TCR)isdefinedas

TCR(ppm/ºC)=

whereR25isthetemperatureat25ºC,andRistheresistanceattemperatureT,asTvariesfrom‑55ºCto+125ºC.Wewilldoasimpleexampleillustratingtheeffectoftoleranceonperformance.WewillrevisittheLEDinterfaceinitiallypresentedinFigure1.23.BackinChapter1,weassumedtheVCEofthePN2222was0.3V,thedesiredLEDoperatingpointwas1.9V10mA,andwecalculatedtheresistorvaluetobe(3.3-1.9-0.3V)/10mA=110 .

Figure1.23.Opencollectorusedtointerfacealightemittingdiode.TheLEDdatasheetspecifiesthelightintensityislinearwithcurrent.SowewilldeterminetheeffectoftoleranceonLEDcurrent.InparticularwewillinvestigateanE245%-resistorwithnominalvalueof110.Thisresistorcanvaryfrom104.5to115.5,causingtheLEDcurrenttovaryfrom9.5to10.5mA.Inthiscasethe±5%resistancetoleranceconvertedsimplytoa±5%variabilityinlightintensity.ToconsidertheeffectoftoleranceinthePN2222transistor,weneedtoknowthepossiblerangeforVCE.Atsaturation,theVCEhasamaximumof0.3V.AssumingaworstcaseofVCEequalto0,theLEDcurrentcouldincreaseto12.7mA.PuttingthesetwoeffectstogetherispresentedinTable9.8.Realistically,theVCEprobablyvariesonlyfrom0.2to0.3V,sotheLEDcurrentcouldvaryfrom9.5to11.5mA,or-5%to+15%fromdesired.

VCE=0

VCE=0.1

VCE=0.2

VCE=0.3

R=104.5 13.4 12.4 11.5 10.5

R=110 12.7 11.8 10.9 10.0

R=115.5 12.1 11.3 10.4 9.5

Table9.8.LEDcurrentvarieswithRandVCE.

Table9.8isasimpletwo-dimensionalanalysis.Ifweaddedtheeffectoftemperature,thentheproblembecomesthreedimensional.Asthenumberofparametersincreasessodoesthecomplexityoftheproblem.Itisimportanttodeterminewhichparametershavethelargesteffectonoutput.Theseparameterswillthenrequirelowertolerance.Forexample,assumethecircuithasthreeresistors(R1,R2,andR3)andtheoutputisthevoltageV.Eachresistorhasanerrorduetotoleranceand/ortemperaturedrift(± 1,± 2,and± 3).Wecouldusedifferentialcalculustodeterminetherelativesensitivitytoeachparameter(S1,S2,andS3).Weshouldfocusourmoneyandattentionontheparametershavingthelargesteffectonperformance.WecancalculatetheerrorinV( V)duetoerrorsinR.Letxbethemeasurandandlet xbethedesiredresolution.UsingthesensitivityofVtox,wecandetermineadesignmetric.

S1=∂V/∂R1S2=∂V/∂R2S3=∂V/∂R3

V=±(∂V/∂R1) • 1±(∂V/∂R2) • 2±(∂V/∂R3) • 3

x=(±(∂V/∂R1) • 1±(∂V/∂R2) • 2±(∂V/∂R3) • 3)/(∂V/∂x)≤ x

9.4.DesignforTestabilityDesignfortest(DFT)isimportantandshouldbeincorporatedateverystageofadesign.Wecangreatlyincreasereliabilitybytestingthedevicebeforeitisused,whileitisbeingused,andafterithasbeenused.Havingaplanfortestingshouldbeapartofeveryproject.Insoftwarewecanaddverboseoutput,eventlogging,assertions,diagnostics,resourcemonitoring,testpoints,andfaultinjectionhooks.Theapproachtohardwaretestingmimicsthetwoimportantfactorsforsoftwaretesting:visibilityandcontrol.Visibilityallowsthetestengineertoobservesystembehavior,andcontrolallowsthetestproceduretomanipulateinputstomoduleswithinthesystem.Weneedtoobservetheoutputs,sideeffectsandinternalstates.Animportantparameteristheintrusivenessoftheobservation.Inotherwords,weneedtobecarefultonotintroducedelaysthataffectthedynamicbehavior.Storingdataintoanarray(softwaredump)atruntimecanusuallybeperformedwithdelayoverheadsoflessthan1 s.

Wecanaddhardwarefeaturestooursystemtofacilitatetesting.Thefirstistheabilitytoquicklyandreliableattachhardwaretestequipmenttothedevice.Forlowproductionproductswecanaddtestpointstothecircuittomakeiteasytoattachoscilloscopesandvoltmeters.Forhigherproductionprojectswecanaddtestconnectorsthatdirectlyconnecttotestequipmentlikelogicanalyzersorsignalgenerators.Basically,weneedtocreateknowninputs,andrecordresponsesatstrategicpointsinthesystem.

Theverbosemodesfoundinsomecompilersadddebuggingoutputalongwithdataoutput.Thisapproachmaycausesignificantslowdownindynamicbehavior,soitwillbeimportanttomeasuretimingbehaviorwithandwithoutverbosemode.Foraspectsofthesystemthatarenotrealtime,verboseisanexcellenttestingfeature.Whenyouaddthe“-v”switchwithacommandinUnix,themailprogramwilldisplayitscommunicationwiththelocalmailserver.

Loggingorasoftwaredumpisanexcellenttoolforreal-timesystems.Ifyouplacethefollowingdumpatstrategicplacesinyoursystem,afterashortwhile,the Buffer containsthelast256timesthedebugginginstrumentwasinvoked.Subtracting“3”compensatesforthetimeittakestoruntheinstrumentuptothepointatwhichitreadstheSysTicktimer.Theentireinstrumentis12assemblyinstructionsandrunsinabout16buscycles.Inmostcasesthiswillbeminimallyintrusive.

uint32_tBuffer[256];

uint8_tN=0;

__inlinevoidRecordTime(void)

Buffer[N]=NVIC_ST_CURRENT_R-3;

N++;//8-bitvariablegoes0to255

Faultinjectionistheabilitytointroduceerrorconditionstoseeifthesystemproperlyhandlesthem.Amonitorisadebuggingmechanismtoobserveinternalbehavior.Agoodexampleofamonitorisconfiguringunusedpinsasoutputsandwritingstrategicinformationtothesepinsduringexecution.Ifweconnectthepinstoalogicanalyzer,wehaveaminimallyintrusivedebugginginstrument.OtherexamplesofmonitorsincludestreamingdataoutunusedportssuchasaUART,CAN,orSPI.Wecouldlogdebugdataontoflashdrive.Atestinginterfaceisahardwareorsoftwareconnectionbetweenthesystemandtestprocedure.

Checkpoint9.6:IdentifythreedesignfortestfeaturesonthePCBshownonpage451.

9.5.PrintedCircuitBoardLayoutandEnclosuresBeginwithchoosingaPCBsizeyouneedtoholdallthecomponents.Nextyouchooseaboxwithwhichtoenclosethesystem.Theenclosureisanimportantaspectofthedesign.Itdeterminesthelookandfeelofthedevice.ManymanufacturersgivesuggestedPCBsizesfortheirenclosures.Onewaytomakesureeverythingwillfitistocreateamockup.Onecollectsalltheelectricalandmechanicalcomponentsofthesystem.ThePCBdesignisprintedonpaperandgluedtocardboardorwoodmatchingthethicknessofthePCB.MountingholesaredrilledandcomponentsareattachedtothePCBmockup.Allcomponentsareassembledandmountedintotheenclosuretoverifyproperfit.TheleftsideofFigure9.7showsamockupofanLM3S811-basedrecorderwithanLCDtouchscreen.Therightsideshowsthefinalsystem.Thehardware,software,andPCBlayoutforthissystemcanbedownloadedfromthebookwebsiteasezLCD_811.zip.

Figure9.7.MockupandPCBforanLM3S811systemusingaPacTecenclosure.LayingoutaPCBisanartthatisbestlearnedbypracticeunderthewatchfuleyeofamastercraftsman.So,showyourPCBarttoothersandsolicitfeedback.Formanyyears,ithasbeencommonPCBdesignpracticetoavoid90°cornersinPCBtraces.Manyclaimthata90°turnproducesa¼turninductor,whichwillaffectthefrequencyresponseofthesignals.Thestandardpracticeistouseamiteredcornerreplacinga90°turnwithtwo45°turns.Therealityisthatthesignal-integritybenefitsofavoiding90°anglesareinsignificantatthefrequencies/edge-ratesseeninmicrocontrollercircuits(evenuptoandpast1GHz/100ps).[Johnson,HandGraham,M,High-SpeedDigitalDesign:aHandbookofBlackMagic,PrenticeHall:NewJersey,1993.]However,thereareafewsimplereasonstocontinuetoavoid90°angles.Thereisahigherpossibilityofanacid-trapformingduringetchingontheinsideoftheangle(especiallyinacuteangles).Anacidtrapcausesover-etchingthatcanbeayieldissueinPCboardswithsmalltracewidths.Routingat45°typicallyreducesoveralltracelengthanduseslessboardarea.MostPCBdesignersthinkitlooksbetter.LookinggoodisanimportantfactorforanyonewhoappreciatestheartofPCBlayout.

ThisparagraphcontainssuggestionstoconsiderwhenlayingoutaPCB.Makesurethe“SnaptoGrid”modeisactive.Youshouldexperimentwithdifferentsettingsofthesnap.Anetdefineswhichpinsandpadsmustbeconnected.Justlikesoftwarevariables,nets

shouldhavedescriptivenames.Youshouldvisualizetheunconnectednetswhileplacingcomponentsontheboard.Placeallcomponentsinitiallyontheboardtofacilitateinput/outputconnectors.Youalsowanttominimizethelengthsandthecrossingsofthenets.Extratimespentplacingnetswillberecoveredmanytimesoverwhilethedrawingthetraces.Thinkabouthowitwillbesoldered,byplacingthough-holecomponentsonthetopsidesosolderingwilloccuronthebottom.Surfacemountcomponentscangooneitherside.Addtopandbottomsilklabelingtoassistinconstruction,debugging,anddeviceoperation.Forexample,allcomponentsneedlabels(e.g.,U1R1C1J1etc.),shownbothontheboardandthecircuitdiagram.Addtestpointsatstrategicpointstoassistindebugging.Oneinexpensivetestpointcanbemadebyplacingtwoholes0.1inchapartthensolderingaUwireintoit.YoucanalsopurchasetestpointhardwaresuchasKeystoneElectronics5000through5004,whichusesa0.090inpadwith0.043inhole.Onewaytoconnectalltracesistogoleft-rightononesideandup-downontheotherside.Table9.7showsthetracewidthisafunctionofthelengthandcurrentinthetrace.1oz/ft2isatypicaltracethickness.

Length TemperatureRise

Current Thickness Resistance TraceWidth

5” 1C 100mA 1oz/ft2 1 2mil

5” 1C 200mA 1oz/ft2 0.47 5mil

5” 1C 500mA 1oz/ft2 0.13 20mil

5” 1C 1A 1oz/ft2 0.05 50mil

5” 1C 2A 1oz/ft2 0.02 120mil

Table9.7.Minimumtracewidthforvariouscurrentlevels

Inordertoverifyallcomponentswillfit,youmustanalyzedimensionsin3-D.YoucanuseadesigntoollikeAutoCAD,oryoucanfabricateamockupwithactualparts.YoucancutcardboardintheshapeofthePCB,drillholesinthecardboard,andgluethepartsontothecardboard.Figure9.8showsanembeddedsystemusingaLM3S8962,placedinanenclosure.

Figure9.8.AnEEGsystemdesignedwithaLM3S8962thatfitsintoacase(designedbyKatyLoeffler).Itisimportanttoreaddatasheetsandapplicationnotesforeachcomponentofthesystem.Themanufacturerwillsuggesttipsandreferencedesignsfortheirparts.Figures9.9and9.10showthesuggestedlayoutsforTexasInstrumentsmicrocontrollers.InFigure9.9,thelayoutsshowconnectionstopowerandgroundonthemicrocontroller.Itisrecommendedtoplaceacapacitorasclosetothechipaspossible.ThecirclesinFigure9.9representviastothepowerandgroundplanes.A)isthebestpracticebecausethereisminimalinductancebetweencapacitor,pinsandpowerplanes.B)isnotrecommendedbecausethedistancefrompinstoviasincreasesinductanceinpowerrails.C)isacceptablebecausethedistancefrompintoviaisshort.

Figure9.9.Capacitorlayoutsuggestions(SystemDesignGuidelinesforStellaris® Microcontrollers,AN01283).

Figure9.10showsasuggestedcrystallayout,whereanimportantobjectiveistominimizetheeffectivecapacitancefromOSC0togroundandfromOSC1togroundinherentintheboardlayoutitself(calledCboard).Whencalculatingthecapacitorvaluesneededfortheoscillatoronemustaccountforthisboardcapacitance.Thevaluesofthesecapacitorsalsodependonthecrystalloadcapacitance,CL.AtypicalequationisC1=C2=2×(CL-Cboard).ThismeansoneshouldnotplacepowerorgroundplanesunderthecrystalortheOSC0andOSC1signals.

Figure9.10.Crystallayoutsuggestion(SystemDesignGuidelinesforStellaris® Microcontrollers,AN01283).

9.6.Exercises9.1FindacombinationofanE12capacitorandE24resistorwithafrequencyresponse,1/(2 RC)of1000Hz.Selecttheresistorascloseto100k aspossible.

9.2FindacombinationofanE12capacitorandE24resistorwithafrequencyresponse,1/(2 RC)of12kHz.Selecttheresistorascloseto100k aspossible.

9.3FindtwoE24resistorsthatcanbecombinedtocreatearesistanceascloseto5.12k aspossible.

9.4FindtwoE24resistorsthatcanbecombinedtocreatearesistanceascloseto25.6k aspossible.

9.5Assumethesystemrequires50mAat3.3V.WhatisthedropoutvoltageofanLM317LZ?Usethisregulatortodesigna3.3Vregulatedpowersupply.Whatistherangeofinputvoltagesthatcanbeusedforyourcircuit?

9.6Assumethesystemrequires25mAat2.5V.WhatisthedropoutvoltageofanLM317LZ?Usethisregulatortodesigna2.5Vregulatedpowersupply.Whatistherangeofinputvoltagesthatcanbeusedforyourcircuit?

9.7Alow-powersystemrequiresanaveragecurrentof50 Aat3.7V.Thesystemmustrunfor6months.WhichbatteryinTable9.6wouldyouuse?

9.8Alow-powersystemrequiresanaveragecurrentof25 Aat3.7V.Thesystemmustrunfor3months.WhichbatteryinTable9.6wouldyouuse?

9.9IfyourARMmicrocontrollerrequires50mAtorun,howdoyoubuildthesystemwithanaveragecurrentoflessthan10mA?

9.10Ifyourcrystalhasaloadcapacitanceof18pF,andthePCBhasaboardcapacitanceof6pF,whatcapacitorvalueswouldyouusetobuildtheoscillator?

D9.11WriteaCprogramthatknowstheE24capacitorsvaluesandE192resistorsvalues.Theinputtotheprogramisthedesiredfrequencyresponse,1/(2 RC),andtheoutputisanE192resistorvalueandanE24capacitorvaluethatcombinestocreatethedesiredfrequency.

9.7.LabAssignmentsLab9.1Design,buildandtestaLi-Ionbatterycharger.Assumethebatteryvoltageis3.7V.

Lab9.2Design,buildandtestaNiMHbatterycharger.Assumethebatteryvoltageis4.2V.

Lab9.3Design,buildandtestcompletebattery-poweredembeddedsystem.Beginwitharequirementsdocument.DesignandlayoutaPCB.PlacethecomponentsinanenclosureliketheoneshowninFigure9.8.Includedesignfortestfeatures.

10.DataAcquisitionSystemsChapter10objectivesareto:•Defineperformancecriteriatoevaluateouroveralldataacquisitionsystem

•Introducespecificationsnecessarytoselectthepropertransducer

•Developamethodologyfordesigningdataacquisitionsystems

•Analyzethesourcesofnoiseandsuggestmethodstoreducetheireffect

•Illustrateconceptsofthischapterwithcasestudies

Embeddedsystemsaredifferentfromgeneral-purposecomputersinasensethatembeddedsystemshaveadedicatedpurpose.Aspartofthispurpose,manyembeddedsystemsarerequiredtocollectinformationabouttheenvironment.Asystemthatcollectsinformationiscalledadataacquisitionsystem.Inthischapter,wewillusethetwoterms,dataacquisitionsystemandinstrumentinterchangeably.Previouschapterspresentedthebasicbuildingblockstoacquiredataintothecomputer,andinthischapterwewillcombinetheseblocksintodataacquisitionsystems.Sometimestheacquisitionofdataisfundamentalpurposeofthesystem,suchaswithavoltmeter,athermometer,atachometer,anaccelerometer,analtimeter,amanometer,abarometer,ananemometer,anaudiorecorder,oracamera.Atothertimes,theacquisitionofdataisanintegralpartofalargersystemsuchasacontrolsystemorcommunicationsystem.ControlsystemswillbepresentedinVolume3,andcommunicationsystemswillbediscussedinChapter11.

10.1.IntroductionFigure10.1illustratestheintegratedapproachtoinstrumentdesign.Inthissection,webeginwiththeclearunderstandingoftheproblem.Wecanusethedefinitionsinthissectiontoclarifythedesignparametersaswellastoreporttheperformancespecifications.NextinSection10.2,wewilldefinetheparametersanddiscussthephysicsinordertoselectasuitabletransducer.PerformingdifferentiationandintegrationisdescribedinSection10.3.InSection10.4,weputtogethertheanaloganddigitalcomponents,introducedinChapter8,tobuilddataacquisitionsystems.Theuseofperiod/pulse/frequencyasameansofcollectinginformationwasdevelopedinChapter6.Noisecanneverbeeliminated,butwewillstudytechniquesinSection10.5toreduceitseffectonoursystem.TheintegratedapproachtodesignwillbeillustratedusingthecasestudiesinSection10.6.

Themeasurandisthephysicalquantity,property,orconditionthattheinstrumentmeasures.SeeFigure10.2.Themeasurandcanbeinherenttotheobject(likeposition,mass,orcolor),locatedonthesurfaceoftheobject(likethehumanEKG,orsurfacetemperature),locatedwithintheobject(e.g.,fluidpressure,orinternaltemperature),orseparatedfromtheobject(likeemittedradiation.)

Figure10.1.Individualcomponentsareintegratedintoadataacquisitionsystem.Ingeneral,atransducerconvertsoneenergytypeintoanother.Inthecontextofthisbook,thetransducerconvertsthemeasurandintoanelectricalsignalthatcanbeprocessedbythemicrocontroller-basedinstrument.Typically,atransducerhashavingaprimarysensingelementandavariableconversionelement.Theprimarysensingelementinterfacesdirectlytotheobjectandconvertsthemeasurandintoamoreconvenientenergyform.Theoutputofthevariableconversionelementisanelectricalsignalthathopefully

dependsonthemeasurand.Forexample,theprimarysensingelementofapressuretransduceristhediaphragm,whichconvertspressureintoadisplacementofaplunger.Thevariableconversionelementisastraingaugethatconvertstheplungerdisplacementintoachangeinelectricalresistance.Ifthestraingaugeisplacedinabridgecircuit,thevoltageoutputisdirectlyproportionaltothepressure.Sometransducersperformadirectconversionwithouthavingaseparateprimarysensingelementandvariableconversionelement.Theinstrumentationcontainssignalprocessing,whichmanipulatesthetransducersignaloutputtoselect,enhance,ortranslatethesignaltoperformthedesiredfunction,usuallyinthepresenceofdisturbingfactors.Thesignalprocessingcanbedividedintostages.Theanalogsignalprocessingconsistsofinstrumentationelectronics,isolationamplifiers,amplifiers,analogfilters,andanalogcalculations.Thefirstanalogprocessinginvolvescalibrationsignalsandpreamplification.Calibrationisnecessarytoproduceaccurateresults.Anexampleofacalibrationsignalisthereferencejunctionofathermocouple.Thesecondstageoftheanalogsignalprocessingincludesfilteringandrangeconversion.TheanalogsignalrangeshouldmatchtheADCanaloginputrange.Examplesofanalogcalculationsinclude:RMScalculation,integration,differentiation,peakdetection,thresholddetection,phaselockloops,AMFMmodulation/demodulation,andthearithmeticcalculationsofaddition,subtraction,multiplication,division,andsquareroot.Whenperiod,pulsewidth,orfrequencymeasurementisused,wetypicallyuseananalogcomparatortocreateadigitallogicsignaltomeasure(seeFigures8.6and8.20).WhereastheFigure10.1outlineddesigncomponents,Figure10.2showsthedataflowgraphforadataacquisitionsystemorcontrolsystem.Thecontrolsystemusesanactuatortodriveaparameterintherealworldtoadesiredvaluewhilethedataacquisitionsystemhasnoactuatorbecauseitsimplymeasurestheparameterinanonintrusivemanner.

Figure10.2.Signalpathsadataacquisitionsystem.

Thedataconversionelementperformstheconversionbetweentheanaloganddigitaldomains.Thispartoftheinstrumentincludes:hardwareandsoftwarecomputerinterfaces,ADC,DAC,S/H,analogmultiplexer,andcalibrationreferences.TheADCconvertstheanalogsignalintoadigitalnumber.InChapter6,wesawtheperiod,pulsewidth,andfrequencymeasurementapproachprovidesalow-costhigh-precisionalternativetothetraditionalADC.Thedigitalsignalprocessingincludes:dataacquisition(samplingthesignalatafixedrate),dataformatting(scaling,calibration),dataprocessing(filtering,curvefitting,FFT,eventdetection,decisionmaking,analysis),controlalgorithms(openorclosedloop).Thehumaninterfaceincludestheinputandoutputwhichisavailabletothehumanoperator.Theadvantageofcomputer-basedinstrumentationisthatsophisticatedbuteasytouseandunderstanddevicesarepossible.Theinputstotheinstrumentcanbeaudio(voice),visual(lightpens,cameras),ortactile(keyboards,touchscreens,buttons,switches,joysticks,rollerballs).Theoutputsfromtheinstrumentcanbenumericdisplays,CRTscreens,graphs,buzzers,bells,lights,andvoice.Ifthesystemcandeliverenergytotherealworldthenitisclassifiedasacontrolsystem.Controlsystemswillbedevelopedinthenextbook(Volume3).Inthischapter,wefocusondataacquisition.

Wheneverreportingspecificationsofourinstrument,itisimportanttogivethedefinitionsofeachparameter,themagnitudesofeachparameter,andtheexperimentalconditionsunderwhichtheparameterwasmeasured.Thisisbecauseengineersandscientistsapplyawiderangeofinterpretationsfortheseterms.

10.1.1.AccuracyTheinstrumentaccuracyistheabsoluteerrorreferencedtotheNationalInstituteofStandardsandTechnology(NIST)oftheentiresystemincludingtransducer,electronics,andsoftware.Letxmibethevaluesasmeasuredbytheinstrument,andletxtibethetruevaluesfromNISTreferences.Insomeapplications,thesignalofinterestisarelativequantity(liketemperatureordistancebetweenobjects).Forrelativesignals,accuracycanbeappropriatelydefinedmanyways:

Averageaccuracy(withunitsofx)=

Maximumerror(withunitsofx)=max

Standarderror(withunitsofx)=

Inotherapplications,thesignalofinterestisanabsolutequantity.Forthesesituations,wecanspecifyerrorsasapercentageofreadingorasapercentageoffullscale:

Averageaccuracyofreading(%)=

Averageaccuracyoffullscale(%)=

Maximumaccuracyofreading(%)=100max

Maximumaccuracyorfullscale(%)=100max

Observation:Thedefinitionsofthetermsaccuracy,resolution,andprecisionvaryconsiderablyinthetechnicalliterature.Itisgoodpracticetoincludeboththedefinitionsofyourtermsaswellastheirvaluesinyourtechnicalcommunication.

SincetheCelsiusandFahrenheittemperaturescaleshavearbitraryzeroes(e.g.,0˚Cisthefreezingpointofwater),itisinappropriatetospecifytemperatureerrorasapercentageofreadingorasapercentageoffullscalewhenCelsiusandFahrenheitscalesareused.Whenspecifyingtemperatureerror,weshoulduseaverageaccuracy,maximumerror,orstandarderror.Theseerrorshaveunitsof˚Cor˚F.

Typically,wecalibrateaquantitativedataacquisitionsystembydeterminingatransferfunctionthatrelatesthemeasuredvariable,x,torawmeasurementssuchastheADCsample.Accuracyislimitedbytwofactors:resolutionandcalibrationdrift.Calibrationdriftisthechangeinthetransferfunctionoccurringovertimeusedtocalculatethemeasuredvariablefromtherawmeasurements.

10.1.2.ResolutionTheinstrumentresolutionisthesmallestinputsignaldifference,∆xthatcanbedetectedbytheentiresystemincludingtransducer,electronics,andsoftware.Theresolutionofthesystemissometimeslimitedbynoiseprocessesinthetransduceritself(e.g.,thermalimaging)andsometimeslimitedbynoiseprocessesintheelectronics(e.g.,thermistors,RTDs,andthermocouples).

Thespatialresolution(orspatialfrequencyresponse)ofthetransduceristhesmallestdistancebetweentwoindependentmeasurements.Thesizeandmechanicalpropertiesofthetransducerdetermineitsspatialresolution.Whenmeasuringtemperature,ametalprobewilldisturbtheexistingmediumtemperaturefieldmorethanaglassprobe.Hence,aglassprobehasasmallerspatialresolutionthanametalprobeofthesamesize.Noninvasiveimagingsystemsexhibitexcellentspatialresolutionbecausetheinstrumentdoesnotdisturbthemedium,whichisbeingmeasured.Thespatialresolutionofanimagingsystemisthemediumsurfaceareafromwhichtheradiationoriginatesthatiseventuallyfocusedontothedetectorduringtheimagingofasinglepixel,theso-calledinstantaneousfieldofview,IFOV.Whenmeasuringforce,pressure,orflow,thespatialresolutionistheeffectiveareaoverwhichthemeasurementisobtained.Anotherwaytoillustratespatialresolutionistoattempttocollecta2-Dor3-Dimageofthemeasurand.Thespatialresolutionisthedistancebetweenpointsinourimage.

10.1.3.PrecisionPrecisionisthenumberofdistinguishablealternatives,nx,fromwhichthegivenresultisselected.Precisioncanbeexpressedinalternatives,bitsordecimaldigits.Considerathermometerinstrumentwithatemperaturerangeof0to100˚C.Thesystemdisplaystheoutputusing3digits(e.g.,12.3˚C).Inaddition,thesystemcanresolveeachtemperatureTfromthetemperatureT+0.1˚C.Thissystemhas1001distinguishableoutputs,andhencehasaprecisionof1001alternativesorabout10bits.Foralinearsystem,thereisasimplerelationshipbetweenrange(rx),resolution(∆x)andprecision(nx).Rangeisequaltoresolutiontimesprecision

rx(100˚C)=∆x(0.1˚C)•nx(1001alternatives)

where“range”isthemaximumminusminimumtemperature,andprecisionisspecifiedintermsofnumberofalternatives.Table10.1illustratestherelationshipbetweenalternativesanddecimaldigits.

Alternatives Decimaldigits

1000 3

2000 3½

4000 3¾

10000 4

Table10.1.Definitionofdecimaldigits.

Observation:Agoodruleofthumbtorememberis210•n≈103•n.

10.1.4.ReproducibilityandRepeatabilityReproducibility(orrepeatability)isaparameterthatspecifieswhethertheinstrumenthasequaloutputsgivenidenticalinputsoversomeperiodoftime.Thisparametercanbeexpressedasthefullrangeorstandarddeviationofoutputresultsgivenafixedinput,wherethenumberofsamplesandtimeintervalbetweensamplesarespecified.Oneofthelargestsourcesofthistypeoferrorcomesfromtransducerdrift.Statisticalcontrolisasimilarparameterbasedonaprobabilisticmodelthatalsodefinestheerrorsduetonoise.Theparameterincludesthenoisemodel(e.g.,normal,chi-squared,uniform,saltandpepper)andtheparametersofthemodel(e.g.,average,standarddeviation).

10.2.TransducersInthissection,wewillstartwithquantitativeperformancemeasuresforthetransducer.Next,specifictransducerswillbeintroduced.Ratherthangiveanexhaustivelistofallavailabletransducers,theintentinthissectionistoillustratetherangeofpossibilities,andtoprovidespecificdevicestouseinthedesignsectionslaterinthechapter.

10.2.1.StaticTransducerSpecificationsTheinputormeasurandisx.Theoutputisy.Atransducerconvertsxintoy.Inthissubsection,weassumetheinputparameter,x,isconstantorstatic.

Figure10.3.Transducersinthisbookconvertaphysicalsignalintoanelectricalsignal.Theinputxandtheoutputycanbeeitherabsoluteordifferential.Anabsolutesignalrepresentsaparameterthatexistsinasingleplaceatasingletime.Adifferentialsignalisderivedfromthedifferencebetweentwosignalsthatexistatdifferentplacesoratdifferenttimes.Voltageisindeeddefinedasapotentialdifference,butwhenthevoltageisreferredtoground,weconsideritanabsolutequantity.Ontheotherhand,ifthesignalisrepresentedbythevoltagedifferencebetweentwopointsneitherofwhichareground,thenweconsiderthesignalasdifferential.Table10.2illustratesfourtypesoftransducers.

Type Input->output

Example

Absolute→absolute

x→y Thermistorconvertsabsolutetemperaturetoaresistance

Relative→absolute

∆x→y Massbalanceconvertsmassdifferencetoanangle

Absolute→relative

x→∆y Straingaugeconvertsdisplacementtoaresistancedifference

Relative→relative

∆x→∆y Thermocoupleconvertstemperaturedifferencetovoltagedifference

Table10.2.Fourtypesoftransducers.

Thestaticsensitivityistheslope,m,ofthestraightlinethroughthestaticcalibrationcurvethatgivesminimalmeansquarederror.Letxi,yibetheinput/outputsignalsofthetransducerasshowninthefigurebelow.Thelinearityisameasureofthestraightnessofthestaticcalibrationcurve.Letyi=f(xi)bethetransferfunctionofthetransducer.Alineartransducersatisfies:

f(ax1+bx2)=af(x1)+bf(x2)

foranyarbitrarychoiceoftheconstantsaandb.Letyi=mxi+bbethebestfitlinethroughthetransducerdata.Linearity(ordeviationfromit)asafigure-of-meritcanbeexpressedaspercentageofreadingorpercentageoffullscale.Letymaxbethelargesttransduceroutput.

Averagelinearityofreading(percent)=

Averagelinearityoffullscale(percent)=

Twodefinitionsforsensitivityareusedfortemperaturetransducers.Thestaticsensitivityis:

Ifthetransducerislinearthenthestaticsensitivityistheslope,m,ofthestraightlinethroughthestaticcalibrationcurvewhichgivestheminimummeansquarederror.Ifxiandyirepresentmeasuredinput/outputresponsesofthetransducer,thentheleastsquaresfittoyi=mxi+bis

m=andb=

Thermistorscanbemanufacturedtohavearesistancevalueat25˚Crangingfrom4Ωto20MΩ.Becausetheinterfaceelectronicscanjustaseasilyconvertanyresistanceintoavoltage,a20MΩthermistorisnotmoresensitivethana30Ωthermistor.Inthissituation,itmakesmoresensetodefinefractionalsensitivityas:

= (units1/˚C) =

Unfortunately,transducersareoftensensitivetofactorsotherthanthesignalofinterest.Environmentalissuesinvolvehowthetransducerinteractswithitsexternalsurroundings(e.g.,temperature,humidity,pressure,motion,acceleration,vibration,shock,radiationfields,electricfieldsandmagneticfields.)Specificityisameasureofrelativesensitivityofthetransducertothedesiredsignalcomparedtothesensitivityofthetransducerstotheseotherunwantedinfluences.Atransducerwithagoodspecificitywillrespondonlytothesignalofinterestandbeindependentofthesedisturbingfactors.Ontheotherhand,a

transducerwithapoorspecificitywillrespondtothesignalofinterestaswellastosomeofthesedisturbingfactors.Ifallthesedisturbingfactorsaregroupedtogetherasnoise,thenthesignal-to-noiseratio(S/N)isaquantitativemeasureofthespecificityofthetransducer.

Theinputrangeistheallowedrangeofinput,x.Theinputimpedanceisthephasorequivalentofthesteadystatesinusoidaleffort(voltage,force,pressure)inputvariabledividedbythephasorequivalentofsteadystateflow(current,velocity,flow)inputvariable.Theoutputsignalstrengthofthetransducercanbespecifiedbytheoutputresistance,Rout,andoutputcapacitance,Cout.

Figure10.4.Outputmodelofatransducer.Theinputimpedanceofathermalsensorisameasureofthethermalperturbationthatoccursduetothepresenceoftheprobeitselfinthemedium.Forexample,athermocoupleneedleinsertedintoalaser-irradiatedmediumwillaffectthemediumtemperaturebecauseheatwillconductdownthestainlesssteelshaft.Athermocouplehasalowinputimpedance(whichisbad)becausethetransduceritselfloads(reduces)themediumtemperature.Ontheotherhand,aninfrareddetectormeasuressurfacemediumtemperaturewithoutphysicalcontact.Infrareddetectorsthereforehaveaveryhighinputimpedance(whichisgood)becausethepresenceofthetransducerhasnoeffectonthetemperaturetobemeasured.Inthecaseoftemperaturesensors,thedrivingforceforheattransferisthetemperaturedifference,∆T.Theresultingheatflow,q,canbeexpressedusingFourier’slawofthermalconduction:

wherekistheprobethermalconductivity,andAistheprobesurfaceareaandaistheradiusofasphericaltransducer.Thesteadystateinputimpedanceofasphericaltemperatureprobecanthusbeapproximatedby:

Again,theapproximationintheaboveequationassumesasphericaltransducer.Similardiscussionscanbeconstructedforthesinusoidalinputimpedance.Sincemostthermaleventscanbeclassifiedasstepeventsratherthansinusoidallyvaryingevents,mostresearchersprefertheuseoftimeconstanttodescribethetransientbehavioroftemperaturetransducers.

Sometransducersarecompletelypassive(e.g.,thermocouple,EKGelectrode),andothersareactiverequiringexternalpower(e.g.ultrasoniccrystals,straingauge,microphone,andthermistors.)Electricalisolationisacriticalfactorinmedicalinstrumentation.Sometransducersareinherentlyisolated(e.g.,thermistors,thermocouples,microphones),whileothersarenotisolated(e.g.,EKGelectrodes,pacemakers,bloodpressurecatheters.)Minimizationoferrorsisimportantforallinstruments.Thesensitivitytodisturbingfactors(electricfields,magneticfields,radiation,vibration,shock,acceleration,temperature,humidity)mustbedeterminedbeforeadevicecanbeused.

Thezerodriftisthechangeinthestaticsensitivitycurveintercept,b,asafunctionoftimeorotherfactor(seetheFigure10.5).Thesensitivitydriftisthechangeinthestaticsensitivitycurveslope,m,asafunctionoftimeorsomeotherfactor.Thesedriftfactorsdeterminehowoftenthetransducermustbecalibrated.Forexample,thermistorshaveadriftmuchlargerthanthatofRTDsorthermocouples.Transducersmaybeagedathightemperaturesforlongperiodsoftimetoimprovetheirreproducibility.

Figure10.5.Thetwotypesoftransducerdrift:sensitivitydriftandzerodrift.Thetransducerisoftenacriticaldeviceinvolvingboththecostandperformanceoftheentiresystem.Aqualitytransducermayproducebettersignalsbutatanincreasedcost.Animportantmanufacturingissueistheavailabilityofcomponents.Theavailabilityofadevicemaybeenhancedbyhavingasecondsource(morethanonemanufacturerproducesthedevice).Theuseofstandardcablesandconnectorswillsimplifytheconstructionofyoursystem.Thepowerrequirements,sizeandweightofthedeviceareimportantinsomesystems,andthusshouldbeconsideredwhenselectingatransducer.

10.2.2.DynamicTransducerSpecificationsThetransientresponseisthecombinationofthedelayphase,theslewingphase,andtheringingphaseasshowninFigure10.6.Thetotaltransientresponseisthetimefortheoutput,y(t),toreach99%ofitsfinalvalueafterastepchangeininput,x(t)=u0(t).

Figure10.6.Thestepresponseoftenhasdelay,slewingandringingphases.Thetransientresponseofatemperaturetransducertoasuddenchangeinsignalinputcansometimesbeapproximatedbyanexponentialequation(assumingfirst-orderresponse):

y(t)=yf+(y0-yf)e-t/

wherey0andyfaretheinitialandfinaltransduceroutputsrespectively.Thetimeconstant, ,ofatransduceristhetimetoreach63.2%ofthefinaloutputaftertheinputisinstantaneouslyincreased.Thistimeisdependentonboththetransducerandtheexperimentalsetup.Manufacturersoftenspecifythetimeconstantofthermistorsandthermocouplesinwell-stirredoil(fastest)orstillair(slowest).Inyourapplications,onemustconsiderthesituation.Ifthetransducerisplacedinahighflowliquidlikeanarteryorawaterpipe,itmaybereasonabletousethestirredoiltimeconstant.Ifthetransducerisinairorembeddedinasolid,thenthermalconductioninthemediumwilldeterminethetimeconstantalmostindependentlyofthetransducer.

Thefrequencyresponseisastandardtechniquetodescribethedynamicbehavioroflinearsystems.Lety(t)bethesystemresponsetox(t).Let

x(t)=Asin( t)y(t)=Bsin( t+ ) =2πf

ThemagnitudeB/Aandthephase responsesarebothdependentonfrequency.Differentialequationscanbeusedtomodellineartransducers.Letx(t)bethetimedomaininputsignal.LetX(j )bethefrequencydomaininputsignal.Lety(t)bethetimedomainoutputsignal.LetY(j )bethefrequencydomainoutputsignal.

Classification differentialequation gainresponse phaseresponse

ZEROORDER y(t)=mx(t) Y/X=m=staticsensitivity

FIRSTORDER y’(t)+ay(t)=bx(t) Y/X=b/sqrt(a2+ 2) =arctan(- /a)

SECONDORDER

y”(t)+ay’(t)+by(t)=cx(t)

TIMEDELAY y(t)=x(t-T) Y/X=exp(-j T)

Table10.3.Classificationsofsimplelinearsystems.

10.2.3.NonlinearTransducersNonlinearcharacteristicsincludehysteresis,saturation,bang-bang,breakdown,anddeadzone.Hysteresisiscreatedwhenthetransducerhasmemory.WecanseeintheFigure10.7thatwhentheinputwaspreviouslyhighitfallsalongthehighercurve,andwhentheinputwaspreviouslylowitfollowsalongthelowercurve.Hysteresiswillcauseameasurementerror,becauseforanygivensensoroutput,y,theremaybetwopossiblemeasurandinputs.Saturationoccurswhentheinputsignalexceedstheusefulrangeofthetransducer.Withsaturation,thesensordoesnotrespondtochangesininputvaluewhentheinputiseithertoohighortoolow.Breakdowndescribesasecondpossibleresultthatmayoccurwhenintheinputexceedstheusefulrangeofthetransducer.Withbreakdown,thesensoroutputchangesrapidly,usuallytheresultofpermanentdamagetothetransducer.Hysteresis,bangbanganddeadzonealloccurwithintheusefulrangeofthetransducer.Bangbangisasuddenlargechangeintheoutputforasmallchangeintheinput.Ifthebangbangoccurspredictably,thenitcanbecorrectedforinsoftware.Adeadzoneisaconditionwherealargechangeintheinputcauseslittleornochangeintheoutput.Deadzonescannotbecorrectedforinsoftware,thusifpresentwillcausemeasurementerrors.

Figure10.7.Nonlineartransducerresponses.

Therearemanywaystomodelnonlineartransducers.Anonlineartransducercanbedescribedasapiecewiselinearsystem.Thefirststepistodividetherangeofxintoafinitesubregions,assumingthesystemislinearineachsubregion.Thesecondstepistosolvethecoupledlinearsystemssothatthesolutioniscontinuous.Anothermethodtomodelanonlinearsystemistouseempiricallydeterminednonlinearequations.Thefirststepinthisapproachistoobservethetransducerresponseexperimentally.Givenatableofxandyvalues,thesecondstepistofittheresponsetoanonlinearequation.Engineerscalltheseempiricalfitsperformancemaps.

Athirdapproachtomodelanonlineartransducerusesalookuptablelocatedinmemory.Thismethodisconvenientandflexible.Letxbethemeasurandandybethetransduceroutput.Thetablecontainsxvaluesandthemeasuredyvalueisusedtoindexintothetable.Sometimesasmalltablecoupledwithlinearinterpolationachievesequivalentresultstoalargetable.ThespreadsheetTherm12.xls,whichcanbefoundonthewebsite,isanexampleofthisapproach.

Anonmonotonicresponseisaninput/outputfunctionthatdoesnothaveamathematicalinverse.Forexample,iftwoormoreinputvaluesyieldthesameoutputvalue,thenthetransducerisnonmonotonic.Softwarewillhaveadifficulttimecorrectinganonmonotonictransducer.Forexample,theSharpGP2Y0A21YKIRdistancesensorhasatransferfunctionasshowninFigure10.8.Ifyoureadatransducervoltageof2V,youcannottelliftheobjectis3cmawayor12cmaway.

Figure10.8.TheSharpIRdistancesensorexhibitsnonmonotonicbehavior.

10.2.4.PositionTransducersOneofthesimplestmethodstoconvertpositionintoanelectricalsignalusesapositionsensitivepotentiometer.Thesedevicesareinexpensivetobuildandaresensitivetosmalldisplacements.Thetransducerisconstructedfromapotentiometer.Thefixedpartofthepotentiometeriscalledtheframe,andthemovablepartisthearmature.Thearmatureisfreetomoveupanddownalongthemeasurementaxis,seeFigure10.9.Theframeisfixedandthearmatureisattachedtotheobjectbeingmeasured.Thetotalelectricalresistanceofthetransducerisfixed,buttheresistancetotheslidearmvarieswithdistance,d.

Rout=Rmax*d/dmax

wheredmaxisdistanceatfullscaleandRmaxistheresistanceatfullscale.Ifthematerialinthepotentiometerhasuniformresistance,thenRoutwillbelinearlyrelatedtodisplacement,d.Thedisadvantagesofthistransducerareitslowfrequencyresponse,itshighmechanicalinputimpedance,anditdegenerateswithtime.Nevertheless,thistypeoftransducerisadequateformanyapplications.ThistransducerwillbeinterfacedtwowayslaterinExample10.3.

Figure10.9.Potentiometer-basedpositionsensor.ThetransducerinFigure10.8usesIRlighttomeasuredistancetoareflectingobject.Thesesensorsrequireanonuniformpower,soplacinga10 Fnearthepowerlineofthesensorreducesnoiseonothercomponents.Iftheobjectismorethan6cmaway,theoutputvoltageisinverselyrelatedtovoltage.IfNistheADCsample,thendistancecanbecalculatedas

d=c/Nwherecisacalibrationconstant

Anothermethodtomeasurethedistancebetweentwoobjectsistotransmitaultrasonicwavefromoneobjectattheotherandlistenforthereflection(Figure10.10).Theinstrumentmustbeabletogeneratethesoundpulse,heartheechoandmeasurethetime,tin,betweenpulseandecho.Ifthespeedofsound,c,isknown,thenthedistance,d,canbecalculated.Ourmicrocontrollersalsohavemechanismstomeasurethepulsewidthtin.

d=ctin/2

Figure10.10.Anultrasonicpulse-echotransducermeasuresthedistancetoanobject,Ping))).

10.2.5.SoundTransducers

Amicrophoneisatypeofdisplacementtransducer.Soundwaves,whicharepressurewavestravellinginair,causeadiaphragmtovibrate,andthediaphragmmotioncausesthedistancebetweencapacitorplatestochange.Thisvariablecapacitancecreatesavoltage,whichcanbeamplifiedandrecorded.Theelectretcondensermicrophone(ECM)isaninexpensivechoiceforconvertingsoundtoanalogvoltage.Electretmicrophonesareusedinconsumerandcommunicationaudiodevicesbecauseoftheirlowcostandsmallsize.Forapplicationsrequiringhighsensitivity,lownoise,andlinearresponse,wecouldusethedynamicmicrophone,liketheonesusedinhigh-fidelityaudiorecordingequipment.TheECMcapsuleactsasanacousticresonatorforthecapacitiveelectretsensorshowninFigure10.11.TheECMhasaJunctionFieldEffectTransistor(JFET)insidethetransducerprovidingsomeamplification.ThisJFETrequirespowerassuppliedbytheR1resistor.ThislocalamplificationallowstheECMtofunctionwithasmallercapsulethantypicallyfoundwithothermicrophones.ECMdevicesarecylindricallyshaped,haveadiameterrangingfrom3to10mm,andhaveathicknessrangingfrom1to5mm.

Figure10.11.PhysicalandelectricalviewofanECMwithJFETbuffer(Vccdependsonmicrophone)AnECMconsistsofapre-charged,non-conductivemembranebetweentwoplatesthatformacapacitor.Thebackplateisfixed,andtheotherplatemoveswithsoundpressure.Movementoftheplateresultsinacapacitancechange,whichinturnresultsinachangeinvoltageduetothenon-conductive,pre-chargedmembrane.Anelectricalrepresentationofsuchanacousticsensorconsistsofasignalvoltagesourceinserieswithasourcecapacitor.Themostcommonmethodofinterfacingthissensorisahigh-impedancebuffer/amplifier.AsingleJFETwithitsgateconnectedtothesensorplateandbiasedasshowninFigure10.11providesbufferingandamplification.ThecapacitorCprovideshigh-passfiltering,sothevoltageattheoutputwillbelessthan±100mVfornormalvoice.Audiomicrophonesneedadditionalamplificationandband-passfiltering.Typicalaudiosignalsexistfrom100Hzto10kHz.ThepresenceoftheR1resistoriscalled“phantombiasing”.Theelectrethastwoconnections:GndandSignal/bias.Typically,themetalliccapsuleisconnectedtoGnd.

10.2.6.ForceandPressureTransducers

Acommondevicetomeasureforceandpressureisthestraingauge.Asawireisstretcheditslengthincreasesanditscross-sectionalareadecreases.Thesegeometricchangescauseanincreaseintheelectricalresistanceofthewire,R.Thetransducerisconstructedwithfoursetsofwiresmountedbetweenastationarymember(frame)andamovingmember(armature.)Asthearmaturemovesrelativetotheframe,twowiresarestretched(increaseinR1R4),andtwowiresarecompressed(decreaseinR2R3),asshowninFigure10.13.Thestraingaugeisadisplacementtransducer,suchthatachangeintherelativepositionbetweenthearmatureandframe,∆x,causesachangeinresistance,∆R.Thesensitivityofastraingaugeiscalleditsgaugefactor.

Figure10.12.Straingaugesusedforforceorpressuremeasurement.ThegaugefactorforanAdvancestraingaugeis2.1.Thetypicalresistance,R,is120Ω.Ifthegaugeisbondedontoamaterialwithaspringcharacteristic:

F=-kx

thenthetransducercanbeusedtomeasureforce.Thewireseachhaveasignificanttemperaturedrift.Whenthefourwiresareplacedintoabridgeconfiguration,thetemperaturedependencecancels.Ahighgain,highinputimpedance,highCMRRdifferentialamplifierisrequired.

Figure10.13.Fourstraingaugesareplacedinabridgeconfiguration.Forceandpressuresensorscanalsobemadefromsemiconductormaterials.Thesesiliconsensorscanbemademuchsmallerthanstraingauges,buttendtomuchmorenonlinear.Aforcesensingresistor(FSR)ismadewitharesistivefilmandconvertsforceintoresistance.Thesesensorsarelowcostandeasytointerface,buttendtobequitenonlinear.

10.2.7.TemperatureTransducersThermistorsareapopulartemperaturetransducermadefromaceramic-likesemiconductor.ANTC(negativetemperaturecoefficient)thermistorismadefromcombinationsofmetaloxidesofmanganese,nickel,cobalt,copper,iron,andtitanium.Amixtureofmilledsemiconductoroxidepowdersandabinderisshapedintothedesiredgeometry.Themixtureisdriedandsintered(underpressure)atanelevatedtemperature.Thewireleadsareattachedandthecombinationiscoatedwithglassorepoxy.Byvaryingthemixtureofoxides,arangeofresistancevaluesfrom30Ωto20MΩ(at25˚C)ispossible.Table10.4liststhetradeoffsbetweenthermistorsandthermocouples.

Thermistors Thermocouples

Moresensitive Moresturdy

Bettertemperatureresolution

Fasterresponse

Lesssusceptibletonoise

Inert,interchangeableVvs.Tcurves

Lessthermalperturbation

Requireslessfrequentcalibration

Doesnotrequireareference

Morelinear

Table10.4.Tradeoffsbetweenthermistorsandthermocouples.

Aprecisionthermometer,anohmmeter,andawaterbatharerequiredtocalibratethermistorprobes.Thefollowingempiricalequationyieldsanaccuratefitoveranarrowrangeoftemperature:

or

whereTisthetemperatureinKelvin,andRisthethermistorresistanceinohms.0degreesCelsiusis273.15degreesKelvin.Itispreferabletousetheohmmeterfunctionoftheeventualinstrumentforcalibrationpurposessothatinfluencesoftheresistancemeasurementhardwareandsoftwareareincorporatedintothecalibrationprocess.

Thefirststepinthecalibrationprocessistocollecttemperature(measuredbyaprecisionthermometer)andresistancedata(measuredbytheohmmeterprocessoftheinstrument).Thethermistor(s)tobecalibratedshouldbeplacedasclosetothesensingelementoftheprecisionthermometeraspossible.Thewaterbathcreatesastableyetcontrollableenvironmentinwhichtoperformthecalibration.SeethespreadsheetTherm12.xlsonthebookwebsiteforanexamplethermistorcalibration.

Athermocoupleisconstructedbyspotweldingtwodifferentmetalwirestogether.Probe

transducersincludeaprotectivecasingwhichsurroundsthethermocouplejunction.Probescomeinmanyshapesincludingroundtips,conicalneedlesandhypodermicneedles.Barethermocouplejunctionsprovidefasterresponsebutaremoresusceptibletodamageandnoisepickup.Ungroundedprobesallowelectricalisolationbutarenotasresponsiveasgroundedprobes.Commercialthermocoupleshavebeenconstructedin16to30gaugehypodermicneedles—a30gaugeneedlehasanoutsidediameterofabove0.03cm.Barethermocouplescanbemadefrom30-µmwireproducingatipwithan80µmdiameter.Aspotweldisproducedbypassingalargecurrentthroughthemetaljunctionthatfusesthetwometalstogether.Ifthewiresformaloop,andthejunctionsareatdifferenttemperatures,thenacurrentwillflowintheloop.ThisthermaltoelectricalenergyconversioniscalledtheSeebeckeffect(seeFigure10.14).Ifthetemperaturedifferenceissmall,thenthecurrent,I,islinearlyproportionaltothetemperaturedifferenceT1-T2.

Figure10.14.Whenthetwothermocouplejunctionsareatdifferenttemperaturescurrentwillflow.Iftheloopisbroken,andanelectricalvoltageisapplied,thenheatwillbeabsorbedatonejunctionandreleasedattheother.ThiselectricaltothermalenergyconversioniscalledthePeltiereffect,seeFigure10.15.Ifthevoltageissmall,thentheheattransferredislinearlyproportionaltothevoltage,V.

Figure10.15.Whenvoltageisappliedtotwothermocouplejunctionsheatwillflow.Iftheloopisbroken,andthejunctionsareatdifferenttemperatures,thenavoltagewilldevelopbecauseoftheSeebeckeffect.Ifthetemperaturedifferenceissmall,thenthevoltage,V,isnearlylinearlyproportionaltothetemperaturedifferenceT1-T2.AnamplifierisrequiredtomatchthetransduceroutputrangetotheADCrange,seeFigure10.16.Undermostconditionswewilladdananaloglowpassfilter.

Type-Thermocouple µV/˚Cat20˚C

Usefulrange˚C

Comments

T-Copper/Constantan 45 -150to+350 Moistenvironment

J-Iron/Constantan 53 -150to+1000

Reducingenvironment

K-Chromel/alumel 40 -200to+1200

Oxidizingenvironment

E-Chromel/constantan 80 0to+500 Mostsensitive

RS-Platinum/platinum-rhodium

6.5 0to+1500 Corrosiveenvironment

C–Tungsten/rhenium 12 0to2000 Hightemperature

Table10.5.Temperaturesensitivityandrangeofvariousthermocouples.

Figure10.16.Athermocoupleconvertstemperaturedifferencetovoltage.Thermocouplesarecharacterizedby:1)lowimpedance(resistanceofthewires),2)lowtemperaturesensitivity(45µV/˚Cforcopper/constantan),3)low-powerdissipation,4)fastresponse(becauseofthemetal),5)highstability(becauseofthepurityofthemetals),and6)interchangeability(againbecauseofthephysicsandthepurityofthemetals).

Ifthetemperaturerangeislessthan25˚C,thenthelinearapproximationcanbeusedtomeasuretemperature.LetNbethedigitalsamplefromtheADCfortheunknownmediumtemperature,T1.Acalibrationisperformedundertheconditionsofaconstantreferencetemperature:typically,oneusestheextremesofthetemperaturerange(TminandTmax).Aprecisionthermometersystemisusedtomeasurethe“truth”temperatures.LetNminandNmaxbethedigitalsamplesatTminandTmaxrespectively.Thenthefollowingequationcanbeusedtocalculatetheunknownmediumtemperaturefromthemeasureddigitalsample.

T1=Tmin+(N-Nmin)•

Becausethethermocoupleresponseisnotexactlylinear,theerrorsintheabovelinearequationwillincreaseasthetemperaturerangeincreases.Forinstrumentswithalargertemperaturerange,aquadraticequationcanbeused,

T1=H0+H1•N+H2•N2

whereH0H1andH2aredeterminedbycalibrationoftheinstrumentovertherangeofinterest.

10.3.DiscreteCalculusGivenamethodtomeasureposition,wecanusecalculustodeterminevelocityandacceleration.

Thecontinuousderivativecanbeexpressedasalimit.

Themicrocontrollercanapproximatethevelocitybecalculatingthediscretederivative.

where∆tisthetimebetweenvelocitysamples.Atthispointweintroduceashorthandtodescribedigitalsamples.x(n)isthecurrentsample,x(n-1)istheprevioussample,andx(n-2)isthesampletwotimesago.Whendescribingcalculationsperformedonsampleddataitiscustomarytousethisnotation.Theuseofnasthetimevariableemphasizesthesignalexistsonlyatdiscretetimes.

Thiscalculationofderivativeisverysensitivetoerrorsinthemeasurementofx(n).Amorestablecalculationaveragestwoormorederivativetermstakenoverdifferenttimewindows.Ingeneralwecandefinesucharobustcalculationas

Iftheintegersp,m,andcareallpositive,thiscalculationcanbeperformedinreal-time.Thefirsttermisthederivativeoverthelargetimewindowofp∆t.Thesecondwindowtermhasasmallersizeofm∆t.Itnormallyfitsentireinsidethefirstwithc>0andc-m<p.Thecoefficientsaandbcreatetheweightforcombiningtheshortandlongintervals.Witha=b=1,p=3,m=1,andc=1,weget:

Theaccelerationcanalsobeapproximatedbyadiscretederivative.

Observation:Intheabovecalculationsofderivative,asingleerrorinoneofthex(t)inputtermswillpropagatetoonlyafinitenumberoftheoutputcalculations.

Observation:Althoughthecentraldifferencecalculationofv(t)=(x(t+∆t)-x(t-∆t))/2∆tistheoreticallyvalid,wecannotuseitforreal-timeapplications,becauseitrequiresknowledgeaboutthefuture,x(t+∆t),whichisunavailableatthetimev(t)isbeingcalculated.

Similarly,wecanperformintegrationofvelocitytodetermineposition.

Themicrocontrollercanperformadiscreteintegrationbysummation.

Therearetwoproblemswiththisapproach.Thefirstdifficultyisdeterminingx(0).Thesecondproblemistheaccumulationoferrors.Ifoneiscalculatingvelocityfrompositionandanerroroccursinthemeasurementofx(n),thenthaterroraffectsonlytwocalculationsofv(n).Unfortunately,ifoneiscalculatingpositionfromvelocityandanerroroccursinthemeasurementofv(n),thenthaterrorwillaffectallsubsequentcalculationsofx(n).

Observation:Intheabovecalculationofintegration,asingleerrorinoneofthex(t)inputtermswillpropagateintoallremainingoutputcalculations.

Thefollowingfunction,whichisquitesimilartothederivative,isactuallyalowpassdigitalfilter.

Furthermore,ifyousampleaninputataregularrate,fs,thenthefollowingisadigitallowpassfilterwithacutoffofaboutfs/2k.Thisfiltercalculatestheaverageofthelastksamples(seeProgram8.10).

Checkpoint10.1:Ifyousampleat1000Hz,howdoyoucreateadigitallowpassfilterwithacutoffof10Hz?

10.4.DataAcquisitionSystemDesign

10.4.1.IntroductionandDefinitionsBeforedesigningadataacquisitionsystem(DAS)wemusthaveaclearunderstandingofthesystemgoals.WecanclassifysystemasaQuantitativeDAS,ifthespecificationscanbedefinedexplicitlyintermsofdesiredrange(rx),resolution(∆x),precision(nx),andfrequenciesofinterest(fmintofmax).Ifthespecificationsaremorelooselydefined,weclassifyitasaQualitativeDAS.Examplesofqualitativesystemsincludethosewhichmimicthehumansenseswherethespecificationsaredefinedusingtermslike“soundsgood”,“lookspretty”,and“feelsright.”Otherqualitativesystemsinvolvethedetectionofevents.Wewillconsidertwoexamples,aburglardetector,andaninstrumenttodiagnosecancer.Forbinarydetectionsystemslikethepresence/absenceofaburglarorthepresence/absenceofcancer,wedefineatruepositive(TP)whentheconditionexists(thereisaburglar)andthesystemproperlydetectsit(thealarmrings.)Wedefineafalsepositive(FP)whentheconditiondoesnotexist(thereisnoburglar)butthesystemthinksthereis(thealarmrings.)Afalsenegative(FN)occurswhentheconditionexists(thereisaburglar)butthesystemdoesnotthinkthereis(thealarmissilent.)Atruenegative(TN)occurswhentheconditiondoesnotexist(thepatientdoesnothavecancer)andthesystemproperlydetectsit(theinstrumentsaysthepatientisnormal.)Prevalenceistheprobabilitytheconditionexists,sometimescalledpre-testprobability.Inthecaseofdiagnosingthedisease,prevalencetellsuswhatpercentageofthepopulationhasthedisease.Sensitivityisthefractionofproperlydetectedevents(aburglarcomesandthealarmrings)overthetotalnumberofevents(numberofrobberies.)Itisameasureofhowwelloursystemcandetectanevent.Fortheburglardetector,asensitivityof1meanswhenaburglarbreaksinthealarmwillgooff.Forthediagnosticinstrument,asensitivityof1meanseverysickpatientwillgettreatment.Specificityisthefractionofproperlyhandlednon-events(apatientdoesn’thavecancerandtheinstrumentclaimsthepatientisnormal)overthetotalnumberofnon-events(thenumberofnormalpatients.)Aspecificityof1meansnopeoplewillbetreatedforacancertheydon’thave.Thepositivepredictivevalueofasystem(PPV)istheprobabilitythattheconditionexistswhenrestrictedtothosecaseswheretheinstrumentsaysitexists.Itisameasureofhowmuchwebelievethesystemiscorrectwhenitsaysithasdetectedanevent.APPVof1meanswhenthealarmrings,thepolicewillcomeandarrestaburglar.Similarly,aPPVof1meansifourinstrumentsaysapatienthasthedisease,thenthatpatientissick.Thenegativepredictivevalueofasystem(NPV)istheprobabilitythattheconditiondoesnotexistswhenrestrictedtothosecaseswheretheinstrumentsaysitdoesn’texist.ANPVof1meansifourinstrumentsaysapatientdoesn’thavecancer,thenthatpatientisnotsick.Sometimesthetruenegativeconditiondoesn’treallyexist(howmanytimesadaydoesaburglarnotshowupatyourhouse?)Iftherearenotruenegatives,onlysensitivityandPPVarerelevant.

Prevalence=(TP+FN)/(TP+TN+FP+FN)

Sensitivity=TP/(TP+FN)

Specificity=TN/(TN+FP)

PPV=TP/(TP+FP)

NPV=TN/(TN+FN)

Thetransducerconvertsthephysicalsignalintoanelectricalsignal.TheamplifierconvertstheweaktransducerelectricalsignalintotherangeoftheADC(e.g.,0to+3V).Theanalogfilterremovesunwantedfrequencycomponentswithinthesignal.TheanalogfilterisrequiredtoremovealiasingerrorcausedbytheADCsampling.Theanalogmultiplexerisusedtoselectonesignalfrommanysources.ThesampleandholdisananaloglatchusedtokeeptheADCinputvoltageconstantduringtheADCconversion.Theclockisusedtocontrolthesamplingprocess.InherentindigitalsignalprocessingistherequirementthattheADCbesampledonafixedtimebasis.Thecomputerisusedtosaveandprocessthedigitaldata.Adigitalfiltermaybeusedtoamplifyorrejectcertainfrequencycomponentsofthedigitizedsignal.Themultipleaccesscircularqueueisaconvenientdatastructuretousewiththedigitalfilter.

10.4.2.UsingNyquistTheorytoDetermineSamplingRateTherearetwoerrorsintroducedbythesamplingprocess.VoltagequantizingiscausedbythefinitewordsizeoftheADC.TheprecisionisdeterminedbythenumberofbitsintheADC.IftheADChasnbits,thenthenumberofdistinguishablealternativesis

nz=2n

Timequantizingiscausedbythefinitediscretesamplinginterval.TheNyquistTheoremstatesthatifthesignalissampledatfs,thenthedigitalsamplesonlycontainfrequencycomponentsfrom0to0.5fs.Conversely,iftheanalogsignaldoescontainfrequencycomponentslargerthan½fs,thentherewillbeanaliasingerror.Aliasingiswhenthedigitalsignalappearstohaveadifferentfrequencythantheoriginalanalogsignal.Simplyput,ifonesamplesasinewaveatasamplingrateoffs,

V(t)=Asin(2πft+ )

isitpossibletodetermineAfand fromthedigitalsamples?NyquistTheorysaysthatiffsisstrictlygreaterthantwicef,thenonecandetermineAfand fromthedigitalsamples.Inotherwords,theentireanalogsignalcanbereconstructedfromthedigitalsamples.Butiffslessthanorequaltof,thenonecannotdetermineAfand .Inthiscase,theapparentfrequency,aspredictedbyanalyzingthedigitalsamples,willbeshiftedtoafrequencybetween0and½fs.

Inthisexample,thesamplingrateisfixedat1600Hzandthesignalfrequencyisvaried.Whensamplingrateisexactlytwicetheinputfrequency,theoriginalsignalmayormaynotbeproperlyreconstructed.Inthisspecificcase,itisfrequencyshifted(aliased)toDCandlost.

100Hzsinewave(properlysampled)400Hzsinewave(properlysampled)

800Hzsinewave(aliased)1500Hzsinewave(aliased)

Figure10.17.Aliasingdoesnotoccurwhenthesamplingrateismorethantwicethesignalfrequency.Whensamplingrateisslowerthantwicetheinputfrequency,theoriginalsignalcannotbeproperlyreconstructed.Itisfrequencyshifted(aliased)toafrequencybetween0and½fs.Inthiscasethe1500Hzwavewasaliasedto100Hz.

Thechoiceofsamplingrate,fs,isdeterminedbythemaximumusefulfrequencycontainedinthesignal.Onemustsampleatleasttwicethismaximumusefulfrequency.Fastersamplingratesmayberequiredtoimplementadigitalfilterandotherdigitalsignalprocessing.

fs>2fmax

Eventhoughthelargestsignalfrequencyofinterestisfmax,theremaybesignificantsignalmagnitudesatfrequenciesabovefmax.Thesesignalsmayarisefromtheinputx,fromaddednoiseinthetransducerorfromaddednoiseintheanalogprocessing.Oncethesamplingrateischosenatfs,thenalowpassanalogfiltermayberequiredtoremovefrequencycomponentsabove½fs.Adigitalfiltercannotbeusedtoremovealiasing.

Aninterestingquestionarises:howdowedeterminethemaximumfrequencycomponentinourinput?Ifweknowenoughaboutoursystemwemightbeabletoderiveanequationtodeterminethemaximumfrequency.Forexample,ifamechanicalsystemconsistsofamass,frictionandaspring,thenwecanwriteadifferentialequationrelatingtheappliedforcetothepositionoftheobject.Thesecondwaytofindthemaximumfrequencycomponentinoursignalistomeasureitwithaspectrumanalyzer.

ValvanoPostulate:Iffmaxisthelargestfrequencycomponentoftheanalogsignal,thenyoumustsamplemorethantentimesfmaxinorderforthereconstructeddigitalsamplestolookliketheoriginalsignalwhenplottedonavoltageversustimegraph.

10.4.3.HowManyBitsDoesOneNeedfortheADC?ThechoiceoftheADCprecisionisacompromiseofvariousfactors.ThedesiredresolutionofthedataacquisitionsystemwilldictatethenumberofADCbitsrequired.Ifthetransducerisnonlinear,thentheADCprecisionmustbelargerthantheprecisionspecifiedintheproblemstatement.Forexample,letybethetransduceroutput,andletxbetherealworldsignal.Assumefornow,thatthetransduceroutputisconnectedtotheADCinput.Lettherangeofxberx.Lettherangeofybery.Lettherequiredprecisionofxbenx.Theresolutionsofxandyare∆xand∆yrespectively.Letthefollowingdescribethenonlineartransducer.

y=f(x)

TherequiredADCprecision,ny,(inalternatives)canbecalculatedby:

∆x=rx/nx

∆y=minf(x+∆x)-f(x)forallxinrx

ny=ry/∆y

Forexample,considerthenonlineartransducery=x2.Therangeofxis0≤x≤1.Thus,therangeofyisalso0≤y≤1.Letthedesiredresolutionbe∆x=0.01.nx=rx ∆x=100alternativesorabout7bits.Fromtheaboveequation,∆y=min(x+0.01)2-x2=min0.02x+0.0001=0.0001.Thus,ny=ry ∆y=10000alternativesoralmost15bits.

Checkpoint10.2:Whatistherelationshipbetweennxandnyifthetransducerislinear?

10.4.4.SpecificationsfortheAnalogSignalProcessing

IngeneralwewishtheanalogsignalprocessingtomapthefullscalerangeofthetransducerintothefullscalerangeoftheADC.IftheADCprecisionisN=2ninalternatives,andtheoutputimpedanceofthetransducerisRout,thenweneedaninputimpedancelargerthanN*Routtoavoidloadingthesignal.Weneedtheanalogcircuittopassthefrequenciesofinterest.Whenconsideringnoise,wedeterminethesignalequivalentnoise.Forexample,considerthethermistorcircuitinFigure10.16.IfwewishtoconsidernoiseonsignalVout,wecalculatetherelationshipbetweeninputtemperatureTandthesignalVout.Next,wedeterminethesensitivityofthesignaltotemperature,dVout/dT.IfthenoiseisVn,thenthetemperatureequivalentnoiseisTn=Vn/(dVout/dT).Ingeneral,wewishallequivalentnoisestobelessthanthesystemresolution.

Ananaloglowpassfiltermayberequiredtoremovealiasing.Thecutoffofthisanalogfiltershouldbelessthan½fs.Sometransducersautomaticallyremovetheseunwantedfrequencycomponents.Forexample,athermistorisinherentlyalowpassdevice.Othertypesoffilters(analoganddigital)maybeusedtosolvethedataacquisitionsystemobjective.Oneusefulfilterisa60Hzbandrejectfilter.

Inordertopreventaliasing,onemustknowthefrequencyspectrumoftheADCinputvoltage.Thisinformationcanbemeasuredwithaspectrumanalyzer.Typically,aspectrumanalyzersamplestheanalogsignalataveryhighrate(>1MHz),performsaDiscreteFourierTransform(DFT),anddisplaysthesignalmagnitudeversusfrequency.AsdefinedinFigure10.2,z(t)istheinputtotheADC.Let|Z(f)|bethemagnitudeoftheADCinputvoltageasafunctionoffrequency.Thereare3regionsinthemagnitudeversusfrequencygraphshowninFigure10.18.

Figure10.18.Topreventaliasingthereshouldbenomeasurablesignalabove½fs.WewillclassifyanysignalwithanamplitudelessthantheADCresolution,∆z,tobeundetectable.Thisregionislabeled“Undetectable”.Undetectablesignalscannotcausealiasingregardlessoftheirfrequency.

WewillclassifyanysignalwithanamplitudelargerthantheADCresolutionatfrequencieslessthan½fstobeproperlysampled.Thisregionislabeled“Properlysampled”.Itisinformationinthisregionthatisavailabletothesoftwarefordigitalprocessing.

ThelastregionincludessignalswithamplitudeabovetheADCresolutionatfrequenciesgreaterthanorequalto½fs.Signalsinthisregionwillbealiased,whichmeanstheirapparentfrequencieswillbeshiftedintothe0to½fsrange.

Mostspectrumanalyzersgivetheoutputindecibelsfullscale(dBFS).ForanADCsystemwitharangeof0to3V,thefullscalepeak-to-peakamplitudeforanyACsignalis3V.IfVistheDFToutputmagnitudeinvolts

dBFS=20log10(V/3)

Table10.6calculatestheADCresolutionindBFS.ForarealADC,theresolutionwillbeafunctionofotherfactorsotherthanbits.Forexample,theMAX124612-bitADChasaminimumSignal-to-Noise+DistortionRatio(SINAD)of70dB,meaningitisnotquite12bits.ThetypicalSINADis73dB,whichisslightlybetterthan12bits.

Bits dBFS

8 -48.2

9 -54.2

10 -60.2

11 -66.2

12 -72.2

13 -78.3

14 -84.3

Table10.6.ADCresolutionindBFS,assumingfullscaleisdefinedaspeak-to-peakvoltage.

Aliasingwilloccurif|Z|islargerthantheADCresolutionforanyfrequencylargerthanorequalto½fs.Inordertopreventaliasing,|Z|mustbelessthantheADCresolution.Ourdesignconstraintwillincludeasafetyfactorof≤1.Thus,topreventaliasingwewillmake:

|Z|< ∆zforallfrequencieslargerthanorequalto½fsThisconditionusuallybecanbesatisfiedbyincreasingthesamplingrateorincreasingthenumberofpolesintheanaloglowpassfilter.Wecannotremovealiasingwithadigitallowpassfilter,becauseoncethehighfrequencysignalsareshiftedintothe0to½fsrange,wewillbeunabletoseparatethealiasedsignalsfromtheregularones.Todetermine ,thesumofallerrors(e.g.,ADC,aliasing,andnoise)mustbelessthanthedesiredresolution.

10.5.AnalysisofNoiseTheconsiderationofnoiseiscriticalforallinstrumentationsystems.Thesuccessofaninstrumentdoesdependoncarefultransducerdesign,precisionanalogelectronics,andcleversoftwarealgorithms.Butanysystemwillfailifthesignalisoverwhelmedbynoise.Fundamentalnoiseisdefinedasaninherentandnonremovableerror.Itexistsbecauseoffundamentalphysicalorstatisticaluncertainties.Wewillconsiderthreetypesoffundamentalnoise:

Thermalnoise,WhitenoiseorJohnsonnoise

Shotnoise

1/fnoiseAlthoughfundamentalnoisecannotbeeliminated,therearewaystoreduceitseffectonthemeasurementobjective.Ingeneral,addednoiseincludesthemanydisturbingexternalfactorsthatinterferewithorareaddedtothesignal.Wewillconsiderthreetypesofaddednoise:

Galvanicnoise

Motionartifact

Electromagneticfieldinduction

10.5.1.ThermalNoiseThermalfluctuationsoccurinallmaterialsattemperaturesaboveabsolutezero.Brownianmotion,therandomvibrationofparticles,isafunctionofabsolutetemperature.Astheparticlesvibrate,thereisanuncertaintyastothepositionandvelocityoftheparticles.Thisuncertaintyisrelatedtothethermalenergy

Theabsolutetemperature,T(K)

Boltzmann’sconstant,k=1.6710-23joules/K

Uncertaintyinthermalenergy 1/2kT

Becausetheelectricalpowerofaresistorisdissipatedasthermalpower,theuncertaintyinthermalenergyproducesanuncertaintyinelectricalenergy.Theelectricalenergyofaresistordependson

Resistance,R(Ω)

voltage,V(volts)

Time,(sec).

Electricalpower=V2/R(watts)

Electricalenergy=V2*time/R(watt-sec)

Byequatingthesetwoenergieswecanderive(handwave)anequationforvoltagenoisesimilartotheempiricalfindingsofJ.B.Johnson.In1928,hefoundthattheopencircuitroot-mean-square(RMS)voltagenoiseofaresistorisgivenby:

VJ2=4kTR∆ where∆ =fmax-fminwherefmax-fministhefrequencyinterval,orbandwidthoverwhichthemeasurementwastaken.Forinstance,ifthesystembandwidthisDCto1000Hzthen∆ is1000cycles/sec.Similarly,ifthesystemisabandpassfrom10kHzto11kHz,then∆ isalso1000cycles/sec.Theterm“whitenoise”comesfromthefactthatthermalnoisecontainsthesuperpositionofallfrequenciesandisindependentoffrequency.Itisanalogoustoopticswhere“whitelight”isthesuperpositionofallwavelengths.

1Hz 10Hz 100Hz 1kHz 10kHz 100kHz 1MHz

10kΩ 14nV 45nV 142nV 448nV 1.4µV 4.5µV 14µV

100kΩ 45nV 142nV 448nV 1.4µV 4.5µV 14µV 45µV

1MΩ 142nV 448nV 1.4µV 4.5µV 14µV 45µV 142µV

Table10.7.Whitenoiseforresistorsat300K=27˚C.

Interestingly,onlyresistivebutnotcapacitiveorinductiveelectricaldevicesexhibitthermalnoise.Thusatransducerwhichdissipateselectricalenergywillhavethermalnoise,andatransducerwhichsimplystoreselectricalenergywillnot.

Figure10.19definesroot-mean-squaredasthesquarerootofthetimeaverageofthevoltagesquared.RMSnoiseisproportionaltonoisepower.ThecrestfactoristheratioofpeakvaluedividedbyRMS.Thepeakvalueis1/2ofthepeaktopeakamplitude,andcanbeeasilymeasuredfromrecordeddata.FromTable10.8,weseethatthecrestfactorisabout4.Thecrestfactorcanbedefinedforothertypesofnoise.

Percentofthetimethepeakisexceeded

Crestfactor(peak/RMS)

1.0 2.6

0.1 3.3

0.01 3.9

0.001 4.4

0.0001 4.9

Table10.8.Crestfactorforthermalnoise.

Figure10.19.Root-mean-squared(RMS)isatimeaverageofthevoltagesquared.Observation:Whenmeasuredwithaspectrumanalyzer,theresponseisuniformamplitudeatallfrequencies,whichiswhyitisclassifiedaswhitenoise.

10.5.2.ShotNoiseShotnoisearisesfromthestatisticaluncertaintyofcountingdiscreteevents.Thermalcameras,radioactivedetectors,photomultipliers,andO2electrodescountindividualphotons,gammarays,electrons,andO2particlesrespectivelyastheyimpingeuponthetransducer.Letdn/dtbethecountrateofthetransducer.Lettbethemeasurementintervalorcounttime.Theaveragecountis

n=dn/dt•t

Ontheotherhand,thestatisticaluncertaintyofcountingrandomeventsis√n.Thustheshotnoiseis

shotnoise=

Thesignaltonoiseratio(S/N)is

S/N=

Thesolutionsaretomaximizethecountrate(bymovingcloserorincreasingthesource)andtoincreasethecounttime.Thereisacleartradeoffbetweenaccuracyandmeasurementtime.

10.5.3.1/f,Flicker,orPinkNoisePinknoiseissomewhatmysterious.Theoriginof1/fnoiselacksrigoroustheory.Itispresentindevicesthathaveconnectionsbetweenconductors.Garrettdescribesitasafluctuatingconductivity.Itisofparticularinteresttolowbandwidthapplicationsduetothe1/fbehavior.Wirewoundresistorsdonothave1/fnoise,butsemiconductors(likeMOSFETs)andcarbonresistorsdo.Oneoftheconfusingaspectsof1/fnoiseisitsbehaviorasthefrequencyapproaches0Hz.ThenoiseatDCisnotinfinitebecausealthough1/fisinfiniteatDC,∆ iszero.Garrettgivesanequationtocalculatethe1/fnoiseofacarbonresistor.

whereVcisthe1/fvoltagenoiseinvolts

fisthefrequencyinHz

RistheresistanceinΩ

IistheaverageDCcurrentinamps

∆ isthesystembandwidthinHz

f(Hz) Vc(µV)

1 316

10 100

100 32

1000 10

Table10.9.VcversusfrequencyforR=10kΩ,I=1mA,∆ =1kHz.

10.5.4.GalvanicNoiseThecontactbetweendissimilarmetalswillinduceavoltage,duetotheelectrochemistryatthemetal-metalinterface.Voltageswillalsodevelopwhenaconductiveliquidcontactsametal.Thisproblemusuallyarisesasametalsurfacewithinaconnectoroxidizes(corrosionduetomoisture).Thematerialsleastsusceptibletocorrosionaresilver,graphite,goldandplatinum.Forthisreason,weusegold-platedconnectorpinsandsockets.

10.5.5.MotionArtifact

Motioncanintroduceerrorsinmanyways.AccordingtoFaraday’sLaw,aconductingwirethatmovesinamagneticfieldwillinduceanEMF.Thisvoltageerrorisproportionaltothestrengthofthemagneticfield,thelengthofthewirethatismoving,thevelocityofthemotion,andtheanglebetweenthevelocityandthefield.Anotherproblemoccurringwithmovingcablesistheconnectorimpedancemaychangeordisconnect.Accelerationofthetransducerwillinduceforcesinsidethedeviceoftenaffectingitsresponse.

10.5.6.ElectromagneticfieldinductionUsually,thelargestsourceofnoiseiscausedbyelectromagneticfieldinduction.AccordingtoFaraday’sLaw,changingmagneticfieldscaninduceavoltageintoourcircuits.Thechangingmagneticfieldmustpassthroughawireloop,drawnastheshadedareainFigure10.20.Thisvoltagenoise(Vm)isproportionalto,thestrengthofthemagneticfield,B(wb/m2),theareaoftheloopS(m2),andageometricfactor,K(volts/wb.)ThedrawingontheleftofFigure10.20illustratesthephysicalsituationcausingmagneticfieldpickup.AtypicalsituationcausingmagneticfieldnoiseoccurswhenACpowerbeingdeliveredtoalow-impedanceload,suchasamotor.ThevoltageV1isthe120VAC60Hzpowerline,andVsisasignalinourinstrument.Thealternatingcurrent(I1)willcreateamagneticfield,B.Thismagneticfield(B)alsoalternatesasitflowsthroughthelooparea(S)formedbythewiresinourcircuit(suchastheleadwiresbetweenthetransducerandtheinstrumentbox.)Thiswillinduceacurrent(Im)alongthewire,causingavoltageerror(Vm).Wecantestforthepresenceofmagneticfieldpickupbydeliberatelychangingtheloopareaandobservingthemagnitudeofthenoiseasafunctionofthelooparea.ThedrawingontherightofFigure10.20illustratesanequivalentcircuitwecanusetomodelmagneticfieldpickup.Basically,wecanmodelmagneticfieldinducednoiseasamutualinductancebetweenanACcurrentflowandourelectronics.

Figure10.20.Magneticfieldnoisepickupcanbemodeledasatransformer.

ThesecondwayEMfieldscancoupleintoourcircuitsisviatheelectricfield.Changingelectricfieldswillcapacitivelycoupleintotheleadwires.ThedrawingontheleftofFigure10.21illustratesthephysicalsituationcausingelectricfieldpickup.Thealternatingvoltage(V1)willcreateanelectricfield.Thiselectricfieldalsotraversesnearthewiresinourcircuit(suchastheleadwiresbetweenthetransducerandtheinstrumentbox.)Thiswillinduceadisplacementcurrent(Id)alongthewire.Wecantestforthepresenceofelectricfieldpickupbyplacingashieldseparatingourelectronicsfromthesourceofthefieldandobservingthemagnitudeofthenoise.ThedrawingontherightofFigure10.21illustratesanequivalentcircuitwecanusetomodelelectricfieldpickup.Basically,wecanmodelelectricfieldinducednoiseasastraycapacitancebetweenanACvoltageandourelectronics.

Figure10.21.Electricfieldnoisepickupcanbemodeledasastraycapacitance.Observation:SometimestheEMfieldsoriginatefrominsidetheinstrumentbox,suchashighfrequencydigitalclocksandswitchingpowersupplies.

10.5.7.TechniquestoMeasureNoiseTherearetwoobjectiveswhenmeasuringnoise.Thefirstobjectiveistoclassifythetypeofnoise.Inparticular,wewishtoknowifthenoiseisbroadband(i.e.,allfrequencies,likewhitenoise)ordoesthenoisecertainspecificfrequencies(e.g.,60,120,180Hz,…like60HzEMfieldpickup).Thetypeofnoiseisofgreatimportancewhendeterminingwherethenoiseiscomingfrom.Classifyingthenoisetypeisessentialindevelopingastrategyforreducingtheeffectofthenoise.Thesecondobjectiveistoquantifythemagnitudeofthenoise.Quantifyingthenoiseishelpfulindeterminingifachangetothesystemdesignhasincreasedordecreasedthenoise.ThemeasurementresolutionofmanydataacquisitionsystemsislimitedbynoiseratherthanbyADCprecisionandsoftwarealgorithms.Forthesesystems,quantitativenoisemeasurementsareanimportantperformanceparameteroftheinstrument.

Digitalvoltmeter(DVM)inACmode.Root-mean-squared(RMS)isdefinedthesquarerootofthetime-averageofthevoltagesquared(Figure10.19).Ifyouremovetheinputsignal,theoutputofthesystemcontainsjustnoise.Becausetheresistanceloadisusuallyconstant,squaringthevoltageresultsinasignalproportionaltonoisepower.Theaveragingcalculationgivesameasurerelatedtoaveragepower,andthesquarerootproducesaresultwithunitsinvolts.RMSnoiseofasignalcanbemeasuredwithaDVMusingACmode.MostDVMsinACmodeperformadirectmeasurementofRMS;hencethismethodisthemostprecise.Forexample,a3½digitDVMhasaprecisionofabout11bits.AcalibratedvoltmeterinACmodewillbethemostaccuratequantitativemethodtomeasurenoise.

Analogoscilloscope.Thesecondmethodistoconnectthesignaltoanoscilloscopeandmeasurethepeak-to-peaknoiseamplitude,asillustratedinFigure10.22.ThecrestfactoristheratioofpeakvaluedividedbyRMS.Thepeakvalueis½ofthepeak-to-peakamplitude,andcanbeestimatedfromthescopetracing.FromTable10.8,weseeforwhitenoisethatthecrestfactorisabout4,sowecanapproximatetheRMSnoiseamplitudebydividingthepeak-to-peaknoiseby8.Becausethequantitativeassessmentofnoisewithascoperequiresvisualobservation,thismethodcanonlybeusedtoapproximatethequantitativelevelofnoise.Onetheotherhand,oscilloscopehaveveryhighbandwidth,andthereforetheyaregoodforclassifyinghighfrequencynoise.Whitenoiseand1/fnoiselookrandom,likeleftgraphinFigure10.22.Forwhiteand1/fnoise,thescopetriggerwillnotbeabletocapturearepeatingwaveform.NoisefromEMfieldsontheotherhandarerepeatingandcanbetriggeredbythescope.Infacttheline-triggersettingonthescopecanbeusedspecificallytoseeifthenoiseiscorrelatedtothe60Hz120VACpowerline.Inparticular,60Hznoisewilltriggerwhenusingtheline-triggersettingofthescope.Theshapeofthenoisevariesdependingontherelativestrengthsofthefundamentalandharmonicfrequencies.Thegraphontherightisaperiodicwavewithafundamentalplusa50%strengthfirstharmonic.

Figure10.22.Quantifyingnoisemymeasuringpeak-to-peakamplitude.

Figure10.23.Classifyingnoisemymeasuringtheamplitudeversusfrequencywithaspectrumanalyzer.Spectrumanalyzer.Thethirdmethodusesaspectrumanalyzer,whichcombinesacomputer,high-speedADCsamplingandtheFast-Fourier-Transform(FFT),asillustratedinFigure10.23.Beingabletoseethenoiseinthefrequencydomainisparticularlyusefultoclassifythetypeofnoise.Both1/fandwhitenoisecomponentsexistinatypicalanalogamplifier,buttheamplitudeisusuallysmallcomparedtoEMfieldnoise.The1/fandwhitenoiselevelsoccurringinelectronicscanbereducedbydroppingthetemperatureorspendingmoremoneybuyingabettercomponents.Typically,weseethefundamentalandmultipleharmonicsforEMfieldnoise.Forexample,60Hznoisealsoincludescomponentsat120,180,240Hzetc.

10.5.8.TechniquestoReduceNoiseItismuchsimplertoreducenoiseearlyinthedesignprocess.Conversely,itcanbequiteexpensivetoeliminatenoiseafteraninstrumenthasbeenbuilt.Therefore,itisimportanttoconsidernoiseateverystageofthedevelopmentcycle.Wecandividenoisereductiontechniquesintothreecategories.Thefirstcategoryinvolvesreducingnoisefromthesource.Youcanenclosenoisysourcesinagroundedmetalbox.Ifacablecontainsahighfrequencynoisesignal,thenthatsignalcouldbefiltered.MagneticandelectricfieldstrengthdependsondI/dtanddV/dt.Sowheneverpossible,youshouldlimittherise/falltimesofnoisysignals,asshowninFigure10.24.Forexample,thesquarewaveontheleftwillradiatemorenoisethanthesmoothsignalontheright.WhenoperatingaGPIOoutputpinin8mAdrivemode,wecansetthecorrespondingbitintheSLRregistertolimittheslewrateontheoutput.

Figure10.24.Limitingrise/falltimescanreduceradiatednoise.

Motorshavecoilstocreateelectromagnets,andnoisecanbereducedbylimitingthedI/dtinthecoil.Cableswithnoisysignalsshouldbetwistedtogether,sotheradiatedmagneticfieldswillcancel.Thesecablesshouldalsobeshieldedtoreduceelectricfieldradiation,andthisshieldshouldbegroundedonbothsides.

Thesecondcategoryofnoisereductioninvolveslimitingthecouplingbetweenthenoisesourceandyourinstrument.Wheneverpossiblemaximizethedistancebetweenthenoisysourceandthedelicateelectronics.Alltransducercablesshouldusetwistedwire,asshowninFigure10.25.

Figure10.25.Propercablingcanreducenoisewhenconnectingaremotetransducerorwhenconnectingtwoinstruments.Forsituationswherearemotesensorisattachedtoaninstrument,thegroundshieldshouldbeconnectedonlyontheinstrumentside.Ifthecableisconnectedbetweentwoinstruments,thenconnectthegroundshieldonbothinstruments.Forhighfrequencysignalscoaxialcableisrequired.Theshieldshouldbeelectricallyinsulatedtoeliminatedirectelectricalconnectiontootherdevices.Ifnoisysignalsmustexistinthesamecableaslow-levelsignals,thenseparatethetwowithagroundwireinbetween.Wheneverpossiblereducethelengthofacable.Similarlyminimizethelengthofleadsthatextendbeyondthegroundshield.Placethedelicateelectronicsinagroundedcase.Youcanuseopticalortransformerisolationcircuitstoseparatethenoisygroundfromthegroundofthedelicateelectronics.

Thelastcategoryinvolvestechniquesthatreducenoiseatthereceiver.Thebandwidthofthesystemshouldbeassmallaspossible.Inparticular,youcanuseananaloglowpassfiltertoreducethebandwidth,whichwillalsoreducethenoise.Youcanaddfrequency-rejectdigitalfilterstoreducespecificnoisefrequencieslike60,120,180Hz.Youshouldusepowersupplydecouplingcapacitorsoneachchiptoreducethenoisecouplingfromthepowersupplytotheelectronics.Figure10.26illustrateshowEMfieldnoisewillaffectourinstrument.IfthecablehastwistedwiresthenId1shouldequalId2.Theinputimpedanceoftheamplifierisusuallymuchlargerthanthesourceimpedanceofthesignal.Thus,V1-V2=Rs1Id1-Rs2Id2.

Figure10.26.Capacitivelycoupleddisplacementcurrents.Somecapacitorshaveafoilwrapsurroundingitscylindricalshape.Thisfoilshouldbegrounded.FormoreinformationaboutnoiserefertoHenryOtt,NoiseReductionTechniquesinElectronicSystems,Wiley,1988orRalphMorrison,GroundingandShieldingTechniques,Wiley,1998.

10.6.DataAcquisitionCaseStudiesWeintroducedthedesignprocessbackinChapter1.Inthissection,wewillpresentthedesignsoffourdataacquisitionsystems.Thefirstexampleisaquantitativemeasurementoftemperature,thesecondexampleisaqualitativemeasurementofelectrocardiogramsignals,andthethirdexamplecomparesmeasurementsmadewiththeADCversustimingmeasurementsusinginputcapture.Thelastexampleinputsfromamicrophoneandoutputstoaspeaker.Intheanalysisphase,wedeterminetherequirementsandconstraintsforourproposedsystem.Inthehigh-leveldesignphase,wedefineourinput/output,breakthesysteminmodules,andshowtheinterconnectionusingdataflowgraphs.Intheengineeringdesignphasewedesignthehardware/softwaresubcomponentsusingtechniquessuchassimulation,mechanicalmockups,andcallgraphs.WedefinespecificI/Osignals,analogcircuits,powersources,noisefilters,softwarealgorithms,datastructures,andtestingprocedures.Duringtheimplementationandtestingphaseswebuildandtestthemodules.Modularityallowsforconcurrentdevelopment.

Example10.1.Designaninstrumentthatmeasurestemperaturewitharangeof30to40˚C,andaresolutionis0.01˚C.Thefrequencyrangeofinterestis0to20Hz.

Solution:Thefirstdecisiontomakeisthechoiceoftransducer.TheRTDhasalinearresistanceversustemperatureresponse.RTDsareexpensive,butagoodchoiceforeaseofcalibration,interchangeability,andaccuracy.Thermocouplesareinexpensiveandagoodchoiceforlargetemperatureranges,harshmeasurementconditions,fastresponsetime,interchangeability,andlargetemperatureranges.Interchangeabilitymeanswecanbuymultipletransducersandtheywillallhavesimilartemperaturecurves.ThermistorswillbeusedinthisdesignbecausetheyareinexpensiveandhavebettersensitivitythanRTDsandthermocouples.Athermometerbuiltusingathermistorwillbehardertomass-producebecauseeachtransducermustbeseparatelycalibratedinordertocreateanaccuratemeasurement.Fromafirstglance,wemightexpecta10-bitADCwillgenerateatemperatureresolutionof0.01˚C.Recallthatrangeequalsresolutiontimesprecision.Ontheotherhand,becausethethermistorisnonlinear,wewillneedtoverifytheresolutionspecificationismet.Figure10.27showstheblockdiagramoftheinstrument,whichalsoillustratesthedataflowinoursystem.

Figure10.27.Dataflowgraphofatemperaturemeasurementsystemusingathermistor.

Theresistancebridgeisaclassicmeanstoconvertresistancetovoltage.Table10.10isusedduringthedesignphasetoshowthesignalvaluesastheytransversetheelectronics.A+2.50Vreferencedrivesthebridge.ThevalueofresistorR1ischosentoeliminateerrorsduetoself-heatingthethermistor(100kΩ).ThefirsttwocolumnsofTable10.10givethethermistorcalibration.Sincewewillbeusingrail-to-railelectronics,weneedtohaveallvoltagesbetweenthe0to+3.3Vrange.Wecanmakethebridgeoutput(V1-V2)positivebyselectingthevalueofresistorR2lessthanthethermistorresistanceatthemaximumtemperature(18.3kΩ).Forthisthermistor,R2isselectedat18k becauseitisastandardresistorlessthan18.3kΩ.Next,wechoosethegainoftheamplifiertomaptheminimumtemperatureintothe+3VlimitoftheADC(17.5).UsinganINA122orAD627,againresistorof16k createsthedesiredgainof17.5.Sincethethermistorisnonlinear,wewilltabulateexplicitvaluestodeterminetheADCprecisionrequired(Table10.10).Therearetwopossibleapproachestothedesignoftheamplifier.Ifweuseaninstrumentationamp,theinputimpedancewillbelargeenoughnottoaffectthebridge.Ifweuseasingleopampdifferentialamp,thentheamplifierwillloadthebridgeandaffectthebridgeresponse.Inthissystem,theinstrumentationampwillbeused.ThefirsttwocolumnsofTable10.10showtheresistancetemperaturecalibrationofthethermistor.Thethirdcolumn,V1,isthevoltageacrossthethermistor.Thefourthcolumnistheoutputofthebridge,V1-V2.Vout,theoutputoftheinstrumentationamp,isshowninthefifthcolumn.TheADCvaluegivesthedigitaloutputofa10-bitconverter,andthelastcolumnwillbecalculatedbyoursoftwareasadecimalfixed-pointwithresolution0.01˚C.Weusethistableintwoways.Initially,weusetheoreticalvaluestodesigntheelectronicsandsoftware.Duringtheimplementationphase,wesubstituteresistorswithstandardvaluestobringdownthecost.Duringthetestingphase,wemeasureactualvaluestoverifyproperoperation.Measuredvaluesforthelasttwocolumnswillbestoredinsoftwareasacalibrationtable.Tomeasuretemperature,thesoftwaremeasurestheADCvalue,andthenusesatablelook-upandlinearinterpolationtogetthedecimalfixed-pointtemperature(lastcolumn).Thefixed-pointnumberisoutputtoanOLEDdisplay.

Observation:ThereisanExcelworksheetnamedTherm10.xlsthatwasusedtocreatethisdesign.Youcanfinditonthebookwebsite.

T(°C) Rt(kΩ) V1(V) V1-V2(V) Vout(V) ADC T(0.01˚C)

30.0 28.241 0.551 0.169 2.961 1009 3000

32.0 25.837 0.513 0.132 2.309 787 3200

34.0 23.665 0.478 0.097 1.698 579 3400

36.0 21.700 0.446 0.064 1.127 384 3600

38.0 19.921 0.415 0.034 0.594 202 3800

40.0 18.307 0.387 0.005 0.096 32 4000

Table10.10.Signalsastheypassthroughthetemperaturedataacquisitionsystem.

InordertopreventnoiseintheADCsamples,thenoisemustbelessthantheresolution.TheresolutionofV1-V2isitsrange(0.169V)dividedbyitsprecision(1024),whichisabout0.1mV.Againasafetyfactorof1/2isincluded.Thusinthefrequencyrange0to20Hz,themaximumallowablenoisereferredtotheinputofthedifferentialampshouldbe

amplifiernoise≤=50V

Atwo-polelowpassanalogfilter(Figure8.25)isusedtopassthetemperaturesignalhavingfrequenciesfrom0to20Hz,rejectnoisehavingfrequenciesabove½fs.Inordertopreventaliasing,ZmustbelessthantheADCresolutionforallfrequencieslargerthanorequalto½fs.Asanextrameasureofsafety,wemaketheamplitudelessthan0.5∆zforfrequenciesabove½fs.Thus,

|Z|<0.5∆z=3/2048≈1mV

Theeffectiveoutputimpedanceofthebridgeis18k .TheinputimpedanceofthedifferentialampmustbehighenoughtonotaffecttheADCconversion.

Zin>18k •2n+1=36M

Figure10.28.Amplifierandlowpassfilter.

Todeterminetheresolutionweworkbackwards,asillustratedinTable10.11.Thebasicapproachtoverifyingthetemperatureresolutionistoworkbackwardsthroughthecircuit,showingthatachangeinADCvalueof1correspondstoatemperaturechangeof0.01˚C.

ADC V3(V) V1-V2(V) V1(V) RT(kΩ) T(°C) ∆T(˚C)

1023 2.997 0.1713 0.5526 28.3774 29.892

1022 2.994 0.1711 0.5524 28.3664 29.901 0.0087

512 1.500 0.0857 0.4671 22.9752 34.679

511 1.497 0.0855 0.4669 22.9651 34.690 0.0102

1 0.003 0.0002 0.3815 18.0093 40.391

0 0.000 0.0000 0.3814 18.0000 40.404 0.0124

Table10.11.Equationscalculatedinreversetoshowthattheresolutionmeetsthedesignspecification.

TherearethreepossibleapproachestoconvertingADCsampletotemperature(thelasttwocolumnsofTable10.10).First,wecouldfitthetransferfunctiontoapolynomialequation,andsavethecoefficientsofthatequationasthecalibrationfile.Thisapproachperformswellforsimplesituations.AplotofthisdataisshownasFigure10.29.Second,wecouldcalculatethetemperatureoutputforeachpossibleADCandsaveitina1024-entrylookuptable.ThisconversionisfastbecausewejustneedtousetheADCdatatoindexintothebigtable.Thismethodisfast,butrequiresalotofROM.Thethirdapproach,showninProgram10.1,usesatableofpaired(ADC,T)data.Thesepointsaredeterminedfromexperimentalcalibration.TofindthecorrespondingtemperatureforagivenADCvalue,theprogramfirstsearchesthetableforapairofADC-valuesthatsurroundtheinput.Extraentrieswereaddedatthebeginningandendofthetabletoguaranteethesearchstepwillalwaysbesuccessful.Then,ituseslinearinterpolationtocalculatethetemperature,giventhe53entriesinthetableandtheADCinput.Theoutputresultisafixed-pointnumberwitharesolutionof0.01˚C.

Typically,theresolutionofthisthermometerwilldependonanalognoiseratherthanADCresolution.Theaccuracywilldependonbothresolutionandcalibrationdrift.DriftisdefinedasachangeovertimeofthedatainFigure10.29andthenumbersinProgram10.1.

Figure10.29.Transferfunctionbetweensampled10-bitADCandfixed-pointtemperature.

Thecalibrationdatain ADCdata and Tdata arestoredinEEPROM.Ingeneral,weperformtime-criticaltaskslikeADCsamplinginthebackground,andnoncriticalfunctionslikeconversiontotemperatureandOLEDoutputintheforeground.Therefore,theinterruptserviceroutinepassesthemeasuredtemperaturetotheforegroundthroughaFIFOqueue,andthemainprogramhastheresponsibilityofoutputtingtheresulttotheOLED.Thecompletionofthissystemisleftasalaboratoryexercise.

//tableofmultipleunsigned(x,y),piece-wiselinearfunction

uint16_tconstADCdata[53]=0,32,49,65,82,99,116,133,150,167,184,

202,220,237,255,273,291,310,328,346,365,

384,403,422,441,460,480,499,519,539,559,

579,599,619,640,660,681,702,723,744,765,

787,808,830,852,874,896,918,941,964,986,1009,1024

;

uint16_tconstTdata[53]=4000,4000,3980,3960,3940,3920,3900,3880,

3860,3840,3820,3800,3780,3760,3740,3720,3700,3680,3660,3640,3620,

3600,3580,3560,3540,3520,3500,3480,3460,3440,3420,

3400,3380,3360,3340,3320,3300,3280,3260,3240,3220,

3200,3180,3160,3140,3120,3100,3080,3060,3040,3020,3000,3000

;

Program10.1.Calibrationdataforthermistor.

Checkpoint10.3:WhatwouldbethetemperatureresolutioniftheADCprecisionweredecreasedfrom10to8bits?

Tomeasuretemperatureresolution,weusethestudent’st-testtodetermineifthesystemisabletodetectthechange.Tousethestudent’sttestweneedtomakethefollowingassumptions:

1)Errorsinonedatasetareindependent,notcorrelatedtoerrorsintheotherdataset;

2)Errorsineachdatasampleareindependent,notcorrelatedtoerrorswithinthatset;

3)Errorsarenormallydistributed;

4)Varianceisunknown;

5)Variancesinthetwosetsareequal.

Ifarandomvariable,X,isnormallydistributedwithameanis andastandarddeviationofσ,thentheprobabilitythatitfallsbetween±1σis68%.I.e.,

P( -σ<X< +σ)=0.68

Similarly,

P( -1.96σ<X< +1.96σ)=0.95

P( -2σ<X< +2σ)=0.954

P( -2.58σ<X< +2.58σ)=0.99

P( -3σ<X<+3σ)=0.9997

Thesquareofthestandarddeviationiscalledvariance,σ2.Inmostsituations,wedonotknowthemeanandstandarddeviation,sowecollectdataandestimatethem.Inparticular,wetakemultiplemeasurementsassumingthetemperatureisconstant.LetXiberepeatedmeasurementsunderthesameconditions,andNisthenumberofmeasurements(e.g.,N=10).

TheN-1termisusedinthecalculationofSbecausethereareN-1degreesoffreedom.Theseexpressionsareunbiasedestimatesof andσ,meaningasthesamplesizeincreasestheestimatesapproachtruth.Formally,wesaytheexpectedvalueof is ,orE( )=.Similarly,theexpectedvalueofS2isσ2,orE(S2)= σ2.Forexample,wecollecttwosetsofdata(e.g.,10measurementsineachset,N=10),andwewanttoknowifthemeansoftwosamplesetsaredifferent.Considerthemeasurementsinthetwodatasetsasthesumofthetruevalueplusanerror:

X01= 0+e0iXi1= 1+e1iAssumption1statesthate0iarenotcorrelatedtoe1i.Assumption2statesthate0iarenotcorrelatedtoe0jande1iarenotcorrelatedtoe1j.Thermalnoisewillsatisfytheseassumptions.WeemployateststatistictotestthehypothesisH0:µ0=µ1.First,weestimatethemeansandvariancesofthedata(assumingequalsizedsamples)

Fromthese,wecalculatetheteststatistict:

Thetwosetsofdata,together,have2N-2degreesoffreedom.Thestudent’sttable,shownasTable10.12,hastwodimensions.Intheverticaldirection,wespecifythedegreesoffreedom,df.Forexample,ifthereare10datapointsineachdataset,thendfequals18.Inthehorizontaldirectionweselecttheprobabilityofbeingcorrect.Forexample,ifwewishtobe99%sureofthetest,thenweselectthe99%column.Selectingtherowandthecolumnallowsustopickanumberthreshold.Forexample,thenumberinthedf=18row,confidence=99%columnis2.878.

ThismeansifH0istrue,thentheprobabilityoft<-2.878=0.005andtheprobabilityoft>2.878=0.005.

Therefore,theprobabilityof-2.878<t<2.878=0.99(confidenceintervalof99%)

Ifwecollectdataandcalculatetsuchthattheteststatistictisgreaterthan2.878orlessthan‑2.878,thenweclaim“werejectthehypothesisH0”.Iftheteststatistictisbetween-2.878and2.878wedonotclaimthehypothesistobetrue.Inotherwordswehavenotproventhemeanstobeequal.Rather,wesay“wedonotrejectthehypothesisH0”.

confidence 80% 90% 98% 99% 99.8% 99.9%

dfp=

0.10 0.05 0.01 0.005 0.001 0.0005

8 1.397 1.860 2.896 3.355 4.501 5.041

9 1.383 1.833 2.821 3.250 4.297 4.781

10 1.372 1.812 2.764 3.169 4.144 4.587

11 1.363 1.796 2.718 3.106 4.025 4.437

12 1.356 1.782 2.681 3.055 3.930 4.318

13 1.350 1.771 2.650 3.012 3.852 4.221

14 1.345 1.761 2.624 2.977 3.787 4.140

15 1.341 1.753 2.602 2.947 3.733 4.073

16 1.337 1.746 2.583 2.921 3.686 4.015

17 1.333 1.740 2.567 2.898 3.646 3.965

18 1.330 1.734 2.552 2.878 3.610 3.922

19 1.328 1.729 2.539 2.861 3.579 3.883

20 1.325 1.725 2.528 2.845 3.552 3.850

21 1.323 1.721 2.518 2.831 3.527 3.819

22 1.321 1.717 2.508 2.819 3.505 3.792

23 1.319 1.714 2.500 2.807 3.485 3.767

24 1.318 1.711 2.492 2.797 3.467 3.745

25 1.316 1.708 2.485 2.787 3.450 3.725

26 1.315 1.706 2.479 2.779 3.435 3.707

27 1.314 1.703 2.473 2.771 3.421 3.690

28 1.313 1.701 2.467 2.763 3.408 3.674

29 1.311 1.699 2.462 2.756 3.396 3.659

30 1.310 1.697 2.457 2.750 3.385 3.646

40 1.303 1.684 2.423 2.704 3.307 3.551

50 1.299 1.676 2.403 2.678 3.261 3.496

60 1.296 1.671 2.390 2.660 3.232 3.460

80 1.292 1.664 2.374 2.639 3.195 3.416

100 1.290 1.660 2.364 2.626 3.174 3.390

120 1.289 1.658 2.358 2.617 3.160 3.373

∞ 1.282 1.645 2.326 2.576 3.090 3.291

Table10.12.Student’stdistributiontable.

Figure10.30.ResolutionmeansifthetemperatureincreasesbyT,thesystemwillprobablynotice.

Example10.2.Designasystemtomeasuretheelectricalactivityintheheart.Inparticular,wewishtomeasureheartrate.

Solution:Biopotentialsareimportantmeasurementsinmanyresearchandclinicalsituations.Biopotentialsareelectricvoltagesproducedbyindividualcellsandcanbemeasuredontheskinsurface.Thestatusoftheheart,brain,muscles,andnervescanbestudiedbymeasuringbiopotentials.Electrodes,whichareattachedtotheskin,interfacethemachinetothebody.Electronicinstrumentationamplifiesandfiltersthesignal.Forexample,Figure10.31showsanormalLeadIIelectrocardiogram,orEKG,whichismeasuredwiththepositiveterminalattachedtotheleftarm,thenegativeterminalattachedtotherightarm,andgroundconnectedtotherightleg.Eachwaverepresentsoneheartbeat,andtheshapeandrhythmofthiswavecontainsalotofinformationaboutthehealthandstatusoftheheart.

Figure10.31.NormalII-leadelectrocardiogram.

Therearetwotypesofelectrodesusedtorecordbiopotentials.Nonpolarizableelectrodeslikesilver/silverchlorideinvolvethefollowingchemicalreactionintheelectrodeattheelectrode/tissueinterface:

AgCl+e-↔Ag+Cl-

Anonpolarizableelectrodehasalowelectricalimpedancebecauseelectronscanfreelypasstheelectrode/tissueinterface.Intheelectrode,currentflowsbymovingelectrons,butinthetissuecurrentflowsbyphysicalmotionofchargedions(e.g.,Na+,K+andCl-).Asilver/silverchlorideelectrodedoesincludeahalf-cellpotentialof0.223V,butsince

biopotentialsarealwaysmeasuredwithtwoelectrodes,thesehalf-cellpotentialscancel.Ontheotherhand,ifyoutriedtousetheseelectrodestomeasureDCvoltages,thentheabovechemicalreactionwouldsaturateandfail.Fortunately,biopotentialsmeasuredproducedbymusclesandnervesareAConlyandhavenoDCcomponent.

Polarizableelectrodes,madefrommetalslikeplatinumgoldorsilver,haveahighelectricalimpedancebecauseelectronscannotfreelypasstheelectrode/tissueinterface.Chargecandevelopattheelectrode/tissueinterfaceeffectivelycreatingacapacitivebarrier.Displacementcurrentcanflowacrossthecapacitor,allowingtheACbiopotentialstobemeasuredbytheelectronics.Themetallicelectrodesalsoincludeahalf-cellpotential,butagain,thesepotentialswillcancel.

Ag++e-↔Ag

ThegraphicaldisplayofEKGversustimeisanexampleofaqualitativedataacquisitionsystem.Themeasurementofheartrateisquantitative.TheparametersofanEKGamplifierinclude:highinputimpedance(largerthan1M ),highgain,0.05to100Hzbandpassfilterandgoodcommonmoderejectionratio.TheEKGsignalisabout±1mV,soanoverallgainofabout2000willproducearangeof0to+3VonV3.ThedataflowgraphofthissystemissimilartoFigure10.27.ThisEKGamp(Figure10.32)beginswithapreampstagehavingagoodCMRR,highinputimpedance,andagainof10.Ifthesystemisbatteryoperated,thenitdoesnotneedathirdorgroundelectrode.Pin5oftheAD627istheanalogground,whichinthiscircuitisthe1.233Vreferencevoltage.TheAD627israil-to-rail.A0.05HzpassivehighpassfilteriscreatedbyR4andC4.Low-leakagecapacitorsforC1,C2,andC4arecriticalforeliminationofDCoffsetdrift.Apolypropyleneorpolystyrenewouldbeagoodchoice,butaC0Gceramicisacceptable.Theremaininggainisperformedwithanoninvertingamplifier(U2a).TheLPFisimplementedasa2-poleButterworthLPF.The153Hzcutoffwaschosenbecauseitisgreaterthan100Hzandcanbeimplementedwithstandardcomponents.IfthesignalV1saturates,youcanreducethegainofthepreampandincreasethegainoftheamp.

Figure10.32.Abattery-powerEKGinstrument.Program10.2showsthereal-timedataacquisitionand60Hzdigitalnotchfilter.Thedesignofdigitalfilterswillbepresentedinthesecondvolumeofthisbook.TheADCsamplingoccursinthebackgroundandthedataarepassedtotheforegroundusingaFIFOqueue.ThelargepulseintheEKG,originatingfromthecontractionoftheventricles,iscalledtheR-waveanditoccursonceaheartbeat.Program10.3showstheforegroundprocess,wheretherearefourcalculationstepsperformedontheEKGdata.Alowpassfilterfollowedbyahighpassfiltercaptureanarrowbandofinformationaround8Hz.Thesquarefunctioncalculatespowerandthe200mswidemovingaveragegivesanoutputveryspecificfortheR-wave.Hysteresisisimplementedwithtwothresholds.Aheartbeatiscounted( RCount++ )whenthemovingaveragegoesbelowthe LOW threshold,thenabovethe HIGH threshold.Thissoftwareusesacombinedfrequency-periodmethodtocalculateheartrate.ThealgorithmtomeasureheartratesearchesforR-wavesina5-secondinterval. Rfirst isthetime(in1/120secunits)ofthefirstR-waveand Rlast isthetime(alsoin1/120secunits)ofthelastR-wave.( RCount-1 )isthenumberofbeat-to-beatintervalsbetween Rfirst and Rlast .Thenumber7200istheconversionbetweenthesampleperiod(1/120sec)andoneminute.Forexampleat72BPM,therewillbe6R-wavesdetectedinthe5-secondinterval,making( Rcount-1) equalto5andthedifference Rlast-Rfirst willbe500.FormoreinformationonEKGsystems,seeWebster’sbookMedicalInstrumentation,publishedbyWiley1997,orPanandTompkins,“AReal-TimeQRSDetectionAlgorithm,”IEEETransactionsonBiomedicalEngineering,pp.230-236,March1985.

Warning:IfyouaregoingtobuildanEKG,pleasehaveatrainedengineerverifythesafetyofyourhardwareandsoftwarebeforeyouattachpeopletoyourmachine.

voidADC3_Handler(void)int16_tdata;

ADC_ISC_R=0x08;//acknowledgeADCsequence3

data=ADC_SSFIFO3_R-512;//twoscomplement

Fifo_Put(data);//passtoforeground

Program10.2.Real-timesamplingofEKG(seeprogram8.8),sampledat120Hz.

int16_tData;//ADCsample,-512to+512,10-bitsignedADCsample

int16_tx[50];//sampledEKG,120Hz

int16_ty[50];//lowpassfilter,120Hz

int16_tz[50];//highpassfilter,120Hz

int16_tw[50];//squaredresult,R-wavepower,120Hz

int16_tRwav;//movingaverageofR-wavepower,energy

uint16_tn=25;//25,26,…,49

uint16_tTrigger;

#defineHIGH100//triggerwhenoverthis

#defineLOW20//resetwhenunderthis

uint16_tRcount;//numberofR-waves

uint16_tRfirst;//timeoffirstR-wave

uint16_tRlast;//timeoflastR-wave

uint16_tHeartRate;//unitsbpm

voidmain(void)uint16_ttime;//units1/120sec

int16_tlpfSum=0,hpfSum=0,RwavSum=0;//usesProgram8.8

ADC_InitTimer0ATriggerSeq3(0,49,8332);//120Hzsampling

Fifo_Init();

Trigger=0;//lookingforHIGH

for(;;)

Rcount=0;

Rlast=0;

for(time=0;time<600;time++)//120Hz,every5second

while(Fifo_Get(&Data));//Getdatafrombackgroundthread

Plot(Data);//drawvoltageversustimeplot

n++;if(n==50)n=25;

x[n]=x[n-25]=Data;//newdata

lpfSum=lpfSum+x[n]-x[n-4];

y[n]=y[n-25]=lpfSum/4;//LowPassFilter

hpfSum=hpfSum+y[n]-y[n-10];

z[n]=z[n-25]=y[n]-hpfSum/10;//HighPassFilter

w[n]=w[n-25]=(z[n]*z[n])/10;//Powercalculation

RwavSum=RwavSum+w[n]-w[n-24];//200mswidemovingaverage

Rwav=RwavSum/24;

if(Trigger)

if(Rpow<LOW)

Trigger=0;//foundlow

else

if((Rpow>HIGH)&&((time-Rlast)>30))//maxHR=240bpm

Trigger=1;//foundhigh

if(Rcount)

Rlast=time;//marktimeoflastR-wave,units1/120sec

else

Rfirst=time;//marktimeoffirstR-wave

Rcount++;

if(Rcount>=2)

HeartRate=(7200*(int32_t)(Rcount-1))/(int32_t)(Rlast-Rfirst);

else

HeartRate=0;

Output(HeartRate);//displayresults

Program10.3.Measurementofheartrate.

Checkpoint10.4:WhatisthetheoreticalheartrateresolutionofthisapproachwhentheHRis60BPM?

Commonerror:TherearetworeasonsforEKGcircuitstofail.ThefirstpoorcontactbetweentheskinandelectrodecausingareductioninCMRR,andsecondisresistiveleakageincapacitorC4.Cleantheskinwell,usenewelectrodesandselectahighqualityC4.

Example10.3.Designasystemtomeasureposition.Therangeis0to2cmandtheresolutionatleast0.01cm.

Solution:ABournsSSHA20B20300potentiometercanbeusedtoconvertpositionintoresistance,seeFigure10.33.Inthisparticulartransducertheresistancebetweenpins1and3isfixedat20kΩ,whiletheresistancebetweenpins2and3(Rout)variesfrom0to20kΩ.

Rout=5•x

wheretheunitsofRoutandxareink andcmrespectively.TointerfacethistransducertothemicrocontrollerwedrivethepotentiometerwithastableDCvoltageusingaprecisionvoltagereference.Ifweweretodrivethecircuitwiththe+3.3Vpower,thenanynoiserippleinthepowerlinewouldcoupledirectlyintothemeasurement.Rather,weimplementthe3.00Vwithaprecisionreference.TheADCproducesadigitaloutputdependentonitsanaloginput.Thepositionresolutionis2cm/1024,whichisabout0.002cm.

Figure10.33.PotentiometerinterfaceusinganADC.

Anotherapproachtointerfacethistransducertothemicrocontrollerwouldbetouseastablemultivibrator.Theperiodofa555timeris0.693•CT•(RA+2RB).Inourcircuit,RAisRout,RBis1kΩ,andCTis2.2µF.GivenafixedRB,CT,theperiodofthesquarewave,Pout,isalinearfunctionofRout.Ourmicrocontrollershavearichsetofmechanismstomeasurefrequency,pulsewidthorperiod.TochangetheslopeandoffsetoftheconversionbetweenRoutandPout,thevaluesofRBandCTcanbeadjusted.Eventhoughtheperioddoesnotincludezero,theprecisionofthismeasurementisover3000alternativesormorethan11bits.Theprecisioncanbeimprovedbyincreasingthecapacitor,CT,ordecreasingtheperiodonthemeasurementclock.Thepositionresolutionis2cm/3049,whichisabout0.001cm.Theactualresolutionwillprobablybedominatedbynoiseandnotperiodmeasurementresolution.

Figure10.34.Potentiometerinterfaceusinginputcapture.

Example10.4.Designasystemthatcaninputandoutputsound.

Solution:TheelectretmicrophonewasdescribedpreviouslyinFigure10.11asalow-cost,small-sizetransducercapableofconvertingsoundintovoltage.ManyelectretdatasheetssuggestanR1of2k ,butsignal-to-noiseratiocanbeimprovedbyusinga10k resistor.BecausetheoutputofaHPFwouldnormallyincludepositiveandnegativevoltages,wewillneedawaytooffsetthecircuitsoallvoltagesexistfrom0to+3.3V,allowingtheuseofasinglesupplyandrail-to-railopamps.R2andR3provideanoffsetfortheHPF,sothesignalV2willbethesoundsignalplus1.65V.TheeffectiveimpedancefromV2togroundis11k ,sotheHPFcutoffis1/(2 *1 F*11k )=14Hz.Thegainofthesystemis1+R6/R5,whichwillbe101.ThecapacitorC2willmakethesignalV3betheamplifiedsoundplus1.65V.ThegainisselectedsotheV3signalis1.65±1Vforthesoundswewishtorecord.ThecapacitorC3providesalittlelowpassfiltering,causingtheamplifiergaintodroptooneforfrequenciesabove1/(2 *220pF*100k )=7.2kHz.AbetterLPFwouldbetoaddanactiveLPF.TheLPFwouldneeda1.65VoffsetliketheoneinFigure10.35.Ifwewishtoprocesssoundwithfrequencycomponentsfrom100to5kHz,thenweshouldsampleatorabove10kHz.Ifwesampledsoundwitha12-bitADC,weshouldselecta12-bitDACtooutputthesound.Wecouldimprovesignaltonoisebyreplacingthe+3.3VconnectedtoR1andR2inFigure10.35withaLM4041adjustablereferenceandcreatealownoise3.0Vvoltage.

Figure10.35.Anelectretmicrophonecanbeusedtorecordsound(PD3onTM4C123).

TheLM4041CILPisashuntreferenceusedtomaketheanalogreferencerequiredbytheMAX535312-bitDAC.ThisDACwaspreviouslyinterfacedinExample7.2.TheMC34119audioamp,firstshowninFigure9.2,canbeusedtoamplifytheDACoutputprovidingthecurrentneededtodriveatypical8 speaker.Thegainoftheaudioamplifieris2*R11/R10,whichforthiscircuitwillbeone.Thismeansa2-Vpeak-to-peaksignaloutoftheDACwilltranslatetoa2-Vpeak-to-peaksignalonthespeaker.ThemaximumpowerthattheMC34119candelivertothespeakeris250mW,sothesoftwareshouldlimitthesoundsignalbelow1.4Vrmswhendrivingan8 speaker.Thequalityofsoundcanbeincreasedbyselectingabetterspeakerandplacingthespeakerintoanenclosure.Formoreinformationonhowtodesignaspeakerbox,performawebsearchon“speakerenclosure”.

SoftwareinProgram7.2canbeusedtointerfacetheMAX535312-bitDAC.Program10.4performsthesoundinputandoutput.Thesamplingrateis10kHz.TheADCcodewaspreviouslyshownasProgram8.8.

voidADC0Seq3_Handler(void)int32_tdata;

ADC0_ISC_R=0x08;//acknowledgeADCsequence3

data=ADC0_SSFIFO3_R-2048;//sound,-2048to+2047

//process,filter,recordetc.

DAC_Out(data);//output,Program7.2

voidmain(void)PLL_Init();//80MHzTM4C123

ADC0_InitTimer0ATriggerSeq3PD3(7999);//sampleat10kHz

DAC_Init(2048);//Program7.2

while(1)

WaitForInterrupt();

Program10.4.Real-timesoundoutputinput/output.

Figure10.36.A12-bitDACandanaudioamplifierallowthemicrocontrollertooutputsound.

10.7.Exercises10.1Foreachtermgiveadefinitionin16wordsorless.

a)Measurandb)Transducerc)Hysteresis

d)Saturatione)Breakdownf)Bangbang

g)Deadzoneh)Phantombiasi)Prevalence

j)Triplepointk)Positivepredictivevaluel)Negativepredictivevalue

m)Impedanceloadingn)Crestfactor

10.2Foreachtransducerparametergiveadefinitionin16wordsorless.

a)Accuracyb)Linearityc)Sensitivityd)Specificitye)Inputimpedance

f)Driftg)Firstorderh)Slewratei)Secondorderj)Frequencyresponse

10.3Foreachinstrumentparametergiveadefinitionin16wordsorless.

a)Accuracyb)Maximumerror

c)Resolutiond)Precision

e)Reproducibilityf)Signaltonoiseratio

10.4Foreachtransducergiveitsmeasurand.

a)Thermistorb)LVDT

c)Electretd)Straingauge

e)Thermocouplef)Ultrasoniccrystal

g)Shaftencoderh)RTD

i)ADXL202j)Ag-AgClelectrode

10.5Foreachconceptgiveadefinitionin16wordsorless.

a)NyquistTheoryb)Aliasing

c)Voltagequantizationd)Timequantization

e)Timejitterf)Samplingrate

10.6Fortypeofnoisegiveadefinitionin16wordsorless.Alsogiveonewaytoreducetheeffectofthisnoise

a)Whitenoiseb)1/fnoise

c)Magneticfieldnoised)Electricfieldnoise

10.7Giveashortanswerin16wordsorless.

a)Howdowechoosethesamplingrate?

b)Howdowechoosethegainofouramplifier?

c)WhendoweneedaS/Hmodule?

d)Whendoesadataacquisitionsystemhavealiasing?

10.8Thefollowinginput/outputdatawascollectedforadistancetransducer(0,5),(1,7),(2,8),(3,11),(4,14),wherethefirstnumberisdistanceincm,andthesecondnumberisresistanceink .Calculateaveragelinearityofreading(percent)andaveragelinearityoffullscale(percent).Calculatethesensitivityofthistransducer.Isthistransducermonotonicornonmonotonic?

10.9Thefollowinginput/outputdatawascollectedforadistancetransducer(0,10),(1,12),(2,14),(3,12),(4,10),wherethefirstnumberisdistanceincm,andthesecondnumberisresistanceink .Calculateaveragelinearityofreading(percent)andaveragelinearityoffullscale(percent).Calculatethesensitivityofthistransducer.Isthistransducermonotonicornonmonotonic?

10.9bExplainhowanultrasonicdistancetransduceristemperaturesensitive.

10.10Letx(n)beasampleddatarepresentingtheangularpositionofashaft.Itisstoredasa0.01radiandecimalfixed-pointnumber.E.g.x(n)rangesfrom0to628.Thisdataissampledevery10ms.Deriveequationsforangularvelocityandangularacceleration.

10.11Considerthenonlineartransducery=x2.Therangeofxis1≤x≤2.Thus,therangeofyis1≤y≤4.Letthedesiredresolutionbe∆x=0.01.nx=rx/∆x=100alternativesorabout7bits.HowmanyADCbitswillbeneeded?

10.12Considerthelineartransducery=2x+1.Therangeofxis0≤x≤1.Thus,therangeofyis1≤y≤3.Letthedesiredresolutionbe∆x=0.01.nx=rx/∆x=100alternativesorabout7bits.HowmanyADCbitswillbeneeded?

10.13Whatistheresolutionofa13-bitADCindBFS?

10.14Howmuchwhitenoiseina10M resistorifthesystembandwidthis100kHz?

10.15Howmuchwhitenoiseina100M resistorifthesystembandwidthis10MHz?

10.16Howcanyouexperimentallydifferentiatebetweenwhitenoiseandpinknoise?

10.17Whatdoyouseeifyoutouchthemeasurementprobeofanoscilloscope?Explain.

D10.18Designacomputerbaseddataacquisitionsystemthatmeasurespressure.ThepressuretransducerisbuiltwithfourresistivestraingaugesplacedinaDCbridge.Whenthepressureiszero,eachgaugehasa120Ωresistancemakingthebridgeoutput,y,zero.Whenpressureisappliedtothetransducertwogaugesarecompressed(whichlowerstheirresistance)andtwoareexpanded(whichincreasestheirresistance.)Atfullscalepressure(p=±100dynes/cm2),eachresistorchangesby±2 .Thetransducer/bridgeoutputimpedanceistherefore120Ω.Youmayassumethetransducerislinear.Thedesiredpressureresolutionis1dyne/cm2.Thefrequenciesofinterestare0to100Hz,andthe2poleButterworthanaloglowpassfilterwillhaveacutoff(gain=0.707)frequencyof100Hz(youwilldesignitinb).Intermsofchoosingasamplingrate,youmayassumetheLPFremovesallsignalsabove100Hz.

D10.19Designatemperatureacquisitionsystemusingatype-Tthermocouple.Therangeis1≤T≤200˚C.Thedesiredresolutionis∆T=1˚C.ThefrequenciesofinterestareDCto5Hz.Showtheanaloginterfaceanddataacquisitionsoftware.

D10.20Oneofthedifficultiesinmassproducingatransduceristocreatemultiplecopiesthatcanbeusedinterchangeably.Inthispressuremeasurementdevice,theR3nodeisthetransducerthatissensitivetopressure.AssumeR3equals100 (1+mP+ T),where100misthepressuresensitivity,andpistheinputpressure.TheR4nodeisadummydevicebutisnotaffectedbypressure.R4equals100 (1+ T).BothR1andR4haveequaltemperaturesensitivity, R3/ T= R4/ T=100 .AssumetheINAgainis10.TheresistorsR1andR2aretheotherpartofthebridge.

a)AssumeR1andR2arenottemperaturedependentShowthattheoutputismuchlesssensitivetotemperaturethanifR4wereafixedvalue.

b)IfR2isreplacedwithapotentiometerwecanadjusttheoffset.DeriveanequationofshowingtheoutputvoltageasafunctionofpandR2.

c)IfR1isreplacedwithapotentiometerwecanadjustthesensitivity.DeriveanequationofshowingtheoutputvoltageasafunctionofpandR1.

D10.21Thisproblemdealswiththeclassificationandreductionofnoise.

a)Describeasingleexperimentalprocedure(measurement)whichcouldidentify(ordifferentiate)thetype(s)ofnoiseexistingonthecircuit.

b)Foreachofthesethreetypesofnoise(whitenoise,60Hznoise,or1/fnoise),giveatypicaloutcomeoftheexperimentalprocedure.

c)Giveoneapproach(otherthananalogordigitalfiltering)whichwillreducewhitenoise.

d)Giveoneapproach(otherthananalogordigitalfiltering)whichwillreduce60Hznoise.

e)Giveoneapproach(otherthananalogordigitalfiltering)whichwillreduce1/fnoise.

D10.22Atemperaturetransducerhasthefollowingrelationship

R=200+10TwhereRisinΩandTisin˚C.

Theproblemspecificationsarerangeis30≤T≤50˚C.Theresolutionshouldbe0.01˚C.Thefrequenciesofinterestare0to100Hz.Thetransducerdissipationconstantis20mW/˚C.TheADCrangeis0to+3voltsandthesamplingrateis1000Hz.

a)HowmanyADCbitsarerequired?

b)Whatisthemaximumallowablenoiseattheampoutput?

c)Designtheanalogamplifier/filter.

D10.23Designawinddirectionmeasurementinstrument.Youaregivenatransducerthathasaresistancethatislinearlyrelatedtothewinddirection.Asthewinddirectionvariesfrom0to360degrees,thetransducerresistancevariesfrom0to1000Ω.Thefrequenciesofinterestare0to0.5Hz,andthesamplingratewillbe1Hz.

a)ShowtheanaloginterfacebetweenthetransducerandtheADC.Onlythe+3.3Vsupplycanbeused.Showhowtheanalogcomponentsarepowered.Givechipnumbersbutnotpinnumbers.Specifythetypeandtoleranceofresistorsandcapacitors.

b)Writetheinitializationandfunctionthatmeasuresthewinddirectionandreturnsa16-bitunsignedresultwithunitsofdegrees.I.e.,thevaluevariesfrom0to359.Youdonothavetowritesoftwarethatsamplesat1Hz,simplyafunctionthatmeasureswinddirectiononce.

10.8.LabAssignmentsLab10.1.ThisexperimentwilluseathermistorandtheADCtoconstructadigitalthermometer.Thetemperaturerangeshouldbe20to40˚C.Ifthecurrenttemperatureisabovetheupperlimitinthespecifiedrange,aredLEDshouldbeturnedon.Youcantestthisfeaturebyshortingthethermistorleadstogether(zeroresistance.)Ifitisbelowthelowerlimitofthespecifiedrange,ayellowLEDshouldbeturnedon.Similarly,youcantestthisfeaturebydisconnectingonewireofthethermistor(infiniteresistance.)Otherwise,agreenLEDwillstayonindicatingthetemperatureiswithinthespecifiedrange.Thetemperaturemeasurementswillbedisplayedasfixed-pointnumbersonanLCDorOLED.Designyoursystemwiththebestpossibleresolution.Thetemperaturecomponentis0to1Hz.Experimentallyverifynoiselevel,time-constant,andaccuracyofyoursystem.

Lab10.2.Theobjectiveofthislabistobuildadigitalsoundrecorderforhumanspeech.YouwillfirstneedtointerfaceanexternalRAMtostorethedata.Next,youwillneedtodesignananalogcircuitthatinterfacesamicrophonetotheADCofthemicrocontroller.Investigatethefrequencycomponentsofhumanspeechanddesignyoursystemtopassthesefrequencies.Youwillalsoneedamechanismtoplaybacktherecordedsound,sodesignanaudioamplifierthatinterfacesaDACtoaspeaker.Yourhumaninterfaceshouldincludeabuttonstotriggersoundrecording,stoprecording,startplayback,andstopplayback.

Lab10.3.Designathermocouple-basedthermometerwitharangeof0to50˚C.Youcanuseanicebucketforthereference,oryoucoulddesignthethermistor-basedthermometerofLab10.1anduseittocompensateforthecoldjunctionofthethermocouple.Designyoursystemwiththebestpossibleresolution.Thetemperaturemeasurementswillbedisplayedasfixed-pointnumbersonanLCDorOLED.Thetemperaturecomponentis0to1Hz.Experimentallyverifynoiselevel,time-constant,andaccuracyofyoursystem.

Lab10.4.Designadigitalscaleusingastraingaugeforcetransducer.Selecttherangeofthescaletomatchthelinearrangeofyourforcetransducer.Youcanbuildaforcetransducerusingaslidepotandaspring.Designyoursystemwiththebestpossibleresolution.Theforcemeasurementswillbedisplayedasfixed-pointnumbersonanLCDorOLED.Experimentallyverifynoiselevel,time-constant,andaccuracyofyoursystem.

Lab10.5.Designtwodigitalpositionmeasurementsystemsusingaslidepotentiometerasthetransducer.Selecttherangeofthescaletomatchthelinearrangeofyourtransducer.Thepositionmeasurementswillbedisplayedasfixed-pointnumbersonanLCD.ThefirstsystemwillusetheADC,andthesecondsystemwilluseanastablemultivibrator(TLC555)andinputcapture.Designyoursystemswiththebestpossibleresolution.Experimentallyverifynoiselevel,time-constant,andaccuracyofbothsystems.

Lab10.6.Designanautorangingvoltmeter.Thethreerangesare0to2V,0to0.2V,and0to0.02V.Thehardware/softwaresystemautomaticallyadjuststherangeprovidingthebestpossiblemeasurementresolution.DisplayboththenumericalandgraphicalresultsonanLCDorOLEDdisplay.WriteagraphicaldevicedriverfortheLCDorOLEDanduseittographthetime-varyingvoltageinrealtime.Experimentallydeterminethemeasurementresolutionforeachrangeandcompareittotheexpectedtheoreticalresolution.Analyzethevarioussourcesofmeasurementerrorinyoursystem.

Lab10.7.DesignadataacquisitionsystemusinganIRdistancesensor.SeeFigure10.8.Designasystemforstaticmeasurementoftiltandadynamicmeasurementofmotion.

Lab10.8.Designadataacquisitionsystemusinganaccelerometer.Ifthesensorisnotmoving,itresponsetotheearth’sgravityandcanbeusedtodeterminethetiltangle.Ifthesensorismovingitresponsestobothgravityandtheaccelerationofthesensor.Designasystemforstaticmeasurementoftiltandadynamicmeasurementofmotion.

Lab10.9.Designadataacquisitionsystemusingaforcesensingresistor(FSR).Thesensorresistancedependsonthepressurebeingappliedtothesensingarea.WhennopressureisappliedtotheFSRitsresistancewillbelargerthan1MΩ.Designastaticmeasurementofpressureandtestitwithstandardweights.

11.IntroductiontoCommunicationSystemsChapter11objectivesareto:•Introducebasicconceptsofnetworks

•Presentmaster/slave,ringandmultidropnetworksbasedonUARTs

•Introducetheconceptofwirelesscommunications

Thegoalofthischapteristoprovideabriefintroductiontocommunicationsystems.Communicationtheoryisarichlydevelopeddiscipline,andmuchofthecommunicationtheoryisbeyondthescopeofthisbook.Nevertheless,thetrendinembeddedsystemsistoemploymultipleintelligentdevices,thereforetheinterconnectionwillbeastrategicfactorintheperformanceofthesystem.Thesedeviceswillbedevelopedbydifferentmanufacturers,thustheinterconnectionnetworkmustbeflexible,robust,andreliable.Consequently,thischapterfocusesonimplementingcommunicationsystemsappropriateforembeddedsystems.Thecomponentsofanembeddedsystemtypicallycombinedtosolveacommonobjective,thusthenodesonthecommunicationnetworkwillcooperatetowardsthatsharedgoal.Inparticular,requirementsofanembeddedsystem,ingeneral,involverelativelylowbandwidth,staticconfiguration,andalowprobabilityofcorrupteddata.Wewillintroducetheinternetofthings(IoT)bydesigningatwosystemsthatconnecttotheinternet,onewiredandonewireless.

11.1.FundamentalsIntheserialinterfacingchapter,weconsideredthehardwareinterfacesbetweencomputers.Inthischapter,wewillbuildonthoseideasandintroducetheconceptsofnetworksbyinvestigatingacoupleofsimplenetworks.Acommunicationnetworkincludesboththephysicalchannel(hardware)andthelogicalprocedures(software)thatallowusersorsoftwareprocessestocommunicatewitheachother.Thenetworkprovidesthetransferofinformationaswellasthemechanismsforprocesssynchronization.ItisconvenienttovisualizethenetworkinahierarchicalfashionasshowninFigure11.1.

Figure11.1.Alayeredapproachtocommunicationsystems.TheInternationalStandardsOrganization(ISO)definesa7-layermodelcalledtheOpenSystemsInterconnection(OSI),describedinChapter9ofVolume3.Figure11.1showsasimple3-layermodel.

Atthelowestlevel,framesaretransferredbetweenI/Oportsofthetwo(ormore)computersalongthephysicallinkorhardwarechannel.Atthenextlogicallevel,theoperatingsystem(OS)ofonecomputersendsmessagesorpacketstotheOSontheothercomputer.Themessageprotocolwillspecifythetypesandformatsofthesemessages.Typically,errordetectionandcorrectionishandledatthislevel.Messagestypicallycontainfourfields:

1)Addressinformationfield

physicaladdressspecifyingthedestination/sourcecomputers

logicaladdressspecifyingthedestination/sourceprocesses

2)Synchronizationorhandshakefield

Physicalsynchronizationlikesharedclock,startandstopbits

OSsynchronizationlikerequestconnectionoracknowledge

Processsynchronizationlikesemaphores

3)Datafield

ASCIItext(raworcompressed)

Binary(raworcompressed)

4)Errordetectionandcorrectionfield

Verticalandhorizontalparity

Checksum

Blockcorrectioncodes(BCC)

Observation:Communicationsystemsoftenspecifybandwidthintotalbits/sec,buttheimportantparameteristhedatatransferrate.

Observation:Oftenthebandwidthislimitedbythesoftwareandnotthehardwarechannel.

Atthehighestlevel,weconsidercommunicationbetweenusersortasks.Aprocessisacompletesoftwaretaskthathasawell-definedgoal.Forexample,whenafileistobeprintedonanetworkprinter,theOScreatesaprocessthat

1)establishesconnectionwiththeremoteprinter,

2)readsblocksfromtheharddiskdriveandsendsthedatatotheprinter

mayhavetomanipulategraphics/colorsforthespecificprinter

networkdriverwillbreakthedataintomessagepackets,

3)disconnecttheprinter.

Manyembeddedsystemsrequirethecommunicationofcommandordatainformationtoothermodulesateitheranearoraremotelocation.Becausethefocusofthisbookisembeddedsystems,wewilllimitourdiscussionwithcommunicationwithdeviceswithinthesameroom.Afullduplexchannelallowsdatatotransferinbothdirectionsatthesametime.Inahalfduplexsystem,datacantransferinbothdirectionsbutonlyinonedirectionatatime.Halfduplexispopularbecauseitislessexpensive(2wires)andallowstheadditionofmoredevicesonthechannelwithoutchangetotheexistingnodes.

Information,suchastext,sound,picturesandmovies,canbeencodedindigitalformandtransmittedacrossachannel,asshowninFigure11.2.Thechannelwillhaveamaximuminformationperseconditcantransmit,calledchannelcapacity.Inordertoimprovetheeffectivebandwidthmanycommunicationsystemswillcompresstheinformationatthesource,transmitthecompressedversion,andthendecompressthedataatthedestination.Compressionessentiallyremovesredundantinformationinsuchawaythatthedecompresseddataisidentical(lossless)orslightlyalteredbutsimilarenough(lossy).Forexample,a400pixels/inchphotocompressedusingtheJPEGalgorithmwillbe5to30timessmallerthantheoriginal.Aguidedmediumfocusesthetransmissionenergyintoawell-definedpath,suchascurrentflowingalongcopperwireofatwistedpaircable,orlighttravelingalongafiberopticcable.Conversely,anunguidedmediumhasnofocus,andtheenergyfielddiffusesasinpropagates,suchassoundorEMfieldsinairorwater.Ingeneral,forcommunicationtooccur,thetransmittermustencodeinformationasenergy,thechannelmustallowtheenergytomovefromtransmittertoreceiver,andthereceivermustdecodetheenergybackintotheinformation,seeFigure11.2.Inananalogcommunicationsystem,energycanvarycontinuouslyinamplitudeandtime.Adigitalcommunicationsignalexistsatafinitenumberofenergylevelsfordiscreteamountsoftime.Alongtheway,theenergymaybelostduetoattenuation.Forexample,asimpleV=I*Rvoltagedropisinactualityalossofenergyaselectricalenergyconvertedtothermalenergy.AsecondexampleofattenuationisanRFcablesplitter.Foreachsplitter,therewillbe50%attenuation,wherehalftheenergygoesleftandtheotherhalfgoesrightthroughthesplitter.Unguidedmediawillhaveattenuationastheenergypropagatesinmultipledirections.Attenuationcausesthereceivedenergytobelowerinamplitudethanthetransmittedenergy.Asecondproblemisdistortion.Thetransfergainandphaseinthechannelmaybefunctionoffrequency,time,oramplitude.Distortioncausesthereceivedenergytobedifferentshapethanthetransmittedenergy.Athirdproblemisnoise.Thenoiseenergyiscombinedwiththeinformationenergytocreateanewsignal.WhitenoiseandEMfieldnoisewerediscussedinChapter10.Crosstalkisaproblemwhereenergyinonewirecausesnoiseinanadjacentwire.Wequantifynoisewithsignal-to-noiseratio(SNR),whichistheratiooftheinformationsignalpowertonoisepower.

Checkpoint11.1:WhydowemeasureSNRaspowerandnotvoltage?

Figure11.2.Informationisencodedasenergy,buterrorscanoccurduringtransmission.Wecanmakeaninterestinganalogybetweentimeandspace.AcommunicationsystemallowsustransferinformationfrompositionAtopositionB.AdigitalstoragesystemallowsustransferinformationfromtimeAtotimeB.Manyoftheconcepts(encoding/decodinginformationasenergy,noise,errordetection/correction,security,andcompression)applyinananalogousmannertobothtypesofsystems.

Checkpoint11.2:Wemeasuretheperformanceofacommunicationsystemasbandwidthinbits/sec.Whatistheanalogousperformancemeasureofadigitalstoragesystem?

Errorscanoccurwhencommunicatingthroughachannelwithattenuation,distortionandaddednoise.Ifthereceiverdetectsanerror,itcansendanegativeacknowledgementsothetransmitterwillretransmitthedata.TheCANandZigBeeprotocolshandlethisdetection-retransmissionprocessautomatically.NetworksbasedontheUARTportwillneedtodefineandimplementerrordetection.I.e.,wecanaddanadditionalbittotheserialframeforthepurposeofdetectingerrors.Withevenparity,thesumofthedatabitsplustheparitybitwillbeanevennumber.TheframingerrorintheUARTcanalsobeusedtosignifythedatamaybecorrupted.TheCANnetworksendsalongitudinalredundancycheck,LRC,whichistheexclusiveorofthebytesintheframe.TheZigBeenetworkaddsachecksum,whichisthesumofallthedata.

Therearemanywaystoimprovetransmissioninthechannel,reducingtheprobabilityoferrors.Thefirstdesignchoiceistheselectionoftheinterfacedriver.ForexampleRS422islesslikelytoexhibiterrorsthanRS232.Ofcoursehavingadriverwillbemorereliablethannothavingadriver.Thesecondconsiderationisthecable.PropershieldingcanimproveSNR.Forexample,Cat6Ethernetcableshaveaseparatorbetweenthefourpairsoftwistedwire,whichreducethecrosstalkbetweenlinesascomparedtoCat5ecable.Ifwecanseparateoreliminatethesourceofaddednoise,theSNRwillimprove.Reducingthedistanceandreducingthebandwidthoftenwillreducetheprobabilityoferror.Ifwemusttransmitlongdistances,wecanusearepeater,whichacceptstheinputandretransmitsthedataagain.

Errorcorrectingcodesarebeyondthescopeofthisbook.However,wecanpresenttwosimpleerrorcorrectingcodes.Thefirsterrorcorrectingcodeinvolvessendingthreecopiesofeachdata.Thereceiverwillcomparethethreeversionsreceivedandmajorityvotewilldecidewhichvaluetouse.AseconderrorcorrectingcodeusesbothparityandLRC.Forexample,assumewewishedtosendthemessage“Ciao”.EncodedasASCIIcharactersthedataare0x43,0x69,0x61,and0x6F.Thefirststepistodisplaythebinarydatain2-D.

Byte0 Byte1 Byte2 Byte3

Bit7 0 0 0 0

Bit6 1 1 1 1

Bit5 0 1 1 1

Bit4 0 0 0 0

Bit3 0 1 0 1

Bit2 0 0 0 1

Bit1 1 0 0 1

Bit0 1 1 1 1

ThesecondstepistoaddanevenparitytoeachbyteandaddaLRCattheend.NoticethattheevenparityistheexclusiveORofeachbitintheverticalcolumnandtheLRCistheexclusiveORofeachbitinthehorizontalrow.TheparitybitfortheLRC(ortheLRCbitfortheparity)willbetheexclusiveORofallthedatabits.

Byte0 Byte1 Byte2 Byte3 LRC

Parity 1 0 1 0 0

Bit7 0 0 0 0 0

Bit6 1 1 1 1 0

Bit5 0 1 1 1 1

Bit4 0 0 0 0 0

Bit3 0 1 0 1 0

Bit2 0 0 0 1 1

Bit1 1 0 0 1 0

Bit0 1 1 1 1 0

Now,ifanyonebitinthis9-rowby5-columnmatrixisflipped,wecandeterminewhichbyteisinerrorbytheparityandwhichbitisinerrorbytheLRC.Ratherthanaskingforretransmission,wesimplycorrecttheerror.Theseareverysimpleerrorcorrectingcodes,buttheyillustratethatwecansendmorebitsthantheminimumandusethoseextrabitsinacreativewaytoeitherdetectorcorrecterrors.

11.2.CommunicationSystemsBasedontheUARTsInthissection,wewillpresentthreecommunicationsystemsthatutilizetheUARTport.Ifthedistancesareshort,halfduplexcanbeimplementedwithsimpleopencollectororopen-draindigital-levellogic.Opendrainlogichastwooutputstates:lowandoff.Intheoffstatetheoutputisnotdrivenhighorlow,itjustfloats.The10k pull-upresistorwillpassivelymakethesignalhighifnoneoftheopendrainoutputsarelow.ThemicrocontrollercanmakeitsTxDserialoutputsbeopendrain(ODEontheLM3S/LM4F/TM4C).Thismodeallowsahalfduplexnetworktobecreatedwithoutanyexternallogic(althoughpull-upresistorsareoftenused).Threefactorswilllimittheimplementationofthissimplehalfduplexnetwork:1)thenumbernodesonthenetwork,2)thedistancebetweennodes;and3)presenceofcorruptingnoise.InthesesituationsahalfduplexRS485driverchipliketheSP483madebySipexorMaximcanbeused.

Thefirstcommunicationsystemismaster-slaveconfiguration,wherethemastertransmitoutputisconnectedtoallslavereceiveinputs,asshowninFigure11.3.Thisprovidesforbroadcastofcommandsfromthemaster.Allslavetransmitoutputsareconnectedtogetherusingwire-oropendrainlogic,allowingfortheslavestorespondoneatatime.TheODE(OpenDrainEnable)intheslavesshouldbesettoactivateopendrainmodeontransmitters.Thelow-leveldevicedriverforthiscommunicationsystemisidenticaltotheUARTdriverdevelopedinProgram5.9.WhenthemasterperformsUARToutputitisbroadcasttoalltheslaves.Therecanbenoconflictwhenthemastertransmits,becauseasingleoutputisconnectedtomultipleinputs.Whenaslavereceivesinput,itknowsitisacommandfromthemaster.Intheotherdirection,however,apotentialproblemexistsbecausemultipleslavetransmittersareconnectedtothesamewire.Iftheslavesonlytransmitafterspecificallybeingtriggeredbythemaster,nocollisionscanoccur.

Figure11.3.Amaster-slavenetworkimplementedwithmultiplemicrocontrollers.

Checkpoint11.3:WhatvoltagelevelwillthemasterRxDobserveiftwoslavessimultaneouslytransmit,onemakingitalogichighandtheotheralogiclow?

Thenextcommunicationsystemisaringnetwork.Thisisthesimplestdistributedsystemtodesign,becauseitcanbeconstructedusingstandardserialports.Infact,wecanbuildaringnetworksimplybychainingthetransmitandreceivelinestogetherinacircle,asshowninFigure11.4.BuildingaringnetworkisamatterassimpleassolderingaRS232cableinacirclewithoneDB9connectorforeachnode.Messageswillincludesourceaddress,destinationaddressandinformation.IfcomputerAwishestosendinformationtocomputerC,itsendsthemessagetoB.ThesoftwareincomputerBreceivesthemessage,noticesitisnotforitself,anditresendsthemessagetoC.ThesoftwareincomputerCreceivesthemessage,noticesitisforitself,anditkeepsthemessage.Althoughsimpletobuild,thissystemhasslowperformance(responsetimeandbandwidth),anditisdifficulttoadd/subtractnodes.

Figure11.4.Aringnetworkimplementedwiththreemicrocontrollers.Checkpoint11.4:Assumetheringnetworkhas10nodes,thebaudrateis100,000bits/sec,andthereare10bits/frame.Whatisaveragetimeittakestosenda10bytemessagefromonecomputertoanother?

Thethirdcommunicationsystemisaverycommonapproachtodistributedembeddedsystems,calledmulti-drop,asshowninFigure11.5.Totransmitabytetotheothercomputers,thesoftwareactivatestheSP483driverandoutputstheframe.Sinceitishalfduplex,theframeisalsosenttothereceiverofthecomputerthatsentit.Thisechocanbecheckedtoseeifacollisionoccurred(twodevicessimultaneouslyoutputting.)Ifmorethantwocomputersexistonthenetwork,weusuallysendaddressinformationfirst,sothattheproperdevicereceivesthedata.Manycollisionscanbeavoidedlookingtoseeifthereceiverisactivebeforetransmitting.

Figure11.5.AmultidropnetworkiscreatedusingahalfduplexserialchannelimplementedwithopendrainlogicorwithRS485drivers.Checkpoint11.5:Howcanthetransmitterdetectacollisionhadcorrupteditsoutput?

Checkpoint11.6:Howcanthereceiverdetectacollisionhadcorrupteditsinput?

Therearemanywaystocheckfortransmissionerrors.Youcouldusealongitudinalredundancycheck(LRC)orhorizontalevenparity.Theerrorcheckbyteissimplytheexclusive-ORofallthemessagebytes(excepttheLRCitself).Thereceiveralsoperformsanexclusive-ORonthemessageaswellastheerrorcheckbyte.Theresultwillequalzeroiftheblockhasbeentransmittedsuccessfully.Anotherpopularmethodischecksum,whichissimplythemodulo256(8-bit)ormodulo65536(16-bit)sumofthedatapacket.Inaddition,eachbytecouldhave(butdoesn’thaveto)includeevenparity.

Therearetwomechanismsthatallowthetransmissionofvariableamountsofdata.Someprotocolsusestart(STX=0x02)andstop(ETX=0x03)characterstosurroundavariableamountofdata.Thedisadvantageofthis“terminationcode”methodisthatbinarydatacannotbesentbecauseadatabytemightmatchtheterminationcharacter(ETX).Therefore,thisprotocolisappropriateforsendingASCIIcharacters.Anotherpossibilityistouseabytecounttospecifythelengthofamessage.Manyprotocolsuseabytecount.TheZigBeeframes,forexample,haveabytecountineachframe.

11.3.WirelessCommunicationThedetailsofhowwirelesscommunicationoperatesarebeyondthescopeofthisbook.Nevertheless,theinterfacingtechniquespresentedinthisbookaresufficienttoimplementwirelesscommunicationbyselectingawirelessmoduleandinterfacingittothemicrocontroller.Ingeneral,oneconsidersbandwidth,distance,topologyandsecuritywhendesigningawirelesslink.Bandwidthisthefundamentalperformancemeasureforacommunicationsystem.Inthisbook,wedefinebandwidthofthesystemastheinformationtransferrate.However,whencharacterizingthephysicalchannel,bandwidthcanhavemanydefinitions.Ingeneral,thebandwidthofachannelistherangeoffrequenciespassedbythechannel(CommunicationNetworksbyLeon-Garcia).LetGx(f)bethegainversusfrequencyofthechannel.WhenconsideringEMfieldstransmittedacrossspace,wecandefineabsolutebandwidthasthefrequencyintervalthatcontainsallofthesignal’sfrequencies.Half-powerbandwidthistheintervalbetweenfrequenciesatwhichGx(f)hasdroppedtohalfpower(‑3dB).Letfcbethecarrierfrequency,andPxbethetotalsignalpoweroverallfrequencies.TheequivalentrectangularbandwidthisPx/Gx(fc).Thenull-to-nullbandwidthisthefrequencyintervalbetweenfirsttwonullsofGx(f).TheFCCdefinesfractionalpowercontainmentbandwidthasthebandwidthwith0.5%ofsignalpoweraboveandbelowtheband.TheboundedpowerspectraldensityisthebanddefinedsothateverywhereoutsideGx(f)musthavefallentoagivenlevel.Thepurposeofthislististodemonstratetothereaderthat,whenquotingperformancedata,wemustgivebothdefinitionoftheparameterandthedata.IfweknowthechannelbandwidthWinHzandtheSNR,wecanusetheShannon–HartleyChannelCapacityTheoremtoestimatethemaximumdatatransferrateCinbits/s:

Forexample,consideratelephonelinewithabandwidthWof3.4kHzandSNRof38dB.ThedimensionlessSNR=10(38/10)=6310.UsingtheChannelCapacityTheorem,wecalculateC=3.4kHz*log2(1+6310)=43kbits/s.

OneofthesimplestmoduleswecanuseforwirelessembeddedsystemsistheChipconCC2500,whichisalow-power2.4-GHzRFtransceiverwithaSPIinterface.TheCC2500isintendedfor2400to2483.5MHzIndustrial,ScientificandMedical(ISM)applications.ThecomputerinterfaceusesaSPIprotocol,theclockcircuitisbasedonanexternalcrystal,andtheantennacircuitmustbetunedforthe2.4GHzfrequency.TheeZ430-RF2500fromTexasInstrumentsisalow-costdevelopmenttoolbasedaroundtheMSP430andtheChipconCC2500low-powertransceiver.TheCC430microcontrollerincludesbothanMSP430andRF.

Example11.1.Designasystemthatcancommunicateat1000bytes/secbetweentwomicrocontrollerswithinthesameroomwithoutsecurity.

Solution:Thislowbandwidthcanbesolvedwitharadio-frequency(RF)linkwithoutthecomplexitiesnecessarytosupportBluetoothor802.11.ThisshortdistanceisclassifiedasaShortRangeDevice(SRD).TherearemanyRFcommunicationmodulesthatcouldhavebeenused.AsillustratedinFigure11.6,theCC2500interfacehasthreeparts.Thesystemwilloperateupto500Kbits/sec,andthechipimplementsdual64-bitFIFOsfortransmitandreceive.Basically,themicrocontrolleronthelefttransmitsdataviaitsSPI,andthemicrocontrollerontherightreceivesthedatawithitsSPI.Itisatransceiver,meaningdatacanflowacrossthelinkinbothdirections.

Figure11.6.Blockdiagramofawirelesslinkbetweentwomicrocontrollersystems.

ZigBeeisanotherlow-costwirelesssolutionforembeddedsystems.Thenameisderivedfromthebehaviorofhoneybees.Honeybeesdistributedacrossalargeopenfieldimplementameshnetworkinordertocommunicateinformationtotheirhive.Theydothisbymessagerelaying.Abeedistantfromthehivewillflyaparticularzigzagpatternthatrepresentstheinformation.Asecondbeenearerthehivewillrepeatthepattern.Therelaycontinuesuntiltheinformationreachesthehive.ZigBeeisastandardthatdefinesasetofcommunicationprotocolsforlow-data-rate,verylow-power,short-rangewirelessnetworking.Itcanoperateunderbatterypower,andlastforyearsbecausetherearemultipletypesoflow-powersleepmodes.Itisanappropriatesolutionforsensornetworks,meterreading,industrialautomation,securitysystems,andpatientmonitoring.ZigBeeisanextensionoftheIEEE802.15.4bstandard.ItislowercostandlowerperformancethanBluetoothorIEEE801.11b,asshowninTable11.1.Therangevaluesinthistablerepresentperformanceindoors–outdoors.ZigBeemodulescomeinavarietyofpowerversusperformancemodels.Inotherwords,youcanrunatlowerpowerifyouarewillingtosacrificedistanceanddatarate.

Datarate Range Wirelessapplications

ZigBee 20to250Kbps

10–100m SensorNetworks

Bluetooth 1to3Mbps 2–10m Headset,mouse

IEEE802.11b 1to11Mbps 38–140m Internetconnection

IEEE802.11g 1to54Mbps 38–140m

Internetconnection

IEEE802.11n 1to72Mbps 70–250m Internetconnection

Table11.1.Comparisonofwirelessprotocols.

TheZigBeeprotocolislayered.Thetoplayeristheapplicationlayer,implementedastheuserprogram.Atthislayersoftwareinonenodesendsmessagestoanothernode.Thissectionwillfocusonthislayer,becausewewillpurchaseaZigBeemodulethatperformsthelowerlayersautomatically.Thesecondlayeristheapplicationsupportsublayer(APS).BelowtheAPSisthenetworklayer(NWK).BelowtheNWKisthemediaaccesscontrol(MAC)layer.BelowtheMACisthephysicallayer,whichincludestheRFtransmitterandreceiver.FormoreinformationabouthowZigBeeworks,seehttp://www.zigbee.org/.YoualsocoulddoawebsearchonXBee,whichisalow-costimplementationofZigBee.

ZigBeeisapersonalareanetwork(PAN),asshowninFigure11.7.Therecanbeacoordinator,devicesthatsupportallfunctions,ordevicesthatsupportsomefunctions.AtypicalapplicationofZigBeeisaremotesensornetwork.ConsidereachofthenodesinFigure11.7iscapableofcollectingsensordata.ThesensordatabasecanbecentralizedatthePANcoordinatorordistributedacrosstheentiresystem.

Figure11.7.ZigBeeusedtocreatearemotesensornetwork.Onelow-costimplementationofZigBeeistheXBeemodulefromDigi(formallyMaxStream).ThesemodulestakeZigBeeandwrapitintoasimple-to-useserialcommandset,calledATcommands.Thesemodulesallowaveryreliableandsimplecommunicationbetweenmicrocontrollerswithaserialport.Bothpoint-to-pointandmulti-pointnetworksaresupported.Thehardwareinvolvesinterfacing3.3VfullduplexserialchanneltotheXBeemodule,asshowninFigure11.8.

Figure11.8.Circuitdiagramfora2-nodeZigBeenetwork.PowerandgroundsaresharedbetweenthemicrocontrolleranditsXBeebutnotbetweenXBees.ThefullsoftwaresolutiontothisnetworkwillbeleftasLab11.2.Inthissimpleconfiguration,therecanbeupto256nodesonthenetwork.Theinitializationsoftwaremustestablishthebaudrateat9600bits/sec.Thesystembandwidththereforewillbeontheorderof960bytes/sec.The<CR>symbolreferstothecarriagereturn,character13,existingasone8-bitASCIIcharacter.ToplacetheXBeeinATcommandmode,weexecutesteps1to5overandoveruntilwegetanOKresponse.

1.Sendadummycharacterlike‘X’.

2.Wait1.1second(greaterthantheonesecondguardtime).

3.Output“+++”.

4.Wait1.1second(greaterthantheonesecondguardtime).

5.Waitforresponse,shouldbeOK<CR>.

Eachnodemustestablishitsaddress(calledmy)andthedestinationaddress.Forexample,ifthiscomputerisataddress0x01andwishestosendpacketstothecomputerataddress0x02,thenthiscomputerexecutestheseATcommands.Thesoftwareshouldwait20msaftereachATcommand.WhenanATcommandisexecutedcorrectly,theXBeerespondswithOK<CR>.OneofthesimplestmodesisApplicationProgrammingInterface(API)mode1,whichallowsthenodestosendandreceivepackets:

ATDL02<CR>Setsdestinationaddressto2(numbergiveninhexadecimal)

ATDH0<CR>Setsdestinationhighaddressto0

ATMY01<CR>Setsmyaddressto1(numbergiveninhexadecimal)

ATAP1<CR>SetsAPImode1(packets)

ATCN<CR>Endscommandmode

Theothernodeinthepoint-to-pointconnectionperformsasimilarinitialization,butobviouslywiththemyanddestinationaddressesreversed.UsingtheAPImodesimplifiestheapplicationsoftware,becauseallmessagerouting,errordetection/retransmission,andmessageacknowledgementoccursatlowerlevelsautomatically.Adatatransmissionframehasthefollowingformat:

0x7E,LengthHi,LengthLo,0x01,ID,DestHi,DestLo,0x00,b1,…,bn,Chksum

AllAPImode1framesbeginwith0x7E.Thenexttwobytesarethelengthofthemessage,whichwillbethenumberofbytesafterthelengthandbeforethechecksum.Thelengthdoesnotincludethefourbytescomprisingthe0x7E,whichisthelengthitselfandthechecksum.Thefourthbyte0x01signifiesthisisatransmitdatapacket.TheIDshouldbeusedasamessagesequencenumber.I.e.,asthiscomputersendspacketstothedestinationcomputer,theIDissequencedas1,2,…255,1,2,…AnIDof0isnotused.Thissequencenumberguaranteesthepacketsarriveatthedestinationinthesameorderastheyweresent.

ThetwobytesDestHi,DestLospecifythedestinationnodeaddress.Thehighbyteshouldbezeroforthisconfiguration.Thenextbyteprovidesoptionsfortheframe,whichshouldbe0.Bytesb1throughbnarethedatatobetransmitted.Becausethereisaframelength,thisdatacanbeformattedhoweveryouwish.Thelastbyteoftheframewillbeachecksum.Letsumbethe8-bitadditionofallbytesnotincluding0x7Edelimiterandthelength.WecalculateChksumas0xFF-sum.InthiswaythereceivercanaddupallthebytesafterthelengthandincludingtheChksuminordertogettheresult0xFF.

Forexample,assumewewishtosendthemessage“He”tonode2.Alsoassumethisisthe254thframesent,sotheIDwillbe0xFD.Themessagehasalengthof7.Thechecksumiscalculatedas

0xFF‑(0x01+0xFD+0x00+0x02+0x00+0x48+0x65)=0x52.TheoscilloscoperecordingforthisframeisshowninFigure11.9.

Figure11.9.APItransmitframemeasuredontheDinpinoftheXBeemodule.Whenthetransmittedframeisproperlydelivered,theXBeesendsanacknowledgmenttothetransmitter.Thelengthisalways3bytes.TheAPIcodeis0x89.TheIDmatchesthecorrespondingvalueofthetransmittedframe.Thenextbyteisastatusfield,and0x00meanssuccess.Astatusof0x01meansnoacknowledgementreceived,whichmaymeanthedestinationnodedoesnotexistoristurnedoff.Astatusof0x02meansCCAfailureand0x03meansthemessagewaspurged.TheChksumbyteiscalculatedinthesamemannerasalltheAPIframes.

0x7E,0x00,0x03,0x89,ID,0x00,Chksum

Figure11.10showsascopetracewhenanacknowledgeframewasreportedtothetransmitter.TheIDofthisframeis0xC1.Thechecksumforthisframeis0xFF-(0x89+0xC1+0x00)=0xB5.

Figure11.10.APIacknowledgeframemeasuredontheDoutpinoftheXBeemodule.ThereceiverwithamyaddressmatchingthedestinationaddressofthetransmittedframewillbegiventhatframeasanAPIpackettype0x81.Adatareceiveframehasthefollowingformat:

0x7E,LengthHi,LengthLo,0x81,SourceHi,SourceLo,RSSI,0x00,b1,…,bn,Chksum

Thesourcefieldidentifiesthenodethatsentthemessage.TheRSSI(receivedsignalstrengthindictor)isthedecimalequivalentmeasureofthesignal.Forexample0x3Ameansthereceivedsignalstrengthis58dBm.Theoptionfieldshouldbezero.Bytesb1throughbncontainsthedata.Figure11.11showsascopetraceofthereceivedmessagecorrespondingtothetransmittedmessageinFigure11.9

Figure11.11.APIreceiveframemeasuredontheDoutpinoftheXBeemodule.

11.4.InternetofThings

11.4.1.BasicConceptsWiththeproliferationofembeddedsystemsandthepervasivenessoftheinternet,itisonlynaturaltoconnectthetwo.Theinternetofthings(IoT)isthecombinationofembeddedsystems,whichhavesensorstocollectdataandactuatorstoaffectthesurrounding,andtheinternet,whichprovidesforubiquitousremoteandsecurecommunication.Thissectionwillnotdescribehowtheinternetworks,butratherwewilldiscussboththegeneralandspecificapproachesforconnectingembeddedsystemstotheinternet.ForanexcellentdescriptionoftheTCP/IP(TransmissionControlProtocol/InternetProtocol)protocolthereaderisreferredtoW.RichardStevens,TCP/IPIllustrated,Volume1:TheProtocols.Forageneraldescriptionoftheinternetofthings,seeVasseurandDunkels,InterconnectingSmartObjectswithIP.Thesetwobooksprovidesgoodoverviewsofnetworktechnologiesusedforconnectingdevices.

Figure11.12illustratesthedistributedapproachtakenwiththeinternetofthings.Adistributedsolutiondeploysmultiplesensorsandactuatorsconnectedbytheinternet.Anothernamegivenforanembeddedsystemconnectedtotheinternetissmartobject.Smartobjectsincludesensorstocollectdata,processingtodetecteventsandmakedecisions,andactuatorstomanipulatethelocalenvironment.Therearemanyreasonstoconsideradistributedsolution(network)overacentralizedsolution.Oftenmultiplesimplemicrocontrollerscanprovideahigherperformanceatlowercostcomparedtoonecomputerpowerfulenoughtoruntheentiresystem.Someembeddedapplicationsrequireinput/outputactivitiesthatarephysicallydistributed.Forreal-timeoperationtheremaynotbeenoughtimetoallowcommunicationbetweenaremotesensorandacentralcomputer.Anotheradvantageofdistributedsystemisimproveddebugging.Forexample,wecoulduseonenodeinanetworktomonitoranddebugtheothers.Often,wedonotknowthelevelofcomplexityofourproblematdesigntime.Similarly,overtimethecomplexitymayincreaseordecrease.Adistributedsystemcanoftenbedeployedthatcanbescaled.Forexample,asthecomplexityincreasesmorenodescanbeadded,andifthecomplexityweretodecreasenodescouldberemoved.Table11.2listssomeexistingapplicationsandthethingstheysenseorcontrol.

Figure11.12.Theinternetofthingsplacesinput,outputandprocessing

atmultiplelocationsconnectedtogetherovertheinternet.IndustrialAutomationFactories,machines,shipping

EnvironmentWeather,pollution,publicsafety

SmartGridElectricpower,energydelivery

SmartCitiesTransportation,hazards,publicservices

SocialNetworksIdeas,politics,sales,andcommunication

HomeNetworksLighting,heat,security,information

BuildingNetworksEnergy,hazards,security,maintenance

StructuralMonitorsBridges,roads,building

HealthCareHeartfunction,medicaldata,remotecare

LawenforcementCrime,publicsafetyTable11.2.Applicationsofsmartobjects.

Challenges.Onalocalscale,thedesignofsmartobjectsfacesthesamechallengesexistinginallembeddedsystems:power,size,reliability,longevity,andcost.Luckilythedeploymentofbillionsofmicrocontrollersintothemarkethascreatedatechnologyracetoreducepower,sizeandcostwhileincreasingtheperformance.Atthemicrocontrollerlevelthingsaregettingsmaller,butatthenetworklevel,complexityisincreasingandprotocolsareconstantlychangingastheworld’sthirstforinformationandcommunicationrapidlygrows.

Standardization.Theexistenceofstandardsallowsforawidevarietyofobjectstocommunicatewitheachother.Adheringtoastandardwillincreasetheacceptanceofourdevicebycustomers,andallowourcustomerstoapplyourdevicetosolveproblemsweneverenvisioned.uIPisalight-weightimplementationoftheIPstackspecificallydesignedtooperatewiththeavailablememoryresourcesofsmartobjects.InthissectionwewillstartwithamicrocontrollerwiththehardwareandsoftwaretoimplementTCP/IPprotocols,andbuildourapplicationontopofthisstandard.

Interoperabilitymeansourdevicecanfunctionwithawiderangeofotherdevicesmadewithdifferenttechnologies,soldbydifferentvendors,andproducedbydifferentcompanies.

Evolutionisprocessofhownewtechnologiesareintroducedintothemarket.Ifthereisoneconstantinthisworld,itisthatthingswillchange.Everythousandyears,onebigdiscoveryfundamentlychangeshowweoperate(fire,language,metaltools).Morefrequently,changeisintroducedgraduallysuchthatthosetechnologiesthatgiveusacompetitiveadvantagesurvive.Ifwebuildourbusinessmodelonthepremiseevolutionarychange,thenwecanbenimbletodeploynewtechnologywhenitprovideslowercostand/orbetterperformance.

Stability.Eventhoughtechnologywilladvance,ourcustomersdemandproductsthatworkreliably,foralongtime,andinamannerwithwhichtheyarecomfortable.Overthelast50yearsautomotivetechnologyhasdrasticallyimproved,butthedrivingexperience,howwedrive,hasremainedalmostconstant.

Abstraction.Youwillnoticetheapproachinthissectiondifferswidelyfromtheotherexamplesinthisbook.Therestofthebookdeploysabottomupapproach.Withbottom-upeducation,thedetailsarefirstexplained,sothereisnomagic,andthenabstractionoccursbyencapsulatingthatwefullyunderstand.Inthissectionwewillpurchasehardwareandsoftwarewithcapabilitiestocommunicatewiththeinternet,andusethisabstractionwithoutfullyunderstandinghowsomeofthelowerlevelsoperate.

Scalability.ARMreportsover50milliondeviceswithanARMcorehavebeenshippedfrom1993to2013,andpredictsanother50billionbeforetheendofthisdecade.Inordertobeeffectiveandprofitable,weneedtodevelopsystemsthatcanscale.

Security.Becauseembeddedsystemsaredeployedinlife-criticalsituations,andbecausethequalityofserviceaffectourprofits,wemustprotectthesystemfromadeterminedadversary.Achainisonlyasstrongasitsweakestlink.Securitycannotbeobtainedsimplybyoperatinginsecret,becauseoncethesecretisout,thesystemwillbeextremelyvulnerable.“Securitybyobscurity”isaverypoordesignmethod.Securityinvolvesmorethanencryptingthedata.Thefirstaspectofsecurityisconfidentiality.Wemustdecidewhatitmeanstoview/changethedataandwhohastherighttoread/write.Authenticationisthemeanstoensuretheidentityofthesenderiscorrect.Confidentialitywillrequirebothlogicalandphysicalmeasurestoprotectagainanattack.Encryptionmakesitharderforanunauthorizedpartytoviewamessage.Thesecondaspectisdataintegrity.FormostoftheapplicationslistedinTable11.2itisimportantthatdatareachtherightfulrecipientinanunalteredfashion.Tosupportnetworkintegrityweneedtechniquesthatsupportbothdetectionandprevention.Thethirdaspectisavailability.Asecurecommunicationnotonlyrequiresthecorrectdataarriveatthecorrectplace,butalsoatthecorrecttime.ADenialofService(DoS)attackattemptstobreachtheavailabilityofthenetwork.Forwirednetworks,wecanreroutetrafficalongmultiplepaths.Withwirelessnetworks,wecanchannelhopbyswitchingchannelsonapseudorandomfashion,makingitharderforanattackertojam.Formoreinformationonsecurity,seeFrankStajano,SecurityforUbiquitousComputing.

11.4.2.LayeredModel

Mostnetworksprovideanabstractionthathideslow-leveldetailsfromhigh-leveloperations.Thisabstractionisoftendescribedaslayers.TheInternationalStandardsOrganization(ISO)definesa7-layermodelcalledtheOpenSystemsInterconnection(OSI),asshownontheleftofFigure11.13.Itprovidesastandardwaytoclassifynetworkcomponentsandoperations.ThePhysicallayerincludesconnectors,bitformats,andameanstotransferenergy.ExamplesincludeRS232,controllerareanetwork(CAN),modemV.35,T1,10BASE-T,100BASE-TX,DSL,and802.11a/b/g/nPHY.TheDatalinklayerincludeserrordetectionandcontrolacrossasinglelink(singlehop).Examplesinclude802.3(Ethernet),802.11a/b/g/nMAC/LLC,PPP,andTokenRing.TheNetworklayerdefinesend-to-endmulti-hopdatacommunication.TheTransportlayerprovidesconnectionsandmayoptimizenetworkresources.TheSessionlayerprovidesservicesforend-userapplicationssuchasdatagroupingandcheckpoints.ThePresentationlayerincludesdataformats,transformationservices.TheApplicationlayerprovidesaninterfacebetweennetworkandend-userprograms.

Figure11.13.TheTCP/IPmodelhasfourlayers.TheTCP/IPmodeloftheInternetdoesnotadheretosuchastrictlayeredstructure,butdoesrecognizefourbroadlayers:scopeofthesoftwareapplication;theend-to-endtransportconnection;theinternetworkingrange;andthedirectlinksasshownontherightofFigure11.13.ExamplesofapplicationsincludeTelnet,FTP(FileTransferProtocol),andSMTP(SimpleMailTransferProtocol).ExamplesoftransportincludeTCP(TransmissionControlProtocol)andUDP(UserDatagramProtocol).TCPprovidesreliable,ordereddeliveryofdatafromasoftwaretaskononecomputertoanothersoftwaretaskrunningonanothercomputer.ForapplicationsthatdonotrequirereliabledatastreamserviceUDPcanbeused.UDPprovidesadatagramservicethatemphasizesreducedlatencyoverreliability.ExamplesofnetworkincludeIP(InternetProtocol),ICMP(InternetControlMessageProtocol)andIGMP(InternetGroupManagementProtocol).Ethernetisthephysicallinkexploredlaterinthissection.Inthissectionwewilldevelopprojectsattheapplicationlayer.Thecommunicationofbitshappenatthephysicallayer,framesatthedatalinklayer,packetsordatagramsatthenetworklayer,segmentsatthetransportlayer,andmessagesattheapplicationlayer.

11.4.3.MessageProtocolsThelayeredformatcanbeseeninthemessagepacketformats,asoverviewedinFigure11.14.Atthelowestlevelarephysicalframes.Ethernetframescontainaheader,46to1500bytesofpayload,andatrailer.Theheaderincludesaddress,typeandlengthinformation.Ifthethereislessthan46bytesofEthernetdata,zerosareadded(padding)tomaketheEthernetpayloadatleast46bytes.Thetrailerincludeserrorchecking(CRC).AttheIPlevel,packetsincludeaheaderandpayload.TheheaderofanIPpacketincludesa32-bitdestinationIPaddress,typicallyshownasfour8-bitnumbers(e.g.,176.31.244.1).SomeoftheseIPaddressesarereservedforcommunicatingwithinnodesonalocalnetwork.TheDomainNameSystem(DNS)hostcanbeusedtotranslatedomainnamestoIPaddresses.ComputersthatcommunicateonlywitheachotherviaTCP/IP,butarenotconnectedtotheInternet,neednothavegloballyuniqueIPaddresses.IPaddressesforprivatenetworksarelistedinTable11.3.TheseIPaddressescouldbeusedforembeddedsystemsthatuseTCP/IPtocommunicate,butarenotconnectedtotheinternet.

Start End Numberofaddresses

10.0.0.0 10.255.255.255 224

172.16.0.0 172.31.255.255 220

192.168.0.0 192.168.255.255 216

Table11.3.PrivateIPaddresses.

Becauseofthegrowthoftheinternet,the32-bitIPaddress(IPv4)isbeingreplacedwitha128-bitaddress(IPv6),whichwillprovideforabout3∙1038addresses.TheIPheaderis20to40bytesandcontainsthesourceIPaddress,destinationIPaddress,andlength.

TheUDPheaderis8bytesandcontainsthesourceport,destinationport,length,andchecksum,seeTable11.4.TheIPaddressspecifiesthenode,andportsareaddresseswithinthesourceanddestinationnodes.

Sourceport:16-bitnumberoftheprocessthatsentthepacket,couldbezero

Destinationport:16-bitnumberoftheprocesstoreceivethepacket.

Length:16-bitnumberspecifyingthesizeinbytesofthedatatofollow

Checksum:16-bitmoduloadditionofalldata,UDPheader,andIPheaderTable11.4.UDPheaderformat.

TheTCPheaderis20byteswiththepossibilityofadditionalandoptionalinformation,seeTable11.5.Thesequenceandacknowledgmentnumbersallowthereceivertoproperlysortsegmentsofdatathatwerereceivedoutoforder.TheflagsspecifydifferentmodesoftheTCPcommunication.TheSYNflagmeansthefirstofasequenceofpackets,andtheFINflagmeansthelast.TheRSTflagterminatesaconnection.TheURGflagmeanstheurgentpointerspecifiesapieceofdatatheapplicationurgentlyneeds.

Figure11.14.Overviewofmessagepacketsusedatvariouslayers.

Sourceport:16-bitnumberoftheprocessthatsentthepacket,couldbezero

Destinationport:16-bitnumberoftheprocesstoreceivethepacket.

Sequencenumber:32-bitnumberdefiningthepositionofthisdata

Acknowledgement:32-bitnumberofthenextdataexpectedtobereceived

Hlen:4-bitfieldoftheheadersize(includingoptions)dividedby4

Flags:6-bitfieldwithFIN,SYN,RST,PSH,ACK,andURG

Window:16-bitnumberspecifyingthenumberofbytesthereceivercanaccept

Checksum:16-bitmoduloadditionofalldata,TCPheader,andIPheader

Urgentpointer:16-bitfieldpointingtoaplaceinthestreamurgentlyneededTable11.5.TCPheaderformat.

11.4.4.Webserver

Thisfirstapplicationcreatesawebserverthatmaintainsawebpagedisplayinglocaldata,seeFigures11.15and11.16.Thecomponentsofthesystemareasensorandsensorinterface(Figure10.28andProgram10.1),anEK-TM4C1294XLLaunchPad,TexasInstrumentsTivaware,andarouterconnectedtotheInternet.TheDynamicHostConfigurationProtocolserverprovidesanIPaddress,andistypicallyinitiatedviaaDHCPbroadcast,whenitconnects.DHCPprovidedtheaddress192.168.0.107,alocaladdressonitsnetwork.ThisexamplewasbuiltontopoftheuIPstackdeliveredaspartofTivaware.First,youneedtodownloadTivaware.Ifirstrantheenet_uipexamplefoundinthe TivaWare_C_Series-2.1.0.12573\examples\boards\ek-tm4c1294xl\enet_uip folder.Icopiedthisexample,andchangedthewebserverasshowninProgram11.1.

Figure11.15.ThethermistormeasurestemperatureandtheLaunchPadservespagestotheinternet.

Figure11.15.ThethermistormeasurestemperatureandtheLaunchPadservespagestotheinternet.

Program11.1showsthecodeyouneedtomodifytocreateyourownremotesensorsmartobject.Whenanothernodesendsarequesttothisserver,thisnodewillrespondwithhtmlcodetorenderthepage.Thepageisdividedintothreeparts.Thefirstpart( default_page_buf1of3 )andlastpart( default_page_buf3of3 )arefixed.Theapplicationcallbackfunction, httpd_appcall ,isinvokedwhenthewebpageisrequested.Thiscallbackfunctioncallsourapplicationfunction Board_Update whichcollectssensordatafromthethermistorandrebuildsthemiddlepartofthehtmlcode( default_page_buf2of3 ).Themetacodeautomaticallyrefreshesevery5seconds.constchardefault_page_buf1of3[]=

“HTTP/1.0200OK\r\n”

“Server:UIP/1.0(http://www.sics.se/~adam/uip/)\r\n”

“Content-type:text/html\r\n\r\n”

“<!DOCTYPEHTMLPUBLIC\”-//W3C//DTDHTML4.01Transitional//EN”

“http://www.w3.org/TR/html4/loose.dtd\”>”

“<html><head>”

“<metahttp-equiv=\“refresh\”content=\“5\”>”

“<title>EmbeddedSystems</title></head>”

“<body><center>”

“<h1>EmbeddedSystems:Real-TimeInterfacing”

“toARMCortexMMicrocontrollers</h1>”

“<p>Thisisanexamplefromthebook,Section11.4InternetofThings</p>”

“<p>Formoreinformationsee“

“<ahref=\“http://users.ece.utexas.edu/~valvano/arm/outline.htm\”>”

“<b>thebookwebsite</b></a>.”

“<hrwidth=\“75%\”>”

“<p>Athermistorisconfiguredfortemperaturemeasurement,“

“witharangefrom0to50C.“

“FordetailsofanalogcircuitseethebookFigure10.28.“

“TheanalogsignalissampledonPE3/Ain0.“

“The12-bitdigitalsampleisconvertedtotemperatureusingtablelookup“

“andlinearinterpolation.</p>“

“<p>Thetemperatureis“;

uint32_tconstbuf1of3_Size=(sizeof(default_page_buf1of3)-1);

chardefault_page_buf2of3[]=“12.01”;

uint32_tbuf2of3_Size=(sizeof(default_page_buf2of3)-1);

constchardefault_page_buf3of3[]=

”C.</p>”

“<hrwidth=\“75%\”>”

“<p>Thiswebpageisservedbyasmallwebserverrunningontopof“

“the<ahref=\“http://www.sics.se/~adam/uip/\”><b>&micro;IPembeddedTCP/IP“

“stack</b></a>.</center></body></html>”;

uint32_tconstbuf3of3_Size=(sizeof(default_page_buf3of3)-1);

voidBoard_Update(void)uint32_tdata,temperature;

data=ADC0_InSeq3();//12-bitADC,0to4095

temperature=ADC2Temperature(data);//temperature,0.01C

Fix2Str(temperature,default_page_buf2of3);//5ASCIIcharacters

buf2of3_Size=5;//inthiscaseitisfixedsize(butitcouldvary)

Program11.1.ThethermistormeasurestemperatureandtheLaunchPadservespagestotheinternet.ToruntheinternetexamplesdescribedinthissectiondownloadandunziptheIoTexamplesintoexamples\boardssothedirectorypathlookslikethis

TivaWare_C_Series-2.1.0.12573

examples

boards

CC31xxxx

ek-tm4c1294xl-enet_uip_temperature

ek-tm4c123gxl-boost-cc3100_basic_wifi_UDP

ek-tm4c123gxl-boost-cc3100_starter

ek-tm4c1294xl-boost-cc3100_starter

11.4.5.UDPcommunicationoverwifiTheapproachforimplementingasmartobjectoverwifiistobeginwithahardware/softwareplatformthatimplementsIEEE801.11wifi.TheCC3100BOOSTisaboosterpackthatcanbeusedwiththeMSP430LaunchPad,theTM4C123LaunchPad,theTM4C1294LaunchPad,orwithaCC31XXEMUBOOSTemulationmodule,seeFigure11.16.Theemulationmodulecanbeusedearlyinaprojecttodevelopwirelessapplicationsusinga“generic”microcontroller.Afteraprototypeisconfigured,theprojectcanselectamicrocontrolleranddesigntheactualsmartobject.InthisdesignwewilluseeitherofthetwoTM4CLaunchPadsanddevelopasolutionthattransmitsUDPpacketsfromonesmartobjecttoanother.UDPissimplerthanTCPandappropriateforapplicationsrequiringsimplicityandspeed.Furthermore,touseUDPtheapplicationmusttoleratelostoroutoforderpackets.UDPprovidesabest-effortdatagramdeliveryservice.

Figure11.16.TheCC3100boosterpacketprovidesIEEE802.11wirelessconnectivity.TheactualTCP/IPsoftwarestackresidesinfirmwareontheboosterpackitself.Therefore,whenusinganyofthewirelessboosterpacksthefirststepistoupgradethefirmware.OnewaytoupgradethefirmwareistousetheCC31XXEMUBOOSTemulationmodule.Theexamplesofthissectionranonversion3.3boosterpackswithoutneedingtoupgradethefirmare.

Program11.2showstheclientsoftware,whichsamplestheADCandsendsUDPpackets.Line1specifiesthenameoftheaccesspoint(AP)towhichthenodewillconnect.ThereisamechanismusingSmartConfigtoautomatethisdiscovery,butinthisexampleInamedtheAPValvanosoIusedamanualmethodtodefinetheconnectionbetweenthenodeandAP.TheUDPpayloadwillhaveatypefield,whichisdefinedinline2.ThedestinationIPaddressishard-codedinline3.Forthisapplication,theserverwasatIPaddressaddress192.168.0.101,whichinhexisC0.A8.00.65.Theportnumber,whichisa16-bitvaluedefiningwhichprocessintheservershouldreceivethedata,isspecifiedinline4.Therearealonglistofregisteredportnumbersthathavespecialpurposes,soIchoseaportnumberlargerthan1024toavoidselectinganyofthesespecialpurposeportnumbers.Lines5and6definethepayloadfortheUDPpacket.Line15setsthebusclockto50MHz.ThePLLneedstobeactivefortheADCtooperate.Line16initializestheADCchannel7usingPD0.Line17initializestheCC3100.Afterexecutingline18wewillbeconnectedandhaveIPaddress.Line19willreturnthenetworkconfiguration.Lines21-24definetheaddressandporttowhichtheUSPpacketwillbesent.Line25definesandopensasocket.Inthisexampleweleavethesocketopen,butitisoktoclosethesocket,gointolow-powermode,andreopentheconnectionaftersleeping.Lines26-29willsampletheADCandcreateanewmessage.Line30sendstheUDPpacketthroughtheopensocket.Thewaitinline32definestherateatwhichpacketsaresent.Eachofthewififunctionswillreturnasuccessflag(errorcode).Inthissimpleprogramweignoredthereturnvalues,assumingitwasok.Intheversionontheweb,theprocessisrestartedonerror.

#defineSSID_NAME“Valvano”//APtoconnectto1

#defineATYPE‘a’//analogdatatype2

#defineIP_ADDR0xC0A80065//serverIP3

#definePORT_NUM5001//Portnumbertobeused4

#defineBUF_SIZE12//5

UINT8uBuf[BUF_SIZE];//UDPpacketpayload6

intmain(void)

UINT8IsDHCP=0;

_NetCfgIpV4Args_tipV4;

SlSockAddrIn_tAddr;

UINT16AddrSize=0;

INT16SockID=0;

UINT32data;

unsignedcharlen=sizeof(_NetCfgIpV4Args_t);

initClk();//PLL50MHz,ADCneedsPPLactive15

ADC0_InitSWTriggerSeq3(7);//Ain7isonPD016

sl_Start(0,0,0);//InitializingtheCC3100device17

WlanConnect();//connecttoAP18

sl_NetCfgGet(SL_IPV4_STA_P2P_CL_GET_INFO,&IsDHCP,&len,//19

(unsignedchar*)&ipV4);//20

Addr.sin_family=SL_AF_INET;//21

Addr.sin_port=sl_Htons((UINT16)PORT_NUM);//22

Addr.sin_addr.s_addr=sl_Htonl((UINT32)IP_ADDR);//23

AddrSize=sizeof(SlSockAddrIn_t);//24

SockID=sl_Socket(SL_AF_INET,SL_SOCK_DGRAM,0);//25

while(1)

uBuf[0]=ATYPE;//analogdatatype26

uBuf[1]=‘=’;//27

data=ADC0_InSeq3();//0to4095,Ain7isonPD028

Int2Str(data,(char*)&uBuf[2]);//6digitnumber29

sl_SendTo(SockID,uBuf,BUF_SIZE,0,//30

(SlSockAddr_t*)&Addr,AddrSize);//31

ROM_SysCtlDelay(ROM_SysCtlClockGet()/25);//40ms32

Program11.2.ClientsoftwarethatmeasuresADCdataandsendsUDPpackets.Program11.3showstheserversoftware,whichacceptsUDPpacketsandplotsthedataonanST7735graphicsLCD.Line1specifiesthenameoftheaccesspoint(AP)towhichthenodewillconnect.TheclientandserverusethesameAP,whichInamedValvano,soIusedthemanualmethodtodefinetheconnectionbetweenthenodeandAP.TheUDPpayloadwillhaveatypefield,whichisdefinedinline2.Lines16,22-25configurethewificonnectioninasimilarwayastheclient.Lines17-20initializetheST7735LCDandoutputawelcomemessage.Line21configurestheLCDgraphicsroutinesspecifyingtherangeonthey-axisoftheplot.RawADCdatawillbeplottedversustime.Lines26-29defineanIPaddressandporttouse.Line31definesandopensasocket,andlines32-33bindtheporttothatsocket.Lines34-35receiveaUDPpacket.Justliketheclient,weleavethesocketopen.Ifwewishedtosavepower,wecouldclosethesocket,gointolow-powermode,andreopentheconnectionaftersleeping.Lines36-51decodethepacketandplotthedataontheLCD.

#defineSSID_NAME“Valvano”//APtoconnectto1

#defineATYPE‘a’//analogdatatype2

#defineIP_ADDR0xC0A80065//serverIP3

#definePORT_NUM5001//Portnumbertobeused4

#defineBUF_SIZE12//5

UINT8uBuf[BUF_SIZE];//UDPpacketpayload6

intmain(void)

UINT8IsDHCP=0;

_NetCfgIpV4Args_tipV4;

SlSockAddrIn_tAddr,LocalAddr;

UINT16AddrSize=0;

INT16SockID=0;

INT16Status=1;//ok

UINT32data;

unsignedcharlen=sizeof(_NetCfgIpV4Args_t);

initClk();//PLL50MHz,ADCneedsPPLactive16

ST7735_InitR(INITR_REDTAB);//Initialize17

ST7735_OutString(“InternetofThings\n”);//18

ST7735_OutString(“EmbeddedSystems\n”);//19

ST7735_OutString(“Vol.2,Valvano”);//20

ST7735_PlotClear(0,4095);//rangefrom0to409521

sl_Start(0,0,0);//InitializingtheCC3100device22

WlanConnect();//connecttoAP23

sl_NetCfgGet(SL_IPV4_STA_P2P_CL_GET_INFO,&IsDHCP,&len,//24

(unsignedchar*)&ipV4);//25

LocalAddr.sin_family=SL_AF_INET;//26

LocalAddr.sin_port=sl_Htons((UINT16)PORT_NUM);//27

LocalAddr.sin_addr.s_addr=0;//28

AddrSize=sizeof(SlSockAddrIn_t);//29

while(1)

SockID=sl_Socket(SL_AF_INET,SL_SOCK_DGRAM,0);//31

Status=sl_Bind(SockID,(SlSockAddr_t*)&LocalAddr,//32

AddrSize);//33

Status=sl_RecvFrom(SockID,uBuf,BUF_SIZE,0,//34

(SlSockAddr_t*)&Addr,(SlSocklen_t*)&AddrSize);//35

if((uBuf[0]==ATYPE)&&(uBuf[1]==‘=’))//36

inti,bOk;uint32_tplace;//37

data=0;bOk=1;//38

i=4;//ignorepossiblenegativesign39

for(place=1000;place;place=place/10)//40

if((uBuf[i]&0xF0)==0x30)//ignorespaces41

data+=place*(uBuf[i]-0x30);//42

else//43

if((uBuf[i]&0xF0)!=‘‘)//44

bOk=0;//45

//46

//47

i++;//48

//49

if(bOk)//50

ST7735_PlotLine(data);//51

ST7735_PlotNextErase();//51

Program11.3.ServersoftwarethatreceivesUDPpacketsandplotsresultsontheLCD.SinceUDPtransmissionis“besteffort”wecouldloosepacketsorreceivepacketsoutoforder.Inthissimpleexamplewewillnotknowifeitheroftheseerrorsweretooccur.Ifwewishedtohaveamorereliabletransmission,wecouldhaveusedTCP.Program11.2line25wouldhavespecifiedasocketstreaminsteadofadatagram.TocreateaTCPcommunication,usetheexamplesoftwareinthe tcp_socket folder.

SockID=sl_Socket(SL_AF_INET,SL_SOCK_STREAM,0);//TCPsocket

11.4.6.OtherCC3100ApplicationsThissectionliststhesampleapplicationsarealsoprovidedforMSP430F5739,TM4C123GH6PMandSimpleLinkStudio.ThesourcecodefortheseexamplescanbefoundintheexamplesdirectoryafterdownloadingCC3100SDK,theSimpleLinkWi-FiCC3100SoftwareDevelopmentKit(SDK)fromtheTIwebsite.Formoredetailsoneachexample,seethedocsfolderincludedintheCC3100SDKdownload.TheCC3100comespreloadedwithCC3100BoosterPackcomespreloadedwithOutofBoxHTMLpages.Outofboxdemohighlightsthefollowingfeatures:SimpleWLANConnectionUsingSmartConfig,andeasyaccesstoCC3100usingmDNSandHTTPServer.

AntennaSelection.Thisisareferenceimplementationforantenna-selectionschemerunningonthehostMCU,toenableimprovedradioperformanceinsidebuildings

ConnectionPolicies.ThisapplicationdemonstratestheusageoftheCC3100profilesandconnection-policies.

SendEmail.ThisapplicationsendsanemailusingSMTPtoauser-configurableemailaddressatthepushofabutton.

EnterpriseNetworkConnection.ThisapplicationdemonstratestheprocedureforconnectingtheCC3100toanenterprisenetwork.

FileDownload.ThisapplicationdemonstratesfiledownloadfromacloudservertotheonboardserialFlash.

FileSystem.ThisapplicationdemonstratestheuseofthefilesystemAPItoreadandwritefilesfromtheserialFlash.

GetTime.ThisapplicationconnectstoanSNTPcloudserverandreceivestheaccuratetime.

GetWeather.Thisapplicationconnectsto‘OpenWeatherMap’cloudserviceandreceivesweatherdata.

GettingStartedinAPMode.ThisapplicationconfigurestheCC3100inAPmode.Itverifiestheconnectionbypingingtheconnectedclient.

GettingStartedinStationMode.ThisapplicationconfigurestheCC3100inSTAmode.ItverifiestheconnectionbypingingtheconnectedAccessPoint.

HTTPServer.Thisapplicationdemonstratesusingtheon-chipHTTPServerAPIstoenablestaticanddynamicwebpagecontent.

IPConfiguration.ThisapplicationdemonstrateshowtoenablestaticIPconfigurationinsteadofusingDHCP.

mDNS.ThisapplicationregistersthemDNSserviceforbroadcastingandattemptstogettheservicebythenamebroadcastedbyanotherdevice.

ModeConfiguration.ThisapplicationdemonstratesswitchingbetweenSTAandAPmodes.

NWPFilters.ThisapplicationdemonstratestheconfigurationofRx-filteringtoreducetheamountoftraffictransferredtothehost,andtoachievelowerpowerconsumption.

NWPPowerPolicy.Thisapplicationshowshowtoenabledifferentpowerpoliciestoreducepowerconsumptionbasedonusecaseinthestationmode.

P2P(Wi-FiDirect).ThisapplicationconfiguresthedeviceinP2P(Wi-FiDirect)modeanddemonstrateshowtocommunicatewitharemotepeerdevice.

ProvisioningAP.ThisapplicationdemonstratestheuseoftheonChipHTTPserverforWi-FiprovisioninginAPMode,buildinguponexampleapplication7.8above.

ProvisioningwithSmartConfig.ThisapplicationdemonstratestheusageofTI’sSmartConfig™Wi-Fiprovisioningtechnology.TheWi-FiStarterApplicationforiOSandAndroidisrequiredtousethisapplication.Itcanbedownloadedfromfollowinglink:http://www.ti.com/tool/wifistarterorfromtheAppleAppstoreandGooglePlay.

ProvisioningwithWPS.ThisapplicationdemonstratestheusageofWPSWi-FiprovisioningwithCC3100.

ScanPolicy.Theapplicationdemonstratesthescan-policysettingsinCC3100.

SPIDiagnosticsTool.ThisisadiagnosticsapplicationfortroubleshootingthehostSPIconfiguration.

SSL/TLS.TheapplicationdemonstratestheusageofcertificateswithSSL/TLSforapplicationtrafficprivacyanddeviceoruserauthentication

TCPSocket.TheapplicationdemonstratessimpleconnectionwithTCPtraffic.

TransceiverMode.TheapplicationdemonstratestheCC3100transceivermodeofoperation.

UDPSocket.TheapplicationdemonstratessimpleconnectionwithUDPtraffic.

XMPPClient.TheapplicationdemonstratesinstantmessagingusingacloudbasedXMPPserver.

ThesewerethestepsIusedtocreatetheUDPcommunicationexampleinSection11.4.5.Ibeganwiththestarterapplication, ek-tm4c123gxl-boost-cc3100_starter .Ifirstchanged SSID_NAME tomatchouraccesspoint

#defineSSID_NAME“Valvano”//APtoconnectto

Next,Icompiled,downloadedandranthisapplicationontotwoLaunchPad+CC3100systems,observingtheoperatingonPuTTy.TheinterpreteroutputshouldshowithasconnectedandshowstheIPassignedtothesetwonodesbytheAP.IcouldrunthepingcommandtocheckthewificonnectiontomyAP.

OnceIwassuremytwoLaunchPad+CC3100systemscouldcommunicatewithmyAP,Imadeacopyofthestarterapplicationbycopy-pastingtheentirefolder.Irenamedthisnewfolderto ek-tm4c123gxl-boost-cc3100_basic_wifi_UDP .IopenedthenewprojectinthecompilerIDEandopenedthemain.cfromthe udp_socket examplefolder.Iaddedand/ormergedthesourcecodefrommain.cof udp_socket intostarter.cofthenewproject.Theeventhandlersandthemainprojectneededmerging,butthe BsdUdpClient and BsdUdpServer functionsweresimplyadded.IchangedtheIPaddresstomatchtheaddressgiventotheserver.

#defineIP_ADDR0xC0A80068

Ithenloadedaversionthatcalledtheclient(sendUDP)ononesystem

while(1)BsdUdpClient(PORT_NUM);

andloadedaversionthatcalledtheserver(receiveUDP)ontheothersystem

while(1)BsdUdpServer(PORT_NUM);

Iranthetwosystemsinthedebuggertoseethatpacketswerebeingsent.IdidnotuseSmartConfig,becauseIknewthenameoftheAP.Thelaststepwastomodifytheclientandserversotheclientcollectsdataandtheserverdisplaysit.

11.5.Exercises11.1Foreachtermgiveadefinitionin32wordsorless.

a)Master-slaveb)Multidrop

c)Channelcapacityd)Ringnetwork

e)Lossycompressionf)Crosstalk

g)Stuffbitsh)Frequencyshiftkey

i)Phaseencodingj)Compression

11.2Foreachpairoftermscompareandcontrastin32wordsorless.

a)Fullduplexversushalfduplex

b)Losslessversuslossycompression

c)Guidedversusunguidedmedia

d)LRCversuschecksum

e)Originateversusanswer

11.3Whatfundamentalpropertyisusedtotransmitinformationthroughachannel?

11.4Consideratelephonelinewithachannelbandwidthof2kHzandSNRof40dB.Whatisthemaximumdataratepossible?

11.5Consideratelephonelinewithachannelbandwidthof4kHzandSNRof30dB.Whatisthemaximumdataratepossible?

11.6Assumenode0x31wishestosendthemessage“Hello”tonode0x32.LetID=0.LetRSSI=0x3A.

a)GivetheAPImode1framenode0x31sendstoitsXBeemodule.Includetheentiremessagefrom0x7Euptoandincludingthechecksum.

b)GivetheAPImode1frame0x32willreceivefromitsXBeemodule.

11.7Assumenode0x41wishestosendthemessage“Ciao”tonode0x42.LetID=0x12.LetRSSI=0x3A.

a)GivetheAPImode1framenode0x41sendstoitsXBeemodule.Includetheentiremessagefrom0x7Euptoandincludingthechecksum.

b)GivetheAPImode1frame0x42willreceivefromitsXBeemodule.

D11.8.Theobjectiveofthisproblemistodesignacommunicationnetworkusingfoursinglechipmicrocontrollersplacedinaring.Developamessageprotocolthatsupportsaddressingandvariablelengthdata.PerformtheI/Ointhebackgroundusinginterruptsynchronization.

11.6.LabAssignmentsLab11.1.Theoverallgoalofthislabistodesign,implementandtestapeer-to-peercommunicationsystem.Peer-to-peermeanspeopleontwocomputerscommunicatewithoutthepeopleontheothercomputersseeingtheinformation.Thesystemmustusearing-connectedRS232serialchannel(Figure11.3),mustuseinterrupt-drivenI/O,andmusthavealayeredsoftwareconfiguration.Thelowest-levelsoftwareperformsserialI/O.Themiddle-levelsoftwaresendsmessagepacketsfromonecomputertoanother.Thehighestlevelsoftwareinterfaceswiththehumanoperator(keypad/LCD)andprovidesamechanismtocreateapeer-to-peerconnection.Inalayeredsystem,softwareinonelayercanonlycallroutineswithinthatlayerorthelayerimmediatelybelowit.Youneedawaytoseewhoisonthenetwork,andawaytorequest/accept/terminateconnectionbetweentwooperators.Localoperatorinput/outputwilloccurviaakeypadandLCD.Youmayassumeallnodesonthesystemarewillingtocooperateandnotperformmaliciousactivity.Ontheotherhanditispossiblethatanothercomputeronthenetworkmaynotbepluggedin,orthenetworkconnectionmaybebroken.

Thecommunicationsystembetweentwoormoremicrocontrollerswillbedesignedinthreelayers.Thefirstlayer,thephysicallayerisimplementedbytheUARThardwareandtheinterrupt-drivendevicedriver.Thesecondlayermayconsistofasimplifiedbinarysynchronouscommunicationprotocol(BSC).Atthislevelmessagepacketswillbetransmittedbetweenthetwomachines.ThehighestlevelwillbeakeypadinterpreterandanLCDdisplay.TheLCDshouldshowinteractivefeedbacktotheoperatorcreatesmessagestobesentanddisplaysmessagesreceived.

Lab11.2.Theobjectiveofthislabistodesignapoint-to-pointZigBeenetworkusingtheXBeemodule.Eachmicrocontrollerwillhaveakeypadandatwo-lineLCDdisplay.Whentheoperatortypesonthekeypad,charactersaredisplayedononelineoftheLCD.Whentheoperatorhitsthesendkey,thosecharactersaresentandeventuallydisplayedonthesecondlineoftheLCDoftheothermicrocontroller.Debugthesysteminabottom-upmanner.WritesimplemainprogramsintheRxandTxsystemstotestthelow-levelfunctionalityoftheinterface.Addappropriatedebugginginstruments,suchasprofilesanddumps.Connectunusedpinsfrombothdevicestoasinglelogicanalyzerandrecordathreadprofile(whichprogramrunswhen)asastreamofdataispassedfromtheTxsystemtotheRxsystem.Writeamainprogramtomeasuremaximumbandwidthofyourchannelwithoutusinghardwareflowcontrol.Furthermore,youshoulddeterminewhichcomponentlimitsbandwidth.Modifythetransmittersoitoutputspseudodataasfastaspossible.ModifythereceiversoitdoesnotdisplaydataontheLCD.Rather,thereceiverwillcheckforthispatternofcharactersandcountthenumberoferrors.Addminimallyintrusivedebugginginstrumentstodetermineifandwheredataislost.Measurethemaximumrangeofyoursystem.Inparticular,findthemaximumdistancewherethesystemperformsreliablecommunication.

Appendix1.Glossary1/fnoiseAfundamentalnoiseinresistivedevicesarisingfromfluctuatingconductivity.Sameaspinknoise.

2’scomplement(seetwo’scomplement).

60HznoiseAnaddednoisefromelectromagneticfieldscausedbyeithermagneticfieldinductionorcapacitivecoupling.

accumulatorHigh-speedmemorylocatedintheprocessorusedtoperformarithmeticorlogicalfunctions.AnyoftheregistersR0toR12ontheARMCortex-Mprocessorcanbeusedasaccumulators.

accuracyAmeasureofhowcloseourinstrumentmeasuresthedesiredparameterreferredtotheNIST.

acknowledgeClearingtheinterruptflagbitthatrequestedtheinterrupt.

activethreadAthreadthatisintheready-to-runcircularlinkedlist.Itiseitherrunningorisreadytorun.

actuatorElectro-mechanicalorelectro-chemicaldevicethatallowscomputercommandstoaffecttheexternalworld.

ADCAnalogtodigitalconverter,anelectronicdevicethatconvertsanalogsignals(e.g.,voltage)intodigitalform(i.e.,integers).

addressbusAsetofdigitalsignalsthatconnecttheCPU,memoryandI/Odevices,specifyingthelocationtoreadorwriteforeachbuscycle.Seealsocontrolbusanddatabus.

addressdecoderAdigitalcircuithavingtheaddresslinesasinputandaselectlineasoutput(seeselectsignal)

agingAtechniqueusedinpriorityschedulersthattemporarilyincreasesthepriorityoflowprioritytreadssotheyarerunoccasionally.(Seestarvation)

aliasingWhendigitalvaluessampledatfscontainfrequencycomponentsabove½fs,thentheapparentfrequencyofthedataisshiftedintothe0to½fsrange.SeeNyquistTheory.

alternativesThetotalnumberofpossibilities.E.g.,an8-bitnumberschemecanrepresent256differentnumbers.An8-bitdigitaltoanalogconverter(DAC)cangenerate256differentanalogoutputs.

anodeThepositivesideofadiode.Currententerstheanodesideofadiode.Contrastwithcathode.

answermodemThedevicethatreceivesthetelephonecall.

anti-reset-windupEstablishinganupperboundonthemagnitudeoftheintegralterminaPIDcontroller,sothistermwillnotdominate,whentheerrorsarelarge.

arithmeticlogicunit(ALU)Componentoftheprocessorthatperformsarithmeticandlogicoperations.

armActivatesothatinterruptsarerequested.Triggerflagsthatcanrequestinterruptswillhaveacorrespondingarmbittoallowordisallowthatflagtorequestinterrupts.Contrasttoenable.

armatureThemovingstructureinarelay,thepartthatmoveswhentherelayisactivated.Contrasttoframe.

ASCIIAmericanStandardCodeforInformationInterchange,acodeforrepresentingcharacters,symbols,andsynchronizationmessagesas7bit,8-bitor16-bitbinaryvalues.

assemblerSystemsoftwarethatconvertsanassemblylanguageprogram(humanreadableformat)intoobjectcode(machinereadableformat).

assemblydirectiveOperationsincludedintheprogramthatarenotexecutedbythecomputeratruntime,butratherareinterpretedbytheassemblerduringtheassemblyprocess.Sameaspseudo-op.

assemblylistingInformationgeneratedbytheassemblerinhumanreadableformat,typicallyshowingtheobjectcode,theoriginalsourcecode,assemblyerrors,andthesymboltable.

asynchronousbusAcommunicationprotocolwithoutacentralclockwhereisthedataistransferredusingtwoorthreecontrollinesimplementingahandshakedinteractionbetweenthememoryandthecomputer.

asynchronousprotocolAprotocolwherethetwodeviceshaveseparateanddistinctclocks

atomicSoftwareexecutionthatcannotbedividedorinterrupted.Oncestarted,anatomicoperationwillruntoitscompletionwithoutinterruption.Onmostcomputerstheassemblylanguageinstructionsareatomic.AllinstructionsontheARM®Cortex-Mprocessorareatomicexceptstoreandloadmultiple, STMLDM .

autoinitializationTheprocessofautomaticallyreloadingtheaddressregistersandblocksizecountersattheendofapreviousblocktransfer,sothatDMAtransfercanoccurindefinitelywithoutsoftwareinteraction.

availabilityTheportionofthetotaltimethatthesystemisworking.MTBFisthemeantimebetweenfailures,MTTRisthemeantimetorepair,andavailabilityisMTBF/(MTBF+MTTR).

bandwidthIncommunicationsystems,theinformationtransferrate,theamountofdatatransferredpersecond.Sameasthroughput.Inanalogcircuits,thefrequencyatwhichthegaindropsto0.707ofthenormalvalue.Foralowpasssystem,thefrequencyresponserangesfrom0toamaximumvalue.Forahighpasssystem,thefrequencyresponserangesfromaminimumvaluetoinfinity.Forabandpasssystem,thefrequencyresponserangesfromaminimumtoamaximumvalue.Comparetofrequencyresponse.

bandwidthcouplingModuleAisconnectedtoModuleB,becausedataflowsfromAtoB.

bang-bangAcontrolsystemwheretheactuatorhasonlytwostates,andthesystem“bangs”allthewayinonedirectionor“bangs”allthewayintheother,sameasbinarycontroller.

bank-switchedmemoryAmemorymodulewithtwobanksthatinterfacestotwoseparateaddress/databuses.Atanygiventimeonememorybankisattachedtooneaddress/databustheotherbankisattachedtotheotherbus,butthisattachmentcanbeswitched.

basisSubsetfromwhichlinearcombinationscanbeusedtoreconstructtheentireset.

baudrateIngeneral,thebaudrateisthetotalnumberofbits(information,overhead,andidle)pertimethataretransmitted.Inamodemapplicationitisthetotalnumberofsoundspertimearetransmitted

bi-directionalDigitalsignalsthatcanbeeitherinputoroutput.

biendianTheabilitytoprocessnumbersinbothbigandlittle-endianformats.

bigendianMechanismforstoringmultiplebytenumberssuchthatthemostsignificantbyteexistsfirst(inthesmallestmemoryaddress).Seealsolittleendian.

binaryAsystemthathastwostates,onandoff.

binarycontrollerSameasbang-bang.

binaryrecursionArecursivetechniquethatmakestwocallstoitselfduringtheexecutionofthefunction.Seealsorecursion,linearrecursion,andtailrecursion.

binarysemaphoreAsemaphorethatcanhavetwovalues.Thevalue=1meansOKandthevalue=0meansbusy.Comparetocountingsemaphore.

bipolartransistorEitheraNPNorPNPtransistor.

bipolarsteppermotorAsteppermotorwherethecurrentflowsinbothdirections(in/out)alongtheinterfacewires;astepperwithfourinterfacewires.Contrasttounipolarsteppermotor.

bitBasicunitofdigitalinformationtakingonthevalueofeither0or1.

bitrateTheinformationtransferrate,giveninbitspersecond.Sameasbandwidthandthroughput.

bittimeThebasicunitoftimeusedinserialcommunication.Withserialchannelbittimeis1/baudrate.

blind-cycleAsoftware/hardwaresynchronizationmethodwherethesoftwarewaitsaspecifiedamountoftimeforthehardwareoperationtocomplete.Thesoftwarehasnodirectinformation(blind)aboutthestatusofthehardware.

blockcorrectioncode(BCC)Acode(e.g.,horizontalparity)attachedtotheendofamessageusedtodetectandcorrecttransmissionerrors.

blockedthreadAthreadthatisnotscheduledforrunningbecauseitiswaitingonanexternalevent.

blockingsemaphoreAsemaphorewherethethreadswillblock(sootherthreadscanperformusefulfunctions)whentheyexecutewaitonabusysemaphore.Contrasttospinlocksemaphore.

BoardSupportPackage(BSP)AsetofsoftwareroutinesthatabstracttheI/Ohardwaresuchthatthesamehigh-levelcodecanrunonmultiplecomputers.

borrowDuringsubtraction,ifthedifferenceistoosmall,thenweuseaborrowtopasstheexcessinformationintothenexthigherplace.Forexample,indecimalsubtraction36-27requiresaborrowfromtheonestotensplacebecause6-7istoosmalltofitintothe0to9rangeofdecimalnumbers.

boundedwaitingTheconditionwhereonceathreadbeginstowaitonaresource,thereareafinitenumberofthreadsthatwillbeallowedtoproceedbeforethisthreadisallowedtoproceed.

break-before-makeInadouble-throwrelayordouble-throwswitch,thereisonecommoncontactandtwoseparatecontacts.Break-before-makemeansasthecommoncontactmovesfromoneofseparatecontactstoanother,itwillbreakoff(finishbouncingandnolongertouch)thefirstcontactbeforeitmakes(beginstobounceandstartstotouch)theothercontact.AformCrelayhasabreak-before-makeoperation.

breakortrapAbreakoratrapisadebugginginstrumentthathaltstheprocessor.Witharesidentdebugger,thebreakiscreatedbyreplacingspecificopcodewithasoftwareinterruptinstruction.Whenencountereditwillstopyourprogramandjumpintothedebugger.Therefore,abreakhaltsthesoftware.Theconditionofbeinginthisstateisalsoreferredtoasabreak.

breakdownAtransducerthatstopsfunctioningwhenitsinputgoesaboveamaximumvalueorbelowaminimumvalue.Contrasttodeadzone.

breakpointTheplacewhereabreakisinserted,thetimewhenabreakisencountered,orthetimeperiodwhenabreakisactive.

brushedDCmotorAmotorwherethecurrentreversalsareproducedwithbrushesbetweenthestatorandrotor.TheyarelessexpensivethanbrushlessDCmotors.

brushlessDCmotor(BLDC)Amotorwherethecurrentreversalsareproducedwithshaftsensorsandanelectroniccontroller.TheyarefasterandmorereliablethanbrushedDCmotors.

bufferedI/OAFIFOqueueisplacedinbetweenthehardwareandsoftwareinanattempttoincreasebandwidthbyallowingbothhardwareandsoftwaretoruninparallel.

burnTheprocessofprogrammingaROM,PROMorEEPROM.

burstDMAAnI/Osynchronizationschemethattransfersanentireblockofdataallatoncedirectlyfromaninputdeviceintomemory,ordirectlyfrommemorytoanoutputdevice.

busAsetofdigitalsignalsthatconnecttheCPU,memoryandI/Odevices,consistingofaddresssignals,datasignalsandcontrolsignals.Seealsoaddressbus,controlbusanddatabus.

busbandwidthThenumberofbytestransferredpersecondbetweentheprocessorandmemory.

businterfaceunit(BIU)Componentoftheprocessorthatreadsandwritesdatafromthebus.TheBIUdrivestheaddressandcontrolbuses.

busy-waitsynchronizationAsoftware/hardwaresynchronizationmethodwherethesoftwarecontinuouslyreadsthehardwarestatuswaitingforthehardwareoperationtocomplete.Thesoftwareusuallyperformsnoworkwhilewaitingforthehardware.Sameasgadfly.

byteDigitalinformationcontainingeightbits.

calibrationdriftAchangeoccurringovertimeinthetransferfunctionusedtocalculatethemeasuredvariablefromtherawmeasurements.Forexample,assumewehaveavoltmeterthatusestheequationV=C1*n/1024+C2tocalculatevoltageVinmVfromarawADCvaluen.C1andC2arecalibrationconstantsdeterminedbymeasurementsonknownvoltages.CalibrationdriftoccurswhenthecorrectvaluesforC1andC2varyovertime.

carrierfrequencytheaverageormidvaluesoundfrequencyinthemodem.

carryDuringaddition,ifthesumistoolarge,thenweuseacarrytopasstheexcessinformationintothenexthigherplace.Forexample,indecimaladdition36+27requiresacarryfromtheonestotensplacebecause6+7istoobigtofitintothe0to9rangeofdecimalnumbers.

cathodeThenegativesideofadiode.Currentexitsthecathodesideofadiode.Contrasttoanode.

cathoderaytube(CRT)terminalAnI/Odeviceusedtoinputdatafromakeyboardandoutputcharacterdatatoascreen.Theelectricalinterfaceisusuallyasynchronousserial.

causalThepropertywheretheoutputdependsonthepresentandpastinputs,butnotonanyfutureinputs.

ceilingEstablishinganupperboundontheresultofanoperation.Seealsofloor.

certificationAprocesswhereagoverningbody(e.g.,FDA,NASA,FCC,DODetc.)givesapprovalfortheuseofthedevice.Itusuallyinvolvesdemonstratingthedevicemeetsorexceedssafetyandperformancecriteria.

channelThehardwarethatallowscommunicationtooccur.

checksumThesimplesumofthedata,usuallyinfiniteprecision(e.g.,8,16,24bits).

closedloopcontrolsystemAcontrolsystemthatincludessensorstomeasurethecurrentstatevariables.Theseinputsareusedtodrivethesystemtothedesiredstate.

CMOSAdigitallogicsystemcalledcomplementarymetaloxidesemiconductor.Ithaspropertiesoflowpowerandsmallsize.Itspowerisafunctionofthenumberoftransitionspersecond.Itsspeedisoftenlimitedbycapacitiveloading.

cohesionAcohesivemoduleisonesuchthatallpartsofthemodulearerelatedtoeachothertosatisfyacommonobjective.

commandsignalsThelinesthatspecifygeneralinformationaboutthecurrentcycle;signalsthatspecifywhetherornottoactivateduringthiscycle;thespecifictimesfortheriseandfalledgesareuncertain.Contrasttotimingsignals.

commonanodeLEDdisplayAdisplaywithmultipleLEDs,configuredwithalloftheLEDanodesconnectedtogether,thereareseparateconnectionstothecathodes(currentflowsinthecommonanodeandouttheindividualcathodes).

commoncathodeLEDdisplayAdisplaywithmultipleLEDs,configuredwithalloftheLEDcathodesconnectedtogether,thereareseparateconnectionstotheanodes(currentflowsintheindividualanodesandoutthecommoncathode).

commonmodeForasystemwithdifferentialinputs,thecommonmodepropertiesaredefinedassignalsappliedtobothinputssimultaneously.Contrasttodifferentialmode.

commonmodeinputimpedanceCommonmodeinputvoltagedividedbycommonmodeinputcurrent.

commonmoderejectionratioForadifferentialamplifier,CMRRistheratioofthecommonmodegaindividedbythedifferentialmodegain.AperfectCMRRwouldbezero.

compilerSystemsoftwarethatconvertsahigh-levellanguageprogram(humanreadableformat)intoobjectcode(machinereadableformat).

complexinstructionsetcomputer(CISC)Acomputerwithmanyinstructions,instructionsthathavevaryinglengths,instructionsthatexecuteinvaryingtimes,manyinstructionscanaccessmemory,instructionsthatcanreadandwritememoryinthesamebuscycle,fewerandmorespecializedregisters,andmanydifferenttypesofaddressingmodes.ContrasttoRISC.

compressionratioTheratioofthenumberoforiginalbytestothenumberofcompressedbytes.

concurrentprogrammingAsoftwaresystemthatsupportstwotaskstobeactiveatthesametime.Acomputerwithinterruptsimplementsconcurrentprogramming.

conditioncoderegister(CCR)RegisterintheprocessorthatcontainsthestatusofthepreviousALUoperation,aswellassomeoperatingmodeflagssuchastheinterruptenablebit.

controlbusAsetofdigitalsignalsthatconnecttheCPU,memoryandI/Odevices,specifyingwhentoreadorwriteforeachbuscycle.Seealsoaddressbusanddatabus.

controlcouplingModuleAisconnectedtoModuleB,becauseactionsinAaffectthecontrolpathinB.

controlunit(CU)Componentoftheprocessorthatdeterminesthesequenceofoperations.

cooperativemulti-taskingAschedulerthatcannotsuspendexecutionofathreadwithoutthethread’spermission.Thethreadmustcooperateandsuspenditself.Sameasnonpreemptivescheduler.

countingsemaphoreAsemaphorethatcanhaveanysignedintegervalue.Thevalue>0meansOKandthevalue≤0meansbusy.Comparetobinarysemaphore.

CPUboundAsituationwheretheinputoroutputdeviceisfasterthanthesoftware.InotherwordsittakeslesstimefortheI/Odevicetoprocessdata,thanforthesoftwaretoprocessdata.ContrasttoI/Obound.

CPUcycleAmemorybuscyclewheretheaddressandR/Warecontrolledbytheprocessor.OnmicrocontrollerswithoutDMA,allcyclesareCPUcycles.ContrasttoDMAcycle.

crispinputAninputparametertothefuzzylogicsystem,usuallywithunitslikecm,cm/sec, Cetc.

crispoutputAnoutputparameterfromthefuzzylogicsystem,usuallywithunitslikedynes,wattsetc.

criticalsectionLocationswithinasoftwaremodule,whichifaninterruptweretooccuratoneoftheselocations,thenanerrorcouldoccur(e.g.,datalost,corrupteddata,programcrash,etc.)Sameasvulnerablewindow.

cross-assemblerAnassemblerthatrunsononecomputerbutcreatesobjectcodeforadifferentcomputer.

cross-compilerAcompilerthatrunsononecomputerbutcreatesobjectcodeforadifferentcomputer.

cyclestealDMAAnI/Osynchronizationschemethattransfersdataonebyteatatimedirectlyfromaninputdeviceintomemory,ordirectlyfrommemorytoanoutputdevice.

cyclestretchTheactionwheresomememorycyclesarelongerallowingtimeforcommunicationwithslowermemories,sometimesthememoryitselfrequeststheadditionaltimeandsometimesthecomputerhasapreprogrammedcyclestretchforcertainmemoryaddresses

DACDigitaltoanalogconverter,anelectronicdevicethatconvertsdigitalsignals(i.e.,integers)toanalogform(e.g.,voltage).

dataacquisitionsystemAsystemthatcollectsinformation,sameasinstrument.

databusAsetofdigitalsignalsthatconnecttheCPU,memoryandI/Odevices,specifyingthevaluethatisbeingreadorwrittenforeachbuscycle.Seealsoaddressbusandcontrolbus.

datacommunicationequipment(DCE)Amodemorprinterconnectedaserialcommunicationnetwork.

dataterminalequipment(DTE)Acomputeroraterminalconnectedaserialcommunicationnetwork.

deadzoneAconditionofatransducerwhenalargechangeintheinputcauseslittleornochangeintheoutput.Contrasttobreakdown.

deadlockAscenariothatoccurswhentwoormorethreadsareallblockedeachwaitingfortheotherwithnohopeofrecovery.

decibelAmeasureoftherelativeamplitudeoftwovoltages:dB=20log10(V1/V2).Itisalsoreferstotherelativeamplitudeoftwopowers:dB=10log10(P1/P2).

defuzzificationConversionfromthefuzzylogicoutputvariablestothecrispoutputs.

deskcheckingordryrunWeperformadeskcheck(ordryrun)bydetermininginadvance,eitherbyanalyticalalgorithmorexplicitcalculations,theexpectedoutputsofstrategicintermediatestagesandfinalresultsforasetoftypicalinputs.Wethenrunourprogramcancomparetheactualoutputswiththistemplateofexpectedresults.

devicedriverAcollectionofsoftwareroutinesthatperformI/Ofunctions.

differentialmodeForasystemwithdifferentialinputs,thedifferentialmodepropertiesaredefinedassignalsappliedasadifferencebetweenthetwoinputs.Contrasttocommonmode.

differentialmodeinputimpedanceDifferentialmodeinputvoltagedividedbydifferentialmodeinputcurrent.

digitalsignalprocessingProcessingofdatawithdigitalhardwareorsoftwareafterthesignalhasbeensampledbytheADC,e.g.,filters,detectionandcompression/decompression.

directmemoryaccess(DMA)theabilitytotransferdatabetweentwomodulesonthebus,thistransferisusuallyinitiatedbythehardware(deviceneedsservice)andthesoftwareconfiguresthecommunication,butthedataistransferredwithoutexplicitsoftwareactionforeachindividualpieceofdata

directionregisterSpecifieswhetherabi-directionalI/Opinisaninputoranoutput.Wesetadirectionregisterbitto0(or1)tospecifythecorrespondingI/Opintobeinput(oroutput.)

disarmDeactivatesothatinterruptsarenotrequested,performedbyclearingthearmbit.

DiscreteFourierTransform(DFT)Atechniquetoconvertdatainthetimedomaintodatainthefrequencydomain.Ndatapointsaresampledatfs.Theresultingfrequencyresolutionisfs/N.

DMADirectMemoryAccessisasoftware/hardwaresynchronizationmethodwherethehardwareitselfcausesadatatransferbetweentheI/Odeviceandmemoryattheappropriatetimewhendataneedstobetransferred.Thesoftwareusuallycanperformotherworkwhilewaitingforthehardware.Nosoftwareactionisrequiredforeachindividualbyte.

DMAcycleAmemorybuscyclewheretheaddressandR/WarecontrolledbytheDMAcontroller.ContrasttoCPUcycle.

doublebyteTwobytescontaining16bits.Sameashalfword.

double-polerelayTwoseparateandcompleterelays,whichareactivatedtogether.Contrasttosinglepole.

double-poleswitchTwoseparateandcompleteswitches.Thetwoswitchesareelectricallyseparate,butmechanicallyconnected.Suchthatbothswitchesareactivatedtogether.Contrasttosinglepole.

double-throwrelayArelaywiththreecontactconnections,onecommonandtwothrows.Thecommonwillbeconnectedtoexactlyofonethetwothrows(seesingle-throw).

double-throwswitchAswitchwiththreecontactconnections.Thecentercontactwillbeconnectedexactlyoneoftheothertwocontacts.Contrastwithsingle-throw.

doublewordTwowordscontaining64bits.

downloadTheprocessoftransferringobjectcodefromthehost(e.g.,thePC)tothetargetmicrocontroller.

drop-outAnerrorthatoccursafterarightshiftoradivide,andtheconsequenceisthatanintermediateresultlosesitsabilitytorepresentallofthevalues.E.g.,I=100*(N/51)canonlyresultinthevalues0,100,or200,whereasI=(100*N)/51properlycalculatesthedesiredresult.

dualaddressDMADirectmemoryaccessthatrequirestwobuscyclestotransferdatafromaninputdeviceintomemory,orfrommemorytoanoutputdevice.

dualportmemoryAmemorymodulethatinterfacestotwoseparateaddress/databuses,andallowsbothsystemsread/writeaccessthedata.

dutycycleForaperiodicdigitalwave,itisthepercentageoftimethesignalishigh.WhenanLEDdisplayisscanned,itisthepercentageoftimeeachLEDisactive.Amotorinterfacedusingpulse-width-modulationallowsthecomputertocontroldeliveredpowerbyadjustingthedutycycle.

dynamicallocationDatastructuresliketheTCBthatarecreatedatruntimebycallingmalloc()andexistuntilthesoftwarereleasesthememoryblockbacktotheheapbycallingfree().Contrasttostaticallocation.

dynamicRAMVolatileread/writestoragebuiltfromacapacitorandasingletransistorhavingalowcost,butrequiringrefresh.ContrastwithstaticRAM.

EEPROMElectricallyerasableprogrammablereadonlymemorythatisnonvolatileandeasytoreprogram.EEPROMcanbeerasedandreprogrammedmultipletimes.

embeddedcomputersystemAsystemthatperformsaspecificdedicatedoperationwherethecomputerishiddenorembeddedinsidethemachine.

emulatorAnin-circuitemulatorisanexpensivedebugginghardwaretoolthatmimicstheprocessorpinouts.Todebugwithanemulator,youwouldremovetheprocessorchipandattachtheemulatorcableintotheprocessorsocket.Theemulatorwouldsensetheprocessorinputsignalsandrecreatetheprocessoroutputssignalsonthesocketasifanactualprocessorwereactuallythere,runningatfullspeed.Insidetheemulatoryouhaveinternalread/writeaccesstotheregistersandprocessorstate.Mostemulatorsallowyoutovisualize/recordstrategicinformationinreal-timewithouthaltingtheprogramexecution.YoucanalsoremoveROMchipsandinserttheconnectorofaROM-emulator.Thistypeofemulatorislessexpensive,anditallowsyoutodebugROM-basedsoftwaresystems.

EPROMSameasPROM.Electricallyprogrammablereadonlymemorythatisnonvolatileandrequiresexternaldevicestoeraseandreprogram.ItisusuallyerasedusingUVlight.

eraseTheprocessofclearingtheinformationinaPROMorEEPROM,usingelectricityorUVlight.Theinformationbitsareusuallyallsettologic1.

EVBEvaluationBoard,aboard-levelproductusedtodevelopmicrocontrollersystems.

evenparityAcommunicationprotocolwherethenumberofonesinthedataplusaparitybitisanevennumber.Contrastwithoddparity.

externalfragmentationAconditionwhenthelargestfileormemoryblockthatcanbeallocatedislessthanthetotalamountoffreespaceonthediskormemory.

fanoutThenumberofinputsthatasingleoutputcandriveifthedevicesareallinthesamelogicfamily.

FastFourierTransform(FFT)Afasttechniquetoconvertdatainthetimedomaintodatainthefrequencydomain.Ndatapointsaresampledatfs.Theresultingfrequencyresolutionisfs/N.Mathematically,theFFTisthesameastheDFT,justfaster.

FETFieldeffecttransistor,alsoJFET.

filterInthedebuggingcontext,afilterisaBooleanfunctionorconditionaltestusedtomakerun-timedecisions.Forexample,ifweprintinformationonlyiftwovariablesx,yareequal,thentheconditional(x==y)isafilter.Filterscaninvolvehardwarestatusaswell.

finiteimpulseresponsefilter(FIR)Adigitalfilterwheretheoutputisafunctionofafinitenumberofcurrentandpastdatasamples,butnotafunctionofpreviousfilteroutputs.

FiniteStateMachine(FSM)Anabstractdesignmethodtobuildamachinewithinputsandoutputs.Themachinecanbeinoneofafinitenumberofstates.Whichstatethesystemisinrepresentsmemoryofpreviousinputs.Theoutputandnextstateareafunctionoftheinput.Theremaybetimedelaysaswell.

fixed-pointAtechniquewherecalculationsinvolvingnonintegersareperformedusingasequenceofintegeroperations.E.g.,0.123*xisperformedindecimalfixed-pointas(123*x)/1000orinbinaryfixed-pointas(126*x)>>10.

flashEEPROMElectricallyerasableprogrammablereadonlymemorythatisnonvolatileandeasytoreprogram.FlashEEPROMsaretypicallylargerthanregularEEPROM.

floatingAlogicstatewheretheoutputdevicedoesnotdrivehighorpulllow.Theoutputsofopencollectorandtristatedevicescanbeinthefloatingstate.SameasHiZ.

floorEstablishingalowerboundontheresultofanoperation.Seealsoceiling.

followerAnanalogcircuitwithgainequalto1,largeinputimpedanceandsmalloutputimpedance.Sameasvoltagefollower.

frameAcompleteanddistinctpacketofbitsoccurringinaserialcommunicationchannel.

frameThefixedstructureinarelayortransducer.Contrasttoarmature.

framingerrorAnerrorwhenthereceiverexpectsastopbit(1)andtheinputis0.

frequencyresponseThefrequencyatwhichthegaindropsto0.707ofthenormalvalue.Foralowpasssystem,thefrequencyresponserangesfrom0toamaximumvalue.Forahighpasssystem,thefrequencyresponserangesfromaminimumvaluetoinfinity.Forabandpasssystem,thefrequencyresponserangesfromaminimumtoamaximumvalue.Sameasbandwidth.

frequencyshiftkey(FSK)Amodemthatmodulatesthedigitalsignalsintofrequencyencodedsinewaves.

friendlyFriendlysoftwaremodifiesjustthebitsthatneedtobemodified,leavingtheotherbitsunchanged,makingtoeasiertocombinemodules.

fullduplexchannelHardwarethatallowsbits(information,errorchecking,synchronizationoroverhead)totransfersimultaneouslyinbothdirections.Contrastwithsimplexandhalfduplexchannels.

fullduplexcommunicationAsystemthatallowsinformation(data,characters)totransfersimultaneouslyinbothdirections.

functionaldebuggingTheprocessofdetecting,locating,orcorrectingfunctionalandlogicalerrorsinaprogram,typicallynotinvolvingtime.Theprocessofinstrumentingaprogramforsuchpurposesiscalledfunctionaldebuggingoroftensimplydebugging.

fuzzificationConversionfromthecrispinputstothefuzzylogicinputvariables.

fuzzylogicBooleanlogic(true/false)thatcantakeonarangeofvaluesfromtrue(255)tofalse(0).Fuzzylogicandiscalculatedastheminimum.Fuzzylogicoristhemaximum.

gadflyAsoftware/hardwaresynchronizationmethodwherethesoftwarecontinuouslyreadsthehardwarestatuswaitingforthehardwareoperationtocomplete.Thesoftwareusuallyperformsnoworkwhilewaitingforthehardware.Sameasbusywait.

gaugefactorThesensitivityofastraingaugetransducer,i.e.,slopeoftheresistanceversusdisplacementresponse.

gibibyte(GiB)230or1,073,741,824bytes.Comparetogigabyte,whichis1,000,000,000bytes.

halfduplexchannelHardwarethatallowsbits(information,errorchecking,synchronizationoroverhead)totransferinbothdirections,butinonlyonedirectionatatime.Contrastwithsimplexandfullduplexchannels.

halfduplexcommunicationAsystemthatallowsinformationtotransferinbothdirections,butinonlyonedirectionatatime.

halfwordTwobytescontaining16bits.Sameasdoublebyte.

handshakeAsoftware/hardwaresynchronizationmethodwherecontrolandstatussignalsgobothdirectionsbetweenthetransmitterandreceiver.Thecommunicationisinterlockedmeaningeachdevicewillwaitfortheother.

hardreal-timeAsystemthatcanguaranteethataprocesswillcompleteacriticaltaskwithinacertainspecifiedrange.Indataacquisitionsystems,hardreal-timemeansthereisanupperboundonthelatencybetweenwhenasampleissupposedtobetaken(every1/fs)andwhentheADCisactuallystarted.Hardreal-timealsoimpliesthatnoADCsamplesaremissed.

heartbeatAdebuggingmonitor,suchasaflashingLED,weaddforthepurposeofseeingifourprogramisrunning.

hexadecimalAnumbersystemthatusesbase16.

HiZAlogicstatewheretheoutputdevicedoesnotdrivehighorpulllow.Theoutputsofopencollectorandtristatedevicescanbeinthefloatingstate.Sameasfloating.

holdtimeWhenlatchingdataintoadevicewitharisingorfallingedgeofaclock,theholdtimeisthetimeaftertheactiveedgeoftheclockthatthedatamustcontinuetobevalid.Seesetuptime.

hookAnindirectfunctioncalladdedtoasoftwaresystemthatallowstheusertoattachtheirprogramstorunatstrategictimes.Theseattachmentsarecreatedatruntimeanddonotrequirerecompiling.

hysteresisAconditionwhentheoutputofasystemdependsnotonlyontheinput,butalsoonthepreviousoutputs,e.g.,atransducerthatfollowsadifferentresponsecurvewhentheinputisincreasingthanwhentheinputisdecreasing.

I/OboundAsituationwheretheinputoroutputdeviceisslowerthanthesoftware.InotherwordsittakeslongerfortheI/Odevicetoprocessdata,thanforthesoftwaretoprocessdata.ContrasttoCPUbound.

I/OdeviceHardwareandsoftwarecomponentscapableofbringinginformationfromtheexternalenvironmentintothecomputer(inputdevice),orsendingdataoutfromthecomputertotheexternalenvironment(outputdevice.)

I/OportAhardwaredevicethatconnectstheinternalsoftwarewithexternalhardware.

IIHInputcurrentwhenthesignalishigh.

IILInputcurrentwhenthesignalislow.

immediateAnaddressingmodewheretheoperandisafixeddataoraddressvalue.

impedanceloadingAconditionwhentheinputofstagen+1ofananalogsystemaffectstheoutputofstagen,becausetheinputimpedanceofstagen+1istoosmallandtheoutputimpedanceofstagenistoolarge.

impedanceTheratiooftheeffort(voltage,force,pressure)dividedbytheflow(current,velocity,flow).

incrementalcontrolsystemAcontrolsystemwheretheactuatorhasmanypossiblestates,andthesystemincrementsordecrementstheactuatorvaluedependingoneitherinerrorispositiveornegative.

indexedAnaddressingmodewherethedataoraddressvaluefortheinstructionislocatedinmemorypointedtobyanindexregister.

infiniteimpulseresponsefilter(IIR)Adigitalfilterwheretheoutputisafunctionofaninfinitenumberofpastdatasamples,usuallybymakingthefilteroutputafunctionofpreviousfilteroutputs.

inputbiascurrentDifferencebetweencurrentsofthetwoopampinputs.

inputcaptureAmechanismtosetaflagandcapturethecurrenttime(TCNTvalue)ontherising,fallingorrising&fallingedgeofanexternalsignal.Theinputcaptureeventcanalsorequestaninterrupt.

inputimpedanceInputvoltagedividedbyinputcurrent.

inputnoisecurrentCurrentnoiserefereedtotheopampinputs.

inputnoisevoltageVoltagenoiserefereedtotheopampinputs.

inputoffsetcurrentAveragecurrentintothetwoopampinputs.

inputoffsetvoltageVoltagedifferencebetweenthetwoopampinputsthatmakestheoutputzero.

instructionregister(IR)Registerinthecontrolunitthatcontainstheopcodeforthecurrentinstruction.

instrumentAninstrumentisthecodeinjectedintoaprogramfordebuggingorprofiling.Thiscodeisusuallyextraneoustothenormalfunctionofaprogramandmaybetemporaryorpermanent.Aprintstatementaddedtoyoursourcecodeisanexampleofapermanentinstrument,becauseremovalrequireseditingandrecompiling.

instrumentAembeddedsystemthatcollectsinformation,sameasdataacquisitionsystem.

instrumentationThedebuggingprocessofinjectingorinsertinganinstrument.

instrumentationampAdifferentialamplifieranalogcircuit,whichcanhavelargegain,largeinputimpedance,smalloutputimpedance,andagoodcommonmoderejectionration.

internalfragmentationStoragethatisallocatedfortheconvenienceoftheoperatingsystembutcontainsnoinformation.Thisspaceiswasted.

interruptAsoftware/hardwaresynchronizationmethodwherethehardwarecausesaspecialsoftwareprogram(interrupthandler)toexecutewhenitsoperationtocomplete.Thesoftwareusuallycanperformotherworkwhilewaitingforthehardware.

interruptflagAstatusbitthatissetbythetimerhardwaretosignifyanexternaleventhasoccurred.

interruptmaskAcontrolbitthat,ifprogrammedto1,willcauseaninterruptrequestwhentheassociatedflagisset.Sameasarm.

interruptserviceroutine(ISR)Programthatrunsasaresultofaninterrupt.

interruptvector32-bitvaluesinROMspecifyingwherethesoftwareshouldexecuteafteraninterruptrequest.Thereisauniqueinterruptvectorforeachtypeofinterruptincludingreset.

intrusiveThedebuggeritselfaffectstheprogrambeingtested.Seenonintrusive.

InverseDiscreteFourierTransform(IDFT)Atechniquetoconvertdatainthefrequencydomaintodatainthetimedomain.ThereareNdatapointsandthesamplingrateisfs.Theresultingfrequencyresolutionisfs/N.

invocationcouplingModuleAisconnectedtoModuleB,becauseAcallsB.

IOHOutputcurrentwhenthesignalishigh.ThisisthemaximumcurrentthathasavoltageaboveVOH.

IOLOutputcurrentwhenthesignalislow.ThisisthemaximumcurrentthathasavoltagebelowVOL.

isolatedI/OAconfigurationwheretheI/Odevicesareinterfacedtothecomputerinamannerdifferentthanthewaymemoriesareconnected.

jerkThechangeinacceleration;thederivativeoftheacceleration.Thirdderivativeofshaftangle.

JohnsonnoiseAfundamentalnoiseinresistivedevicesarisingfromtheuncertaintyaboutthepositionandvelocityofindividualmolecules.Sameasthermalnoiseandwhitenoise.

Karnaughmaptabularrepresentationoftheinput/outputrelationshipforacombinationaldigitalfunction,theinputspossibilitiesareplacedintherowandcolumnlabels,andtheoutputvaluesareplacedinsidethetable

kibibyte(KiB)210or1024bytes.Comparetokilobyte,whichis1000bytes.

latchAsanoun,itmeansaregister.Asaverb,itmeanstostoredataintotheregister.

latchedinputportAninputportwherethesignalsarelatched(saved)onanedgeofanassociatedstrobesignal.

latencyInthisbooklatencyusuallyreferstotheresponsetimeofthecomputertoexternalevents.Forexample,thetimebetweennewinputbecomingavailableandthetimetheinputisreadbythecomputer.Forexample,thetimebetweenanoutputdevicebecomingidleandthetimetheinputisthecomputerwritesnewdatatoit.TherecanalsobealatencyforanI/Odevice,whichistheresponsetimeoftheexternalI/Odevicehardwaretoasoftwarecommand.

LCDLiquidCrystalDisplay,wherethecomputercontrolsthereflectanceortransmittanceoftheliquidcrystal,characterizedbyitsflexibledisplaypatterns,lowpower,andslowspeed.

LEDLightEmittingDiode,wherethecomputercontrolstheelectricalpowertothediode,characterizedbyitssimpledisplaypatterns,mediumpower,andhighspeed.

light-weightprocessSameasathread.

linearfilterAfilterwheretheoutputisalinearcombinationofitsinputs.

linearrecursionArecursivetechniquethatmakesonlyonecalltoitselfduringtheexecutionofthefunction.Linearrecursivefunctionsareeasiertoimplementiteratively.Wedrawtheexecutionpatternasastraightorlinearpath.Seealsorecursion,binaryrecursion,andtailrecursion.

linearvariabledifferentialtransformer(LVDT)Atransducerthatconvertspositionintoelectricvoltage.

littleendianMechanismforstoringmultiplebytenumberssuchthattheleastsignificantbyteexistsfirst(inthesmallestmemoryaddress).Contrastwithbigendian.

loaderSystemsoftwarethatplacestheobjectcodeintothemicrocontroller’smemory.IftheobjectcodeisstoredinEPROM,theloaderisalsocalledaEPROMprogrammer.

LocalAreaNetwork(LAN)Aconnectionbetweencomputersconfinedtoasmallspace,suchasaroomorabuilding.

logicanalyzerAhardwaredebuggingtoolthatallowsyoutovisualizemanydigitallogicsignalsversustime.Reallogicanalyzershaveatleast32channelsandcanhaveupto200channels,withsophisticatedtechniquesfortriggering,savingandanalyzingthereal-timedata.

LSBTheleastsignificantbitinanumbersystemisthebitwiththesmallestsignificance,usuallytheright-mostbit.WithsignedorunsignedintegersthesignificanceoftheLSBis1.

maintenanceProcessofverifying,changing,correcting,enhancing,andextendingasystem.

makebeforebreakinadouble-throwrelayordouble-throwswitch,thereisonecommoncontactandtwoseparatecontacts.Makebeforebreakmeansasthecommoncontactmovesfromoneofseparatecontactstoanother,itwillmake(finishingbouncing)thesecondcontactbeforeitbreaksoff(startbouncing)thefirstcontact.AformDrelayhasamakebeforebreakoperation.

mailboxAformalcommunicationstructure,similartoaFIFOqueue,wherethesourcetaskputsdataintothemailboxandthesinktaskgetsdatafromthemailbox.Themailboxcanholdatmostonepieceofdataatatime,andhastwostates:mailboxhasvaliddataormailboxisempty.

markAdigitalvalueoftrueorlogic1.Contrastwithspace.

maskAsaverb,maskistheoperationthatselectscertainbitsoutofmanybits,usingthelogicalandoperation.Thebitsthatarenotbeingselectedwillbeclearedtozero.Whenusedasanoun,maskreferstothespecificbitsthatarebeingselected.

MealyFSMAFSMwheretheboththeoutputandnextstateareafunctionoftheinputandstate

measurandAsignalmeasuredbyadataacquisitionsystem.

mebibyte(MiB)220or1,048,576bytes.Comparetomegabyte,whichis1,000,000bytes.

membershipsetsFuzzylogicvariablesthatcantakeonarangeofvaluesfromtrue(255)tofalse(0).

memoryAcomputercomponentcapableofstoringandrecallinginformation.

memory-mappedI/OAconfigurationwheretheI/Odevicesareinterfacedtothecomputerinamanneridenticaltothewaymemoriesareconnected.

microcomputerAsmallelectronicdevicecapableofperforminginput/outputfunctionscontainingamicroprocessor,memory,andI/Odevices,wheresmallmeansyoucancarryit.

microcontrollerAsinglechipmicrocomputerliketheTIMSP430,Freescale9S12,Intel8051,PIC16,ortheTexasInstrumentsTM4C123.

mnemonicThesymbolicnameofanoperationcode,likemovstrpush .

modemAnelectronicdevicethatMOdulatesandDEModulatesacommunicationsignal.Usedinserialcommunicationacrosstelephonelines.

monitorAmonitorisadebuggerfeaturethatallowsustopassivelyviewstrategicsoftwareparametersduringthereal-timeexecutionofourprogram.Aneffectivemonitorisonethathasminimaleffectontheperformanceofthesystem.Whendebuggingsoftwareonawindows-basedmachine,wecanoftensetupadebuggerwindowthatdisplaysthecurrentvalueofcertainsoftwarevariables.

MooreFSMAFSMwheretheboththeoutputisonlyafunctionofthestateandthenextstateisafunctionoftheinputandstate

MOSFETMetaloxidesemiconductorfieldeffecttransistor.

MSBThemostsignificantbitinanumbersystemisthebitwiththegreatestsignificance,usuallytheleft-mostbit.Ifthenumbersystemissigned,thentheMSBsignifiespositive(0)ornegative(1).

multipleaccesscircularqueueMACQAdatastructureusedindataacquisitionsystemstoholdthecurrentsampleandafinitenumberofprevioussamples.Itisalwaysfullandnewdataoverwritestheoldest.

multi-threadedAsystemwithmultiplethreads(e.g.,mainprogramandinterruptserviceroutines)thatcooperatetowardsacommonoverallgoal.

mutualexclusionormutexThreadsynchronizationwhereatmostonethreadatatimeisallowedtoenter.

negativefeedbackAnanalogsystemwithnegativegainfeedbackpaths.Thesesystemsareoftenstable.

negativelogicAsignalwherethetruevaluehasalowervoltagethanthefalsevalue,indigitallogictrueis0andfalseis1,inTTLlogictrueislessthan0.7voltsandfalseisgreaterthan2volts,inRS232protocoltrueis-12voltsandfalseis+12volts.Contrastwithpositivelogic.

nibble4binarybitsor1hexadecimaldigit.

nonatomicSoftwareexecutionthatcanbedividedorinterrupted.MostlinesofCcoderequiremultipleassemblylanguageinstructionstoexecute,thereforeaninterruptmayoccurinthemiddleofalineofCcode.Theinstuctionsstoreandloadmultiple, STMLDM ,arenonatomic.

nonintrusiveAcharacteristicwhenthepresenceofthecollectionofinformationitselfdoesnotaffecttheparametersbeingmeasured.Nonintrusivenessisthecharacteristicorqualityofadebuggerthatallowsthesoftware/hardwaresystemtooperatenormallyasifthedebuggerdidnotexist.Intrusivenessisusedasameasureofthedegreeofperturbationcausedinprogramperformancebyaninstrument.Forexample,aprintstatementaddedtoyoursourcecodeandsingle-steppingareveryintrusivebecausetheysignificantlyaffectthereal-timeinteractionofthehardwareandsoftware.Whenaprograminteractswithreal-timeevents,theperformanceissignificantlyaltered.Ontheotherhand,aninstrumentthattogglesanLEDonandoff(requiringjust1µstoexecute)ismuchlessintrusive.Alogicanalyzerthatpassivelymonitorstheaddressanddatabyiscompletelynonintrusive.Anin-circuitemulatorisalsonon-intrusivebecausethesoftwareinput/outputrelationshipswillbethesamewithandwithoutthedebuggingtool.

nonlinearfilterAfilterwheretheoutputisnotalinearcombinationofitsinputs.E.g.,median,minimum,maximumareexamplesofnonlinearfilters.Contrasttolinearfilter.

nonpreemptiveschedulerAschedulerthatcannotsuspendexecutionofathreadwithoutthethread’spermission.Thethreadmustcooperateandsuspenditself.Sameascooperativemulti-tasking.

nonreentrantAsoftwaremodulewhichoncestartedbyonethread,shouldnotbeinterruptedandexecutedbyasecondthread.AnonreentrantmodulesusuallyinvolvenonatomicaccessestoglobalvariablesorI/Oports:readmodifywrite,writefollowedbyread,oramultistepwrite.

nonvolatileAconditionwhereinformationisnotlostwhenpowerisremoved.Whenpowerisrestored,thentheinformationisinthestatethatoccurredwhenthepowerwasremoved.

NyquistTheoremIfainputsignaliscapturedbyanADCattheregularrateoffssamples/sec,thenthedigitalsequencecanaccuratelyrepresentthe0to½fsfrequencycomponentsoftheoriginalsignal.

objectcodeProgramsinmachinereadableformatcreatedbythecompilerorassembler.TheS19recordscontainobjectcode.

oddparityAcommunicationprotocolwherethenumberofonesinthedataplusaparitybitisanoddnumber.Contrastwithevenparity.

opampAnintegratedanalogcomponentwithtwoinputs,(V2,V1)andanoutput(Vout),whereVout=K•(V2-V1).Theamphasaverylargegain,K.Sameasoperationalamplifier.

opcode,opcode,oroperationcodeAspecificinstructionexecutedbythecomputer.Theopcodealongwiththeoperandcompletelyspecifiesthefunctiontobeperformed.Inassemblylanguageprogramming,theopcodeisrepresentedbyitsmnemonic,like LDR .Duringexecution,theopcodeisstoredasamachinecodeloadedinmemory.

opencollectorAdigitallogicoutputthathastwostateslowandHiZ.Sameasopendrainandwire-or-mode.

opendrainAdigitallogicoutputthathastwostateslowandHiZ.Sameasopencollectorandwire-or-mode.

openloopcontrolsystemAcontrolsystemthatdoesnotincludesensorstomeasurethecurrentstatevariables.Ananalogsystemwithnofeedbackpaths.

operandThesecondpartofaninstructionthatspecifieseitherthedataortheaddressforthatinstruction.Anassemblyinstructiontypicallyhasanopcodeandanoperand(e.g.,#55).Instructionsthatuseinherentaddressingmodehavenooperandfield.

operatingsystemSystemsoftwareformanagingcomputerresourcesandfacilitatingcommonfunctionslikeinput/output,memorymanagement,andfilesystem.

originatemodemthedevicethatplacesthetelephonecall.

oscilloscopeAhardwaredebuggingtoolthatallowsyoutovisualizeoneortwoanalogsignalsversustime.

outputcompareAmechanismtocauseaflagtobesetandanoutputpintochangewhenthetimermatchesapresetvalue.Theoutputcompareeventcanalsorequestaninterrupt.

outputimpedanceOpencircuitoutputvoltagedividedbyshortcircuitoutputcurrent.

overflowAnerrorthatoccurswhentheresultofacalculationexceedstherangeofthenumbersystem.Forexample,with8-bitunsignedintegers,200+57willyieldtheincorrectresultof1.

overrunerrorAnerrorthatoccurswhenthereceivergetsanewframebutthereceiveFIFOandshiftregisteralreadyhaveinformation.

parallelportAportwhereallsignalsareavailablesimultaneously.E.g.,PortAisan8-bitparallelport.

parallelprogrammingAsoftwaresystemthatsupportstwoormoreprogramsbeingexecutedatthesametime.Acomputerwithmultiplecoresimplementsparallelprogramming.

partiallyasynchronousbusacommunicationprotocolthathasacentralclockbutthememorymodulecandynamicallyextendthelengthofabuscycle(cyclestretch)ifitneedsmoretime

pathexpressionAsoftwaretechniquetoguaranteesubfunctionswithinamoduleareexecutedinapropersequence.Forexample,itforcestheusertoinitializeI/OdevicebeforeattemptingtoperformI/O.

PC-relativeaddressingAnaddressingmodewheretheeffectiveaddressiscalculatedbyitspositionrelativetothecurrentvalueoftheprogramcounter.

performancedebuggingorprofilingTheprocessofacquiringormodifyingtimingcharacteristicsandexecutionpatternsofaprogramandtheprocessofinstrumentingaprogramforsuchpurposesiscalledperformancedebuggingorprofiling.

periodicpollingAsoftware/hardwaresynchronizationmethodthatisacombinationofinterruptsandbusywait.Aninterruptoccursataregularrate(periodic)independentofthehardwarestatus.Theinterrupthandlerchecksthehardwaredevice(polls)todetermineifitsoperationiscomplete.Thesoftwareusuallycanperformotherworkwhilewaitingforthehardware.

PersonalAreaNetwork(PAN)Aconnectionbetweencomputerscontrolledbyasinglepersonorallworkingtowardforawell-definedsingletask.

phaseshiftkey(PSK)aprotocolthatencodestheinformationasphasechangesbetweenthesounds.

photosensorAtransducerthatconvertsreflectedortransmittedlightintoelectriccurrent.

physicalplantThephysicaldevicebeingcontrolled.

PIDcontrollerAcontrolsystemwheretheactuatoroutputdependsonalinearcombinationofthecurrenterror(P),theintegraloftheerror(I)andthederivativeoftheerror(D).

pinknoiseAfundamentalnoiseinresistivedevicesarisingfromfluctuatingconductivity.Sameas1/fnoise.

poleAplaceinthefrequencydomainwherethefiltergainisinfinite.

pollingAsoftwarefunctiontolookandseewhichofthepotentialsourcesrequestedtheinterrupt.

portExternalpinsthroughwhichthemicrocontrollercanperforminput/output.SameasI/Oport.

positivefeedbackAnanalogsystemwithpositivegainfeedbackpaths.Thesesystemswillsaturate.

positivelogicasignalwherethetruevaluehasahighervoltagethanthefalsevalue,indigitallogictrueis1andfalseis0,inTTLlogictrueisgreaterthan2voltsandfalseislessthan0.7volts,inRS232protocoltrueis+12voltsandfalseis-12volts.Contrastwithnegativelogic.

potentiometerAtransducerthatconvertspositionintoelectricresistance.

precisionAtermspecifyingthedegreesoffreedomfromrandomerrors.Foraninputsignal,itisthenumberofdistinguishableinputsignalsthatcanbereliablydetectedbythemeasurement.Foranoutputsignal,itisthenumberofdifferentoutputparametersthatcanbeproducedbythesystem.Foranumbersystem,precisionisthenumberofdistinctordifferentvaluesofanumbersysteminunitsof“alternatives”.Theprecisionofanumbersystemisalsothenumberofbinarydigitsrequiredtorepresentallitsnumbersinunitsof“bits”.

preemptiveschedulerAschedulerthathasthepowertosuspendexecutionofathreadwithoutthethread’spermission.

priorityWhentworequestsforservicearemadesimultaneously,prioritydetermineswhichordertoprocessthem.

privateCanbeaccessedonlybysoftwaremodulesinthatlocalgroup.

privatevariableAvariablethatisusedbyasinglethread,andnotsharedwithotherthreads.

processTheexecutionofsoftwarethatdoesnotnecessarilycooperatewithotherprocesses.

producer-consumerAmultithreadedsystemwheretheproducersgeneratenewdata,andtheconsumersprocessoroutputthedata.

profilingSeeperformancedebugging.

programcounter(PC)Aregisterintheprocessorthatpointstothememorycontainingtheinstructiontoexecutenext.

PROMSameasEPROM.Programmablereadonlymemorythatisnonvolatileandrequiresexternaldevicestoeraseandreprogram.ItisusuallyerasedusingUVlight.

promotionIncreasingtheprecisionofanumberforconvenienceortoavoidoverflowerrorsduringcalculations.

pseudointerruptvectorAsecondaryplacefortheinterruptvectorsfortheconvenienceofthedebugger,becausethedebuggercannotordoesnotwanttheusertomodifytherealinterruptvectors.Theyprovideflexibilityfordebuggingbutincuraruntimedelayduringexecution.

pseudoopOperationsincludedintheprogramthatarenotexecutedbythecomputeratruntime,butratherareinterpretedbytheassemblerduringtheassemblyprocess.Sameasassemblydirective.

pseudo-codeAshorthandfordescribingasoftwarealgorithm.Theexactformatisnotdefined,butmanyprogrammersusetheirfavoritehigh-levellanguagesyntax(likeC)withoutpayingrigorousattentiontothepunctuation.

publicCanbeaccessedbyanysoftwaremodule.

publicvariableAglobalvariablethatissharedbymultipleprogramsorthreads.

pulsewidthmodulationAtechniquetodeliveravariablesignal(voltage,power,energy)usinganon/offsignalwithavariablepercentageoftimethesignalison(dutycycle).Sameasvariabledutycycle.

QTheQofabandpassfilter(passesfmintofmax)isthecenterpassfrequency(fo=(fmax+fmin)/2)dividedbythewidthofthepassregion,Q=fo/(fmax-fmin).TheQofabandrejectfilter(rejectsfmintofmax)isthecenterrejectfrequency(fo=(fmax+fmin)/2)dividedbythewidthoftherejectregion,Q=fo/(fmax-fmin).

quadratureamplitudemodem(QAM)aprotocolthatusedboththephaseandamplitudetoencodeupto6bitsontoeachbaud.

qualitativeDASADASthatcollectsinformationnotintheformofnumericalvalues,butratherintheformofthequalitativesenses,e.g.,sight,hearing,smell,tasteandtouch.AqualitativeDASmayalsodetectthepresenceorabsenceofconditions.

quantitativeDASADASthatcollectsinformationintheformofnumericalvalues.

RAMRandomAccessMemory,atypeofmemorywhereistheinformationcanbestoredandretrievedeasilyandquickly.Sinceitisvolatiletheinformationislostwhenpowerisremoved.

rangeIncludesboththesmallestpossibleandthelargestpossiblesignal(inputoroutput).Thedifferencebetweenthelargestandsmallestinputthatcanbemeasuredbytheinstrument.Theunitsareintheunitsofthemeasurand.Whenprecisionisinalternatives,range=precision•resolution.Sameasspan

readcycledataflowsfromthememoryorinputdevicetotheprocessor,theaddressbusspecifiesthememoryorinputdevicelocationandthedatabuscontainstheinformationatthataddress

readdataavailableThetimeinterval(start,end)duringwhichthedatawillbevalidduringareadcycle,determinedbythememorymodule

real-timeAcharacteristicofasystemthatcanguaranteeanupperbound(worstcase)onlatency.

real-timesystemAsystemwheretime-criticaloperationsoccurwhenneeded.

recursionAprogrammingtechniquewhereafunctioncallsitself.

reducedinstructionsetcomputer(RISC)Acomputerwithafewinstructions,instructionswithfixedlengths,instructionsthatexecutein1or2buscycles,onlyloadandstorecanaccessmemory,instructionsthatcannotreadandwritememoryinthesamebuscycle,manyidenticalgeneralpurposeregisters,andalimitednumberofaddressingmodes.ContrasttoCISC.

reentrantAsoftwaremodulethatcanbestartedbyonethread,interruptedandexecutedbyasecondthread.Areentrantmoduleallowboththreadstoproperlyexecutethedesiredfunction.Contrastwithnon-reentrant.

registersHigh-speedmemorylocatedintheprocessor.TheregistersintheARM ®Cortex-MincludeR0throughR15.

relayAmechanicalswitchthatcanbeturnedonandoffbythecomputer.

reliabilityTheabilityofasystemtooperatewithinspecifiedparametersforastatedperiodoftime.Givenintermsofmeantimebetweenfailures(MTBF).

reproducibility(orrepeatability)Aparameterspecifyinghowconsistentovertimethemeasurementiswhentheinputremainsfixed.

requirementsdocumentAformaldescriptionofwhatthesystemwilldoinaverycompleteway,butnotincludinghowitwillbedone.Itshouldbeunambiguous,complete,verifiable,andmodifiable.

resetvectorThe32-bitvalueatmemorylocations0x0000.0004specifyingwherethesoftwareshouldstartafterpoweristurnedonorafterahardwarereset.

resistancetemperaturedevice(RTD)Alineartransducerthatconvertstemperatureintoelectricresistance.

resolutionForaninputsignal,itisthesmallestchangeintheinputparameterthatcanbereliablydetectedbythemeasurement.Foranoutputsignal,itisthesmallestchangeintheoutputparameterthatcanbeproducedbythesystem,rangeequalsprecisiontimesresolution.Theunitsareintheunitsofthemeasurand.Whenprecisionisinalternatives,range=precision•resolution.

responsetimeSimilartolatency,itisthedelaybetweenwhenthetimeaneventoccursandthetimethesoftwarerespondstotheevent.

ritualSoftware,usuallyexecutedonceatthebeginningoftheprogram,thatdefinestheoperationalmodesoftheI/Oports.

ROMReadOnlyMemory,atypeofmemorywhereistheinformationisprogrammedintothedeviceonce,butcanbeaccessedquickly.Itislowcost,mustbepurchasedinhighvolumeandcanbeprogrammedonlyonce.SeealsoEPROM,EEPROM,andflashEEPROM.

rotorThepartofamotorthatrotates.

roundrobinschedulerAschedulerthatrunseachactivethreadequally.

roundoffTheerrorthatoccursinafixed-pointorfloating-pointcalculationwhentheleastsignificantbitsofanintermediatecalculationarediscardedsotheresultcanfitintothefiniteprecision.

RTDResistancetemperaturedevice,asensorusedtomeasuretemperature,usuallymadefromplatinum

sampleandholdAcircuitusedtolatcharapidlychanginganalogsignal,capturingitsinputvalueandholdingitsoutputconstant.

samplingrateTherateatwhichdataiscollectedinadataacquisitionsystem.

saturationAdevicethatisnolongersensitivetoitsinputswhenitsinputgoesaboveamaximumvalueorbelowaminimumvalue.

scanorscanpointAnyinstrumentusedtoproduceasideeffectwithoutcausingabreak(halt)isascan.Therefore,ascanmaybeusedtogatherdatapassivelyortomodifyfunctionsofaprogram.Examplesincludesoftwareaddedtoyoursourcecodethatsimplyoutputsormodifiesaglobalvariablewithouthalting.Ascanpointistriggeredinamannersimilartoabreakpointbutascanpointsimplyrecordsdataatthattimewithouthaltingexecution.

schedulerSystemsoftwarethatsuspendsandlaunchesthreads.

SchmittTriggerAdigitalinterfacewithhysteresismakingitlesssusceptibletonoise.

scopeAlogicanalyzeroranoscilloscope,hardwaredebuggingtoolsthatallowsyoutovisualizemultipledigitaloranalogsignalsversustime.

SCSISmallComputerSystemsInterface,ahighspeedhandshakingparallelI/Ostandard.

selectsignalTheoutputoftheaddressdecoder(eachmoduleonthebushasaseparateaddressdecoder);aBoolean(true/false)signalspecifyingwhetherornotthecurrentaddressofthebusmatchesthedeviceaddress

semaphoreAsystemfunctionwithtwooperations(waitandsignal)thatprovideforthreadsynchronizationandresourcesharing.

sensitivityThesensitivityofatransduceristheslopeoftheoutputversusinputresponse.ThesensitivityofaqualitativeDASthatdetectseventsisthepercentageofactualeventsthatareproperlyrecognizedbythesystem.

serialcommunicationAprocesswhereinformationistransmittedonebitatatime.

serialcommunicationsinterface(SCI)AFreescaletermforadevicetotransmitdatawithasynchronousserialcommunicationprotocol(sameasUARTandACIA.)

serialperipheralinterface(SPI)Adevicetotransmitdatawithsynchronousserialcommunicationprotocol.SameasSSI.

serialportAnI/Oportwithwhichthebitsareinputoroutputoneatatime.

servoADCmotorwithbuiltincontroller.Themicrocontrollerspecifiesdesiredpositionandtheservoadds/subtractspowertomovetheshafttothatposition.

setuptimeWhenlatchingdataintoaregisterwithaclock,itisthetimebeforeanedgetheinputmustbevalid.Contrastwithholdtime.

shotnoiseAfundamentalnoisethatoccursindevicesthatcountdiscreteevents.

signedtwo’scomplementbinaryAmechanismtorepresentsignedintegerswhere1followedbyall0’sisthemostnegativenumber,all1’srepresentsthevalue-1,all0’srepresentsthevalue0,and0followedbyall1’sisthelargestpositivenumber.

sign-magnitudebinaryAmechanismtorepresentsignedintegerswherethemostsignificantbitissetifthenumberisnegative,andtheremainingbitsrepresentthemagnitudeasanunsignedbinary.

simplexchannelHardwarethatallowsbits(information,errorchecking,synchronizationoroverhead)totransferonlyinonedirection.Contrastwithhalfduplexandfullduplexchannels.

simplexcommunicationAsystemthatallowsinformationtotransferonlyinonedirection.

simulatorAsimulatorisasoftwareapplicationthatsimulatesormimicstheoperationofaprocessororcomputersystem.MostsimulatorsrecreateonlysimpleI/Oportsandoftendonoteffectivelyduplicatethereal-timeinteractionsofthesoftware/hardwareinterface.Ontheotherhand,theydoprovideasimpleandinteractivemechanismtotestsoftware.

singleaddressDMADirectmemoryaccessthatrequiresonlyonebuscycletotransferdatafromaninputdeviceintomemory,orfrommemorytoanoutputdevice.

single-polerelayAsimplerelaywithonlyonecopyoftheswitchmechanism.Contrastwithdoublepole.

single-poleswitchAsimpleswitchwithonlyonecopyoftheswitchmechanism.Oneswitchthatactsindependentfromotherswitchesinthesystem.Contrastwithdouble-pole.

single-throwswitchAswitchwithtwocontactconnections.Thetwocontactsmaybeconnectedordisconnected.Contrastwithdouble-throw.

slewrateThemaximumslopeofasignal.Ifthetime-varyingsignalV(t)isinvolts,theslewrateisthemaximumdV/dtinvolts/s.

softwareinterruptSeetrap.

softwaremaintenanceProcessofverifying,changing,correcting,enhancing,andextendingsoftware.

solenoidAdiscretemotiondevice(on/off)thatcanbecontrolledbythecomputerusuallybyactivatinganelectromagnet.Forexample,electronicdoorlocksonautomobiles.

sourcecodeProgramsinhumanreadableformatcreatedwithaneditor.

spaceAdigitalvalueoffalseorlogic0.Contrastwithmark.

spanSameasrange.

spatialresolutionThevolumeoverwhichtheDAScollectsinformationaboutthemeasurand.

specificityThespecificityofatransduceristherelativesensitivityofthedevicetothesignalofinterestversusthesensitivityofthedevicetootherunwantedsignals.ThesensitivityofaqualitativeDASthatdetectseventsisthepercentageofeventsdetectedbythesystemthatareactuallytrueevents.

spinlocksemaphoreAsemaphorewherethethreadswillspin(runbutdonousefulfunction)whentheyexecutewaitonabusysemaphore.Contrasttoblockingsemaphore.

stabilizeThedebuggingprocessofstabilizingasoftwaresysteminvolvesspecifyingallitsinputs.Whenasystemisstabilized,theoutputresultsareconsistentlyrepeatable.Stabilizingasystemwithmultiplereal-timeevents,likeinputdevicesandtime-dependentconditions,canbedifficulttoaccomplish.Itofteninvolvesreplacinginputhardwarewithsequentialreadsfromanarrayordiskfile.

stackLastinfirstoutdatastructurelocatedinRAMandusedtotemporarilysaveinformation.

stackpointer(SP)AregisterintheprocessorthatpointstotheRAMlocationofthestack.

startbitAnoverheadbit(s)specifyingthebeginningoftheframe,usedinserialcommunicationtosynchronizethereceivershiftregisterwiththetransmitterclock.Seealsostopbit,evenparityandoddparity.

starvationAconditionthatoccurswithapriorityschedulerwherelowprioritythreadsareneverrun.

staticallocationDatastructuressuchasanFSMorTCBthataredefinedatassemblyorcompiletimeandexistthroughoutthelifeofthesoftware.Contrasttodynamicallocation.

staticRAMVolatileread/writestoragebuiltfromthreetransistorshavingfastspeed,andnotrequiringrefresh.ContrastwithdynamicRAM.

statorThepartofamotorthatremainsstationary.Sameasframe.

steppermotorAmotorthatmovesindiscretesteps.

stopbitAnoverheadbit(s)specifyingtheendoftheframe,usedinserialcommunicationtoseparateoneframefromthenext.Seealsostartbit,evenparityandoddparity.

straingaugeAtransducerthatconvertsdisplacementintoelectricresistance.Itcanalsobeusedtomeasureforceorpressure.

stringAsequenceofASCIIcharacters,usuallyterminatedwithazero.

symboltableAmappingfromasymbolicnametoitscorresponding16-bitaddress,generatedbytheassemblerinpassoneanddisplayedinthelistingfile.

synchronousbusacommunicationprotocolthathasacentralclock;thereisnofeedbackfromthememorytotheprocessor,soeverymemorycycletakesexactlythesametime;datatransfers(putdataonbus,takedataoffbus)aresynchronizedtothecentralclock

synchronousprotocolasystemwherethetwodevicessharethesameclock.

synchronousserialinterface(SSI)Adevicetotransmitdatawithsynchronousserialcommunicationprotocol.SameasSPI.

tachometerasensorthatmeasurestherevolutionspersecondofarotatingshaft.

tailrecursionAtechniquewheretherecursivecalloccursasthelastactiontakenbythefunction.Seealsorecursion,binaryrecursion,andlinearrecursion.

thermalnoiseAfundamentalnoiseinresistivedevicesarisingfromtheuncertaintyaboutthepositionandvelocityofindividualmolecules.SameasJohnsonnoiseandwhitenoise.

thermistorAnonlineartransducerthatconvertstemperatureintoelectricresistance.

thermocoupleAtransducerthatconvertstemperatureintoelectricvoltage.

threadTheexecutionofsoftwarethatcooperateswithotherthreads.Athreadembodiestheactionofthesoftware.Oneconceptdescribesathreadasthesequenceofoperationsincludingtheinputandoutputdata.

threadcontrolblockTCBInformationabouteachthread.

three-polerelayThreeseparateandcompleterelays,whichareactivatedtogether(seesinglepole).

three-poleswitchThreeseparateandcompleteswitches.Theswitchesareelectricallyseparate,butmechanicallyconnected.Thethreeswitchesturnedonandofftogether(seesinglepole).

throughputTheinformationtransferrate,theamountofdatatransferredpersecond.Sameasbandwidth.

timeconstantThetimetoreach63.2%ofthefinaloutputaftertheinputisinstantaneouslyincreased.

timeprofileandexecutionprofileTimeprofilereferstothetimingcharacteristicofaprogramandexecutionprofilereferstotheexecutionpatternofaprogram.

timingsignalsThelinesusedtoclockdataontooroffofthebus;signalsthatspecifywhentoactivateduringthiscycle;thespecifictimesfortheriseandfalledgesaresynchronizedtoaclock.Contrasttocommandsignals.

toleranceThemaximumdeviationofaparameterfromaspecifiedvalue.

totalharmonicdistortion(THD)Ameasureoftheharmonicdistortionpresentandisdefinedastheratioofthesumofthepowersofallharmoniccomponentstothepowerofthefundamentalfrequency.

transducerAdevicethatconvertsonetypeofsignalintoanothertype.

trapAtrapissimilartoaregularorhardwareinterrupt:thereisatriggerthatinvokestheexecutionofanISR.OntheCortex-M,therearetwosoftwareinterrupts:supervisorcallandPendSV.Hardwareinterruptsaretriggeredbyhardwareevents,whilesoftwareinterruptsaretriggeredexplicitlybysoftware.ToinvokeaPendSV,thesoftwaresetsbit28oftheNVIC_INT_CTRL_Rregister.Sameassoftwareinterrupt.

tristateThestateofatristatelogicoutputwhenoffornotdriven.

tristatelogicAdigitallogicdevicethathasthreeoutputstateslow,high,andoff(HiZ).

truncationTheactofdiscardingbitsasanumberisconvertedfromoneformattoanother.

two’scomplementAnumbersystemusedtodefinesignedintegers.TheMSBdefineswhetherthenumberisnegative(1)orpositive(0).Tonegateatwo’scomplementnumber,onefirstcomplements(flipfrom0to1orfrom1to0)eachbit,thenadd1tothenumber.

two-polerelaytwoseparateandcompleterelays,whichareactivatedtogether(sameasdoublepole).

two-poleswitchTwoseparateandcompleteswitches.Theswitchesareelectricallyseparate,butmechanicallyconnected.Thetwoswitchesturnedonandofftogetherwhichareactivatedtogether,sameasdouble-pole.

ultrasoundAsoundwithafrequencytoohightobeheardbyhumans,typically40kHzto100MHz.

unbufferedI/OThehardwareandsoftwarearetightlycoupledsothatbothwaitforeachotherduringthetransmissionofdata.

unipolarsteppermotorAsteppermotorwherethecurrentflowsinonlyonedirection(on/off)alongtheinterfacewires;astepperwith5or6interfacewires.

universalasynchronousreceiver/transmitter(UART)Adevicetotransmitdatawithasynchronousserialcommunicationprotocol.

unsignedbinaryAmechanismtorepresentunsignedintegerswhereall0’srepresentsthevalue0,andall1’srepresentsisthelargestpositivenumber.

vectorA32-bitaddressinROMcontainingthelocationoftheinterruptserviceroutines.Seealsoresetvectorandinterruptvector.

velocityfactor(VF)Theratioofthespeedatwhichinformationtravelsrelativetothespeedoflight.

verticalparitythenormalparitybitcalculatedoneachindividualframe,canbeevenoroddparity

VOHThesmallestpossibleoutputvoltagewhenthesignalishigh,andthecurrentislessthanIOH.

VOLThelargestpossibleoutputvoltagewhenthesignalislow,andthecurrentislessthanIOL.

volatileAconditionwhereinformationislostwhenpowerisremoved.

volatileApropertyofavariableinC,suchthatthevalueofthevariablecanchangeoutsidetheimmediatescopeofthesoftwareaccessingthevariable.

voltagefollowerAnanalogcircuitwithgainequalto1,largeinputimpedanceandsmalloutputimpedance.Sameasfollower.

vulnerablewindowLocationswithinasoftwaremodule,whichifaninterruptweretooccuratoneoftheselocations,thenanerrorcouldoccur(e.g.,datalost,corrupteddata,programcrash,etc.)Sameascriticalsection.

whitenoiseAfundamentalnoiseinresistivedevicesarisingfromtheuncertaintyaboutthepositionandvelocityofindividualmolecules.SameasJohnsonnoiseandthermalnoise.

wire-or-modeAdigitallogicoutputthathastwostateslowandHiZ.Sameasopencollector.

wordFourbytescontaining32bits.

workstationApowerfulgeneralpurposecomputersystemhavingapriceinthe$3Kto50Krangeandusedforhandlinglargeamountsofdataandperformingmanycalculations.

writecycledataissentfromtheprocessortothememoryoroutputdevice,theaddressbusspecifiesthememoryoroutputdevicelocationandthedatabuscontainstheinformation

writedataavailabletimeinterval(start,end)duringwhichthedatawillbevalidduringawritecycle,determinedbytheprocessor

writedatarequiredtimeinterval(start,end)duringwhichthedatashouldbevalidduringawritecycle,determinedbythememorymodule

XON/XOFFAprotocolusedbyprinterstofeedbacktheprinterstatustothecomputer.XOFFissentfromtheprintertothecomputerinordertostopdatatransfer,andXONissentfromtheprintertothecomputerinordertoresumedatatransfer.

ZTransformAtransformequationconvertingadigitaltime-domainsequenceintothefrequencydomain.Inboththetimeandfrequencydomainitisassumedthesignalisbandlimitedto0to½fs.

zeroAplaceinthefrequencydomainwherethefiltergainiszero.

Appendix2.SolutionstoCheckpointsCheckpoint1.1:Amicrocomputerisasmallcomputerthatincludesaprocessor,memory,andinput/output.Amicroprocessorisasmallprocessorthatincludesregisters,ALU,controlunitandabusinterfaceunit.Amicrocontrollerisasinglechipmicrocomputer.

Checkpoint1.2:FlashROMishigherdensitybecauseitrequiresfewertransistorsperbit.

Checkpoint1.3:Aninputportishardwarethatispartofthecomputer,anditisthechannelthroughwhichinformationentersintothecomputer.Aninputinterfaceincludeshardwarecomponentsexternaltothecomputer,theinputport,andsoftware,whichalltogetherperformtheinputfunction.

Checkpoint1.4:Typicalinputdevicesincludethekeysonthekeyboard,mouseanditsbuttons,touchpad,DVDreader,andmicrophone.USBdrives,Ethernet,andwirelesscanbeusedforinputandoutput.

Checkpoint1.5:TypicaloutputdevicesincludetheLEDsonthekeyboard,monitor,speaker,printer,CDburner,andspeaker.Thefloppydiskcanbeusedforinputandoutput.

Checkpoint1.6:CU(controlunit)BIU(businterfaceunit)andALU(arithmeticlogicunit)areallpartoftheprocessor.DMAstandsfordirectmemoryaccess,whichisahigh-speedmechanismtomovedatadirectlyfrominputtomemory,ormovedatadirectlyfrommemorytooutput.

Checkpoint1.7:Anembeddedsystemisamicrocomputerwithmechanical,chemicalandelectricaldevicesattachedtoit,programmedforaspecificdedicatedpurpose,andpackagedupasacompletesystem.

Checkpoint1.8:Westoretemporaryinformationlikepictures,soundrecordings,andtextmessages.

Checkpoint1.9:Westorenonvolatileinformationlikeprograms,addressbook,music,andcalendar.

Checkpoint1.10:Thesoftwareinthealarmclockmustmaintaintimeusingareal-timeclock,outputthecurrenttimeonthedisplay,respondtobuttonpushesupdatingparametersasrequired,checkandseeifthecurrenttimematchesthealarmtime.

Checkpoint1.11:Arequirementisadetailedperformanceparameterthatthesystemmustsatisfy,generallyderivedfromtheoverallobjectiveofthesystem.Aconstraintisaconditiondefininghowthesystemwillbedeveloped,generallyrestrictingtherangeofsolutionsfromwhichthesystemwillbebuilt.

Checkpoint1.12:Iftwomodulesoutputtothesameport,thenthesecondmodulewillundothefunctionofthefirstone.Forexampleifonemodulesays“go”andtheotheronesays“stop”,thentheorderofexecutiondeterminestheresultingfunction.Asimilarerrorcanoccurforinputports.

Checkpoint1.13:WhenIstartthewait,Iobservethecurrenttime.Next,Iaddttothistime,callingitendTime.Lastly,IwaitforthetimeonthewaittopassendTime.

Checkpoint1.14:0and1VareconsideredlowbecausetheyarelesstheVIL.2,3,4,and5VareconsideredhighbecausetheyaremorethanVIH.LM3S/LM4F/TM4Cinputsare5Vtolerant.

Checkpoint1.15:Wemustcheckfourinequalities.Yes,VOL(0.5V)islessthanVIL(0.8V),VOH(4.4V)isgreaterthanVIH(2V),IOH(4mA)isgreaterthanIIH(20µA),andIOL(4mA)isgreaterthanIIL(0.4mA).

Checkpoint1.16:No,onlythreeofthefourinequalitiesaretrue.VOL(0.4V)islessthanVIL(1.5V),VOH(2.4V)isNOTgreaterthanVIH(3.5V),IOH(0.4mA)isgreaterthanIIH(1µA),andIOL(4mA)isgreaterthanIIL(1 A).

Checkpoint1.17:Thecorrectresistorvalueis(3.3-2-0.5V)/20mA=40 .

Checkpoint1.18:2½decimaldigitsis200alternatives,whichisabout8bits.

Checkpoint1.19:10binarybitsis1024alternatives,whichisabout3decimaldigits.

Checkpoint1.20:6½decimaldigitsis2,000,000alternatives,whichisabout21bits.

Checkpoint1.21:Theruleofthumbsays260isabout1018,whichis18decimaldigits.24is16,whichisabout1½decimaldigits.Together,wehave19½decimaldigits.

Checkpoint1.22:First,breakintonibbles1110,1110,1011,thenconverteach,0xEEB.

Checkpoint1.23:First,converteachhexdigitoneatatime0011100000000000,thencombinetoget2_0011100000000000.

Checkpoint1.24:Eachhexdigitneeds4bits,soatotalof20bitswillberequired

Checkpoint1.25:2_01101010equals64+32+8+2=106.

Checkpoint1.26:0x23equals2*16+3=35.

Checkpoint1.27:37equals32+4+1=2_00100101=0x25.

Checkpoint1.28:202equals128+64+8+2=2_11001010=0xCA.

Checkpoint1.29:Theyarethesame,bothequally53(3*16+5=.48+5)

Checkpoint1.30:-31equals–128+64+32+1=2_11100001=0xE1.

Checkpoint1.31:Thecharacter‘0’isrepresentedinASCIIas0x30.

Checkpoint1.32:ConvertingeachcharactertoASCIIyields“48656C6C6F20576F726C6400”

Checkpoint1.33:Theintegerpartrangesfrom-32768to+32767,sothevalueofthefixed-pointnumberrangesfrom-32.768to+32.767.

Checkpoint1.34: isabout3142,witharesolutionof0.001.

Checkpoint1.35: isabout804,witharesolutionof1/256.

Checkpoint1.36:Usingacalculator,wefind 5-(161/72)is-0.000043. 5-(682/305)is0.000002.

Checkpoint1.37:y=(1000•x‑53•x1+1000•x2+51•y1-903•y2)/1000.

Checkpoint2.1:0x2200.0000+32*n+4*b=0x2200.0000+32*0x1003+4*5=0x2200.0000+0x20060+0x14=0x2202.0074.

Checkpoint2.2:0x2200.0000+32*n+4*b=0x2200.0000+32*0x1000+4*20=0x2200.0000+0x20000+0x50=0x2202.0050.

Checkpoint2.3:0x4200.0000+32*n+4*b=0x4200.0000+32*3+4*2=0x4200.0000+0x00060+0x08=0x4200.0068.

Checkpoint2.4:Theaddressingmodespecifieswheretheinstructionwillreadorwritedata.

Checkpoint2.5:Itwillaccessthe32-bitvalueat0x2000.0008.R3isnotupdated.

Checkpoint2.6:Itwillaccessthe32-bitvalueat0x2000.0000,thenR3isupdatedto0x2000.0008.

Checkpoint2.7:InC,changethe int32_t to int16_t .Inassembly,change40to20,and LSL#2 to LSL#1 .

Checkpoint2.8:

Checkpoint2.9:Thesumoftwounsigned32-bitnumbersisa33-bitunsignednumber.Considersmallestandlargestvalues.0+0=0,and(232-1)+(232-1)=(233-2).

Checkpoint2.10:Thesumoftwosigned32-bitnumbersisa33-bitsignednumber.Considersmallestandlargestvalues.(-231)+(-231)=(-232),and(231-1)+(231-1)=(232-2).

Checkpoint2.11:Thediscontinuityisbetween-128and+127,whichareadjacentonthewheel.

Checkpoint2.12:Theproductoftwounsigned32-bitnumbersisa64-bitunsignednumber.Considersmallestandlargestvalues.0*0=0,and(232-1)*(232-1)=(264-233+1).

Checkpoint2.13:Theproductoftwosigned32-bitnumbersisa64-bitsignednumber.Considersmallestandlargestproducts.(-231)*(231-1)=(-262-232),and(-231)*(-231)=(262).(262)willnotfitintoasigned63-bitnumber.

Checkpoint2.14:No,becausetheproductgoesinto64bits.SeeCheckpoints2.12and2.13.

Checkpoint2.15:Theconditional BHS isusedwithunsignedcomparisonsand BGEwithsigned.

Checkpoint2.16:Nothinghappensifthesoftwarewritestoaninputport.

Checkpoint2.17:Ifthesoftwarereadsthisoutputportitgetsthevaluesonthepins.Forexample,iftheusermistakenlygroundedtheoutputpin(verybadthingtodo),andthesoftwarewritesa‘1’;whenitreadsitwillget‘0’.

Checkpoint2.18:Sincethereare8bitsinaportand8bitsinthedirectionregister,eachbitcanbeindividuallyprogrammedasinputoroutput.

Checkpoint2.19:ThealternativefunctionforPD5onaLM3S811isTimer1ACapture/Compare.

Checkpoint2.20:Nothinghappens.Sincenoneoftheaddressbitsareselected,noneoftheportbitsareaffected.

Checkpoint2.21:ThebaseaddressforPortDis0x4000.7000.#definePD72(*((volatileuint32_t*)0x40007210))

PD72=0x84;//setsPD7PD2,other6bitsarenotaffected

Checkpoint2.22:ThebaseaddressforPortBis0x4000.5000.#definePB650(*((volatileuint32_t*)0x40005184))

PB650=0x61;//setsPB6PB5PB0,other5bitsarenotaffected

Checkpoint2.23:Itwillstilloperateaccordingtospecifications,butitmaybemoreexpensivetobuildoritmaybehardertoordercomponentstobuildit.

Checkpoint2.24:Itwillnolongeroperateaccordingtospecifications.

Checkpoint2.25:Changethespecificationfrom6MHzto8MHz.Changetheline

SYSCTL_RCC_R+=0x00000540;//10101,configurefor16MHzcrystal

to

SYSCTL_RCC_R+=0x00000380;//01110,configurefor8MHzcrystal

Changetheline(400/8is50MHz,soSYSDIV2is7)

SYSCTL_RCC2_R+=(4<<22);//configurefor80MHzclock

to

SYSCTL_RCC2_R+=(7<<22);//configurefor50MHzclock

Checkpoint2.26:Change60000to80000.

Checkpoint3.1:Thepolicyrewardsbadbehavior.Abetterpolicywouldbetorewardgoodbehavior.

Checkpoint3.2:Aprivatefunctionhasnounderlineinthename(e.g., OutNibble ).Apublicfunctionhasthemodulename,underline,thenoperationinthename(e.g., FIFO_Put ).

Checkpoint3.3:Alocalvariablebeginswithalowercaseletter.Aglobalvariablebeginswithanuppercaseletter.

Checkpoint3.4:Alocalvariableiscreatedbydefiningitafteranopenbrace butbeforeanyexecutablecode.E.g.,thisroutineasthreelocalvariables littleVarmediumVarlargeVar

voidProcess(void)

int8_tlittleVar;

int16_tmediumVar;

int32_tlargeVar;

Checkpoint3.5:Aglobalvariableiscreatedbydefiningitoutsidethefunction.E.g.,thisprogramasthreeglobalvariables LittleVarMediumVarLargeVar

int8_tLittleVar;

int16_tMediumVar;

int32_tLargeVar;

voidProcess(void)

Youcanalsodefineglobalvariablesinsidethefunctionusing static .Thescopeofstaticvariablesisrestrictedtothefunction.E.g.,

voidProcess(void)

staticint8_tLittleVar;

staticint16_tMediumVar;

staticint32_tLargeVar;

Checkpoint3.6: static changesalocalvariablefromtemporarytopermanentallocation. static doesnotchangethescopeofalocalvariable. static changesaglobalvariablefrompublictoprivatescope(thisfileonly). static doesnotchangetheallocationofaglobalvariable. static changesafunctionfrompublictoprivatescope(thisfileonly).Itdoesnotchangehowthefunctionrunsorhowitisstored.

Checkpoint3.7: const changesaglobalvariablefromRAMallocationtoROMallocation. const doesnotchangethescopeofavariable.

Checkpoint3.8:InaMooreFSM,theoutputdependsonlyonthestate.InaMealyFSM,theoutputdependsonthestateandtheinput.

Checkpoint3.9:Redefinetheseparameters

#defineBSP_InPortGPIO_PORTA_DATA_R

#defineBSP_InPort_DIRGPIO_PORTA_DIR_R

#defineBSP_InPort_DENGPIO_PORTA_DEN_R

#defineBSP_OutPortGPIO_PORTB_DATA_R

#defineBSP_OutPort_DIRGPIO_PORTB_DIR_R

#defineBSP_OutPort_DENGPIO_PORTB_DEN_R

#defineBSP_GPIO_ENSYSCTL_RCGCGPIO_R

#defineBSP_InPort_Mask0x00000001//bitmaskforPortA

#defineBSP_In_M0x00000030//bitmaskforpins5,4

#defineBSP_In_Shift0x00000004//shiftvalueforInputpins

#defineBSP_OutPort_Mask0x00000002//bitmaskforPortB

#defineBSP_Out_M0x0000006E//bitmaskforpins6-1

#defineBSP_Out_Shift0x00000001//shiftvalueforOutputpins

Checkpoint3.10:Itwillfirstsitup,thenitwillstandup.

Checkpoint3.11: AddIndexFifo(CAN1,100,char,0,1)AddIndexFifo(CAN2,100,char,0,1)AddIndexFifo(CAN3,100,char,0,1)Checkpoint3.12: AddIndexFifo(F1,256,int16_t,0,1)AddIndexFifo(F2,256,int16_t,0,1)Checkpoint3.13:Searchthefreelisttoseeiftheaddress&Heap[SIZE*i] isfree.

Checkpoint3.14:Ignoresizeparameter,return100bytesregardlessoftherequest.

Checkpoint3.15:Aruntimeflagcanbeturnedonduringacheckupprocedurewhilethedeviceisinservice.Acompiletimeflagremovesalldebuggingsothesoftwaresystemisfasterandsmaller.

Checkpoint3.16:Therearethreeadvantagesofleavingtheinstruments.First,thesystemwastestedwiththeinstrumentsandworkstospecificationwiththeinstruments.Thereisnoguaranteethesystemwillstillworkiftheinstrumentsareremoved.Second,theinstrumentscouldprovideruntimecheckstocatchfailuresduringoperation.Third,theinstrumentscouldbeusedduringsystemcheckup(recalibration,diagnosticcheckupetc.)

Checkpoint3.17:Theexistenceoftheinstrumenthasasmallbutinconsequentialeffectonthesystemperformance.Thetimetoexecutetheinstrumentissmallcomparedtothetimebetweenexecutionsoftheinstrument.

Checkpoint3.18:ThebaseaddressofPortAis0x4000.0000.4*23=0x20.

#defineGPIO_PORTA3(*((volatileuint32_t*)0x40000020))

#defineDebug_HeartBeat()(GPIO_PORTA3^=0x08)

Checkpoint3.19:Eachoutputisabout3instructions,soIexpectittobe4to6buscycleswide.ThistimedelayexplainswhyProgram3.18hadameasurementvalue6cycleslargerthanProgram3.17(244-238).

Checkpoint4.1:Eachoutputisabout3instructions,soIexpectittobe4to6buscycleswide.

Checkpoint4.2:Negativelogicmeanswhenwetouchtheswitchthevoltagegoesto0(low).Formally,negativelogicmeansthetruevoltageislowerthanthefalsevoltage.Positivelogicmeanswhenwetouchtheswitchthevoltagegoesto+3.3(high).Formally,positivelogicmeansthetruevoltageishigherthanthefalsevoltage.

Checkpoint4.3:ForPA2,weneedinputwithpull-up.DIRbit2islow(input),AFSELbit2islow(notalternate),PUEbit2high(pull-up)andPDEbit2low(notpull-down).ForPA3,weneedinputwithpull-down.DIRbit3islow(input),AFSELbit3islow(notalternate),PUEbit3low(nopull-up)andPDEbit3high(pull-down).

Checkpoint4.4:Increasingvoltagewillincreasetorque.Toincreasecurrentyouwouldhavetoreplacethemotorwithamotorhavinglowerresistance.

Checkpoint4.5:No,ifyouknowtheinitialposition,youstepslowenough,andthesystemdoesnotloadthemotor.Yesiftheyoudonotknowinitialposition,yousteptoofast,orifyouloadthemotor.

Checkpoint4.6:IfOutgoesfirst,Outgoes1,2,3,andwaitson5,Ingoes2,3.Nowbotharerunning(Outruns6,andInruns4,5,andwaitson6).AfterOutruns6,Infinishes6.

IfIngoesfirst,Inwaitson2.Thenthestepsarethesameasabove.

Checkpoint4.7:Thereis1byteofdataper10bitsoftransmission.So,thereare960bytes/sec.

Checkpoint4.8:divider=1010.0101002.or10and20/64=10.3125.Thebaudrateis6MHz/10.3125/16whichis36.36kHz.

Checkpoint4.9:50,000,000/38400/16is81.3802,whichissimilarto81and24/64. UART0_IBRD_R is81 UART0_FBRD_R is24.Thebaudrateis50MHz/(81+24/64)/16whichis38402bits/sec.

Checkpoint4.10:RXFEissetandclearedbyhardware.ItmeansreceiveFIFOempty.Tomakeit0meanstoputdataintotheFIFO.Softwarecannotclearthisflag.AnincomingUARTframewillclearRXFE.

Checkpoint4.11:TXFFissetandclearedbyhardware.ItmeanstransmitFIFOfull.Tomakeit0meanstogetdatafromtheFIFO.Softwarecannotclearthisflag.AnoutgoingUARTframewillclearTXFF.

Checkpoint4.12:Thedatawillbereceivedinerror(valueswillnotbecorrect).Thereceivercouldappeartogettwoinputframesforeveryoneframetransmitted.Itwillprobablycauseframingerrors(FE).Itwouldcauseparityerrorsifactive.

Checkpoint4.13:Thedatawillbereceivedinerror(valueswillnotbecorrect).Thereceiverwillappeartogetoneinputframeforeveryoneframetransmitted.Itwillprobablynotcauseframingerrors(FE).Itwouldcauseparityerrorsifactive.

Checkpoint5.1:Triggerflagsetbyhardware;thedeviceisarmedbysoftware;thedeviceisenabledforinterruptsintheNVIC;theprocessorisenabledforinterrupts(PRIMASKIbitisclear).

Checkpoint5.2:TheprocessorisenabledforinterruptsbyclearingtheIbitinthePRIMASK.Execute

CPSIEI

Checkpoint5.3:Instructionisfinished;registersR0–R3,R12,LR,PC,andPSRarepushed;PCissetwithinterruptvectoraddress;LR=0xFFFFFFF9.Thelasttwostepscanoccurineitherorder.

Checkpoint5.4:No,iftheaverageproducerrateexceedstheaverageconsumerrate,theFIFOwillalwaysfillregardlessofsize.However,iftheaverageproducerrateislessthantheaverageconsumerrate,theFIFOfullerrorscanbeeliminatedbyincreasingsize.

Checkpoint5.5:Attheendoftheinnernestedprogram,interruptswouldbeenabled.So,thelastpartoftheoutersectionwouldberunningwithinterruptsenabled.Inthisexample Stuff2B runswithinterruptsenabled.

Critical1Critical2

DisableDisable

Stuff1AStuff2A

CallCritical2Enable

Stuff1Breturn

Enable

return

Checkpoint5.6:TheRxFifoisemptywhenthereisnoinputdata.Softwareiswaitingforhardware.

Checkpoint5.7:TheTxFifoisemptywhenthereisnooutputdata.Hardwareiswaitingforsoftware.

Checkpoint6.1:Aninputcaptureeventoccursontheselectededgeonaninputpin.

Checkpoint6.2:Timervalueiscopiedintotheinputcapturelatch,theflagisset,andifarmedaninterruptisrequested.

Checkpoint6.3:SwitchingmicrocontrollersmeanschangingtheI/Opin.

voidTimerCapture_Init(void)

SYSCTL_RCGCTIMER_R|=0x0001;//activatetimer0

SYSCTL_RCGCGPIO_R|=0x0002;//activateportB

Count=0;//allowtimetofinishactivating

GPIO_PORTB_DEN_R|=0x01;//enabledigitalI/OonPB0

GPIO_PORTB_AFSEL_R|=0x01;//enablealtfunctonPB0

TIMER0_CTL_R&=~0x00000001;//disabletimer0Aduringsetup

TIMER0_CFG_R=0x00000004;//configurefor16-bittimermode

TIMER0_TAMR_R=0x00000007;//configureforinputcapturemode

TIMER0_CTL_R&=~(0x000C);//TAEVENTisrisingedge

TIMER0_TAILR_R=0x0000FFFF;//startvalue

TIMER0_IMR_R|=0x00000004;//enablecapturematchinterrupt

TIMER0_ICR_R=0x00000004;//cleartimer0Acaptureflag

TIMER0_CTL_R|=0x00000001;//enabletimer0A

NVIC_PRI4_R=(NVIC_PRI4_R&0x00FFFFFF)|0x40000000;//Timer0A=priority2

NVIC_EN0_R=0x00080000;//enableinterrupt19inNVIC

EnableInterrupts();

voidTimer0A_Handler(void)

TIMER0_ICR_R=TIMER_ICR_CAECINT;//acknowledgetimer0Acapturematch

Count=Count+1;

Checkpoint6.4:SwitchingtimersmeanschangingtheI/Opinandthetimer.

voidTimerCapture_Init(void)

SYSCTL_RCGCTIMER_R|=0x0002;//activatetimer1

SYSCTL_RCGCGPIO_R|=0x0008;//activateportD

Count=0;//allowtimetofinishactivating

GPIO_PORTD_DEN_R|=0x20;//enabledigitalI/OonPD5

GPIO_PORTD_AFSEL_R|=0x20;//enablealtfunctonPD5

TIMER1_CTL_R&=~0x00000001;//disabletimer1Aduringsetup

TIMER1_CFG_R=0x00000004;//configurefor16-bittimermode

TIMER1_TAMR_R=0x00000007;//configureforinputcapturemode

TIMER1_CTL_R&=~(0x000C);//TAEVENTisrisingedge

TIMER1_TAILR_R=0x0000FFFF;//startvalue

TIMER1_IMR_R|=0x00000004;//enablecapturematchinterrupt

TIMER1_ICR_R=0x00000004;//cleartimer0Acaptureflag

TIMER1_CTL_R|=0x00000001;//enabletimer0A

NVIC_PRI5_R=(NVIC_PRI5_R&0xFFFF00FF)|0x00004000;//Timer1A=priority2

NVIC_EN0_R=0x00200000;//enableinterrupt21inNVIC

EnableInterrupts();

voidTimer1A_Handler(void)

TIMER1_ICR_R=TIMER_ICR_CAECINT;//acknowledgetimer1Acapturematch

Count=Count+1;

Checkpoint6.5: TIMER1_ICR_R=TIMER_ICR_CBECINT;//bit10=0x0400

Checkpoint6.6:One-shot/periodic:onthecyclewhenthetimercountsdownto0.

Checkpoint6.7:One-shot/periodic:TnTORISbitisset,interruptmayberequested,theoutputpinistoggled,andADCconversionmaybetriggered.

Checkpoint6.8:PWM:onthecyclewhenthetimerequalsthevalueintheMatchRegisterortheIntervalLoadRegister.

Checkpoint6.9:PWM:outputpincleared(setifinvertingmode)onmatchorset(clearedifinvertingmode)onreload.

Checkpoint6.10:TheVOLofthe7406at40mAwillbe0.7V.Thismeanstherewillbe4.3Vacrossthecoil.

Checkpoint7.1:Thereisalways1startbit.Sowith8databitsandonestop,therewillbe10totalbits.

Checkpoint7.2:Fullduplexisbothdirectionsatthesametime,whilehalfduplexisbothdirections,butonlyonedirectionatatime.

Checkpoint7.3:Withsynchronousserial(SSI),thetransmitterandreceiveroperateonthesameclock,whichisincludedinthecable.Withasynchronousserial(UART),thetransmitterandreceiveroperatewithclocksofsimilarfrequencies,andthereceiverusestransitionsinthedatatosynchronizewiththetransmitter.

Checkpoint7.4:TheMAX3232allowsthesignaltotravelfartherandbelesssensitivetonoise.

Checkpoint7.5:Capacitancereducestheslewrateofthesignalscausingareductioninthemaximumpossiblebaudrate.Theendresultiscapacitancereducesthebandwidthofthechannel.

Checkpoint7.6:Setuptimeisthetimebeforeaclockedgetheinputdatamustbevalid.Holdtimeisthetimeafteraclockedgethedatamustcontinuetobevalid.

Checkpoint7.7:Change SSI0_CPSR_R froma2toa6.

Checkpoint7.8:ThelowertheresistancethehighertheIOH.Thehigherthecurrentthefastertherisetime.Afasterrisetimemeansthebusoperatesfaster.

Checkpoint7.9:Opendrainisthesameasopencollector.Therearetwooutputstatesfloatandlow.Formally,opendrainmeansthedrainpinoftheN-channelMOStransistorisopen.SeeFigure1.20.

Checkpoint7.10:Ifnodevicesendsanacknowledgement,theSDAsignalwillfloathigh,generatinganegativeacknowledgement.

Checkpoint7.11:Halfduplexbecausecommunicationgoesbothdirections,butonlyonewayatatime.

Checkpoint7.12:Master2winsbecausethe0inaddressbit4dominatesoverthe1.

Checkpoint7.13:Iftheybothsendthesameaddressandthesamesequenceofdatabits,bothwillfinishwithoutgettingalost-arbitrationerror.Iftheybothsendthesameaddressandbutdifferentdatavalues,anarbitrationwilloccurduringthedatatransfer,andthemasterwiththesmallerdatavaluewillwinarbitration.

Checkpoint8.1:Speedrequirespower.Thepowergoesupasbandwidthincreases.

Checkpoint8.2:Outputimpedanceisopencircuitvoltagedividedbyshortcircuitcurrent.5V/30mAis167 .

Checkpoint8.3:Theoffsetvoltageis0.5mV.Theerrorwillbe0.5mVtimes100,whichwillbe50mV.

Checkpoint8.4:Theoffsetvoltageis3mV.Theerroris3mVtimes1000,whichwouldbe3V.

Checkpoint8.5.Thegain-bandwidthproductoftheMAX494is500kHz.Thebandwidthofthecircuitwillbe500kHzdividedbythegain,whichwillbe5kHz.ThenoisedensityoftheMAX494is25nV/√Hz.Thenoisewillbe25nV*sqrt(5000),whichisabout1.8 V.

Checkpoint8.6.Theinputimpedanceoftheopampdeterminestheinputimpedanceofanoninvertingamplifier,whichfortheOPA227is10M .

Checkpoint8.7.LM3S:1*1024/3=341or1*1023/3=341.LM4F/TM4C:1*4096/3=1365or1*4095/3=1365.

Checkpoint9.1:Thisisaonedimensionalsearchofatmost24possibilities.ChooseC=1.6nF,so1/(2 *100e3*1.6e-9)=995Hz..

Checkpoint9.2:Thisisatwodimensionalsearchofatmost½242possibilities.ChooseR1=47k andR2=82k .

Checkpoint9.3:Aswitchingorbuck-boostregulatorcanincreasevoltage,e.g.,TPS63002.

Checkpoint9.4:Averagecurrentis500mA-hr/5years*(1day/24hour)*(1year/365days)=20 A.

Checkpoint9.5:Energystorageis20 A*1year*(24hour/1day)*(365days/1year)=175mA-hr.So,choosetheGM041842thathas250mA-hr.

Checkpoint9.6:ThereisaJTAGdebuggerinterface,alogicanalyzerinterface,andtherearetestpoints.

Checkpoint10.1:Set10=1000/2k,sok=50.ForeveryADCsample,calculatetheaverageoftheprevious50samples.

Checkpoint10.2:Ifthesystemislinear,theprecisionsareequal.

Checkpoint10.3:SincetheresponseinFigure10.29isapproximatelylinear,theresolutionwillbe10˚C/256,whichisabout0.04˚C.

Checkpoint10.4:At60BPM,the Rcount willbe4andthedifference Rlast-Rfirst willbe480.Iftheheartrateincreasesalittle,thenthedifferencewillreduceto479,givingameasurementof60.13.Thustheinherentmeasurementresolutionisabout0.12BPM.Becauseintegermathisused,thesystemwillhavearesolutionof1BPM.

Checkpoint11.1:Sinceinformationisencodedasenergy,anddataistransferredatafixedrate,eachenergypacketwillexistforafinitetime.Energypertimeispower.

Checkpoint11.2:Theperformancemeasureforastoragesystemisinformationdensityinbits/cm3.

Checkpoint11.3:Withopencollectoroutputs,thelowwilldominateoverHiZ.Thesignalwillbelow.

Checkpoint11.4:Onaverage,itwilltakeN/2transmissionsforthemessagetogofromonecomputertoanother.Thereare10bits/frame,sothereare10,000bytes/sec.Becausethereare10bytes/message,ittakes1mstotransmitamessage.Becauseithastobesent5times,ittakes5msonaverage.

Checkpoint11.5:Theframesentbyatransmitterisechoedtoitsownreceiver.Ifthedatadoesnotmatch,orifthereareanyframingornoiseerrorsthenacollisionoccurred.

Checkpoint11.6:Paritycouldbeusedtodetectcollisions.Alsothemessagecouldhavechecksumadded.Framingornoiseerrorscanalsoindicateacollision.

ReferenceMaterial

Vectoraddress Number IRQ ISRnameinStartup.s NVIC Prioritybits

0x00000038 14 -2 PendSV_Handler

0x0000003C 15 -1 SysTick_Handler

0x00000040 16 0 GPIOPortA_Handler NVIC_PRI0_R 7–5

0x00000044 17 1 GPIOPortB_Handler NVIC_PRI0_R 15–13

0x00000048 18 2 GPIOPortC_Handler NVIC_PRI0_R 23–21

0x0000004C 19 3 GPIOPortD_Handler NVIC_PRI0_R 31–29

0x00000050 20 4 GPIOPortE_Handler NVIC_PRI1_R 7–5

0x00000054 21 5 UART0_Handler NVIC_PRI1_R 15–13

0x00000058 22 6 UART1_Handler NVIC_PRI1_R 23–21

0x0000005C 23 7 SSI0_Handler NVIC_PRI1_R 31–29

0x00000060 24 8 I2C0_Handler NVIC_PRI2_R 7–5

0x00000064 25 9 PWMFault_Handler NVIC_PRI2_R 15–13

0x00000068 26 10 PWM0_Handler NVIC_PRI2_R 23–21

0x0000006C 27 11 PWM1_Handler NVIC_PRI2_R 31–29

0x00000070 28 12 PWM2_Handler NVIC_PRI3_R 7–5

0x00000074 29 13 Quadrature0_Handler NVIC_PRI3_R 15–13

0x00000078 30 14 ADC0_Handler NVIC_PRI3_R 23–21

0x0000007C 31 15 ADC1_Handler NVIC_PRI3_R 31–29

0x00000080 32 16 ADC2_Handler NVIC_PRI4_R 7–5

0x00000084 33 17 ADC3_Handler NVIC_PRI4_R 15–13

0x00000088 34 18 WDT_Handler NVIC_PRI4_R 23–21

0x0000008C 35 19 Timer0A_Handler NVIC_PRI4_R 31–29

0x00000090 36 20 Timer0B_Handler NVIC_PRI5_R 7–5

0x00000094 37 21 Timer1A_Handler NVIC_PRI5_R 15–13

0x00000098 38 22 Timer1B_Handler NVIC_PRI5_R 23–21

0x0000009C 39 23 Timer2A_Handler NVIC_PRI5_R 31–29

0x000000A0 40 24 Timer2B_Handler NVIC_PRI6_R 7–5

0x000000A4 41 25 Comp0_Handler NVIC_PRI6_R 15–13

0x000000A8 42 26 Comp1_Handler NVIC_PRI6_R 23–21

0x000000AC 43 27 Comp2_Handler NVIC_PRI6_R 31–29

0x000000B0 44 28 SysCtl_Handler NVIC_PRI7_R 7–5

0x000000B4 45 29 FlashCtl_Handler NVIC_PRI7_R 15–13

0x000000B8 46 30 GPIOPortF_Handler NVIC_PRI7_R 23–21

0x000000BC 47 31 GPIOPortG_Handler NVIC_PRI7_R 31–29

0x000000C0 48 32 GPIOPortH_Handler NVIC_PRI8_R 7–5

0x000000C4 49 33 UART2_Handler NVIC_PRI8_R 15–13

0x000000C8 50 34 SSI1_Handler NVIC_PRI8_R 23–21

0x000000CC 51 35 Timer3A_Handler NVIC_PRI8_R 31–29

0x000000D0 52 36 Timer3B_Handler NVIC_PRI9_R 7–5

0x000000D4 53 37 I2C1_Handler NVIC_PRI9_R 15–13

0x000000D8 54 38 Quadrature1_Handler NVIC_PRI9_R 23–21

0x000000DC 55 39 CAN0_Handler NVIC_PRI9_R 31–29

0x000000E0 56 40 CAN1_Handler NVIC_PRI10_R 7–5

0x000000E4 57 41 CAN2_Handler NVIC_PRI10_R 15–13

0x000000E8 58 42 Ethernet_Handler NVIC_PRI10_R 23–21

0x000000EC 59 43 Hibernate_Handler NVIC_PRI10_R 31–29

0x000000F0 60 44 USB0_Handler NVIC_PRI11_R 7–5

0x000000F4 61 45 PWM3_Handler NVIC_PRI11_R 15–13

0x000000F8 62 46 uDMA_Handler NVIC_PRI11_R 23–21

0x000000FC 63 47 uDMA_Error NVIC_PRI11_R 31–29

Table5.1.SomeoftheinterruptvectorsfortheLM3S/TM4C.

CDatatype C99Datatype Precision Range

unsignedchar

uint8_t 8-bitunsigned 0to+255

signedchar int8_t 8-bitsigned -128to+127

char char 8-bit ASCIIcharacters

unsignedint unsignedint compiler-dependent

int int compiler-dependent

unsignedshort

uint16_t 16-bitunsigned 0to+65535

short int16_t 16-bitsigned -32768to+32767

unsignedlong uint32_t unsigned32-bit 0to4294967295L

long int32_t signed32-bit -2147483648Lto2147483647L

float float 32-bitfloat ±10-38to±10+38

double double 64-bitfloat ±10-308to±10+308

DatatypesinC( 1 somecompilerstreatcharasunsigned,sometreatcharassigned)

BITS4to6

0 1 2 3 4 5 6 7

0 NUL DLE SP 0 @ P ` p

B 1 SOH XON ! 1 A Q a q

I 2 STX DC2 “ 2 B R b r

T 3 ETX XOFF # 3 C S c s

S 4 EOT DC4 $ 4 D T d t

5 ENQNAK % 5 E U e u

0 6 ACKSYN & 6 F V f v

7 BEL ETB ‘ 7 G W g w

T 8 BS CAN ( 8 H X h x

O 9 HT EM ) 9 I Y i y

A LF SUB * : J Z j z

3 B VT ESC + ; K [ k

C FF FS , < L \ l |

D CR GS - = M ] m

E SO RS . > N ^ n ~

F SI US / ? O _ o DEL

Table1.10.Standard7-bitASCII.

Parameter

PN2222(IC=150mA)

PN2907(IC=150mA)

2N2222(IC=500mA)

2N2907(IC=500mA)

TIP120(IC=3A)

TIP125(IC=3A)

hfe 100 40 1000

VBEsat 0.6 2 2.5V

VCEatsaturation

0.3 1 2V

Designparametersforthe2N2222andTIP120.

Chip Current Comment

MC3479 0.35A Stepperdriver

L293D 0.6A Dual,diodes

L293 1A Dual

TPIC0107 3A Direction,faultstatus

L6203 5A Dual

H-bridgedrivers

Family Example IOH IOL IIH IIL fanout

StandardTTL 7404 0.4mA 16mA 40µA 1.6mA

10

SchottkyTTL 74S04 1mA 20mA 50µA 2mA 10

LowPowerSchottky

74LS04 0.4mA 4mA 20µA 0.4mA

10

HighspeedCMOS

74HC04 4mA 4mA 1µA 1µA

LM3S/TM4C2mA-drive

TM4C 2mA 2mA 2µA 2µA

LM3S/TM4C4mA-drive

TM4C 4mA 4mA 2µA 2µA

LM3S/TM4C8mA-drive

TM4C 8mA 8mA 2µA 2µA

Table1.4.Theinputandoutputcurrentsofvariousdigitallogicfamiliesandmicrocontrollers.

Figure1.15.Voltagethresholdsforvariousdigitallogicfamilies.Memoryaccessinstructions

LDRRd,[Rn];load32-bitnumberat[Rn]toRd

LDRRd,[Rn,#off];load32-bitnumberat[Rn+off]toRd

LDRRd,[Rn,#off]!;load32-bitnumberat[Rn+off]toRd,preindex

LDRRd,[Rn],#off;load32-bitnumberat[Rn]toRd,postindex

LDRTRd,[Rn,#off];load32-bitnumberunprivileged

LDRRd,=value;setRdequaltoany32-bitvalue(PCrel)

LDRHRd,[Rn];loadunsigned16-bitat[Rn]toRd

LDRHRd,[Rn,#off];loadunsigned16-bitat[Rn+off]toRd

LDRHRd,[Rn,#off]!;loadunsigned16-bitat[Rn+off]toRd,pre

LDRHRd,[Rn],#off;loadunsigned16-bitat[Rn]toRd,postindex

LDRHTRd,[Rn,#off];loadunsigned16-bitunprivileged

LDRSHRd,[Rn];loadsigned16-bitat[Rn]toRd

LDRSHRd,[Rn,#off];loadsigned16-bitat[Rn+off]toRd

LDRSHRd,[Rn,#off]!;loadsigned16-bitat[Rn+off]toRd,pre

LDRSHRd,[Rn],#off;loadsigned16-bitat[Rn]toRd,postindex

LDRSHTRd,[Rn,#off];loadsigned16-bitunprivileged

LDRBRd,[Rn];loadunsigned8-bitat[Rn]toRd

LDRBRd,[Rn,#off];loadunsigned8-bitat[Rn+off]toRd

LDRBRd,[Rn,#off]!;loadunsigned8-bitat[Rn+off]toRd,pre

LDRBRd,[Rn],#off;loadunsigned8-bitat[Rn]toRd,postindex

LDRBTRd,[Rn,#off];loadunsigned8-bitunprivileged

LDRSBRd,[Rn];loadsigned8-bitat[Rn]toRd

LDRSBRd,[Rn,#off];loadsigned8-bitat[Rn+off]toRd

LDRSBRd,[Rn,#off]!;loadsigned8-bitat[Rn+off]toRd,pre

LDRSBRd,[Rn],#off;loadsigned8-bitat[Rn]toRd,postindex

LDRSBTRd,[Rn,#off];loadsigned8-bitunprivileged

LDRDRd,Rd2,[Rn,#off];load64-bitat[Rn+off]toRd,Rd2

LDRDRd,Rd2,[Rn,#off]!;load64-bitat[Rn+off]toRd,Rd2,pre

LDRDRd,Rd2,[Rn],#off;load64-bitat[Rn]toRd,Rd2,postindex

LDMFDRn!,Reglist;loadregfromlistatRn(inc),!updateRn

LDMIARn!,Reglist;loadregfromlistatRn(inc),!updateRn

LDMDBRn!,Reglist;loadregfromlistatRn(dec),!updateRn

STMIARn!,Reglist;storeregfromlisttoRn(inc),!updateRn

STMFDRn!,Reglist;storeregfromlisttoRn(dec),!updateRn

STMDBRn!,Reglist;storeregfromlisttoRn(dec),!updateRn

STRRt,[Rn];store32-bitRtto[Rn]

STRRt,[Rn,#off];store32-bitRtto[Rn+off]

STRRt,[Rn,#off]!;store32-bitRtto[Rn+off],pre

STRRt,[Rn],#off;store32-bitRtto[Rn],postindex

STRTRt,[Rn,#off];store32-bitRtto[Rn+off]unprivileged

STRHRt,[Rn];storeleastsig.16-bitRtto[Rn]

STRHRt,[Rn,#off];storeleastsig.16-bitRtto[Rn+off]

STRHRt,[Rn,#off]!;storeleastsig.16-bitRtto[Rn+off],pre

STRHRt,[Rn],#off;storeleastsig.16-bitRtto[Rn],postindex

STRHTRt,[Rn,#off];storeleastsig.16-bitunprivileged

STRBRt,[Rn];storeleastsig.8-bitRtto[Rn]

STRBRt,[Rn,#off];storeleastsig.8-bitRtto[Rn+off]

STRBRt,[Rn,#off]!;storeleastsig.8-bitRtto[Rn+off],pre

STRBRt,[Rn],#off;storeleastsig.8-bitRtto[Rn],postindex

STRBTRt,[Rn,#off];storeleastsig.unprivileged

STRDRd,Rd2,[Rn,#off];store64-bitRd,Rd2to[Rn+off]

STRDRd,Rd2,[Rn,#off]!;store64-bitRd,Rd2to[Rn+off],pre

STRDRd,Rd2,[Rn],#off;store64-bitRd,Rd2to[Rn],postindex

PUSHReglist;push32-bitregistersontostack

POPReglist;pop32-bitnumbersfromstackintoregisters

ADRRd,label;setRdequaltotheaddressatlabel

MOVSRd,<op2>;setRdequaltoop2

MOVRd,#im16;setRdequaltoim16,im16is0to65535

MOVTRd,#im16;setRdbits31-16equaltoim16

MVNSRd,<op2>;setRdequalto-op2

Branchinstructions

Blabel;branchtolabelAlways

BEQlabel;branchifZ==1Equal

BNElabel;branchifZ==0Notequal

BCSlabel;branchifC==1Higherorsame,unsigned≥

BHSlabel;branchifC==1Higherorsame,unsigned≥

BCClabel;branchifC==0Lower,unsigned<

BLOlabel;branchifC==0Lower,unsigned<

BMIlabel;branchifN==1Negative

BPLlabel;branchifN==0Positiveorzero

BVSlabel;branchifV==1Overflow

BVClabel;branchifV==0Nooverflow

BHIlabel;branchifC==1andZ==0Higher,unsigned>

BLSlabel;branchifC==0orZ==1Lowerorsame,unsigned≤

BGElabel;branchifN==VGreaterthanorequal,signed≥

BLTlabel;branchifN!=VLessthan,signed<

BGTlabel;branchifZ==0andN==VGreaterthan,signed>

BLElabel;branchifZ==1orN!=VLessthanorequal,signed≤

BXRm;branchindirecttolocationspecifiedbyRm

BLlabel;branchtosubroutineatlabel

BLXRm;branchtosubroutineindirectspecifiedbyRm

CBNZRn,label;branchifRnnotzero

CBZRn,label;branchifRnzero

ITxyzcond;ifthenblockwithx,y,zT(true)orF(false)

TBB[Rn,Rm];tablebranchbyte

TBH[Rn,Rm,LSL#1];tablebranchhalfword

Mutualexclusiveinstructions

CLREX;clearexclusive

LDREXcondRt,[Rn,#offset];load32-bitexclusive

STREXcondRd,Rt,[Rn,#offset];store32-bitexclusive

LDREXBcondRt,[Rn];load8-bitexclusive

STREXBcondRd,Rt,[Rn];store8-bitexclusive

LDREXHcondRt,[Rn];load16-bitexclusive

STREXHcondRd,Rt,[Rn];store16-bitexclusive

Miscellaneousinstructions

BKPT#imm;executebreakpoint,debugstate0to255

CPSIEF;clearfaultmaskF=0

CPSIEI;enableinterrupts(I=0)

CPSIDF;setfaultmaskF=1

CPSIDI;disableinterrupts(I=1)

DMB;datamemorybarrier,memoryaccesstofinish

DSB;datasynchronizationbarrier,instructionstofinish

ISB;instructionsynchronizationbarrier,finishpipeline

MRSRd,SpecReg;movespecialregistertoRd

MSRRd,SpecReg;moveRdtospecialregister

NOP;nooperation

SEV;SendEvent

SVC#im8;supervisorcall(0to255)

WFE;waitforevent

WFI;waitforinterrupt

Logicalinstructions

ANDSRd,Rn,<op2>;Rd=Rn&op2(op2is32bits)

BFCRd,#lsb,#width;clearbitsinRn

BFIRd,Rn,#lsb,#width;bitfieldinsert,RnintoRd

ORRSRd,Rn,<op2>;Rd=Rn|op2(op2is32bits)

EORSRd,Rn,<op2>;Rd=Rn^op2(op2is32bits)

BICSRd,Rn,<op2>;Rd=Rn&(~op2)(op2is32bits)

ORNSRd,Rn,<op2>;Rd=Rn|(~op2)(op2is32bits)

TSTRn,<op2>;Rn&op2(op2is32bits)

TEQRn,<op2>;Rn^op2(op2is32bits)

LSRSRd,Rm,Rs;logicalshiftrightRd=Rm>>Rs(unsigned)

LSRSRd,Rm,#n;logicalshiftrightRd=Rm>>n(unsigned)

ASRSRd,Rm,Rs;arithmeticshiftrightRd=Rm>>Rs(signed)

ASRSRd,Rm,#n;arithmeticshiftrightRd=Rm>>n(signed)

LSLSRd,Rm,Rs;shiftleftRd=Rm<<Rs(signed,unsigned)

LSLSRd,Rm,#n;shiftleftRd=Rm<<n(signed,unsigned)

REVRd,Rn;Reversebyteorderinaword

REV16Rd,Rn;Reversebyteorderineachhalfword

REVSHRd,Rn;Reversebyteorderinthebottomhalfword,

;andsignextendsto32bits

RBITRd,Rn;Reversethebitorderina32-bitword

SBFXRd,Rn,#lsb,#width;signedbitfieldandextract

UBFXRd,Rn,#lsb,#width;unsignedbitfieldandextract

SXTBRd,Rm,ROR#n;Signextendbyte

SXTHRd,Rm,ROR#n;Signextendhalfword

UXTBRd,Rm,ROR#n;Zeroextendbyte

UXTHRd,Rm,ROR#n;Zeroextendhalfword

Arithmeticinstructions

ADDSRd,Rn,<op2>;Rd=Rn+op2

ADDSRd,Rn,#im12;Rd=Rn+im12,im12is0to4095

CLZRd,Rm;Rd=numberofleadingzerosinRm

SUBSRd,Rn,<op2>;Rd=Rn-op2

SUBSRd,Rn,#im12;Rd=Rn-im12,im12is0to4095

RSBSRd,Rn,<op2>;Rd=op2-Rn

RSBSRd,Rn,#im12;Rd=im12–Rn

CMPRn,<op2>;Rn–op2setstheNZVCbits

CMNRn,<op2>;Rn-(-op2)setstheNZVCbits

MULSRd,Rn,Rm;Rd=Rn*Rmsignedorunsigned

MLARd,Rn,Rm,Ra;Rd=Ra+Rn*Rmsignedorunsigned

MLSRd,Rn,Rm,Ra;Rd=Ra-Rn*Rmsignedorunsigned

UDIVRd,Rn,Rm;Rd=Rn/Rmunsigned

SDIVRd,Rn,Rm;Rd=Rn/Rmsigned

UMULLRdLo,RdHi,Rn,Rm;Unsignedlongmultiply32by32into64

UMLALRdLo,RdHi,Rn,Rm;Unsignedlongmultiply,withaccumulate

SMULLRdLo,RdHi,Rn,Rm;Signedlongmultiply32by32into64

SMLALRdLo,RdHi,Rn,Rm;Signedlongmultiply,withaccumulate

SSATRd,#n,Rm,shift#s;signedsaturationtonbits

USATRd,#n,Rm,shift#s;unsignedsaturationtonbits

Notes RaRdRmRnRtrepresent32-bitregisters

valueany32-bitvalue:signed,unsigned,oraddress

SifSispresent,instructionwillsetconditioncodes

#im8anyvaluefrom0to255

#im12anyvaluefrom0to4095

#im16anyvaluefrom0to65535

Rd,ifRdispresentRdisdestination,otherwiseRn

#nanyvaluefrom0to31

#offanyvaluefrom-255to4095

labelanyaddresswithintheROMofthemicrocontroller

SpecRegAPSR,IPSR,EPSR,IEPSR,IAPSR,EAPSR,PSR,MSP,PSP,

PRIMASK,BASEPRI,BASEPRI_MAX,FAULTMASK,orCONTROL.

Reglistisalistofregisters.E.g.,R1,R3,R12

op2thevaluegeneratedby<op2>

Examplesofflexibleoperand <op2> creatingthe32-bitnumber.E.g., Rd=Rn+op2

ADDRd,Rn,Rm;op2=Rm

ADDRd,Rn,Rm,LSL#n;op2=Rm<<nRmissigned,unsigned

ADDRd,Rn,Rm,LSR#n;op2=Rm>>nRmisunsigned

ADDRd,Rn,Rm,ASR#n;op2=Rm>>nRmissigned

ADDRd,Rn,#constant;op2=constant , where X and Y arehexadecimaldigits:

producedbyshiftingan8-bitunsignedvalueleftbyanynumberofbits

intheform 0x00XY00XYintheform 0xXY00XY00intheform 0xXYXYXYXY