MOND A Y , MA R CH 9 , 2 009

AT89C51 Description

Now let us talk about this popular 8051 chip. It has on chip ROM

in the form of flash memory. This is ideal for fast development

since flash memory can be erased in seconds compared to the

twenty minutes or more needed for the 8751. For this reason the

AT89C51 is used in place of the 8751 to eliminate the waiting

time needed to erase the chip and thereby speed up the

development time. To use the AT89C51 to develop a

microcontroller-based system requires a ROM burner that

supports flash memory; however, a ROM eraser is not needed.

Notice that in flash memory you must erase the entire contents

of ROM in order to program it again. This erasing of flash is done

by the PROM burner itself . See the following table for its specs

The block diagram shows the internal structure of the

microcontroller AT89C51;

Pin description of this MCU can be seen in the following diagram;

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Learn 8051 (For Beginners)This Blog is made for beginners who wants to learn 8051 based Micro Controller.Starting from simple to advance interfacing techniques of various devices with Micro

Controllers.

Port 0; is a dual-purpose port on pins 32-39 of the 8051 1 C. In

minimum – component designs, it is used as a general purpose I/O Port.

For larger designs with external memory , it becomes a multiplexed

address and data bus.

Port 1; is a dedicated I/O port on pins 1 -8. The pins, designated as P1 .0.

P1 .1 . P1 .2 etc. are av ailable for interfacing to external dev ices as

required. No alternate functions are as signed for Port 1 pins; thus they

are used solely for interfacing to external dev ices. Exceptions are the

8032/8052 ICs. Which use P1 .0 and P1 .1 either as I/O lines or as

external in outs to the third timer.

Port 2; (pints 21 -28) is a dual – purpose port serv ing as general

purpose I/O, or as the high by te of the address bus for designs with

external code memory or more than 256 by tes of external data

memory .

Port 3;is a dual – purpose port on pins 1 0-1 7 . As well as general –

purpose I/O, these pins are multifunctional with each hav ing an

alternate purpose related to special features of the 8051 ).

Now coming to the other pin functions.

PSEN;This is an output pin. PSEN stands for “program store

enable.” In an 8031-based system in which an external ROM

holds the program code, this pin is connected to the OE pin of

the ROM.

VCC;Pin 40 provides supply voltage to the chip. The voltage

source is +5V.

GND; Pin 20 is the Ground pin.

XTAL1 and XTAL2;The 8051 has an on-chip oscillator but requires an

external clock to run it. Most often a quartz cry stal oscillator is

connected to inputs XTALI (pin 1 9) and XTAL2 (pin 1 8). The quartz

cry stal oscillator connected to XTAL1 AND XTAL2 also needs two

capacitors of 30 pF v alue. One side of each capacitor is connected to the

ground as shown in this figure;

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word to see the meaning)

EA; The 8051 family members, such as the 8751, 89C51, or

DS5000. All come with on-chip ROM to store programs. In such

cases, the EA pin is connected to VCC for giving power to save

and erase program from the memory.

RST(RESET);The RST input on pin 9 is the master rest for the

8051. When this signal is brought high for a least two machine

cycles, the 8051 internal registers are loaded with appropriate

values for an orderly system start-up. For normal operation, RST

is low. Figure shows permanent connections of Reset Pin.

ALE; (address latch enable) is an output pin a nd is activ e high. When

connecting an 8031 to external memory , port 0 prov ides both address

and data. In other words, the 8031 multiplexes address and data

through port 0 to sav e pins. The ALE pin is used for de-multiplexing the

address and data by connecting to the G pin of the 7 4LS37 3 chip.Now

we will talk about what other things are inside an AT89C51 MCU;

Registers;In the CPU, registers are used to store information

temporarily . That information could be a by te of data to be processed,

or an address pointing to the data to be fetched. The v ast majority of

89C51 register an address pointing to the data to be fetched. The v ast

majority of 89C51 registers are 8-bit registers. In the 8051 there is

only one data ty pe: 8 bits. The 8 bits of a register are shown in the

diagram from the MSB (most significant bit) D7 to the LSB (least

significant bit) D0. With an 8-bit data ty pe, any data larger than 8 bits

must be broken into 8-bit chunks before it is processed. The most widely

used registers of the 89C51 are A (accumulator), B, R0, R1 , R2, R3, R4,

R5, R6, R7 , DPTR (data pointer), and PC (program counter). All of the

abov e registers are 8-bits, except DPTR and the program counter. The

accumulator, register A, is used for all arithmetic and logic

instructions.

All the registers of 89C51 are;

Timers;Both timer 0 and timer 1 are 16 bits wide. Since the

89C51 has an 8-bit architecture, each 16-bit timer is accessed as

two separate registers of low byte and high byte. Each timer is

discussed separately.

Timer 0 Register;The 16-bit register of time 0 is accesses as low

byte and high byte. The low byte register is called TL0 (timer 0

low byte) and the high byte register is referred to as th0 (timer 0

high byte). These registers can be accesse d like any other

register, such as A,B, R0, R1, R2 etc. For example, the instruction

“TLO= 20” moves the value 500 into TL0, the low byte of timer 0.

These registers can also be read like any other register.

Timer 1 Register;Timer 1 is also 16 bits, and its 16- bit register

is split into two bytes, referred to as TL1 (timer 1 low byte) and

TH1 (timer 1 high byte). These registers are accessible in the

same way as the registers of timer 0.

TMOD(Timer Mode) Register;Both timers 0 and 1 uses the

same register, called TMOD, to set the various timer operation

modes. TMOD is an 8-bit register in which the lower 4 bits are set

aside for timer 0 and the upper 4 bits are set aside for timer 1. In

each case, the lower 2 bits are used to set the timer mode and

the upper 2 bits to specify the operation.

Mode 2 Programming;The following are the characteristics and

operations of mode 2.

1-It is an 8-bit timer; therefore, it allows only values of 00 to FFH

to be loaded into the timer’s register TH.

2-After TH is loaded with the 8-bit value, the 8051 gives a copy

of it to TL. Then the timer must be started. This is done by the

instruction “SETB TR0” for timer 0 and “SETB TR1” for timer 1. This

is just like mode 1.

3-After the timer is started, it starts to count up by incrementing

the TL register. It counts up until it reaches its limit of FFH. When

it rolls over from FFH to 00, it sets high the TF (timer flag). If we

are using timer 0, TF0 goes high; if we are using timer 1, TF1 is

raised.

4-When the TL registers rolls from FFH to 0 and TF is set to 1, TL

is reloaded automatically with the original value kept by the TH

register. To repeat the process, we must simply clear TF and let it

go without any need by the programmer to reload the original

value. This makes mode 2 and auto-reload, in contrast with

mode 1 in which the programmer has to reload TH and TL.

It must be emphasized that mode 2 is an 8-bit timer. However, it

has an auto-reloading capability in auto reload. TH is loaded with

the initial count and a copy of it is given to TL. This reloading

leaves TH unchanged, still holding a copy of original value. This

mode has many applications, including setting the baud rate in

serial communication.

SBUF (Serial Buffer) Register; SBUF is an 8-bit register used

solely for serial communication in the 89C51. For a byte of data

to be transferred via the TxD line, it must be placed in the SBUF

register. Similarly, SBUF holds the byte of data when it is

received by the 89C51 RxD line. SBUF can be accessed like any

other register in the 89C51.The moment a byte is written into

SBUF, it is framed with the start and stop bits and transferred

serially via the TxD pin. Similarly, when the bits are received

serially via RxD, the 89C51 deframes it by eliminating the stop

and start bits, making a byte out of the data received, and then

placing it in the SBUF. Some baud rates are shown below:

Baud Rate= 9600 TH1(Decimal)=-3 TH1(HEX)=FD

Baud Rate= 4800 TH1(Decimal)=-6 TH1(HEX)=FA

Baud Rate= 2400 TH1(Decimal)=-12 TH1(HEX)=F4

Baud Rate= 1200 TH1(Decimal)=-24 TH1(HEX)=E8

SCON(Serial Control) Register;The SCON register is an 8-bit

register used to program the start bit, stop bit, and data bits of

data framing, among other things. Following SCON bits are

explained; SMO SCON.7 Serial port mode specifier

SM1/ SCON.6= Serial port mode specifier

SM2/ SCON.5= Used for multiprocessor communication (Make it 0)

REN/ SCON.4= Set / cleared by software to enable / disable

reception.

TB8/ SCON.3= Not widely used.

RB8/ SCON.2= Not widely used.

T1/ SCON.1= Transmit interrupt flag. Set by hardware at the

beginning of the stop

bit in mode1. Must by cleared by software.

R1/ SCON.0= Receive interrupt flag. Set by hardware halfway

through the stop bit

time in mode1. Must be cleared by software.

Bit-Addressable RAM; Of the 128-byts internal RAM of the 8051,

only 16 bytes of it are bit addressable. The rest must be

accessed in byte format. The bit – addressable RAM locations are

20H to 2FH. These 16 bytes provide 128 bits of RAM bit –

address ability since 16 x 8 = 128. They are addressed as 0 to

127 (in decimal) or 00 to 7FH. Therefore, the bit addresses 0 to 7

are for the first byte of internal RAM location 20H, and 8 to 0FH

are the bit addresses of the second byte, RAM location 21H, and

so on. The last byte of 2FH has bit address byte of 2FH has bit

addresses of 78H to 7FH. Note that internal RAM locations 20-

2FH are both byte-addressable and bit addressable. Notice from

figure that bit addresses 00 – 7FH belong to RAM byte addresses

20 – 2FH, and bit addresses 80 – F7H belong to SFR P0, P1 etc.

Please refer to the following documents for detailed insight of

AT89C51 MCU;

Data Sheet;

www.atmel.com/atmel/acrobat/doc0265.pdf

http://www.filedropper.com/at89c51datasheet

http://www.filedropper.com/flashmcumemorytorg

Tutorials;

http://www.filedropper.com/best8051tutorial

http://www.filedropper.com/8051tutorial

Hardware Manual;

http://www.filedropper.com/atmel8051microcontrollershardware

manual

Posted by Fani at 4:05 PM

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SAMRIDH August 6, 2009 at 11:50 PM

a very nice lesson for beginers

Reply

Anonymous August 26, 2009 at 3:54 PM

hi, your blog's helped me in my understanding of 8051

ucontrollers. However, your links for the tutorials no longer

work. Could you pls upload them again?

thanks

Reply

sagar June 11, 2010 at 11:42 AM

thank ,,, for help me it's really nice one and full information

about the 8051.... it's really super up

Reply