AN52927
PSoC® 3 and PSoC 5LP - Segment LCD Direct Drive
Author: Rajiv Vasanth Badiger
Associated Project: Yes
Associated Part Family: All PSoC 3 and PSoC 5LP families
®
Software: PSoC Creator™ 3.0 SP1
AN52927 demonstrates how easy it is to drive a segment LCD glass using the integrated LCD driver in PSoC 3 and
PSoC 5LP. This application note gives a brief introduction to segment LCD drive features and provides a step-bystep procedure to design Segment LCD applications using the PSoC Creator tool.
Contents
Introduction
Introduction .......................................................................1
Segment LCD Drivers in PSoC .........................................1
Segment LCD Component Configuration ..........................2
PSoC Creator Project ........................................................5
Segment LCD configuration .........................................7
Other Components in the Design ............................... 10
Top Design ................................................................. 11
Firmware ......................................................................... 11
Test Setup ....................................................................... 13
LCD Component Low Power Modes ............................... 13
Using Segment LCD Component in Low Power Mode .... 14
Summary ......................................................................... 15
Appendix A ...................................................................... 16
LCD Operating Principle.................................................. 16
Classification of LCDs ..................................................... 17
Classification (Based on Number of
Backplanes/Commons) .............................................. 17
Classification (Based on Light Source) ....................... 19
Classification (Based on LCD Technology) ................ 20
Classification (Based on Image Produced)................. 21
LCD Viewing Angle ......................................................... 21
Worldwide Sales and Design Support ............................. 23
PSoC Creator, which is used to create PSoC 3 and
PSoC 5LP projects, includes building blocks in the form of
components for different functions. The segment LCD
component is included for the segment LCD drive, which
significantly reduces the design cycle time.
www.cypress.com
This application note takes you through an example
project that uses the segment LCD component. If you are
unfamiliar with the PSoC 3 and PSoC 5LP device, see the
PSoC 3 and PSoC 5 LP Introduction webpage. To learn
the basics of the Segment LCD, see Appendix A.
Segment LCD Drivers in PSoC
PSoC 3 and PSoC 5LP have 64 inbuilt segment LCD
drivers. This gives it the capability to drive up to 768 pixels
(16 commons x 48 segments).
Following are the major features of the segment LCD
drivers in PSoC 3 and PSoC 5LP:
Direct drive with internal bias generation—no other
external hardware is required.
Maximum 64 inbuilt LCD drivers (which includes both
common and segment pin driver).
Supports 14-segment and 16-segment alphanumeric
display, 7-segment numeric display, dot matrix, and
special symbols.
Supports both Type A and Type B waveforms.
Wide LCD bias range (2 V to supply voltage).
Static, 1/3, 1/4, 1/5 bias ratios.
Supports LCD glass with up to 16 common lines.
No CPU intervention in LCD refresh.
Adjustable refresh rate from 10 Hz to 150 Hz.
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PSoC® 3 - Segment LCD Direct Drive
Configurable power modes, which allows power
optimization.
For a more detailed view of the hardware operation of the
LCD drive system, see the Technical Reference Manual.
PSoC Creator Component: Segment
LCD
To ease the application design process, the PSoC Creator
IDE includes a Segment LCD Component that does most
of the work: from setting the LCD drive configuration
registers to LCD refresh. You can find the “Segment LCD”
component in the Component Catalog under “display” in
PSoC Creator.
Segment LCD Component
Configuration
Double-click on the component placed in the Top Design.
There are four tabs, which configure the SegLCD
component completely based on the requirements.
Basic Configuration Tab
Figure 2. Basic Configuration Tab
Instance name
Figure 1. Component Catalog in PSoC Creator
This tab accepts the following:
There are two components: one for static segment LCD
and another for multiplexed segment LCD. In this
application note, only the multiplexed segment LCD
component—segment
LCD—is
discussed.
This
component can also be configured to drive static LCD.
Number of commons and segment lines. The number
of commons can be from 1 to 16. The maximum
number of segment lines will then depend on the
available I/Os in the selected PSoC device.
Bias Type: 1/3, 1/4, or 1/5
Waveform Type: Type A or Type B
Frame Rate or Refresh Rate: 10 Hz to 150 Hz (max
value depends on the driver power mode, waveform
type, and number of commons).
Drive Power Mode: This gives power optimization.
There are three modes, as shown in the following
table.
Table 1. SegLCD Drive Power Modes
Drive Power
Mode
No Sleep
Power
High
Usage Tips
Use under two conditions:
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Document No. 001-52927 Rev. *H
When the PSoC is always in
active mode and no power
saving feature is required
When LCD is not required to be
operational when the device is
put to sleep
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PSoC® 3 - Segment LCD Direct Drive
Drive Power
Mode
Power
Low Power
Using ILO
Low
Low Power
using Ext
32KHz crystal
Low
Usage Tips
Use when extremely low power is
required. In this mode, the SegLCD
component works even when the
PSoC is in the sleep mode. The
SegLCD component takes the
internal low-speed oscillator (ILO)
clock for operation. ILO is low power;
but, it comes at the cost of accuracy.
If accurate LCD refresh rate is not
required, you can use this mode.
Bias Voltage: This defines the contrast of the LCD.
There are two ranges available: 3.0 V and 5.5 V. Note
that these two voltages represent the supply voltage
(VDDA) to PSoC device.
Table 2. SegLCD Bias Voltage
3.0 V supply
Bias Range
Step Size
Figure 3. Driver Power Settings Tab
This mode also gives low power
operation. The only difference
between this mode and “Low Power
using ILO” is that this mode requires
an external 32 kHz crystal for
operation,
thereby
generating
accurate clock for timing LCD
refresh events.
Note that if the mode is set to “Low Power using ILO”,
there are limited values available for frame rate parameter.
This is due to the use of low frequency 1 kHz ILO for the
component. Frame rate is also a function of number of
commons. As the number of common increases,
maximum frame rate decreases.
D r i ve r P o w e r S e t t i n g s
This tab has settings to control the SegLCD component
driver power.
5.5 V supply
2.017 V-3.0 V
2.35 V-5.5 V
27 mV
50 mV
The driver operates in three stages:
High Drive Stage: During this stage, LCD pixel
capacitance is charged quickly using high drive
current at the start of each refresh event.
Low Drive stage: During this stage, power is supplied
to the pixel to sustain the voltage, compensating for
the leakages from the LCD pixel capacitance. This
has less power than high drive stage.
Tristate (Driver Disabled): During this stage, driver is
disabled until the next refresh event. Whatever charge
is stored in the pixel capacitance will cause the LCD
liquid crystal orientation. Note that leakage can occur
during this period. Keeping the driver in this stage for
a long time will affect the display contrast. The
following figure shows the three stages of the LCD
driver.
This step size comes from the 6-bit DAC that is used
to generate these reference voltages. For this reason,
you can get a maximum 64 levels of contrast.
However, the number of levels of contrast actually
depends on the supply voltage VDDA. The full 64 levels
are obtained with a 5.5 V supply, but only 37 levels
are obtained with a 3 V supply. Table 2 gives the
reference for two supply voltages. The SegLCD
component includes the API to adjust this bias level at
run time.
Ganging Commons
This is useful when higher driver strength is required
for driving a larger segment LCD display with higher
capacitance. When this option is selected, two I/Os
will be used to drive a common pin of LCD.
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PSoC® 3 - Segment LCD Direct Drive
Figure 4. Drive Stages
Voltage across pixel driven with 1/3
bias ratio
Vbias
Vbias/3
0V
-Vbias/3
-Vbias
t
Glass Size Option
Enlarged
Vbias
You can leave everything to the component to decide the
drive strength and duration based on the glass size
entered. The component calculates the drive strengths
based on number of commons and the glass size value
which gives information about the amount of glass
capacitance.
-Vbias
t
High
Drive
Low
Drive
TriState
Note that when the drive power mode in the “Basic
Configuration” tab is set to “No Sleep”, the LCD driver will
go through all of the stages—High Drive, Low Drive, and
Tristate. When any of the low power modes is selected in
the Basic Configuration tab, then there will be no Low
Drive stage; the driver will directly transition to Tristate
after High Drive stage.
You can configure the drive power strengths during High
Drive as well as the period of time it remains in each
stage. This gives flexibility in power and performance
optimization. There are eight total modes you can use, as
shown in the following table.
Table 3. SegLCD Driver Modes
Power strength
Mode
Seg=1x, Com=1x
High Drive
Seg=1x, Com=2x
High Drive
Seg=1x, Com=4x
High Drive
Seg=2x, Com=2x
High Drive
Seg=2x, Com=4x
High Drive
Seg=4x, Com=4x
High Drive
Seg=0.06x,
Com=0.06x
Low Drive
Seg=0.06x,
Com=0.12x
Low Drive
Notes
Configurable by the
user
Clear the Advanced check box to use the glass size
option. If advanced is selected, high drive and low drive
time and high drive strength must be manually entered.
Low drive strength will be automatically selected based on
the high drive strength.
Setting High Drive and Low Drive Duration
High Drive and Low Drive duration are the periods when
the LCD drive consumes power. Setting the duration too
high will cause more power to be consumed. Setting it too
low will cause the pixels to be charged for less time over
the active common period and will cause contrast issues.
It becomes even more important when the component is
operated in low power modes and the device is put to
sleep. In these modes, the entire device stays active
during High Drive stage. The LCD component has the
authority to give permission for the device to be put to
sleep. If the sleep command is issued through firmware
and the LCD driver is currently in high drive stage, the
device will not enter sleep mode. The sleep command will
be in pending state as long as the driver is in high drive
stage. Thus, when operating the component in any of the
low power modes, it is important to set the high drive time
as low as possible.
Displa y Helpers
In this tab, LCD functional and hardware specific details
are included. There are three sections on this tab:
Configured
automatically based
on the selected high
drive
As shown in Table 3, there are six power modes in High
Drive and two power modes in Low Drive. These modes
give the ability to adjust the power of the common drive
relative to the segment drive. Because the number of
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commons will be less than the number of segment lines,
AC coupling can occur if the drive strengths of both
segment driver and common driver are same. To avoid
this, programmable drive strengths are given in the driver
system. The driver power strengths given in the previous
table are comparable across the modes as well between
segments and commons. For example, in high drive
mode, Power strength Seg=1x, Com=4x implies common
line drive strength is four times greater than segment line
driver. Also common line drive strength in this mode is four
times stronger than Seg=1x, Com=1x drive mode.
Helpers
Digits information
Pixel mapping table
Helper represents the kind of display features that you
have on an LCD. As shown in Figure 5, helpers include
the LCD display feature such as 7 segment, 14 segment,
and so on. Click on the right arrow button to select a
helper. To add specific number of digits, click the ‘+’
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PSoC® 3 - Segment LCD Direct Drive
button. You can repeat this process for all the display
features.
The next step is to set the common-segment mapping
(pixel mapping) table. This information is available in the
LCD Glass datasheet. Drag the pixel or segment and drop
it on the pixel mapping table. To understand this process
much more clearly, look at the PSoC Creator project
section.
Figure 5. Display Helpers
PSoC Creator Project
In this project, you will learn to:
Configure the segment LCD component based on the
LCD
Use the contrast control feature
As an example, the segment LCD in the Cypress Segment
LCD Drive Kit CY8CKIT-029 will be used.
Resources required for building and testing the project:
PSoC Development Kit—CY8CKIT-001 or any other
custom development platform
LCD Segment Drive Expansion Board Kit—CY8CKIT029
PSoC Creator Development Tool
PSoC Programmer Software
Details of LCD used in CY8CKIT-029:
Segment LCD Glass Features:
Display Technology: Twisted Nematic (TN)
8 common and 16 segment lines
Viewing direction: 6:00
Operating voltage: 3 V
Polarization Mode: Reflective/Positive
Operating Temperature: 0 °C to 50 °C
Various sections of the LCD are as follows.
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PSoC® 3 - Segment LCD Direct Drive
Figure 6. Different Sections of Segment LCD
Figure 7. Segment Naming
The segment and common mapping table is given in the following figure.
Table 4. Segment-Common Mapping
Pin
1
2
3
4
5
6
7
8
COM7 COM7
COM6
COM5
COM6
COM5
COM4
COM3
COM2
COM1
COM0
COM4
COM3
COM2
COM1
COM0
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
T7
S1
S2
COL1
S3
S4
S5
COL2
T1
T4
T2
T3
COL3
COL4
T5
T6
1A
1J
2A
2J
3A
3J
4A
4J
5A
5J
6A
6J
10D
9D
8D
7D
1P
1K
2P
2K
3P
3K
4P
4K
5P
5K
6P
6K
10C
9C
8C
7C
1F
1B
2F
2B
3F
3B
4F
4B
5F
5B
6F
6B
10E
9E
8E
7E
1G
1L
2G
2L
3G
3L
4G
4L
5G
5L
6G
6L
10G
9G
8G
7G
1E
1C
2E
2C
3E
3C
4E
4C
5E
5C
6E
6C
10B
9B
8B
7B
1M
1R
2M
2R
3M
3R
4M
4R
5M
5R
6M
6R
10F
9F
8F
7F
1N
1D
2N
2D
3N
3D
4N
4D
5N
5D
6N
6D
10A
9A
8A
7A
For more information on segment LCD glass and the EBK, see the user guide of CY8CKIT-029. Now, start to configure the
segment LCD component.
www.cypress.com
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PSoC® 3 - Segment LCD Direct Drive
In this LCD, one common line is multiplexed to control
eight segments. Seg=1x, Com=4x is selected for this
project. This is the maximum relative strength drive
available in the driver to balance the total drive strengths
of segment driver and a common line driver.
Segment LCD configuration
Basic Configuration
Figure 8. Basic Configuration
Displa y Helpers
In the Display Helpers tab, you can see the pixel
mapping table with 8 commons and 16 segment lines
listed. This table is prepared based on the information
entered in the basic configuration tab.
Figure 10. Display Helpers Tab
The number of commons and segment lines is set to 8
and 16, respectively. The LCD is driven with 1/4 bias ratio
which is sufficient for this display. The component is
operated in “No Sleep” mode, which means that the LCD
will be non-functional if the device is put into sleep mode.
In this project, device is always kept in active mode. The
bias voltage is set to 3 V. Ganging is not used in this
project. However, if the selected glass has high
capacitance, then the ganging option should be used.
D r i ve r P o w e r S e t t i n g s
Figure 9. Driver Power Settings
You must see the LCD datasheet to configure this tab.
Use the following steps as a guide:
Step 1: Select the display feature: The LCD used in this
project has a 7-segment display section, 14-segment
display section, and several symbols. The Helpers box in
this tab lists all the features supported by the component.
Select the required display feature in the Helpers box one
by one by clicking on right arrow button. It will then be
displayed in the Selected Helpers list box. To delete the
selected helper, use the left arrow button.
The Advanced setting is selected. By default, the high
drive time is set as half of the maximum possible value.
The maximum value in this case is ~1158 us. The high
drive time is set to 25 percent of this value; that is, 290 us.
Low drive time is set to 50 percent of maximum value.
During the rest of the 25 percent of the frame time, the
driver will be disabled. Tune these values depending on
the contrast required and the acceptable power
consumption.
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PSoC® 3 - Segment LCD Direct Drive
Figure 11. Adding Display Helpers
Helper_7Segment_0
Helper_14Segment_0
Helper_Bar_0
Step 3: Edit the segment names of each display
feature according to the LCD used
For special symbols,
(Helper_Bar_0) is used.
bargraph
and
Dial
Helper
Step 2: Add digits: The LCD has four 7-segment digits,
six 14-segment digits, and 16 symbols. Click on the
selected helper and add the required number of
digits/symbols using the “+” button.
It is important that you use the give all the segments of all
display types the same name as the one mentioned in the
LCD datasheet. In this project, the segment names of
14-segment display and all the symbols are changed
based on what is specified in the LCD datasheet. This
helps in avoiding mistakes while filling segment-common
mapping tables. It also helps in easy identification of the
each segment/pixel in firmware. To rename the segments,
click on the segment and type the new name in the text
box.
Figure 13. Renaming Segment Name
Figure 12. Adding More Digits
Four 7-Segment
Digits added
Figure 13 shows that the name of one of the segments of
14-segment display is changed from H14SEG0_L to
H14SEG0_R according to the LCD specification shown in
Figure 7 All other segment names are also changed. The
following figure shows the changed names of a
14-segment display and the symbols.
14-segment configured in component
Six 14segment digits
added
Symbols configured in component
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PSoC® 3 - Segment LCD Direct Drive
Repeat the process of mapping for all segments of display
sections. You can choose any segment row in the pixel
mapping table, as long as you put other corresponding
segments sharing the LCD pin in the same row and do
proper pin assignment. The completed table looks like
this.
In the case of symbols, it does not matter which segment
you want to rename; segment-common mapping must be
done appropriately.
Figure 16. Completed Pixel Mapping Table
Step 4: Fill segment-common mapping table: Drag the
segment from the digits and drop it on the pixel mapping
table. You should see the segment-common mapping
table given in the LCD datasheet (shown in Table 4).
Figure 14. Pixel Mapping Procedure
Segment-common mapping table for your reference
If you want to clear any mapping, right-click on the pixel
and do a reset as shown in Figure 15:
Figure 15. Clearing the Existing Mapping
www.cypress.com
This completes the component configuration. You now
must assign the pins for the segment and common lines.
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PSoC® 3 - Segment LCD Direct Drive
Pin Assignment
For pin assignment, use the .cydwr file. You must make
the hardware compatible with the pins selected here.
Figure 17. Pin Assignment
Here the pins are assigned considering that the Segment
LCD Drive Kit CY8CKIT-029 is plugged to port A of the
PSoC Development Kit CY8CKIT-001.
Other Components in the Design
Delta-Sigma ADC: A potentiometer is connected to port
P0[2] of the device for contrast adjustment. The DeltaSigma component is used to read the potentiometer value.
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PSoC® 3 - Segment LCD Direct Drive
Top Design
Figure 18. Top Design of the Project
Firmware
The component offers several APIs. For details of all the
APIs, see the Segment LCD component datasheet. In this
project:
Text “PSOC” is displayed on the 14-segment display
section
Incrementing values (every second) are printed on the
7-segment display section
3.
In the present project, a medical symbol is blinked
every second. This is done by controlling an individual
pixel. Segment LCD component gives the API:
uint8
LCD_WritePixel(uint16
pixelNumber, uint8 pixelState);
Pixel number can be obtained from the component .h
file. Each every pixel of all the display sections is
uniquely addressed based on the name set while
configuring the component. If 1 is passed to pixel
state, the pixel is turned ON.
Medical symbol blinks every second
This example will give an introduction to APIs associated
with all types of display sections. You will also learn about
the contrast control API.
API to control particular pixel/segment
4.
API to control the contrast of LCD
Segment LCD APIs used in the project:
uint8
1.
This API sets the bias level controlling the contrast of
LCD. This API accepts values between 1 and 64.
API to print string on 14 segment display
Void LCD_WriteString14Seg_1(uint8*
character, uint8 position);
LCD_SetBias(uint8 biasLevel) ;
This prints the string on the 14-segment display with
the specified start position.
2.
API to display number on 7-segment display
void
LCD_Write7SegNumber_0(uint16
value, uint8 position, uint8 mode);
This API prints the specified value on the 7-segment
display with the specified starting position. Mode
value, when set to 1, appends 0 to the MSB positions.
When set to 0, it retains the previous value for MSB
positions.
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PSoC® 3 - Segment LCD Direct Drive
The main.c code is shown here:
/* Increment display count */
DisplayCount++;
if(DisplayCount>9999)
DisplayCount=0;
void main()
{
uint8 ADCValue;
uint8 msCounter=0;
uint16 DisplayCount=0;
uint8 BlinkStatus=0;
}
}
}
/* Starts the Segment LCD */
LCD_Start();
/* Start ADC */
ADC_DelSig_1_Start();
ADC_DelSig_1_StartConvert();
/* Write on 14 segment display section */
LCD_WriteString14Seg_1("PSOC",0);
while(1)
{
/* Give 100ms delay */
CyDelay(100);
ADC_DelSig_1_IsEndConversion
(ADC_DelSig_1_WAIT_FOR_RESULT);
ADCValue=ADC_DelSig_1_GetResult8();
/* Divide the 8bit value by 4 as
SetBias
function accepts 6 bit
value */
ADCValue=ADCValue>>2;
/* Set the LCD bias which controls the
contrast */
LCD_SetBias(ADCValue);
/* Increment counter every 100ms */
msCounter++;
if(msCounter==10)
{
msCounter=0;
/* 1 sec has elapsed, blink medical
symbol and increment count on seven
segment display */
/* Blink Medical Symbol (T3) */
LCD_WritePixel(LCD_T3,BlinkStatus);
BlinkStatus^=0x1; /* Toggle status
*/
/* Update display on 7-segment */
LCD_Write7SegNumber_0(DisplayCount,0
,1);
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PSoC® 3 - Segment LCD Direct Drive
Test Setup
LCD Component Low Power Modes
There are two low power modes include in the Segment
LCD component:
Low power using ILO
In low power modes, LCD operates only in high drive
stage. After the defined high drive time, the LCD driver is
disabled (but timer enabled) and all the LCD pins are
tristated.
Following events occur at time TA and shortly afterward:
Low power using external 32-kHz crystal
With either of these modes, you can put the device to
sleep and keep the LCD functional. The LCD component
is equipped with a timer that gives periodic interrupt to the
device to wake up and refresh the LCD glass. To
understand the component behavior in low power mode,
see the following figure.
Figure 19. Segment LCD Component Behavior in Low
Power Mode
LCD timer issues interrupt. If the device is put in sleep
mode, it wakes up the device. LCD component is
completely powered ON.
LCD component enters high drive stage
Following events occur at time TB and shortly afterward:
LCD driver is disabled; common and segment pins are
tristated.
If the sleep command is issued in firmware between
time instants TA and TB, then the device enters sleep
mode.
Voltage across pixel driven with 1/3
bias ratio
T
If the sleep command is issued between TA and TB
through firmware, the LCD component will keep that
command in pending state as long as it is in High Drive
stage. After the high drive stage is completed at TB, it
gives permission to put the device to sleep.
Vbias
Vbias/3
0V
-Vbias/3
-Vbias
t
Enlarged
Vbias
TriState
High
Drive
TriState
High
Drive
-Vbias
t
TA
www.cypress.com
TB
TA + T
TB + T
TA + 2T
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PSoC® 3 - Segment LCD Direct Drive
3. Clock Settings
Using Segment LCD Component in
Low Power Mode
Figure 21. Clocks Window
Here is the list of things that you need to do in
PSoC creator to use the component in Low Power mode:
1. Set the operating mode of the component to one of
the Low Power modes
In the driver mode settings under the “Basic
Configuration” tab, select either Low Power using ILO
or using external 32 kHz crystal.
Figure 20. Low Power Mode Selection
2. Select the High Drive time based on contrast and
power requirement
As mentioned in “Setting High Drive and Low Drive
duration”, it is important that High Drive time is not set
too high because it will block the device from going into
sleep mode, which leads to higher power consumption.
Keeping a low value of high drive time will cause
contrast issues. Consider contrast and power
requirements when you set the high drive time.
In the .cydwr file, go to the Clocks tab. Click the Edit
button to open the Clock Settings window.
Figure 22. Clock Settings
Settings for component configured in “Low power using
external 32 kHz crystal”:
www.cypress.com
Enable 32 kHz crystal oscillator
Disable PLL for reducing power consumption
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PSoC® 3 - Segment LCD Direct Drive
4.
Firmware
LCD Low power modes are used along with device
sleep mode. It is the responsibility of the user to put
the device to sleep. The LCD component will wake
the device but does not put the device back to sleep
after LCD refresh. For this reason, it is important that
the firmware is structured properly. The following
snippet shows how the code can be structured if the
design has multiple interrupt sources.
void main()
{
/*Enable Global interrupt */
CYGlobalIntEnable;
if ( condition )
{
/*non-LCD interrupt has
triggered */
/* User code */
}
}
}
/* Initialize all the components and
variables used */
/* Start the Segment LCD component
*/
LCD_Start();
LCD_EnableInt();
while(1)
{
/* Put the device to sleep */
CyPmSleep(PM_SLEEP_TIME_NONE,PM_
SLEEP_SRC_LCD);
/* Device wakes up from sleep
*/
/* Check if other interrupts
(non-LCD) in the design have
triggered
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else put the device back to
sleep */
Use this kind of firmware structure when operating the
device in sleep mode with the LCD component configured
in low power mode.
Summary
PSoC 3 and PSoC 5LP offer segment LCD drive as a
value-added feature in addition to other major functions
with its configurable digital and analog hardware. This
application note explains the PSoC 3 and PSoC 5LP
segment LCD drive component through one simple
example project.
About the Author
Name:
Rajiv Vasanth Badiger
Title:
Sr. Applications Engineer
Background:
BE Electronics and Communication
Contact:
[email protected]
Document No. 001-52927 Rev. *H
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PSoC® 3 - Segment LCD Direct Drive
Appendix A
Figure 24. Liquid Crystal – No Excitation
LCD Operating Principle
Rear Polarizer
Front Polarizer
Liquid Crystals
LCD operates based on the light polarization concept.
Polarization is defined based on the electric field vector
direction associated with light. To understand the
fundamental concept, consider the following diagram.
Light Source
Figure 23. Basic LCD Concept
Segment Electrode
Generally, the available light source produces unpolarized
light rays; that is, electric field vector direction is
continuously and rapidly changing. Pass this unpolarized
light through a polarizer sheet, which passes only light
rays whose electric field is aligned vertically. Then, place a
second polarizer, which passes light with horizontal
polarization behind the first polarizer. No light rays will
pass through the second polarizer. If you place some
mysterious material between the two polarizers to control
the polarization of light based on need, this will reorient
the light parallel to the second polarizer or simply pass the
light with the polarization created by the first polarizer
which will be blocked by the second polarizer. This
mysterious material is the liquid crystal by which LCD got
its name.
Liquid crystal orientation is changed by application of
electrical signal across it with the help of electrodesbackplane (common) and segment. To avoid permanent
orientation of liquid crystal, alternating waveform needs to
be applied between the electrodes so that average voltage
across the liquid crystals to be 0. Figure 24 shows the
case when the pixel is in off state, that is, when light
simply passes through the front polarizer. This is the case
when no voltage is given between the electrodes. By
default liquid crystals orient the light by 90°. This is an
example of TN LCD glass which will be explained later in
the document.
To turn ON the pixel, liquid crystals are excited causing it
to align parallel to the electric field as shown in Figure 25.
Figure 25. Liquid Crystal – With Excitation
Rear Polarizer
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Liquid Crystals
Front Polarizer
Light Source
Segment Electrode
Figure 24 shows the basic LCD assembly consisting of
rear polarizer, front polarizer, segment and backplane
electrode, and Liquid crystal. The liquid crystal is
sandwiched between the electrodes and polarizers.
Backplane Electrode
Backplane Electrode
In this case, liquid crystal will not change polarization of
light coming from the rear polarizer and it will be blocked
by the front polarizer. It will be seen as a dark spot on the
LCD front panel.
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PSoC® 3 - Segment LCD Direct Drive
Classification of LCDs
There are many ways LCDs available in market can be classified.
Figure 26. Classification of LCDs
Based on number of
backplanes:
1. Static
2. Multiplexed
Based on LCD Technology:
1. TN
2. STN
3. FSTN
LCD
Based on light source location:
1. Transmissive
2. Reflective
3. Transflective
Based on image:
1. Positive
2. Negative
For the segment to be OFF, the segment and common
signals are kept in phase. This results in 0 Vrms across
the segment and common pin.
Classification (Based on Number of
Backplanes/Commons)
1.
Static
2.
Multiplexed
Static LCD
There is only one backplane or common in static LCDs
and independent control for the segments. The voltage
between the common and the segment pin decides the
state of the pixel. As mentioned earlier alternating
waveform needs to be applied to avoid permanent
damage to the LCD. Following diagram shows the typical
waveforms of static LCD:
Figure 27. Static LCD Waveforms
Vdd
Common
Common
Vdd
OFF Segment
For the segment to be in ON state, segment and common
signal are kept 180° out of phase. If the peak value of the
square waveform is say VDD, then it results in RMS voltage
of VDD across the segment and the common and average
voltage of 0 which is the fundamental requirement.
Multiplexed LCD
In Static LCD, the number of pins and drivers to control
the pixel is equal to the number of segments or pixels LCD
has. This becomes a problem as the complexity of LCD
increases which requires use of higher I/O count LCD
drivers. Instead of having individual control pin for a
segment or pixel, we can multiplex the use of control pin
for other segments/pixels. Then, a question arises while
exciting one pixel; how to avoid affecting another pixel
controlled by same pin. The answer is to use multiple
commons. These types of LCDs are called as multiplexed
LCD. Multiplexed LCDs are specified based on the
number of commons it has.
Vdd
ON Segment
Segments
Vdd
0V
Resulting Voltage
(Common-Segment)
OFF Segment
As an example, consider four segments or pixels in an
LCD. In static LCD, there will be four segment control pins
and one common pin as shown in the following figure.
Therefore, it requires a total of five pins or drivers in a
static configuration.
-Vdd
Vdd
0V
Resulting Voltage
(Common-Segment)
ON Segment
-Vdd
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PSoC® 3 - Segment LCD Direct Drive
B i a s R a t i o / T yp e
Bias is the number of voltage steps applied to the LCD.
The number of voltage levels should be increased with
increase in number of commons of LCD. This is done to
avoid high RMS voltage across the pixel in OFF state.
Figure 28. Static LCD Configuration
Seg1
Seg3
Common
Each multiplexed LCD manufacturer has a recommended
bias ratio for the LCD. Common bias ratio values are 1/2,
1/3, ¼, and 1/5.
Seg4
Seg2
In two common multiplexed LCDs, two segments will
share the same pin. This configuration will result in four
pins or drivers, that is, one less than static configuration.
This advantage becomes clearly evident as the number of
segments increases.
Figure 29. Multiplexed LCD Configuration
D r i ve W a ve f o r m s
Multiplexed LCDs can be driven by two types of
waveforms. These are called Type A and Type B
waveforms.
Type A waveforms take single frame to maintain zero
average voltage across each pixel whereas Type B
waveforms take two frames.
Figure 30. Multiplexed LCD Drive Waveforms
Type A waveforms
Seg1
Type B waveforms
Common
Segment
Com1
0V
Com2
0V
1 Frame
Seg2
D r i vi n g M u l t i p l e x e d L C D s
Before considering the details of drive waveforms, it is
necessary to understand some of the terms associated
with multiplexed LCDs.
Frame Frequenc y
Frame frequency is the rate at which the drive waveform
repeats for each segment line. In simple terms, it is the
rate at which each segment LCD pixel is refreshed. It is
generally in the range of 30 Hz to 150 Hz. High frame
frequency results in higher power consumption and lower
frequency causes flickering on LCD. Therefore, there is a
tradeoff between power consumption and image display
quality.
Duty Ratio/Multiplex Ratio
This term indicates the number of commons or backplanes
multiplexed LCD has. It is specified inverse of number of
commons/backplanes. For example, if multiplexed LCD
has 4 commons, duty ratio is equal to ¼. While driving
multiplexed LCDs, the segments associated with one
common, gets refreshed for 1/(number of commons) of the
frame period; that is Duty Ratio x Frame period.
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Liquid crystals is
energized
Liquid crystals is
not energized
Common - Segment
1 Frame
Liquid crystals is
not energized
Liquid crystals is
energized
Type B waveforms contain fewer edges than Type A
waveforms in a given time duration. This is one major
advantage of Type B waveforms. LCD pixel is modeled as
a capacitor. So there will be current flow from the driver
only during edges. Therefore, a Type B waveform results
in lower power consumption compared to a Type A
waveform.
The drive waveforms for various combinations of Duty
ratio and Bias ratio are discussed earlier in Appendix A. In
all the waveforms, it should be noted that a frame consists
of some number of phases (equal to number of
commons). During each phase, one common is made
active and it is driven with extreme voltage levels. All the
other commons are pulled to intermediate voltage levels;
value depends on the bias ratio. The segments associated
with the active common which needs to be turned ON are
driven with opposite phase signals with respect to active
common. This generates peak RMS voltage across the
pixel necessary to turn it ON, at the same time maintaining
zero average voltage. The segments associated with the
active common which needs to be turned OFF are driven
with in-phase signals. This generates zero RMS voltage
across the pixel.
The segments associated with inactive commons will not
experience high excitation as the inactive commons are
excited with intermediate levels. Thus, the segments
associated with inactive commons will remain unaffected
when other commons are active.
Document No. 001-52927 Rev. *H
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PSoC® 3 - Segment LCD Direct Drive
Adjusting Contrast
Contrast level of LCD is adjusted by the changing the gap
between ON Segment Voltage and OFF segment Voltage.
all the voltages are scaled down by a factor of ¾. This
results in decreased RMS value across pixels. RMS
voltage controls the amount of twist to the light polarization
in the assembly. As the RMS voltage goes down, the
As an example, consider two common LCD drive with ½
change in light polarization due to liquid crystal also goes
bias as shown in Figure 31. To reduce the contrast, all the
down. This results in increased passing of light through
voltage levels are scaled down. In the waveform shown,
the front polarizer which means lower contrast.
Figure 31. Contrast Control
Higher Contrast
Lower Contrast
Vx
¾ Vx
½ Vx
3/8 Vx
COM0
0
Vx
¾ Vx
½ Vx
3/8 Vx
COM1
0
Vx
¾ Vx
SEG
ON
0
Vx
¾ Vx
SEG
OFF
0
∆
Vx
¾ Vx
½ Vx
3/8 Vx
0
-3/8 Vx
-½ Vx
-¾ Vx
-Vx
COM0SEG
ON
½ Vx
3/8 Vx
0
COM0SEG
OFF
-3/8 Vx
-½ Vx
Classification (Based on Light Source)
Transmissive
Reflective
Transflective
All LCDs are passive. They require an external light
source to display information.
T r a n s m i s s i ve L C D
If the light source is required to be behind the rear
polarizer, then it is Transmissive LCD. Figure 24, shown
earlier, is an example of Transmissive LCD. A simplified
form is shown in Figure 32. This type of LCD requires a
backlight source. Depending on backlight brightness it can
be used in either indoor or outdoor applications.
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Document No. 001-52927 Rev. *H
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PSoC® 3 - Segment LCD Direct Drive
Figure 32. Transmissive LCD
Classification (Based on LCD Technology)
Liquid Crystal +
Electrode assembly
Rear
Polarizer
Front
Polarizer
R e f l e c t i ve L C D s
Reflective LCDs are equipped with a reflector behind the
rear polarizer. This reflector allows external light to be
reflected back. These LCDs do not require any backlight.
For this reason, they are used in low power systems. This
also gives good contrast in display under high ambient
light conditions.
Figure 33. Reflective LCD
Liquid Crystal +
Electrode assembly
Rear
Polarizer
Twisted Nematic (TN)
2.
Super Twisted Nematic (STN)
3.
Film Compensated STN (FSTN)
Twisted Nematic (TN)
This type of liquid crystal gives 90° twist to the incoming
light. The two polarizers (rear and front) mounted on the
LCD have direction of polarization perpendicular to the
each other. The example shown in Figure 24 is Twisted
Nematic type of display. This type of display suffers from
lower contrast and smaller viewing angle. Also these
displays are not suitable for LCDs with higher number of
commons.
Super Twisted Nematic (STN)
This type of liquid crystal gives greater than 90° twist to
the incoming light and gives a steeper change in
transmission-voltage curve. With this feature, RMS voltage
of ON LCD pixel can be very close to the RMS voltage of
OFF LCD pixel, still providing acceptable contrast. Due to
this reason, it is suitable for LCDs with higher number of
commons. The typical twist angle is between 180° to 270°.
This has better contrast and viewing angle as compared to
TN displays.
These displays are available in couple of colorations:
Yellow/Green and Grey background with dark blue
characters.
Reflector
Front
Polarizer
T r a n s f l e c t i ve L C D s
Transflective LCDs are equipped with special semireflector which has the capability to reflect as well as pass
fraction of light, thereby allowing both features of
Transmissive and Reflective type LCDs.
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1.
Film Compensated STN (FSTN)
This type is a modification of STN displays. STN displays
have color added that appears in display background due
to birefringence effect. To compensate for this effect,
additional film is introduced in the assembly which
eliminates this coloration. These types of displays, thus,
are available only in White/Grey Background with black
characters. Along with the advantage in eliminating
coloration, these displays also give good contrast and
wide viewing angle.
Document No. 001-52927 Rev. *H
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PSoC® 3 - Segment LCD Direct Drive
Classification (Based on Image Produced)
1.
Positive
2.
Negative
12:00 Clock view
6:00 Clock view
These terms are with reference to a clock. If the bias line
bends towards top portion of the LCD display then it is
referred to as 12:00 Clock or Top view. If the bias angle
bends towards bottom portion, then it is referred to as 6:00
Clock or Bottom view.
P o s i t i ve
This type of LCD displays dark characters on a bright
background. This type of display is suitable for reflective
type LCD with high ambient light conditions or for
Transmissive type LCD with good backlight.
Figure 34 shows the 12:00 view. α is the bias angle and β
is the viewing angle.
Figure 34. 12:00 or Top View
N e g a t i ve
LCD
β
α
This type of LCD displays bright characters on a dark
background. This type of display is suitable for
Transmissive LCDs under good backlight conditions.
LCD Viewing Angle
Similarly, 6:00 view is shown in the following figure.
Viewing angle of LCD depends on the type of liquid crystal
used and the duty cycle. There are two terms associated
with the viewing region - Bias angle and the Viewing
angle.
Figure 35. 6:00 or Bottom View
Bias angle is the angle between the perpendicular axis of
LCD and the line along which best possible view is
obtained. Bias angle is set when the LCD is manufactured.
LCD
Viewing angle refers to angle spacing around the bias line
where satisfactory view is obtained.
α
β
Depending on where the bias line is located, LCD view
positions are referred to as:
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Document No. 001-52927 Rev. *H
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PSoC® 3 - Segment LCD Direct Drive
Document History
®
Document Title: PSoC 3 and PSoC 5LP- Segment LCD Direct Drive – AN52927
Document Number: 001-52927
Revision
ECN
Orig. of
Change
Submission
Date
**
2711107
GDN
05/26/2009
New application note
*A
2765916
GDN
09/18/2009
Changed the figures to latest version of PSoC Creator (Beta 2 Nightly build 5307)
and updated related text with respect to Figure 5.
Description of Change
Changed eight entries in Table 4.
Updated C code for LCDExcercise1 (now display starts from right instead of left)
and the same is updated in project goal.
*B
2958667
RJVB
06/22/2010
Added one more project with Segment LCD Drive EBK CY8CKIT-029
Updated the project with PSoC Creator Beta 4.1
Updated content to include the PSoC 5 device
Added images of test setup for all projects
Added Summary
*C
3010629
RJVB
08/17/2010
Updated the projects on Beta 5.0
Updated the customizer and top design screenshots in the document
Changed the driver mode to low power to add more variety
*D
3178537
RJVB
02/21/2011
Complete rewrite. The document and project are updated to suit PSoC 3 LCD
Driver Hardware.
*E
3296494
RJVB
06/29/2011
Title changed
Moved LCD Basics to Appendix
Updated introduction to include links for PSoC3 Learning
Changed project to be based on CY8CKIT-029; deleted second project
Added section: LCD component low power modes
*F
3444949
NIDH/RJVB
11/22/2011
Updated for PSoC Creator 2.0
Updated template
*G
3809482
VVSK
11/12/2012
Updated for PSoC 5LP
*H
4454392
RKRM
07/24/2014
Updated Abstract
Updated content to address few grammatical issues
Updated the project to PSoC Creator 3.0 SP1
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Document No. 001-52927 Rev. *H
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PSoC® 3 - Segment LCD Direct Drive
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Document No. 001-52927 Rev. *H
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