Display Overview - Atmel AT91SAM9 Application Note

2. Display Overview

2.1 TFT and STN Technologies
2.2 Interface
2.3 Blanking
Application Note
2
The most commonly used display technologies are TFT (Thin Film Transistor) and STN (Super
Twisted Nematic).
STN displays use a passive matrix screen technology that has no active or controlling element
inside the display cell. Pixels are controlled by energizing the appropriate row and column drive
lines of the matrix from outside the display, resulting in a slow frame rate. STN screens have lim-
ited color range and viewing angle (~15 degrees max).
With TFT LCDs, each pixel is controlled by one to four transistors. Typically, one transistor is
used for each of the RGB color channels. Because of this direct control technique, TFT screens
are sometimes called Active-Matrix LCDs. TFT technology provides more accurate color control,
allowing it to display more colors. TFT screens also offer a wider viewing angle range (30 to 70
degrees) than the other popular types of LCDs.
The typical interface of a display is based on analog and digital inputs. The digital lines are a
data bus, a pixel clock, vertical and horizontal synchronization signals and a data enable line.
The voltage input is generally used for contrast control. The LCD Controller drives all these inter-
face lines. If more inputs are required by the display, it may be managed by PIOs (power control,
AC bias on some STN displays, backlight control, etc.)
Contrast Control
Horizontal Synchro
Vertical Synchro
Pixel Clock
For the internal synchronization mechanism, the display may need some dummy data at the
beginning and/or end of a line, and at the beginning and/or end of a frame. This is called
blanking.
Display
Data Enable
Data bus
6300A–ATARM–09-Jul-07