MODULE -15
Basics of LCD
and it's Interface to a microcontroller:
Introduction to LCD:
Liquid Crystal Display (LCD) consists of rod-shaped tiny
molecules sandwiched between a flat piece of glass and an
opaque substrate. These rod-shaped molecules in between
the plates align into two different physical positions based
on the electric charge applied to them. When electric charge
is applied they align to block the light entering through
them, where as when no-charge is applied they become transparent.
Light passing through makes the desired images appear.
This is the basic concept behind LCD displays.
LCDs are most commonly used because of their advantages
over other display technologies. They are thin and flat
and consume very small amount of power compared to LED displays
and cathode ray tubes (CRTs).
LCD Technologies and types:
Some of the LCD technologies are,
Blue Mode STN - This is the basic LCD, which needs
lot of improvement on contrast ratio and viewing angle.
FSTN (Film STN) - Comes with an additional linearization
film to offer better contrast.
CSTN (colour STN) - Layers of color filters are added
to create up to 65,000 colors.
DSTN (Double STN) - Improves contrast and eliminates
any other colors appearing on the screen.
Types based on displayed data:
1. Segment LCD: Displays numbers, letters and fixed
symbols and were used in old style industrial panel display
and such standard where we need to display fixed number
of characters.
2. Graphical LCD: Instead of segments it has pixels
in rows and columns. By energizing set of pixels any character
can be displayed.
3. Color LCD displays: Are of type passive matrix
and Thin film transistor/ active matrix.
Two types of color displays:
Passive Matrix
· Row & Column approach
· Apply small bias to perpendicular lines of electrodes
· Bias strong enough to darken bit at line intersection
· Multiplexed addressing scheme
Advantages: Simple to implement
Disadvantages: Can cause distortion
Active Matrix
· Each cell has its own thin-film transistor (TFT)
· Addressed independently from behind LCD
· Direct addressing scheme
Advantages: Sharp display, better viewing angle, 40:1 contrast
Disadvantages: Need better backlight, complex hardware
Recent passive-matrix displays using new CSTN and DSTN
technologies produce sharp colors rivaling active-matrix
displays.
LCD Module Backlighting:
LCDs unlike LEDS do not produce light, they need some external
light source to view. So most the latest LCD screens used
in notebook computers use a light source. They use CFT or
EL Panel or in recent times white LED as source of light
fixed in the background of LCD.
LED lamp is a better choice over CFT and EL due to its ability
to offer variety of colors, intensity, long life (>100K
Hrs), wide temperature range, and low voltage operation.
Electroluminescent/EL Panel: The EL backlight consumes
less power but require high voltage (120VAC @ 400Hz Typically).
EL panel is also not long lasting and is sensitive to wider
operating temperature range.
Cold Cathode Florescent Lamp: This backlight is most popular
and is suitable for large sized LCD displays. CFT has same
drawbacks of EL panel i.e. short life span and limited temperature
range, and need high voltage to operate (>300VAC @ 30-80KHz)
and is also prone to vibrations.
Different viewing modes:
Reflective: In the reflective mode LCDs use ambient
light to illuminate the display making them more suitable
for outdoor use.
Transmissive: Transmissive mode will depend on internal
backlight and is only viewable in indoors.
Transflective: Transflective mode LCDs employ both
Reflective and Transmissive types and switch based on the
availability of ambient light.
Special characteristics of LCDs:
Liquid Crystals are very sensitive to constant electric
fields. Only AC-voltages should be applied as DC voltages
can cause an electrochemical reaction, which destroys the
liquid crystals irreversibly.
Temperature dependent and in a very cold or hot environment
LCD may not work correctly. This is a reversible effect.
Some displays need temperature compensation circuits to
automatically adjust the applied LC voltage.
Key specifications of LCD display:
Important factors to consider when evaluating an LCD monitor:
1. Resolution: The horizontal and vertical size measured
by number of pixels (e.g., 1024x768).
2. Dot pitch: The distance between the centers of two adjacent
pixels. Smaller dot pitch indicate sharper image.
3. Viewable size: The diagonal size of the LCD display panel
4. Response time: The minimum time LCD takes to change a
pixel's color or brightness.
5. Refresh rate: The rate at which the data is loaded in
the monitor measured with the unit of number of times
per second. Higher the refresh rate lesser is the flickering.
6. Matrix type: Active TFT or Passive.
7. Viewing angle: The angle from which the user can view
the image on the display clearly.
8. Color support: The types and number of colors supported
by the LCD (known as color gamut).
9. Brightness: The amount of light emitted from the display
(known as luminance).
10. Contrast ratio: The ratio of the intensity of the brightest
bright to the darkest dark.
11. Aspect ratio: The ratio of the width to the height (for
example, 4:3, 5:4, 16:9 or 16:10).
12. Gamma correction: It is the name of a nonlinear operation
used to code and decode luminance or
tristimulus values in video or still image systems
Advantages of LCDs:
· Consumes less power and generates less heat.
· Saves lot of space compared picture tubes due to
LCD's flatness.
· Due to less weight and flatness LCDs are highly
portable.
· No flicker and less screen glare in LCDs to reduce
eyestrain.
Drawbacks:
The disadvantages of LCD displays are,
· LCDs cannot form multiple resolution images.
· The contrast ratio for LCD images is lesser than
CRT and plasma displays.
· Due to their longer response time, LCDs show ghost
images and mixing when images change rapidly.
· The narrow viewing angle of an LCD weakens the
image quality in wider viewing angles.
Further links to learn LCD operation and basics
http://kitsrus.com/pdf/k134.pdf
http://www.liquidcrystaltechnologies.com/basop.htm
http://www.hitachi-displays-eu.com/doc/AN-001_Basics_of_LCD_Technologies.pdf
http://www.altadox.com/lcd/knowledge/lcd_liquid_type.htm
http://www.gs-display.com/NRD.htm
http://www.dragonlcd.com/Technical_Info.htm
(Nice pictorial presentation)
http://www.vptech.com/downloadDriverFile.asp?filePath=0
http://www.fujitsu.com/downloads/MICRO/fma/pdf/LCD_Backgrounder.pdf
http://www.computerhope.com/help/lcd.htm
http://www.ecnmag.com/selecting-the-right-display-for.aspx?menuid=580
http://www.wanxindisplay.com/knowledge/lcd_display_modes.htm
LCD interfacing:
LCD are connected to microcontroller through a LCD interface
IC or directly to it's address and databus and few control
pins, Few of the today's latest microcontrollers have built-in
LCD driver.
Also some of the LCDs support serial interface.
Here are few nice articles on LCD interface from the internet
and our own module, you can choose based on your interest
from the below list of LCD interface articles.
LCD interface to ARM Cortex M0 based STM32F0 discovery
embedded board: Here is exclusive module explaining
both hardware connection (wiring) and sample program/code
in C langauge to display alphnumerc text on a simple LCD
display panel. The url is,
www.eeherald.com/section/design-guide/sample_lcd_c_programs.html
Parallel LCD Interfacing - 16 Character x 2 Line LCD
http://www.beyondlogic.org/parlcd/parlcd.htm
LCD interfacing with serial RS-232
http://www.beyondlogic.org/serial/serial3.htm
LCD interfacing with FPGA
http://www.fpga4fun.com/TextLCDmodule.html
LCD interface to a 8051 MCU
http://www.ustr.net/lcd001.shtml
EDE702 Serial LCD Interface IC
http://www.componentkits.com/dslibrary/EDE702.pdf
Character LCD interface code and general information
1. http://ouwehand.net/~peter/lcd/lcd0.shtml#hd44780b
2. http://www.alessioviti.com/lcdprojects/drivedisplay_ita.htm
Interfacing serial LCD interface to Microchip PIC microcontroller
http://www.best-microcontroller-projects.com/serial-lcd.html
For LCD datasheets follow the below URLs
http://www.beyondlogic.org/lcd/lcd.pdf
http://www.larwe.com/technical/lcds.html
Projects based on LCDs:
http://www.best-microcontroller-projects.com/serial-lcd.html
LCD design guide:
http://www.wtec.org/loyola/dsply_jp/c1_s4.htm
http://www.wanxindisplay.com/lcd_knowledge.htm
