WM9712L datasheet(39/77 Pages) WOLFSON

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Production Data

WM9712L

PD Rev 4.5 August 2006

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).

Y+ (15)

X+

(14)

Y- (17)

X- (16)

X+

Y-

pen / finger

Y+

X-

M = (VREF+ - VREF-) RX- / (RX- + RX+)

= proportional to X position

ADC

REF+

REF-

TPVDD

TPGND

WM9712

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.

Y+ (15)

X+

(14)

Y- (17)

X- (16)

X+

Y-

pen / finger

Y+

X-

M = (VREF+ - VREF-) RY- / (RY- + RY+)

= proportional to Y position

ADC

REF+

REF-

TPVDD

TPGND

WM9712

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.

zero power

comparator

Y+ (15)

X+

(14)

Y- (17)

X- (16)

X+

Y-

pen / finger

Y+

X-

TPGND

WM9712

PEN

DOWN

TPVDD

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.

WM9712L Datasheet(PDF) 39 Page - Wolfson Microelectronics plc

WM9712L Datasheet(HTML) 39 Page - Wolfson Microelectronics plc

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