A Liquid Crystal Display Assingment 1 BP SEM 5

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1.0 INTRODUCTION

A liquid crystal display (LCD) is a flat panel display, electronic visual display, video

display that uses the light modulating properties ofliquid crystals (LCs). LCs do not emit lightdirectly.

They are used in a wide range of applications, including computer monitors, television,instrument panels, aircraft cockpit displays, signage, etc. They are common in consumer devices

such as video players, gaming devices, clocks, watches, calculators, and telephones. LCDs have

displaced cathode ray tube (CRT) displays in most applications. They are usually more compact,lightweight, portable, less expensive, more reliable, and easier on the eye. They are available in a

wider range of screen sizes than CRT and plasma displays, and since they do not use phosphors,

they cannot suffer image burn-in.

Monochrome LCD images usually appear as blue or dark gray images on top of a

grayish-white background. Color LCD displays use two basic techniques for producing color:

Passive matrix is the less expensive of the two technologies. The other technology, called thinfilm transistor (TFT) or active-matrix, produces color images that are as sharp as traditional CRT

displays, but the technology is expensive. Recent passive-matrix displays using new CSTN and

DSTN technologies produce sharp colors rivaling active-matrix displays.

1.1 Basic LCD concepts ( function )

LCD televisions produced a black and colored image by selectively filtering a white light.

The light is typically provided by a series ofcold cathode fluorescent lamps (CCFLs) at the back

of the screen, although some displays use white or colored LEDs instead. Millions of individual

LCD shutters, arranged in a grid, open and close to allow a metered amount of the white light

through. Each shutter is paired with a colored filter to remove all but the red, green or blue

(RGB) portion of the light from the original white source. Each shutterfilter pair forms a single
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sub-pixel. The sub-pixels are so small that when the display is viewed from even a short

distance, the individual colors blend together to produce a single spot of color, a pixel. The shade

of color is controlled by changing the relative intensity of the light passing through the sub-

pixels.

Liquid crystals encompass a wide range of (typically) rod-shaped polymers that naturallyform into thin layers, as opposed to the more random alignment of a normal liquid. Some of

these, the liquid crystals, also show an alignment effect between the layers. The particular

direction of the alignment of a liquid crystal can be set by placing it in contact with an alignment

layer or director, which is essentially a material with microscopic grooves in it.

When placed on a director, the layer in contact will align itself with the grooves, and the

layers above will subsequently align themselves with the layers below, the bulk material taking

on the director's alignment. In the case of an LCD, this effect is utilized by using two directors

arranged at right angles and placed close together with the liquid crystal between them. This

forces the layers to align themselves in two directions, creating a twisted structure with eachlayer aligned at a slightly different angle to the ones on either side.

LCD shutters consist of a stack of three primary elements. On the bottom and top of the

shutter are polarizer plates set at right angles. Normally light cannot travel through a pair of

polarizers arranged in this fashion, and the display would be black. The polarizers also carry the

directors to create the twisted structure aligned with the polarizers on either side. As the light

flows out of the rear polarizer, it will naturally follow the liquid crystal's twist, exiting the front

of the liquid crystal having been rotated through the correct angle, that allows it to pass through

the front polarizer. LCDs are normally transparent.

To turn a shutter off, a voltage is applied across it from front to back. the rod-shaped

molecules align themselves with the electric field instead of the directors, destroying the twisted

structure. The light no longer changes polarization as it flows through the liquid crystal, and can

no longer pass through the front polarizer. By controlling the voltage applied across the crystal,

the amount of remaining twist can be selected. This allows the transparency of the shutter to be

controlled. To improve switching time, the cells are placed under pressure, which increases the

force to re-align themselves with the directors when the field is turned off.

Several other variations and modifications have been used in order to improve

performance in certain applications. In-Plane Switching displays (IPS and S-IPS) offer widerviewing angles and better color reproduction, but are more difficult to construct and have slightly

slower response times. IPS displays are used primarily for computer monitors. Vertical

Alignment (VA, S-PVA and MVA) offer higher contrast ratios and good response times, but

suffer from color shifting when viewed from the side. In general, all of these displays work in a

similar fashion by controlling the polarization of the light source.
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The surface of the electrodes that are in contact with the liquid crystal material are treated

so as to align the liquid crystal molecules in a particular direction. This treatment typically

consists of a thin polymer layer that is unidirectional rubbed using, for example, a cloth. The

direction of the liquid crystal alignment is then defined by the direction of rubbing. Electrodes

are made of a transparent conductor called Indium Tin Oxide (ITO).

Before applying an electric field, the orientation of the liquid crystal molecules is

determined by the alignment at the surfaces of electrodes. In a twisted pneumatic device (still the

most common liquid crystal device), the surface alignment directions at the two electrodes are

perpendicular to each other, and so the molecules arrange themselves in a helical structure, or

twist. This reduces the rotation of the polarization of the incident light, and the device appears

grey.

If the applied voltage is large enough, the liquid crystal molecules in the center of the

layer are almost completely untwisted and the polarization of the incident light is not rotated as it

passes through the liquid crystal layer. This light will then be mainly polarized perpendicular tothe second filter, and thus be blocked and the pixel will appear black. By controlling the voltage

applied across the liquid crystal layer in each pixel, light can be allowed to pass through in

varying amounts thus constituting different levels of gray. This electric field also controls

(reduces) the double refraction properties of the liquid crystal.

LCD with top polarizer removed from device and placed on top, such that the top and

bottom polarizers are parallel.

The optical effect of a twisted pneumatic device in the voltage-on state is far less

dependent on variations in the device thickness than that in the voltage-off state. Because of this,

these devices are usually operated between crossed polarizers such that they appear bright with

no voltage (the eye is much more sensitive to variations in the dark state than the bright state).These devices can also be operated between parallel polarizers, in which case the bright and dark

states are reversed. The voltage-off dark state in this configuration appears blotchy, however,

because of small variations of thickness across the device.
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Both the liquid crystal material and the alignment layer material contain ionic

compounds. If an electric field of one particular polarity is applied for a long period of time, this

ionic material is attracted to the surfaces and degrades the device performance. This is avoided

either by applying an alternating current or by reversing the polarity of the electric field as the

device is addressed (the response of the liquid crystal layer is identical, regardless of the polarity

of the applied field).

Displays for a small number of individual digits and/or fixed symbols (as in digital

watches, pocket calculators etc.) can be implemented with independent electrodes for each

segment. In contrast full alphanumeric and/or variable graphics displays are usually implemented

with pixels arranged as a matrix consisting of electrically connected rows on one side of the LC

layer and columns on the other side which makes it possible to address each pixel at the

intersections. The general method of matrix addressing consists of sequentially addressing one

side of the matrix, for example by selecting the rows one-by-one and applying the picture

information on the other side at the coloums row by row

Figure 1 The block diagram of a typical digital LCD TV
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FIGURE 1.1Full operation of LCD


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2.0 COMPLETE SYSTEM LEVEL BLOCK DIAGRAM

A system level blockdiagram for the project entitled Microcontroller Driven

Electroluminescent Display is shown in Figure 1. The inputs from the formula car and the user

are sent to the microcontroller. The microcontroller processes the inputs and makes any updates

to the electroluminescent display indirectly through the LCD controller. The following sections

identify the subsystems shown in Figure 1 as well as their functions.

Figure 2.0 : System Level Block Diagram

AMD-80C31Microcontroller

(Block 1)

Ignition Signal(RPM)Front TireSensor

CoolantTemperature

Oil Pressure

HM62256Static RAM

(Block 5)

DecodingLogic

(Block 2)

AD &Com

SED-1330LCD Controller

(Block 3)

Decoding Logic

ADC0808A/D

(Block 6)

Decoding Logic

SignalConditioning(Block 8)

Conditioned

AnalogSignalsO2Sensor

*

*

*

MM74C923Keypad Encoder

(Block 7)Keypad

ElectroluminescentDisplay

(Block 4)

A 8:15


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2.1 AMDs 80C31 Microcontroller (Block 1)

The 80C31 microcontroller is responsible for deciphering the input signals and updating

the LCD Controller accordingly. The microcontroller block includes program memory and datamemory. They are interfaced trough the multiplexed data bus with the help of a 74ALS573

latch. The microcontroller can receive and send signals over both digital pins and the

address/data bus.

Figure 2.1 : AMDs 80C31 Microcontroller Subsystem Block Diagram (Block 1)

80C31

Microcontroller

with External

Program Memory,

Data Memory, &Latch

(Block 1)

Ignition Signal

Front Tire

Sensor

Oil Pressure

Data Bus

Address Bus

RD

WR

Port 1 & Port 3


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Microcontroller InputsFunction

Ignition Signal The frequency of the ignition signal will be used to determine

the rotations per minute (RPM) of the engine.

Front Tire Sensor The rotational speed of the front tire will be calculated to find

the ground speed of the vehicle. This will also be used in

conjunction with the ignition signal to find the current gear.

Digital Inputs from

Engine Control Unit

Digital input from the engine control unit. It signals low oil

pressure.

Analog Inputs The microcontroller receives up to eight analog inputs from

the ADC0808. The ADC0808 is interfaced to the address/

data bus. Individual inputs will be explained in ADC0808

block.

Keypad Press The keypad encoder can interpret up to 20 keys. It isinterfaced to the address/data bus. When a key is pressed, it

places the binary coded key value on the data bus for the

microcontroller to read.

Table 2.1: Microcontroller Inputs

Microcontroller OutputFunction

Digital Outputs

Port 1&3All digital outputs on port 1 and port 3 will be used for manymiscellaneous tasks handled by the microprocessor.

Output to LCD

Controller

The microcontroller communicates to the LCD controller over

the address/data bus. It specifies what pixels need to be on,

and the LCD controller takes care of the communication to the

display.

Table 2.2: Microcontroller Outputs


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2.2 LCD Controller (Block 3)

The LCD Controller subsystem (Figure 3) interfaces microcontroller to the display. It

has the capabilities of displaying layered text and graphics, scrolling the display in any direction,and partitioning the screen into multiple screens. It is responsible for continually updating the

electroluminescent display and storing the current display in external memory

Figure 2.2 : LCD Controller Subsystem Block Diagram (Block 3)

LCD Controller(Block 3)

RD

WR

RES

A0 XD0-XD3

XECL

XSCL

LP

WF

YDIS

YD

YSCL

CS

VR/W VCEVA0-VA14 VD0-VD7

From DecoderAD[0:7]


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Microcontroller Inputs

to LCD ControllerFunction

A0 Selects the data type to be displayed.

CS Activates the LCD controller when the microcontroller is

accessing external memory that is associated with the LCD

controller. The logic decoding hardware handles the memory

mapping.

AD[0:7] The data bus is used to transfer bi-directional data between the

microcontroller and the LCD controller

RD Control signal used during read cycles.

WR Control signal used during write cycles.

RES Reset signal to initialize LCD controller.

Table 2.3: Microcontroller Inputs to LCD Controller

External RAM InterfaceFunction

VA0VA14 Determines which memory address of the external RAM is

specified.

VD0VD7 Reads or writes to the specified address.

VR/W VRAM R/W Signal

VCE Memory Control Signal

Table 2.4: Controller Outputs to External RAM Interface


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Electroluminescent

Display InterfaceFunction

XD0XD3 X-Driver Data: Data signals to the Electroluminescent

Display

XECL X-driver enable chain clock

XSCL Data shift clock to update data at determined time intervals.

LP Latch pulse so screen is not read while being written to.

WF Frame signal.

YDIS This signal turns off the display if the screen is blanked.

YD Scan start pulse.

YSCL Y-driver shift clock

Table 2.5: Controller Outputs to Electroluminescent Display Interface

2.3 Decoding Logic Subsystems (Block 2)

The decoding logic is required to organize all devices using the address/data bus in

memory space. The decoding logic subsystem block diagram can be seen in figure 4. Specific

memory location are assigned for the LCD controller, A/D write, A/D read, and keypad encoder

in the microcontrollers memory map (figure 5) It is also responsible for creating a 500 kHz

square wave for the A/D conversion. All output signals are explained with their respective

devices.

Figure 2.3: Decoding Logic Subsystem Block Diagram (Block 2)

A (13:15)

GAL26CV12B

Decoding

(Block 2)

LCD Controller Chip Select

A/D Chip Select/Control Signals

Keypad Chip Select

500 kHz Square Wave

for A/D

10 MHz Clock

RD

WR


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Figure 2.5:Memory map partitions created by decoding logic (Block 2)

2.4 Electroluminescent Display Subsystem (Block 4)

This subsystem displays data and graphics based on information received from the LCD

Controller. The only output is what is displayed on the screen. The signal labeled YDIS, from

Table 5, is used to turn off the electroluminescent display when the screen is blank. The voltageregulator maintains a constant voltage for the display and is easily controlled by the YDIS

signal. All other inputs and their functions are listed in Table 5. Figure 6 shows the block

diagram for this subsystem.

Figure 2.6: Electroluminescent Display Subsystem Block Diagram

0000

FFFF

8000

7FFF

A000

9FFF

Keyboard

A/D

LCD Controller

Program

&

Memory

Space

C000


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2.5 Static RAM Subsystem (Block 5)

The external RAM is used to store text, character codes, and bit-mapped graphics data for

the LCD Controller. The LCD Controller determines the address of the desired information andthen reads the data. All of the inputs and outputs are described in Table 4, as well as a block

diagram in Figure 7.

Figure 2.7: Static RAM Subsystem Block Diagram (32,768-word x 8-bit)

2.6 A/D and Signal Conditioning Subsystems (Block 6 & Block 8)

The A/Ds communication with the microcontroller is broken into two processes. To

start the conversion data is written to the memory address that correlates to the A/D and correct

analog signal. The signal number is chosen by A10 through A8 as seen in Figure 8. Once the

conversion is completed the A/D generates an interrupt with the EOC signal. The

microprocessor responds by reading from the memory location associated with the A/D. The

data received from the A/D ranges from 00h = 0V to FFh = 5V. All signal conditioning will be

done off of the board to add flexibility in choosing analog inputs.

Figure 2.8: Multiplexed Input Analog to Digital Subsystem Block Diagram (Block 6) and

Decoding Logic

ADC0808

A/D

(Block 6)

Analog InputsSignal

Conditioning

(Block 8)

ConditionedAnalog Signal

Data Bus

Interrupt Signal

to Microprocessor

EOC

IN[0:7]

A

BC

A8

A9A10

OE

ALE

START

CLK

Vref+

Vref-

+5V

0V


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2.7 Signal Conditioning Subsystem Diagram (Block 8)

InputsFunction

ALE & START Signal initiates A/D conversion.

OE Signal to put converted data on the data bus.

CLOCK 500 MHz clock signal.

A8-A10 Analog signal select lines.

Analog Inputs IN0 - O2 sensor signal

IN1Temperature thermocouple signal

Table 6: Inputs to A/D converter

OutputsFunction

AD[0:7] Outputs digital value of analog signal to data bus.

EOC End of calculations signal. Causes interrupt to handle reading

the data bus.

Table 7: Outputs to A/D converter

2.8 Keypad Encoder Subsystems (Block 7)

When a key is pressed the encoder uses the row and column inputs to decipher what key

was pressed. The encoder then enables the DAV signal, which generates an interrupt signal onthe microcontroller. The microcontroller responds by reading from the memory location

associated with the keypad encoder. The binary key value is placed on AD[0:4] for the

microprocessor to read.


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Figure 2.9: Keypad Encoder Subsystem Block Diagram (Block 7)

InputsFunction

OE Signal to enable key value on the data bus.

ROW Signal to tell encoder what row was pressed.

COLUMN Signal to tell encoder what column was pressed.

OSC Capacitor value sets polling frequency.

KBC Capacitor value sets key debounce period.

Table 2.8: Inputs to keypad encoder

OutputsFunction

AD[0:4] Outputs digital value of key pressed to data bus.

DAV Data available signal. Causes interrupt to handle reading the

data bus

Table 2.9:Outputs to keypad encoder

Keypad EncoderMM74C923

(Block 7)OSCKBC

1uF0.1uF

COLUMN

ROW

OE

KEYPADCHIP SELECT

AD[0:4]

DAV


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3.0 BASICS LCD OPERATION

Liquid crystal displays (LCDs) are a passive display technology. This means they do not

emit light; instead, they use the ambient light in the environment. By manipulating this light,

they display images using very little power. This has made LCDs the preferred technology

whenever low power consumption and compact size are critical.

Liquid crystal (LC) is an organic substance that has both a liquid form and a crystal

molecular structure. In this liquid, the rod-shaped molecules are normally in a parallel array, and

an electric field can be used to control the molecules. Most LCDs today use a type of liquid

crystal called Twisted Nematic (TN).

A Liquid Crystal Display (LCD) consists of two substrates that form a "flat bottle" that

contains the liquid crystal mixture. The inside surfaces of the bottle or cell are coated with a

polymer that is buffed to align the molecules of liquid crystal. The liquid crystal molecules align

on the surfaces in the direction of the buffing. For Twisted Nematic devices, the two surfaces are

buffed orthogonal to one another, forming a 90 degree twist from one surface to the other, see

figure below.

This helical structure has the ability to control light. A polarizer is applied to the front

and an analyzer/reflector is applied to the back of the cell. When randomly polarized light passes

through the front polarizer it becomes linearly polarized. It then passes through the front glass

and is rotated by the liquid crystal molecules and passes through the rear glass. If the analyzer is


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rotated 90 degrees to the polarizer, the light will pass through the analyzer and be reflected back

through the cell. The observer will see the background of the display, which in this case is the

silver gray of the reflector.

The LCD glass has transparent electrical conductors plated onto each side of the glass in

contact with the liquid crystal fluid and they are used as electrodes. These electrodes are made ofIndium-Tin Oxide (ITO). When an appropriate drive signal is applied to the cell electrodes, an

electric field is set up across the cell. The liquid crystal molecules will rotate in the direction of

the electric field. The incoming linearly polarized light passes through the cell unaffected and is

absorbed by the rear analyzer. The observer sees a black character on a sliver gray background,

see figure 2. When the electric field is turned off, the molecules relax back to their 90 degree

twist structure. This is referred to as a positive image, reflective viewing mode. Carrying this

basic technology further, an LCD having multiple selectable electrodes and selectively applying

voltage to the electrodes, a variety of patterns can be achieved.

Many advances in TN LCDs have been produced. Super Twisted Nematic (STN) Liquid

Crystal material offers a higher twist angle (>=200 vs. 90) that provides higher contrast and a

better viewing angle. However, one negative feature is the birefringence effect, which shifts the

background color to yellow-green and the character color to blue. This background color can be

changed to a gray by using a special filter.

The most recent advance has been the introduction of Film compensated Super Twisted

Nematic (FSTN) displays. This adds a retardation film to the STN display that compensates for

the color added by the birefringence effect. This allows a black and white display to be produced.


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xv. Patent No. US 5576867: G. Baur, W. Fehrenbach, B. Staudacher, F. Windscheid, R.Kiefer, Liquid crystal switching elements having a parallel electric field and beta o which

is not 0 or 90 degrees, filed Jan. 9, 1990.
http://books.google.com/?id=Jt1I74g6_28C&pg=PA844&dq=liquid-crystal+1888&q=liquid-crystal%201888http://en.wikipedia.org/wiki/International_Standard_Book_Numberhttp://en.wikipedia.org/wiki/Special:BookSources/9780470512340http://books.google.com/?id=Jt1I74g6_28C&pg=PA844&dq=liquid-crystal+1888&q=liquid-crystal%201888http://books.google.com/?id=Jt1I74g6_28C&pg=PA844&dq=liquid-crystal+1888&q=liquid-crystal%201888http://en.wikipedia.org/wiki/Digital_object_identifierhttp://dx.doi.org/10.1039%2Fa902682ghttp://en.wikipedia.org/wiki/Digital_object_identifierhttp://dx.doi.org/10.1080%2F15421406908084910http://www.invent.org/2009induction/1_3_09_induction_heilmeier.asphttp://www.ieeeghn.org/wiki/index.php/Milestones:Liquid_Crystal_Display,_1968http://www.ieeeghn.org/wiki/index.php/Milestones:Liquid_Crystal_Display,_1968http://www.americanscientist.org/template/AssetDetail/assetid/53321/page/4;jsessionid=aaa6J-GFIciRx2%3Ci%3ELivehttp://www.americanscientist.org/template/AssetDetail/assetid/53321/page/4;jsessionid=aaa6J-GFIciRx2%3Ci%3ELivehttp://www.americanscientist.org/template/AssetDetail/assetid/53321/page/4;jsessionid=aaa6J-GFIciRx2%3Ci%3ELivehttp://www.americanscientist.org/template/AssetDetail/assetid/53321/page/4;jsessionid=aaa6J-GFIciRx2%3Ci%3ELivehttp://www.americanscientist.org/template/AssetDetail/assetid/53321/page/4;jsessionid=aaa6J-GFIciRx2%3Ci%3ELivehttp://www.americanscientist.org/template/AssetDetail/assetid/53321/page/4;jsessionid=aaa6J-GFIciRx2%3Ci%3ELivehttp://www.ieeeghn.org/wiki/index.php/Milestones:Liquid_Crystal_Display,_1968http://www.ieeeghn.org/wiki/index.php/Milestones:Liquid_Crystal_Display,_1968http://www.invent.org/2009induction/1_3_09_induction_heilmeier.asphttp://dx.doi.org/10.1080%2F15421406908084910http://en.wikipedia.org/wiki/Digital_object_identifierhttp://dx.doi.org/10.1039%2Fa902682ghttp://en.wikipedia.org/wiki/Digital_object_identifierhttp://books.google.com/?id=Jt1I74g6_28C&pg=PA844&dq=liquid-crystal+1888&q=liquid-crystal%201888http://books.google.com/?id=Jt1I74g6_28C&pg=PA844&dq=liquid-crystal+1888&q=liquid-crystal%201888http://en.wikipedia.org/wiki/Special:BookSources/9780470512340http://en.wikipedia.org/wiki/International_Standard_Book_Numberhttp://books.google.com/?id=Jt1I74g6_28C&pg=PA844&dq=liquid-crystal+1888&q=liquid-crystal%201888


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7.0 CONCLUSION

LCDs are more energy efficient and offer safer disposal than CRTs. Its low electricalpower consumption enables it to be used in battery-powered electronic equipment. It is an

electronically modulated optical device made up of any number of segments filled with liquid

crystals and arrayed in front of a light source (backlight) or reflector to produce images in colouror monochrome. The most flexible ones use an array of small pixels.

Each pixel of an LCD typically consists of a layer ofmolecules aligned between two transparent

electrodes, and two polarizing filters, the axes of transmission of which are (in most of the cases)

perpendicular to each other. With no actual liquid crystal between the polarizing filters, light

passing through the first filter would be blocked by the second (crossed) polarizer. In most of the

cases the liquid crystal has double refraction.

Short for liquid crystal display, a type of display used in digital watches and many

portable computers. LCD displays utilize two sheets of polarizing material with a liquid crystalsolution between them. An electric current passed through the liquid causes the crystals to align

so that light cannot pass through them. Each crystal, therefore, is like a shutter, either allowing

light to pass through or blocking the light.
http://en.wikipedia.org/wiki/Battery_(electricity)http://en.wikipedia.org/wiki/Electronicshttp://en.wikipedia.org/wiki/Electro-optic_modulatorhttp://en.wikipedia.org/wiki/Liquid_crystalhttp://en.wikipedia.org/wiki/Liquid_crystalhttp://en.wikipedia.org/wiki/Light#Light_sourceshttp://en.wikipedia.org/wiki/Backlighthttp://en.wikipedia.org/wiki/Reflector_(photography)http://en.wikipedia.org/wiki/Monochromehttp://en.wikipedia.org/wiki/Pixelhttp://en.wikipedia.org/wiki/Pixelhttp://en.wikipedia.org/wiki/Moleculehttp://en.wikipedia.org/wiki/Transparency_(optics)http://en.wikipedia.org/wiki/Electrodehttp://en.wikipedia.org/wiki/Polarizerhttp://en.wikipedia.org/wiki/Filter_(optics)http://en.wikipedia.org/wiki/Liquid_crystalhttp://en.wikipedia.org/wiki/Lighthttp://en.wikipedia.org/wiki/Liquid_crystalhttp://en.wikipedia.org/wiki/Double_refractionhttp://www.webopedia.com/TERM/D/digital.htmlhttp://www.webopedia.com/TERM/P/portable.htmlhttp://www.webopedia.com/TERM/P/portable.htmlhttp://www.webopedia.com/TERM/D/digital.htmlhttp://en.wikipedia.org/wiki/Double_refractionhttp://en.wikipedia.org/wiki/Liquid_crystalhttp://en.wikipedia.org/wiki/Lighthttp://en.wikipedia.org/wiki/Liquid_crystalhttp://en.wikipedia.org/wiki/Filter_(optics)http://en.wikipedia.org/wiki/Polarizerhttp://en.wikipedia.org/wiki/Electrodehttp://en.wikipedia.org/wiki/Transparency_(optics)http://en.wikipedia.org/wiki/Moleculehttp://en.wikipedia.org/wiki/Pixelhttp://en.wikipedia.org/wiki/Pixelhttp://en.wikipedia.org/wiki/Monochromehttp://en.wikipedia.org/wiki/Reflector_(photography)http://en.wikipedia.org/wiki/Backlighthttp://en.wikipedia.org/wiki/Light#Light_sourceshttp://en.wikipedia.org/wiki/Liquid_crystalhttp://en.wikipedia.org/wiki/Liquid_crystalhttp://en.wikipedia.org/wiki/Electro-optic_modulatorhttp://en.wikipedia.org/wiki/Electronicshttp://en.wikipedia.org/wiki/Battery_(electricity)


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4.0 DISCUSSION

A cross sectional view of a liquid crystal display is shown below in Figure 1. As can be seen

in the diagram, the display is simply two pieces of extremely flat glass, over coated with a

number of chemical layers, and filled with liquid crystal fluid.

some perspective, the bottom and top glass substrates are typically .043" thick each. The

thin film coatings, SiO2, ITO, and PI, are each a few hundred angstroms thick. The space

between the glass marked "LC Fluid" is about 5 microns thick and is adjusted slightly to

match the characteristics of the chosen fluid. If this drawing was made to scale, it would be

very difficult to see any detail at all between the glass plates.

A liquid crystal display (LCD) is a parallel plate capacitor with a dielectric, in this case the

liquid crystal fluid, between the plates. First we select glass coated with a transparent metal

coating for the electrodes of the display. The glass is usually made of soda lime, but in some

instances it can be a more expensive borosilicate, or because few manufacturers provideborosilicate any more without a fight, aluminasilicate type.

The transparent metal coating can be any thin layer of conductive material, such as gold,

silver or tin. In order to keep the cost down and have a reasonable process window with a

highly transparent coating, the industry has been using indium-tin oxide (ITO) as the

preferred electrode material.

Fig. 1.

Photoresist is then put on top of the ITO coating and a photolithographic process is used to

image the pattern. The exposed patterns are then developed and the glass is sent through an

acid bath where the excess ITO is removed, similar to the way a PC board is made. The

remaining photoresist is then stripped away and the patterned segment and common plane

electrodes remain on the glass.


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The next layer to be applied is the liquid crystal alignment layer. This is usually a polyimide

type material and has been chosen for its environmental stability in high moisture and heat.

More importantly is its ability to cause the molecules of liquid crystal to align their long

axis in the direction in which the polymer has been buffed.

We rub the two halves of the display at right angles to one another and since the liquidcrystal molecules like to arrange themselves parallel to one another, we cause a helical

structure to be formed between the two electrode faces, see Fig 2 below. This helical

structure forms a 90o rotation of the liquid crystal molecules from the top of the display to

the bottom.

After the polymer is rubbed, a thermoplastic seal is printed along the perimeter of one piece

of glass. This is a resin based material with a high curing temperature, about 200oC, that

creates an extremely durable barrier to outside moisture and contamination. Some

manufacturers use a UV cured material for this seal as it speeds up the manufacturing

process.

We then apply a crossover dot, usually a small spot of silver, to connect the common plane

electrode on the top piece of glass to the segment plane which is on the bottom piece of

glass. This is somewhat analogous to a plated thru hole on a PC board.

To make the display uniform in appearance, spacers are then applied. These are usually

glass or plastic spheres that have the desired diameter to produce a fixed gap between the

glass plates. Depending on the liquid crystal used, this gap can be between 4 and 8 microns.

The two halves of the display are then aligned, usually with a three point camera alignment

system for accuracy, and brought together. A very thin, uniform, flat and empty bottle has

been formed with the thermoplastic seal essentially "gluing" the two pieces of glass

together.

A liquid crystal is put inside this bottle by using a vacuum filling technique. The liquid

crystal (the dielectric material of our capacitor) is selected for it's various physical

properties. The application may call for a liquid crystal fluid that has a very low operating

voltage or the display may be used outdoors and require a very wide temperature range.

Display manufacturers have developed several liquid crystal mixtures to fulfill most

applications.

Once the liquid crystal has been put inside the display and the port opening has been sealed,

a polarizer is put on the front of the top glass and a second polarizer is put on the back of the

bottom glass to make sure that the light reaching the eye of the observer is oriented along

the correct axis.

II. Operation of a liquid crystal display


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The liquid crystal molecules are long and thin as

shown in Figure 2 on the right. On the bottom glass

substrate, the PI layer has been rubbed from back to

front, and the molecules are aligned in that

direction. Remember, the direction of this

orientation determines the viewing angle of the

part.

Because the PI layer of the top glass has been

rubbed from right to left, the molecules attached to

the top piece of glass are oriented perpendicular to

the ones at the bottom. This 90o rotation is the

"twist" in a twisted nematic display. Fig. 2

Fig. 3

The liquid crystal molecules between the top

and bottom glass form a spiral structure that

will twist light as it goes through the cell. As

can been seen in Figure 3 on the left, a beam

of light entering from above passes through

the top polarizer along the axis of

polarization.

The light beam goes through the cell and is

twisted as it goes in the same direction as the

twist of the LC fluid.

The light exits the display, and passes

through the polarizer on the bottom glass

which is oriented perpendicular to the

polarizer on the top.


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When a drive signal is applied to the cell

electrodes, an electric field is set up across

the cell. The liquid crystal molecules will

"stand up" to align themselves in the

direction of the electric field.

When the molecules "stand up" the helical

structure is disrupted, and the incoming

linearly polarized light does not "twist" like

it did when the molecules were at rest. The

light is instead blocked by the rear polarizer.

The observer therefore sees a black segment

on the clear background.

Fig. 4

When the electric field is turned off, the molecules relax back to the 90 o twist structure, light

entering the cell is again twisted 90o and the display returns to a transparent state. This is

referred to as a positive image, transmissive viewing mode.

The electro-optic response characteristic of our standard TN cell is asymmetric because only

the "turn-on" state can be activated by an electric field. When the RMS voltage goes to zero,

the twisted structure, which provides the "twist" of the incident light, is restored by the

elastic torques within the LC fluid. We can therefore speed up the "turn-on" time of our

display by increasing the drive voltage waveform (over a very limited range), but the "turn-

off" time is fixed by the relaxation characteristics of the LC fluid.

A Liquid Crystal Display Assingment 1 BP SEM 5

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