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Transcript
LCD Displays
Created March 2008
©Paul R. Godin
Introduction
LCDs are dominating the
display marketplace
◊
◊
◊
portable electronics devices
such as:
◊ digital watches & timers
◊ thermometers & meters
◊ MP3 players
◊ telephones and other
communication devices
◊ cameras
◊ game platforms
laptop & computer monitors
high definition televisions
photo PRGodin
◊
Displays from cellular phones
Advantages of LCDs
LCDs have significant
advantages over other forms
of displays:
◊
◊
◊
◊
◊
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very low power requirement
very compact
low voltage
sharp image (no focus or
distortion problems)
scalable
integrate with digital circuits
photo PRGodin
◊
Display salvaged from a VCR
Disadvantages of LCD displays
sensitive to temperature
variations
◊
◊
◊
◊
◊
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very slow response times with
cold temperatures
risk of permanent damage from
cold or heat
less robust to physical stresses
color displays are susceptible to
bad pixels
poor viewing angles
slow response times
Fixed characters and image
density
photo PRGodin
◊
Damaged display
New product
development strives to
minimize these issues
LCD Configurations
◊ Segment displays
◊ Dot Matrix Displays
◊ Graphic Displays
◊ TFT (Thin Film Transistor) Displays
LCD Diagram
Protective Glass
Polarizer filter 0°
Front glass with
electrodes
Rear glass with liquid
crystal and electrodes
Polarizer filter 90°
Reflective layer
Glass for backlighting
(optional)
Light Fundamentals
◊
Light is electromagnetic energy.
◊
It has physical properties that are similar to both the
photon (particle) model and a wave model.
◊
Light consists of two waves that are perpendicular (90°) to
one another.
Light as a wave
◊ The frequency of the light wave is referenced as
wavelength. Wavelength is distance between
cycles.
C

f
C= Speed of Light
f = frequency
◊ Visible light is between 350 ηM (ultraviolet) and
750 ηM (infrared).
Polarization
◊ Light waves can be filtered using polarized filters.
◊ Polarized filters remove one of the two
perpendicular waves, reducing the intensity of the
light.
Polarization
◊ If a second filter is added perpendicular to the
orientation of the first, all the light is filtered out
and the area appears dark.
Passive Displays
◊ LCDs are considered passive displays as they do
not emit light (except through backlighting).
Instead, they rely on reflecting ambient light.
Reflective layer
Glass for backlighting
(optional)
Polarized displays
◊ LCDs rely on polarization to darken the segments.
Aligned polarization
from two displays
Full polarization
from a 90° angle
between two
displays
Liquid Crystal
◊ Liquid crystal substances are liquids that arrange
themselves in an orderly, crystal-like manner.
◊ There are a variety of liquid crystal compounds
with different properties.
Twisted Nematic Crystal
◊ Twisted Nematic liquid crystals align themselves
in a 90 degree angle. This twists the polarized
light going through the compound.
◊ If exposed to voltage the crystals untwist and
become vertical allowing light to pass through
without twisting.
ITO Traces
◊ Indium Tin Oxide is a transparent conductive
compound that is used with LCDs.
◊ Shaped, conductive traces are placed above and
below the liquid crystal layer.
◊ The matrix displays have long parallel conductors
on the upper side and lower side, but they are
perpendicular to each other.
Basic LCD construction
1-Vertical polarizing filter
2-Transparent layer with ITO (indium
tin oxide) traces
3-Twisted Nematic compound
4-Transparent layer with ITO traces
5-Horizontal filter
6-Reflective backing
Image under CC licence
Liquid Crystal
◊ The polarization angle can be changed by
controlling the twists in the twisted nematic
compound with an applied voltage.
◊ When no voltage is applied, the light which has been
polarized once follows the twist of the compound and
encounters the second polarizing filter with the same
orientation. Light gets through the filters.
◊ When the compound has voltage applied the crystal
structure untwists and the two polarizing filters prevent
light from passing, creating a dark segment.
LCD Drivers
◊ LCDs often come with a microcontroller and
driver circuit which:
◊ Decodes the ASCII code input
◊ Drives the appropriate segments
◊ Retains the values being displayed in memory to refresh
the display
◊ Performs logic functions that control the display
◊ The driver outputs must oscillate. Typical
frequency is approximately 30 Hz.
◊ LCD Drivers typically contain CG-ROM (CG =
Character Generator)
Instruction / Data
◊ Instruction Mode
◊ shift characters left-to-right, or right-to-left
◊ display the cursor
◊ move the cursor or move the characters on the
display
◊ set the number of display lines
◊ direction
◊ etc...
◊ Data Mode
◊ standard ASCII
◊ extended ASCII (Japanese 8-bit for our displays)
Interfacing with LCDs
◊ Most LCD devices operate with CMOS-based logic
devices.
◊ Static sensitive
◊ Sensitive to reverse voltage
◊ Most displays follow a defacto standard for
connectivity and programming.
◊ Charts are available for programming the
displays.
Activity
◊ View specification sheets for display
characteristics
◊ View other reference information on how to
interface displays with the output of electronic
circuits
◊ Lab activity review
END
©Paul R. Godin
prgodin @ gmail.com