wm9713 Wolfson Microelectronics plc, wm9713 Datasheet - Page 67

• Manufacturer Part Number: wm9713
• Description: Ac?97 Audio + Touchpanel Codec
• Manufacturer: Wolfson Microelectronics plc

Pre-Production
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Figure 27 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 R
touchpanel less sensitive to touch, while lowering R
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 R
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 28 Touch Pressure Measurement on 4-wire Touchpanel
Touch pressure can be determined indirectly by measuring the contact resistance R
top and bottom plates. R
measures R
on each plate. The two values are subtracted in the digital domain to obtain the potential difference,
which is proportional to R
To suit different types of touchpanels, the magnitude of I
the PIL control bit.
C
by sending a constant current I
C
C
.
decreases as the touch pressure on the panel increases. The WM9713L
PU
that controls pen-down sensitivity. Increasing R
P
through the touchpanel and measuring the potential
PU
makes it more sensitive.
P
can be set to either 400µA or 200µA using
PP Rev 3.2 September 2008
PU
and the panel, and
WM9713L
C
PU
between the
makes the
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