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AN2869

Surface electrode design

Doc ID 15298 Rev 6

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Surface electrode design

3.1

Touchkey sensor

A touchkey can be either touched or untouched by the user. So the information that is

managed by the microcontroller is a binary one (e.g. ‘0’ for untouched and ‘1’ for touched).

The sensor can be any shape, however it is recommended to use round or oval as these

shapes are the simplest. The libraries and hardware cells automatically compensate for

capacitance differences, but the acquisition time and processing parameters can be

optimized if the electrodes have similar capacitance. For this reason, it is recommended to

use the same shape for all electrodes. The touchkeys can be customized by the drawing on

the panel.

When designing touchkey sensors, two parameters must be taken into account:

The object size to be detected

The panel thickness

Regarding object size (see Figure 10), it is recommended to design a sensor in the same

range as the object to be detected. In most cases, it is a finger.

Figure 10.

Sensor size

AN2869 데이터시트(PDF) 17 Page - STMicroelectronics