MK712

Touch Screen Controller

MDS 712 I

7

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

Resistive Touch Screen (4 Wire)

Resistive touch screens consist of 2 resistive plates that are separated by a small gap. Each plate has an

electrode at each end and when the screen is touched, the two plates are shorted together at that point.

Y PLATE

X PLATE

Electrode

XH

YL

XL

YH

If a voltage is applied, for example, between XL and XH, then a voltage divider is formed on the X

PLATE. When the Y PLATE is touched to the X PLATE, a voltage will be developed on the Y PLATE

that is proportional to distance of the touch from XL and XH. By accurately measuring this voltage, the

position of the touch can be determined.

Power Down/Stopping the Clock

If the screen is untouched, the supply current is under 3µA. If the input clock is stopped, the supply

current is also less than 3µA. When the screen is touched with no clock, TOUCH goes low and INT

goes high, but no further operations occur until the clock starts. A to D conversions are invalid until

after the clock has stabilized at the correct frequency and one full conversion cycle has been

completed.

Input Clock

The MK712 is highly tolerant of different clock frequencies (from 12 to 20 MHz), rise times, and

duty cycles. However, any input clock undershoot or overshoot will cause the MK712 to draw extra

current. This can be eliminated by using a MicroClock clock synthesizer and properly terminating

the clock driver. MicroClock makes low power clock synthesizers with 14.3 MHz power down.

Optimizing Performance when Reading and Writing Registers

Reading and writing the MK712 generates digital noise that may reduce the accuracy of the A to D

converter. This noise has several causes, including board layout, data bus loading, and power supply

voltage. By appropriately timing the register operations, the effects of this noise can be minimized.

After an interrupt is issued, the MK712 allows a minimum of 1 millisecond to elapse before

initiating the conversion cycle. This allows the screen drivers to settle. For best performance,

complete all register operations within this 1ms window after an interrupt.