WM9712CLGEFLSLASHV datasheet(40/78 Pages) WOLFSON

40 / 78 page

WM9712L

Production Data

PD, Rev 4.6, November 2011

When the touchpanel is not being touched, no current flows in the circuit, and the PENDOWN signal

is low. When the panel is touched with a pen or finger, current flows through RPU and the panel, and

the comparator output goes high.

The PENDOWN signal can be read from bit 15 in register 7Ah (labeled PNDN). It can also be

observed on pin 46 (GPIO3 / PENDOWN), if the pin is not used for GPIO (GE3=0). Additionally,

PENDOWN is passed to the GPIO logic block (register 54h, bit 13), where it can generate CPU

interrupts, and / or to wake up the WM9712L from sleep mode (see “GPIO and Interrupt Control”

section).

Y+ (15)

X+

(14)

Y- (17)

X- (16)

X+

Y-

pen / finger

Y+

X-

TPGND

WM9712

TPVDD

ADC

Y- - VX+ = IP RC

= proportional to contact resistance

Figure 9 Touch Pressure Measurement on 4-wire Touchpanel

Touch pressure can be determined indirectly by measuring the contact resistance RC between the top

and bottom plates. RC decreases as the touch pressure on the panel increases. The WM9712L

measures RC by sending a constant current IP through the touchpanel and measuring the potential on

each plate. The two values are subtracted in the digital domain to obtain the potential difference,

which is proportional to RC.

To suit different types of touchpanels, the magnitude of IP can be set to either 400A or 200A using

the PIL control bit.

PRINCIPLE OF OPERATION - FIVE-WIRE TOUCHPANEL

Five-wire touchpanels are connected to the WM9712Las follows:



Top sheet contact = WIPER/AUX4 (pin 12)



Top left corner of bottom sheet = TL (pin 16)



Top right corner of bottom sheet = TR (pin 15)



Bottom left corner of bottom sheet = BL (pin 17)



Bottom right corner of bottom sheet = BR (pin 14)

M = proportional to X position

WM9712

WIPER (12)

TR (15)

ADC

REF+

REF-

TPGND

TPVDD

BR (14)

BL (17)

TL (16)

Figure 10 X Co-ordinate Measurement on 5-wire Touchpanel

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