WM8311 datasheet(135/302 Pages) WOLFSON

135 / 302 page

Pre-Production

WM8311

PP, May 2012, Rev 3.1

135

Touch pressure can only be determined indirectly, using the results of two separate measurements. A

constant current is applied through the plates, and the voltage on each plate is measured. The

difference between the two voltages is proportional to the resistance between the plates, which is a

measure of the pressure being applied to the panel.

The configuration is illustrated in Figure 31. In this example, a constant current flows from the Top

+) connection to the Left (X-) connection. The Right (X+) and Bottom (Y-) points are measured in

turn, and the difference, VX - VY is equal to IP x RC, where IP is the current applied and RC is the

resistance between the plates. The smaller the measured resistance, the greater the pressure being

applied.

Figure 31 Z-axis (Pressure) Measurement on 4-wire Touch Panel

19.6.2 5-WIRE TOUCH PANEL OPERATION

In 5-wire operation, the Touch Panel interface connects to the four separate corners of one sheet and

to a single point on the other sheet. The illustrations show the top sheet for wiper contact and the

bottom sheet for corner contacts, but the reverse is also possible.

The principles of operation are the same for 5-wire and 4-wire modes, but different configuration of

the switching matrix within the Touch Panel controller is required in order to implement the equivalent

functionality.

X-axis measurement is performed by applying a potential difference between the Left and Right sides

of the touch panel. This requires the two Left corners to be connected to one potential and the two

Right corners connected to another. When contact is made between the two sheets, the voltage

present on the Wiper connection is a measure of the X-axis position of the contact. The configuration

is illustrated in Figure 32.

Figure 32 X-axis Measurement on 5-wire Touch Panel

Y-axis measurement is performed by applying a potential difference between the Top and Bottom

sides of the touch panel. This requires the two Top corners to be connected to one potential and the

two Bottom corners connected to another. When contact is made between the two sheets, the voltage

present on the Wiper connection is a measure of the Y-axis position of the contact. The configuration

is illustrated in Figure 33.

WM8311 Datasheet(PDF) 135 Page - Wolfson Microelectronics plc