Production Data
WM8945
TOUCH PANEL OPERATING PRINCIPLES
A typical Touch Panel comprises two conductive sheets, connected via a switch matrix to the Touch
Panel supply voltage. When the Touch Panel is touched (usually with a pen-style pointer), an
electrical contact is made between the two sheets. The switch matrix is used to determine the position
of the pen contact by establishing a potential divider on one of the conductive sheets in either the X-
axis or Y-axis, and measuring the voltage on the other sheet.
Separate configuration is required for each axis measurement; these are configured one after the
other to determine the X and Y co-ordinate positions. Note that, due to the ratiometric measurement
method, the supply voltage does not affect the measurement accuracy in either axis.
Pen Down detection and Z-axis (pressure) measurements are achieved in a similar fashion, by
configuring the switch matrix and taking the appropriate voltage measurement via an ADC.
The Touch Panel interface connects to the Left / Right sides of one sheet and to the Top / Bottom
sides of the other sheet. The illustrations show the top sheet for X-axis and the bottom sheet for Y-
axis, but the reverse is also possible.
X-axis measurement is performed by applying a potential difference between the Left and Right sides
of the touch panel. When contact is made between the two sheets, the voltage present on the Top or
Bottom connections is a measure of the X-axis position of the contact. The configuration is illustrated
in Figure 34.
Figure 34 X-axis Touch Panel Measurement
Y-axis measurement is performed by applying a potential difference between the Top and Bottom
sides of the touch panel. When contact is made between the two sheets, the voltage present on the
Left or Right connections is a measure of the Y-axis position of the contact. The configuration is
illustrated in Figure 35.
Figure 35 Y-axis Touch Panel Measurement
‘Pen Down’ detection uses a zero-power comparator with an internal, programmable pull-up resistor.
When the touch panel is not being touched, no current flows between the touch panel sheets, and the
comparator output is low. When the touch panel is touched, current flows through the panel and
through the pull-up resistor, and the comparator output goes high. The sensitivity of the circuit can be
adjusted using different values of pull-up resistor; a large pull-up resistance leads to the most
sensitive response. The configuration is illustrated in Figure 36.
PD, May 2011, Rev 4.1
89
w
WOLFSON [ WOLFSON MICROELECTRONICS PLC ]
