MK712
TOUCH SCREEN CONTROLLER TOUCH SCREEN CONTROL
IDT™
TOUCH SCREEN CONTROLLER 6
MK712 REV M 122109
Recommended Circuit
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.
