MK712

TOUCH SCREEN CONTROLLER

TOUCH SCREEN CONTROL

IDT™ / ICS™ TOUCH SCREEN CONTROLLER

9

MK712

REV L 070805

Analog to Digital Converter Option

The 12-bit ADC converts X and Y co-ordinates at a rate determined by registers 6 and 7. The converter uses a

ratiometric technique to give absolute co-ordinates on the screen, largely independent of variations in screen

resistance, temperature or power supply voltage. The total voltage applied across the screen is defined as full-scale

for the converter (i.e. 4095) and any point touched on the screen is proportional to this. For example, if the screen

is touched exactly in the center, the converter will read 2047. This feature may allow for the elimination of calibration

upon startup.

However, the full scale voltage is defined at the IC pins and so any parasitic interconnect resistance will be included

in full scale. In addition, the interconnect resistances on the screen also account for about 20% of the total

resistance. This means that approximately the bottom 10% and top 10% of full scale are inaccessible.

The converter is guaranteed to be monotonic, with no missing codes.