Download Direct driving of LCD-modules

Högskolan Dalarna
Avd. Datateknik och Informatik
Lab. Instruction
LCD-Electronics
2001-11-07/pls
Lab. Nr 1
Direct addressing of twisted nematic displays and fundamentals of
cholesteric displays
Aim:
To give insight into:
- the basic electronic properties of an LCD cell
- direct driving of an LCD module
- driving voltages for cholesteric displays
The lab. will be examined in a written report from each lab. group.
1.
Direct DC driving of an LCD pixel.
In the lab. an old LCD module of TN type is used. All the drive electronics and other
circuitry have been removed from the module in order to provide direct measurements.
The LCD module is a simple 3 ½ digit direct addressed display with a common
backplane and 22 segments for characters, as well as extra segments for signs, decimal
points, etc.
The module has connections for the backplane and 4 segments for direct
measurements with external drive voltages.
Tasks
Connect a variable DC voltage to one of the segments and determine what is
characteristic of the segment’s contrast as a function of the voltage.
Investigate visibility from different viewing angles and report the results.
Determine the capacitance for an ordinary pixel and for the decimal point.
Because the capacitance is low it could be important to describe the method of
measurement and any irregularities.
Make a simple drawing of an equivalent electrical circuit for the LCD module. Mark
the pixels which are possible to measure on the lab LCD.
2.
Direct driving of an LCD pixel with balanced DC voltage
DC voltages with only positive polarity are usually applied to LCD modules.
However, LCD cells require that polarity fluctuates if the cell is not to be destroyed. A
simple way to achieve a proper supply voltage is shown in the figure below.
A
40 Hz Osc Input
B
Control Input
C
Output of the Exclusive Or Gate
Resulting Display Waveform
D
Segment
off
Segment
on
Segment
off
B
A
=1
C
D
LCD Segment Plane
LCD Common Plane
Lab 1 LCD Electronics
-1-
Högskolan Dalarna
Avd. Datateknik och Informatik
Lab. Instruction
LCD-Electronics
2001-11-07/pls
Tasks
Breadboard a circuit on a prototype board (IDL-800 Digital Lab) so that the 4
connected pixels can be controlled by 4 switches. The supply voltage to LCD pixels
might be varied and therefore it is best to use a CMOS gate that allows different
supply voltages, e.g. XOR gate CD4030B (\\PERSONAL\WWW\intern\CD4070BC.pdf).
Study the backplane and segment voltages with an oscilloscope and verify that the
shape of the curves is correct. Draw the curve or copy it to the computer.
Vary the frequency to the display and decide at what frequency it begins to flicker.
An LCD display does not require much power, but the changes in polarity do result in
some power consumption. Calculate the theoretical power consumption for a pixel, the
complete LCD module and a 17´ LCD monitor. Compare this with the power
consumption for controller ICL7136.
Hints: For digital circuits in MOS technology the power consumption is normally
calculated as:
P = VDD2*CL*f
Where VDD is the voltage of the power supply, CL is the total capacitive load and f is
the operating frequency (i.e. 1000 MHz for an "ordinary" PC).
3.
Measurement of a direct driven LCD
The LCD module is also connected to controller ICL7136. The controller contains
logic that gives proper supply voltages to the LCD cell as well as an A/D converter
with a voltage range of 0 to 200 mV.
Tasks
Verify that the module works as a voltmeter.
4.
Cholesteric displays
The chiral nematic, or as they are called cholesteric liquid crystals, have three main
structures. Two of these are stable in a voltageless state, while the third only exists
with added voltage. The stable states are called planar and focal conic. Planar is the
reflective state and focal conic is the scattering (ljusspridande) state. When an electric
field is added to the liquid crystals the spirals can be turned up so that a transparent
nematisk fluid is obtained. This is the third state and it is called homeotropic.
The crystal structures for the various states and their colours are shown in figure 1.
The wavelength of the reflected light in the example is 550 nm (green light). To
improve the contrast and colours the back of the LCD is painted black. This means
that the bistable LCD can have two colours, in this case green and black. The green
colour is shown when the crystal is in the planar state and the black colour is shown
when it is the focal conic state.
Lab 1 LCD Electronics
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Högskolan Dalarna
Avd. Datateknik och Informatik
Lab. Instruction
LCD-Electronics
Homeotropic
Focal conic
2001-11-07/pls
Planar
GREEN
Figure 1. Structures in cholesteric liquid crystal.
The colours that appear in the planar phase are due to the molecules arranging
themselves in a spiral shaped structure. When light meets the spirals colours appear
due to Bragg reflection. The pitch (stigningen) of the spiral (p) and the crystal’s
refractive index (brytningsindex) (n) determine what wavelength () will be reflected.
The formula is =n*p.
Drive principles
In contrary to the nematic liquid crystals, which react to rms voltages, the
transformations between the bistable states are achieved with the help of voltage
pulses. The amplitude and length of the pulse of the pulse are the factors that
determine in what state the crystal will end up. The pulses should be bipolar to prevent
ion drift.
When square wave pulses are applied to the liquid crystal four important voltage
thresholds, V1, V2, V3 and V4, are found. Figure 2 shows the connection between the
pulses’ amplitude and reflectiveness. Observe that the figure consists of two curves,
one with the planar state as the start state and the other with the focal conic state as
start state.
Planar
Focal conic
Volt
V1
V2
V3 V4
Figure 2. Reflectance diagram for bistable kolesterisk LCD
Lab 1 LCD Electronics
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Högskolan Dalarna
Avd. Datateknik och Informatik
Lab. Instruction
LCD-Electronics
2001-11-07/pls
The voltages at V1 do not affect the liquid crystal. If the amplitude of the voltage
pulses is increased the reflectance of the planar state cells is linearly reduced, until it is
down at the focal conic level at V2. Between V2 and V3 the cholesteric liquid is in the
focal conic state. Between V3 and V4 the reflectance increases sharply and pulses
with voltages higher than V4 put the cholesteric into the planar state. The values of the
four thresholds are dependent on the length of the voltage pulse, the temperature and
the crystal mixture.
Measuring the voltage-reflectance curve
Material: A cholesteric LCD cell, oscilloscope, various cables and a power source.
The power source must be able to generate pulses that are variable in length. The
amplitude of the pulses must be able to be varied with up to 40V. The pulse should
preferably be bipolar.
The purpose of the lab. is to produce a voltage-reflectance curve for a bistable
cholesteric LCD.
Connect the cell and set the pulse length to 50ms, then vary the amplitude of the
voltage pulses and to optically determine when minimal and full reflectance are
achieved.
Do two series of measurements, one with the planar state (green) as the starting point
and the other with the focal conic state as the starting point. With the help of the
results of the measurements decide the values of the threshold voltages V1, V2, V3
and V4.
Using the results, decide how the power supply should be chosen if the display is to be
driven as a passive matrix. Complete both of the tables below. (NB! Try not to use
negative power supplies, there might be two cases for each polarity).
Positive polarity
Column
voltages
Row
voltages
Writeable row
V
[sel]
Non writeable
V
row [no-sel]
Negative polarity
Column
voltages
Row
voltages
Writeable row
V
[sel]
Non writeable
V
row [no-sel]
Planar (green)
[sel]
Focal conic (black)
[no-sel]
Volt
Volt
Volt
Volt
Volt
Volt
Planar (green)
[sel]
Focal conic (black)
[no-sel]
Volt
Volt
Volt
Volt
Volt
Volt
Positive polarity
Planar (green)
[sel]
Focal conic (black)
[no-sel]
Volt
Volt
V
Volt
Volt
V
Volt
Volt
Negative polarity
Planar (green)
[sel]
Focal conic (black)
[no-sel]
Volt
Volt
V
Volt
Volt
V
Volt
Volt
Task
Determine the conditions between V1, V2, V3 and V4 need to be fulfilled to be able to
drive the display as a passive matrix?
The connection between voltage thresholds and pulse length
Repeat the procedure, using the pulse lengths 10ms, 25ms, 75ms and 100ms.
Task (extra)
Determine what happens to the threshold voltages when the pulse length is varied?
Lab 1 LCD Electronics
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