wm9712l Wolfson Microelectronics plc, wm9712l Datasheet - Page 40

• Manufacturer Part Number: wm9712l
• Description: Ac?97 Audio And Touchpanel Codec
• Manufacturer: Wolfson Microelectronics plc

WM9712L
w
PRINCIPLE OF OPERATION - FIVE-WIRE TOUCHPANEL
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 WM9712L from sleep mode (see “GPIO and Interrupt Control”
section).
Figure 9 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.
Five-wire touchpanels are connected to the WM9712Las follows:
•
•
•
•
•
Figure 10 X Co-ordinate Measurement on 5-wire Touchpanel
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)
C
by sending a constant current I
R
R
X-
pen / finger
Y-
C
C
R
R
.
WIPER (12)
Y+
X+
decreases as the touch pressure on the panel increases. The WM9712L
BR (14)
TR (15)
BL (17)
TL (16)
Y+ (15)
Y- (17)
X- (16)
(14)
X+
VM
V
I
M
P
= proportional to X position
V
V
REF+
REF-
ADC
TPVDD
TPVDD
TPGND
TPGND
P
= proportional to contact resistance
through the touchpanel and measuring the potential
TPGND
ADC
V
WM9712
Y-
TPVDD
- V
P
X+
can be set to either 400µA or 200µA using
= I
P
WM9712
R
C
PD Rev 4.5 August 2006
PU
and the panel, and
Production Data
C
between the
40