WM9712CLGEFLSLASHV datasheet(41/78 Pages) WOLFSON

41 / 78 page

Production Data

WM9712L

PD, Rev 4.6, November 2011

For an X co-ordinate measurement, the top left and bottom left corners of the touchpanel are

grounded internally to the WM9712, 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.

M = proportional to Y position

WM9712

WIPER (12)

TR (15)

ADC

REF+

REF-

TPGND

TPVDD

TPGND

TPVDD

BR (14)

BL (17)

TL (16)

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.

WM9712

WIPER (12)

TR (15)

TPGND

BR (14)

BL (17)

TL (16)

zero power

comparator

VDD

PEN

DOWN

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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