WM9715L
Production Data
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 transmitted through the PENDOWN pin (see “Interrupt Control”
section). Additionally, its state is reflected in the PNDN bit (register 7Ah, bit 15) and the interrupt logic
block (register 54h, bit 13), where it can wake the WM9715L from sleep mode (see “Interrupt Control”
section).
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 WM9715L
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 400µA or 200µA using
the PIL control bit.
PRINCIPLE OF OPERATION - FIVE-WIRE TOUCHPANEL
Five-wire touchpanels are connected to the WM9715Las follows:
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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)
Figure 10 X Co-ordinate Measurement on 5-wire Touchpanel
PD Rev 4.0 December 2007
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WOLFSON [ WOLFSON MICROELECTRONICS PLC ]
