MK712

Touch Screen Controller

MDS 712 I

9

Revision 100899

Printed 11/16/00

Integrated Circuit Systems, Inc.•525 Race St.•San Jose•CA•95126•(408)295-9800tel•www.icst.com

I C R O

XH

XL

YL

MK712

FULL

SCALE

(4095)

X

CO-ORDINATE

BOARD AND WIRE

INTERCONNECT

SCREEN

INTERCONNECT

SCREEN

SCREEN

INTERCONNECT

BOARD AND WIRE

INTERCONNECT

Analog to Digital Converter Operation

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.