Specifications

EE
LCD Flat Panel Interface
Page 3
Active Matrix Displays
In an active matrix flat panel display, each pixel receives output
from an attached transistor to precisely control the display. The
transistors connect in an X, Y grid located on the same substrate as
the pixels. Switching signals drive the transistors in the X grid,
while video signals drive the transistors in the Y grid.
A popular active matrix flat panel display type is the Thin Film
Transistor (TFT). In this display type, a layer of liquid crystal lies
sandwiched between a pair of electrodes and a pair of polarizing
filters positioned at a 90 degree phase difference from each other.
The liquid crystal molecules align naturally in a twisted state with
no particular order or direction. Light entering the display “twists”
as it passes through the layer of liquid crystal and the polarizing
filters. After striking a reflective layer at the back of the display, the
light passes back through the front of the display. A pixel in this
state is not visible from the front of the display.
When current is applied to the liquid crystal molecules, they align
in a single direction, changing the light’s path through the display.
Without the twisting effect provided by the natural state of the
liquid crystal molecules, the polarizing filters, placed at the 90
degree phase difference from each other, block the passage of light
through the display. A pixel in this state is visible from the front of
the display.
Passive Matrix Displays
Technical improvements by leading manufacturers of passive
matrix displays, such as the Super Twist Nematic (STN) type, have
made this type of display an attractive choice for a broadening
variety of applications. These improvements include higher contrast
and brightness, and faster video signal response than was available
with the earlier Twisted Nematic (TN) type displays.
As with active matrix displays, a layer of liquid crystal lies
sandwiched between a pair of electrodes and a pair of polarizing
filters positioned at a 90 degree phase difference from each other.
The liquid crystal molecules, depending on their alignment, control
passage of light through the display. However, in a passive matrix
flat panel display, X grid conductors connect to the lower display