WM9713L datasheet(66/133 Pages) CIRRUS

66 / 133 page

WM9713L

Rev 4.0

PRINCIPLE OF OPERATION – FOUR-WIRE TOUCHPANEL

Four-wire touchpanels are connected to the WM9713L as follows:



Right side contact = X+ (pin 14)



Left side contact = X- (pin 16)



Top side contact = Y+ (pin 15)



Bottom side contact = Y- (pin 17)

The principle of operation is illustrated in Figure 25 through Figure 28. (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

AUX

ADC

REF+

REF-

TPVDD

TPGND

WM9713L

Figure 25 X Co-ordinate Measurement on 4-wire Touchpanel

For an X co-ordinate measurement, the X+ pin is internally switched to TPVDD and X- to TPGND. 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

AUX

ADC

REF+

REF-

TPVDD

TPGND

WM9713L

Figure 26 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.

WM9713L Datasheet(PDF) 66 Page - Cirrus Logic

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