WM8945ECSSLASHR datasheet(89/169 Pages) WOLFSON

89 / 169 page

Production Data

WM8945

PD, May 2011, Rev 4.1

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.

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