E-Paper Displays:A Case Study

E-Paper Displays:A Case Study

Mohamed Imran, Noor Mohamed

Lehrstuhl fur Mikroelektronik,66123, Saarbrucken, Germany

http://www.lme.uni-saarland.de

Abstract. E-paper displays aims to mimic real paper with high re-flectance and low power consumption similar to original paper.Here weintend to study working principle of several E-paper technologies, driv-ing schemes with hardware and software implementations to solve chal-lenges in E-paper ecosystem.We found that understanding different E-paper displays will help us to pursue novel display research to find newmaterials governed by laws of physics and to propose New Driving meth-ods.Comparative studies on E-paper technologies help us to propose newresearch and development on E-paper Displays.We claim that, study onE-paper displays like CH-LCD(Cholestric Liquid crystal Display) en-able us to visualize information effectively.We also speculate next gener-ation contender for E-paper displays, this paper suggests developmentsin Physical Implementations for creating High quality displays involvingLow cost and Low Power consumption.

Keywords: Cholestric Liquid Crystal Displays, Low power consump-tion, Driving Schemes, Passive Matrix addressing, Drivers.

Table of Contents

1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 E-paper technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2.1 Electrophoretic Displays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32.2 Electrowetting Displays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42.3 Cholestric Displays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42.4 Comparison table . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

3 Addressing Schemes for E-Paper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53.1 Active Matrix Addressing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63.2 Passive Matrix Addressing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

4 E-Paper Driver Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104.1 System Architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

5 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

E-Paper Displays 3

1 Introduction

An Electronic display is a converter of electronic information for recognition bybrain via vision. Software and hardware components of Electronic Display(E-Display) enables us to visualize data.E-displays aims to represent the maximumvariance in Human perception while perceiving an information with physicaland environmental considerations.Recent developments in E-Display systems likeLiquid crystal display(LCD) and Organic light emitting diode(OLED) helps torealize the information effectively.Several issues such as wide viewing angle, Lowpower consumption is solved by OLED while LCD is still cost-effective.Though,there exist a well-known problem to mimic hard copy type printed Paper.

Past studies shows that, No single display system can challenge differentenvironmental conditions and characteristics[1].This lead us to have special re-quirements in optical properties such as brightness, contrast ratio, and viewingangle which should be similar or better than printing paper with lower powerconsumption.We asked whether there is better way to present the informationas a replacement for printed paper.This answers our goal to present E-paperdisplays with all readability requirements.

In this paper, We aim to understand the working principle of several E-paper displays, driving schemes with hardware and software implementations tosolve various challenges in E-paper ecosystem.We perform a comparative studydesigned to test our hypothesis on diverse E-paper technologies.To supplementour hypothesis,We look at Cholestric Liquid Crystal Displays(CH-LCD) DrivingScheme as our research example[2].

2 E-paper technologies

Below mentioned three mainstream E-paper technologies which are successful inmaking E-paper displays.

2.1 Electrophoretic Displays

Electrophoretic Ink(E-Ink) technology creates an image that looks like realprinted paper from all angles and lighting condition.Fig.1 explains the work-ing principle, the display is made up of millions of microscopic titanium oxidespheres sandwiched between electrodes within each sphere are positively chargedwhite ink particles and negatively charged black ones.Applying a negative chargeto the bottom electrode repels the black spheres to the top, making the screenappear black at that pixel.Thus, the positive charge moves the white ones to thetop.

There are two main technologies which adopts the electrophoretic theory,namely E-ink’s pearl and Bridgestones QR-LPD.E-ink technology tries to mimicthe original paper as it leverages ambient light.In the past, the refresh speed ofE-ink was low as it took 0.5 to 1 seconds.Although, next generation E-ink displayused active matrix scheme and color filters to refresh display quickly but thistechnology consumed high power.

4 E-Paper Displays:A Case Study

Fig. 1. Electrophoretic Displays

2.2 Electrowetting Displays

Electrowetting display is based on Electrowetting principle[3], which uses thesame underlying technology but replacing the titanium with layers of coloredoil and water.Electrowetting is a microfluidic phenomenon which modifies thesurface tension of liquids on a solid surface with voltage.By applying a voltage,the wetting properties of a hydrophobic surface can be modified and the surfacebecomes increasingly hydrophilic (wettable) as shown in the fig.2

As Electrowetting displays uses colored oil, they can theoretically display upto 16 million colors.This oil and water combination also lets us to switch eachpixel between a colored and a white (blank) state very quickly.Therefore, theswitching speed is so fast to play video very well.This is a huge advantage overElectrophoretic displays, which switches so slowly.It’s response time is 70 timesbetter than the current reflective display.Although, Electrowetting displays hasgood refresh rate and suitable for video applications, still it requires high drivingvoltage.

2.3 Cholestric Displays

Cholestric Liquid crystal displays(CH-LCD) is a promising candidate due itsimpressive features.To cite, it gives better image memory retention even whenpower is off, with ultra-low power consumption.Manufacturing thin and lightweightCH-LCD is easy to display different colors without any color filters.Comparatively,It has good refresh speed[4].Finally, it is flexible and bendable which makes themwell-suited for all hand-held applications like E-books and for advertisementBillboards.One example for CH-LCD based E-papers: it can be used for Publictransportation(trains and buses).This Color electronic paper be used to display

E-Paper Displays 5

Fig. 2. Electrowetting Principle

advertisements inside a train, and the advertisements can be changed dependingon the next station stop.Alternatively, electronic paper can be used to displaydownloaded information from the internet inside the train.At bus stops, insteadof just displaying fixed bus schedules, it can also display information on busesthat are behind schedule because of heavy traffic.

2.4 Comparison table

The table below compares the advantages and challenges in several E-paperdisplays.

E-paper Technology Research Advantages ChallengesElectrophoretic E-ink(PVI) Anti-press capability Color filter neededElectrowetting Liquavista Quick response High driving volt-

age.Ch-LCD (Cholesteric) Kent Display No color filters Temperature de-

pendence.

3 Addressing Schemes for E-Paper

Traditionally, there exists three addressing/driving schemes for display devices:Direct, Matrix, and Raster.The aim of each scheme is to maintain the state of apixel to either black/white or Gray-scale level.In a Matrix addressing display, thecontrol signals are applied to rows and columns of the display.Consider a screensize of M*N pixels, this scheme would require M+N control signals.For activematrix addressing, a capacitor, transistor circuits is used to maintain the stateof the cell whereas Passive matrix addresses without using transistor circuits[5].


3.1 Active Matrix Addressing

Active matrix addressing scheme switches individual elements (pixels) of a dis-play, here each pixel is attached to a transistor and capacitor which activelymaintains the pixel state while other pixels being addressed.

Fig. 3. Active Matrix Addressing(right) and Passive Matrix Addressing(left)

As shown in Figure.3, Given an m*n matrix, the number of connectors neededto address the display is always m+n pixels. Each pixel is attached to a switch-device, which actively maintains the pixel state while other pixels are beingaddressed.There is an advantage in this driving scheme as it prevents crosstalkfrom inadvertently changing the state of an unaddressed pixel.Commonly, usedswitching devices are MOSFET with capacitor.

3.2 Passive Matrix Addressing

As seen from the above Fig.3, Passive matrix addressing is same as AM address-ing but the state of the pixel is maintained without using TFT or any externalstorage capacitors.Passive matrix addressing is extensively used for Cholestric-Liquid crystal displays as they do not need switching component of an activematrix display due to their bistable characteristics.Apart, developing AM driv-ing for a CH-LCD is also cost expensive.Therefore, the addressing of bistabledisplays is done with PM schemes.

Driving Scheme For Cholestric-Liquid Crystal: Physical and Opticalcharacteristics of CH-LCD help us to understand working phenomenon of acholestric cell as it is the workhorse for proposing new driving schemes. Fun-damentally, CH-LC has two stable states(Bi stability)namely Planar and Focalconic states[6].When liquid crystal is sandwiched between two glass plates(cell)

E-Paper Displays 7

Planar states have helical axes perpendicular to cell surface whereas Focal stateshave helical axes randomly scattered in the cell. Fig.4 explains when incidentlight is passed it gets reflected in Planar state and gets weakly scattered in Focalconic state

Fig. 4. Three states of CH-Liquid Crystal

These two states is used to switch on and off CH-LC displays.Yet, no fastdriving of CH-LCD will be realized without any meta-stable states.Recent dis-covery reveals two more states namely: Homeotropic state and transient pla-nar state.In homeotropic state, the applied voltage makes the helical structureto be broken which enables them to align parallel to the applied electric fieldand perpendicular to incident light where as Transient planar state is basedon removal of voltage.If the applied voltage is removed quickly it jumps tothe stable Planar state.In the other way, if the voltage is removed slowly theHomeotropic states jumps to transient planar state and then to Focal conicstate.Transient planar is a state whose helical structure axes perpendicular tothe incident light.Comparatively, We claimed that there is an advantage in usingCH-LCD such as High contrast ratio, Low power consumption and possibilityfor different colors without color filters.For example, having reflective planar andscatter focal conic states with black back plane helps to absorb light which leadsto High contrast ratio.Thus, the characteristics of CH-Liquid cell proves uniquefor any E-paper Display.


Static Driving Scheme As shown in Fig.5, Measured Reflectance Versus ap-plied voltage explains the mechanism to exploit their stable state characteristics.Static Driving Scheme utilizes only stable states without meta-stable states.

Fig. 5. Reflectance Versus Voltage Curve

As Curve ’a’ in Fig.5 is the response of cell in the reflecting state(Planar state)prior to the applied voltage(pulse).For voltage below 20 V, the reflection is notaffected pulse. When voltage is between 20 and 34 V, the reflection decreases.Reflection of the cell reaches its original value for voltage above 46 V. Curve ’b’is the response of the cell in scattering(Focal conic) state prior to the pulse.Inthis case the reflection of the cell is unchanged below 44 Voltage.Then the cellis switched into the reflecting state for a pulse above 50 V.

In this conventional scheme every row is driven independent of other rows.Asshown in Fig.6 the row voltage is either 35 V for selected row or 0 V for unselectedrow.The column voltage is 15 V for obtaining the planar state and 0 V forthe focal conic state.The row and column voltages are AC square pulse andwith opposite phases.Hence, for pixels in selected row the voltage is either 50V for reflecting state(Planar) or 35 V for scattering state(Focal Conic) and forunselected row voltage is either 0 or 15 V. Therefore once a pixel of a multiplexeddisplay is addressed into the reflecting or scattering state, it will remain in thatstate.

Applications developed with Static driving scheme is simple without anycrosstalk.This driving scheme allows high contrast ratio as the former rows don’taffect rows after them.However, Static driving scheme requires 50ms to refreshfrom one row to another this makes them very slow for high video rate.Consider,if we need to drive display consisting of 60 rows(60*50ms) roughly 3 seconds willbe taken by them which make them awful to construct and use.

Dynamic Driving Scheme As conventional driving scheme took 50ms(milliseconds), it makes static driving scheme unpractical.As shown in the Fig.7, thefast transition from meta-stable homeotropic state to transient planar state and

E-Paper Displays 9

Fig. 6. Static Driving Scheme

Fig. 7. Dynamic Driving Scheme- Possible State Transition


the hysteresis between planar state and focal conic state is the principle for Dy-namic driving scheme as proposed by Huang et al[7].The technique is that allpixels are switched to homeotropic state initially.After that all pixels are switchedfrom the homeotropic state to transient state in a very high speed.This intro-duces a short driving time in the range of 1ms.The Driving scheme suggests threestages as shown in the figure. The three stages are:Preparation stage, Selectionstage and Evolution stage.Firstly, in the preparation stage higher driver voltageis added for 60ms to get all pixels into homeotropic state.In the selection stage,if the driving waveform is higher the homeotropic state will be kept. Then in theevolution stage if the pixel was kept in the homeotropic state, then it remains inthe same state until the end of the driving waveform.Alternatively, In selectionstage if the driving wave is less than homeotropic state, the Homeotropic stateis switched to transient planar state.Then in evolution stage it evolves to focalconic texture.Finally, stable focal conic state will be achieved after evolutionstage is turned off.

Although the dynamic driving scheme has increased the driving speed of CH-LCD this introduces high driving voltage.To solve this issue an improvementwith four driving stages was proposed by Kozachenko et al.[8].Thus, several newdriving schemes are developed with high multiplexing level with high addressingspeed and low driving voltage considerations.

4 E-Paper Driver Design

To drive Cholestric-LCD, high voltage is required to change Initial states toHomeotropic state for this special driving circuit need to be designed.A Drivercircuit is used to control another circuit or component such as CH-LCD.In gen-eral, a driver is used to regulate the current flow through a circuit.For our pur-pose, Driver circuit is also used to prevent FPGA from high voltage leak sincecholestric-LCD requires 40 Volts to drive them.Several driving schemes beingproposed, albeit building a right driver circuit will always be a challenge. Nev-ertheless, from our research We constructed a flexible driver circuit to executeall driving schemes[2] .

4.1 System Architecture

As seen from Fig.8, FPGA programming file will be coded and synthesized inVHDL.The downloaded code will generate control signals from FPGA to drivethe row and column drivers respectively.There is level shifter IC used in driverboard to amplify the PWM(Pulse Width Modulation) waves generated fromFPGA.This level shifted waves helps to drive target rows and columns of CH-LCD.We need a Interface between PWM waveform and target CH-LCD withdifferent sets of pin connections.The interface boards acts as plug-in interfacesto connect several configurations in the LCD-panel.We used four independentdriver boards two rows and two columns.

E-Paper Displays 11

Fig. 8. E-Paper Displays: Hardware Implementation

To prevent the high voltage leak we used Transceiver IC between the FPGAoutput pins and Level shifter IC.Similarly, to switch from ON to OFF andvice-versa our driver boards needs a Switching circuit with FET.TransreceiverIC[9],Transreceiver is CMOS-Low-power dual supply translating buffer withthree states.It is used to prevent high cost FPGA from the high voltage cir-cuit.In our model, We had ten input/output pins in the integrated circuit thenswitching circuit had two transistors to control the signals from FPGA to levelshifter through transceiver.Thus, the transistor mechanism gives Low and highvoltage which helps to drive CH-LCD.

5 Discussion

We found that understanding working mechanisms of different E-paper displayswill help us to pursue novel display research to find new materials governedby laws of physics.Also, Our study on E-paper Driving schemes conclude us topropose New Driving methods for High-Quality displays with Low cost and LowPower consumption.Comparative studies on E-paper technologies proves thatthere exists several challenges which need to be solved in E-paper ecosystem.

Our study shows that different technologies are emerging from laboratoriesto compete next generation E-paper displays for different purposes.As an ap-plication scenario, the use of electronic paper for displaying product prices onstore shelves allow us to change prices immediately, enabling stores to easily im-plement discounted shopping hour promotions.Thus, growth of E-paper displayslead us a new paradigm to research and develop state-of-the-art driving schemes.

Although, We claimed Cholestric LCD is a low power display and makingcolor displays is easier with compatible production process they are not wellsuitable for extreme temperatures.One possible solution is to add compensationcircuit to adjust the temperature shift[10].Yet,We need confirmatory studies toconclude which technology would be the next generation contender for E-PaperDisplays.Readers may criticize that E-paper displays find its way for niche ap-plication.Nevertheless, it is important to understand its operation as it leads anew way to unanswered questions and future directions for Display of the future.

Our research implies that studying E-paper displays like CH-LCD presents aneffective way to propose novel driving schemes. It aims to visualize information


effectively under physical and environmental considerations.Whole idea was toappreciate better Physical implementations for High quality displays involvingLow cost and Low Power consumption

References

1. Next Generation Display Technologies for Electronic Books in Japan :JohanBergquist,Nokia-Japan Co., Ltd., Nokia Research Center

2. Qiang Fu, New Driving Schemes of Cholesteric Liquid Crystal Display for HighSpeed and Uniform Gray-Scale Performance. Ph.D. Thesis. 2012, Institute of Mi-croelectronics, University of Saarland.

3. Electrowetting Displays:A paper by Johan Feenstra et al.4. Color Electronic Paper:Fujitsu Laboratories , http://jp.fujitsu.com/group/

labs/en/activities/human-centric-computing/e-paper_p05.html

5. Ernst Lueder:Liquid Crystal Displays: Addressing Schemes and Electro-Optical Ef-fects (Wiley Series in Display Technology)

6. Cholesteric reflective display: Drive scheme and contrast Yang et al. Liquid CrystalInstitute, Kent State University,

7. X.Y. Huang, D.-K Yang etc. Dynamic Drive for Bistable Reflective CholestericDisplays: A rapid addressing. Jounal of SID, 1995. Volume 3, Issue 4, Pages 165-168.

8. A. Koyachenko, P. Oleksenko etc. Histeresis as a Key Factor for the Fast Control ofReflectivity in Cholesteric LCDs. p. 148-151, IDRC. 1997

9. Philips Semiconductors. Data sheet of octal dual supply translating transceiver74LVC4245A

10. J. Gandhi, D.-K. Yang et al.P-81: Temperature Compensation of the DynamicDrive Scheme for Bistable Cholesteric Displays

E-Paper Displays:A Case Study

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