WM9713 datasheet(61/111 Pages) WOLFSON

61 / 111 page

Preliminary Technical Data

WM9713L

PTD, March 2004, Rev 2.2

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.

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 WM9713L from sleep mode (see “GPIO and Interrupt Control”

section).

Figure 20 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 WM9713L

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

µA or 200µA using

the PIL control bit.

PRINCIPLE OF OPERATION - FIVE-WIRE TOUCHPANEL

Five-wire touchpanels are connected to the WM9713L as 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)

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