wm9712l Wolfson Microelectronics plc, wm9712l Datasheet - Page 39

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

Production Data
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The principle of operation is illustrated below (Note: the illustrations assume that the top plate is used
for X and the bottom plate for Y measurements, although the reverse is also possible).
Figure 6 X Co-ordinate Measurement on 4-wire Touchpanel
For an X co-ordinate measurement, the X+ pin is internally switched to VDD and X- to GND. The X
plate becomes a potential divider, and the voltage at the point of contact is proportional to its X co-
ordinate. This voltage is measured on the Y+ and Y- pins, which carry no current (hence there is no
voltage drop in R
Due to the ratiometric measurement method, the supply voltage does not affect measurement
accuracy. The voltage references VREF+ and VREF- are taken from after the matrix switches, so
that any voltage drop in these switches has no effect on the ADC measurement.
Figure 7 Y Co-ordinate Measurement on 4-wire Touchpanel
Y co-ordinate measurements are similar to X co-ordinate measurements, with the X and Y plates
interchanged.
Figure 8 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
R
R
R
R
R
R
X-
pen / finger
X-
Y-
X-
pen / finger
pen / finger
Y+
Y-
Y-
or R
R
R
R
R
R
R
Y+
X+
Y-
Y+
X+
Y+
X+
).
Y+ (15)
Y- (17)
X- (16)
Y+ (15)
Y+ (15)
Y- (17)
X- (16)
Y- (17)
X- (16)
(14)
(14)
X+
(14)
X+
X+
PU
that controls pen-down sensitivity. Increasing R
VM
VM
V
V
M
M
V
V
V
= (V
V
= proportional to X position
REF+
REF+
= (V
REF-
REF-
ADC
= proportional to Y position
ADC
WM9712
REF+
WM9712
REF+
comparator
zero power
TPVDD
- V
- V
TPGND
REF-
REF-
R
PU
) R
PU
) R
WM9712
X-
makes it more sensitive.
Y-
/ (R
/ (R
PEN
DOWN
TPVDD
TPGND
TPVDD
TPGND
X-
Y-
+ R
+ R
X+
Y+
)
)
PD Rev 4.5 August 2006
WM9712L
PU
makes the
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