© 2009 Microchip Technology, Inc.

DS41393A-Page 27

8.4

8-Wire Sensor

An 8-wire resistive touch sensor consist of a Stable and Flex layer, electrically separated by spacer dots. The

layers are assembled perpendicular to each other. The touch position is determined by first applying a voltage

gradient across the flex layer and using the stable layer to measure the flex layer’s touch position voltage. The

second step is applying a voltage gradient across the stable layer and using the flex layer to measure the stable

layer’s touch position voltage.

The measured voltage at any position across a driven axis is predictable. A touch moving in the direction of the

driven axis will yield a linearly changing voltage. A touch moving perpendicular to the driven axis will yield a

relatively unchanging voltage.

The basic decoding of an 8-wire sensor is similar to a 4-wire. The difference is that an 8-wire sensor has four

additional interconnects used to reference sensor voltage back to the controller.

A touch system may experience voltage losses due resistance changes in the bus bars and connection between

the controller and sensor. The losses can vary with product use, temperature, and humidity.

In a 4-wire sensor, variations in the losses manifest themselves as error or drift in the reported touch location. An

8-wire touch sensor automatically adjusts for the changes, with the additional four reference lines. The reference

lines allow the controller to know what the voltage is, at the touch sensor bus bars.

Figure 15: 8-Wire Decoding