WM9715L Datasheet, PDF (41/77 Pages) Wolfson Microelectronics plc – AC’97 Audio and Touchpanel CODEC

Production Data
WM9715L
For an X co-ordinate measurement, the top left and bottom left corners of the touchpanel are
grounded internally to the WM9715, while the top right and bottom right contacts are connected to
TPVDD. The bottom plate becomes a potential divider with a voltage gradient in the X direction. The
voltage at the point of contact is proportional to its X co-ordinate. This voltage is measured on the
TOP pin and converted to a digital value by the ADC.
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 11 Y Co-ordinate Measurement on 5-wire Touchpanel
Y co-ordinate measurements are similar to Y co-ordinate measurements. However, the voltage
gradient on the bottom plate is in the Y direction instead of the X direction. This is achieved by
grounding the bottom left and bottom right corners of the touchpanel, and connecting the top left and
top right contacts to TPVDD.
Figure 12 Pen Down Detection on 5-wire Touchpanel
Pen down detection works in a similar fashion for both 4-wire and 5-wire touchpanels (see Four-Wire
Touchpanel Operation). On a 5-wire touchpanel, all four contacts of the bottom plate are grounded,
and the top plate contact is connected to the internal programmable pull-up resistor, RPU.
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PD Rev 4.0 December 2007
41