Production Data
WM9715L
The principle of operation is illustrated below (Note: the illustrations assume that the top plate is used
for X and the bottom plate for Y measurements, although the reverse is also possible).
Figure 6 X Co-ordinate Measurement on 4-wire Touchpanel
For an X co-ordinate measurement, the X+ pin is internally switched to VDD and X- to GND. The X
plate becomes a potential divider, and the voltage at the point of contact is proportional to its X co-
ordinate. This voltage is measured on the Y+ and Y- pins, which carry no current (hence there is no
voltage drop in RY+ or RY-).
Due to the ratiometric measurement method, the supply voltage does not affect measurement
accuracy. The voltage references VREF+ and VREF- are taken from after the matrix switches, so
that any voltage drop in these switches has no effect on the ADC measurement.
Figure 7 Y Co-ordinate Measurement on 4-wire Touchpanel
Y co-ordinate measurements are similar to X co-ordinate measurements, with the X and Y plates
interchanged.
Figure 8 Pen Down Detection on 4-wire Touchpanel
Pen down detection uses a zero power comparator (effectively a CMOS logic gate) with an internal,
programmable pull-up resistor RPU that controls pen-down sensitivity. Increasing RPU makes the
touchpanel less sensitive to touch, while lowering RPU makes it more sensitive.
PD Rev 4.0 December 2007
39
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WOLFSON [ WOLFSON MICROELECTRONICS PLC ]
