MK712

Touch Screen Controller

MDS 712 I

6

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

Recommended Circuit

YH

INT

XH

XL

TOUCH

YL

GND

CS

RD

GND

D0 - D7

VDD

A0 - A2

CLKIN

VDD

VDD

GND

From

Microprocessor

14.318 MHz

To Power Management

To Interrupt Controller

X HIGH

X LOW

Y HIGH

Y LOW

To

Touch

Screen

0.1µF

0.1µF

22µF

+3.3V or +5V

WR

0.1µF

0.1µF

0.01µF 0.01µF

Pen Bounce

When the screen is untouched, the Y plate is driven high and the X plate is driven low. When the screen is

touched, the X plate is pulled high, which is detected by the MK712. This initiates a conversion (as long as

periodic conversions are enabled). Some de-bounce is provided by the time constant of the screen

decoupling capacitors combined with the screen resistance. However, once conversions have started, pen

bounce will not be detected until after both the X and Y points have been taken.

If the pen is lifted during a conversion, this will also not be detected until the conversion is complete. This is

typically indicated by a point with the correct Y coordinates, but minimum (for that screen) X coordinates.

Screen Decoupling

The component values suggested should give excellent results. However, for screens with a high

resistance operating at a high conversion rate, extra interrupts may be issued. This is because the time

constant of the screen decoupling capacitors and screen resistance is high, resulting in the screen

appearing to be untouched and then touched between conversions. These extra interrupts can be

eliminated by reducing the size of the screen decoupling capacitors on pins XH, XL, YH, and YL.

This will, however, slightly reduce the noise rejection, which could decrease the accuracy of the

conversion.