Introduction to LCDs technology ~ A developer's log book

This article gives an overview of the LCD technologies. It is a first step for who might want to design an embedded system with a color (touch) display.

How does it work

An LCD is an array of Liquid Crystal segments.
These segments are polarized and organized randomly when not in an electrical field.
When an electrical field is applied, the crystals align to the field and gate the amount of light
than can pass through them.

Each LCD segment is aligned with an electric field.
A backlight is required to drive light through aligned crystal.

Types of display

There are passive (STN) and active displays (TFT).
They both consist of a grid of rows and columns electrical signals connected perpendicularly to every segment in the LCD.
An integrated chip controls which row and column are activated to enable/disable the segment at the intersection.

The passive screens apply a small bias current to row/column to generate a field in the intersection.
The active screens have a transistor per segment. This transistor is activated when the row/column of a segment is selected.
Color filters allow the generation of specific color (R, G or B) at a segment which means that a pixel consists of 3 segments.
TFT displays can drive 3 segments (or 1 pixel) per clock with levels of color that depend on the number of data lines (8 bits, 15 bits, 16 bits, 24 bits...).

LCD panel technologies for embedded display

From now on, this article will focus on TFT displays.

Common embedded screen resolutions

Resolution	Pixels	Diagonal
QVGA (Quarter Video Graphics Array)	320x240	3.5”
WQVGA (Wide Quarter VGA)	480x272	4.3”
VGA (Video Graphics Array)	640x480	5.7”
WVGA (Wide VGA)	800x480	7”
SVGA (Super VGA)	800x600	10.4”
XGA (extended graphics array)	1024x768	12.1” or 15”
WXGA (Wide Extended Graphics Array)	1280x800	12.1”

LCD Signals

The TFT LCD displays require these type of timing signals:

VSYNC: Vertical Sync. Used to reset the LCD row pointer to the top of display

HSYNC: Horizontal Sync. Used to reset the LCD column pointer to the edge of the display

D0...Dxx: Data lines. Typically, the encoding of R, G and B color levels.

LCDCLK: LCD Clock. Refresh rate.

The other common signals are:

LCD power signals

LCD backlight power

[Optional] Touchscreen signal (usually 4/5-wires for resistive, I²C link for capacitive)

Frame Buffer

The frame buffer is the memory allocated to store bit mapped image sent to the LCD. This data is used to periodically refresh the display.

The frame buffer can be calculated (Width pixels)*(Height pixels)*(Bytes for colors depth).

Example: For a 24bits SVGA the frame buffer size is 800*600*4 = 1.92 Mb. It gives an idea of the databus speed for a refresh rate of 60 Hz : 1.92*60 = 115.2 Mb/s

Touchscreen technologies

The following array gives a quick overview of the different types of touch screens.

	5-Wire resistive	Capacitive	Projected Capacitive	Surface Acoustic Wave	Infrared
Transmissivity/Clarity	Good 75-85%	Very Good 90-98%	Very Good 90-98%	Very Good 90-98%	Best 95-100%
Activate with any object	Best Any object	Poor Finger or capacitive stylus	Good Same as capacitive + surgical glove	Good Finger, gloved hand, soft/pliable stylus	Very Good Most objects
High sensitivity (Light touch)	Good	Very Good	Very Good	Very Good	Best
Accuracy	Very Good	Good	Best	Very Good	Very Good