MK712
TOUCH SCREEN CONTROLLER TOUCH SCREEN CONTROL
IDT™
TOUCH SCREEN CONTROLLER 7
MK712 REV M 122109
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.
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.
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.
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.
