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False touches on liquid-tolerant CAPSENSE™ design – KBA235658

False touches on liquid-tolerant CAPSENSE™ design – KBA235658

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Community Translation:  耐液性 CAPSENSE™ 設計の誤り – KBA235658

Version: **

A guard sensor (guard line) is a button sensor that prevents false triggers. Users implement this function in the firmware; when it is triggered, other sensors will not report touch.
Follow Section “Layout guidelines for guard sensor” from PSoC™ 4 and PSoC™ 6 MCU CAPSENSE™ design guide.

Some of the expected behaviors are as follows:

  1. If the sensor is properly tuned and has the appropriate firmware, the presence of liquid drops on the guard sensor will trigger the guard sensor. When the guard sensor is triggered, finger touches on other sensors are not reported; therefore, the chances of false touch are eliminated.
  2. If liquid drops are present on the sensor traces or sensors that have a shield hatch layer, the shield hatch prevents the coupling between the liquid drops and ground (GND), and will not generate the extra capacitance. Consequently, a false touch is not reported.
  3. If a finger touch occurs on sensor traces, a false touch may be reported. Such cases will make the tuning process more complex.

For example:

Pattern 1: A, B, C
Pattern 2: D, E, F
Pattern 3: H, G



Figure 1   Example of finger touch and liquid drop cases

Note: The above example is only to explain the expected behaviors with liquid tolerance layout; it’s not the recommended layout.

See the guidelines for the CAPSENSE™ layout described in the design guide.