Detecting Bleed Resistor or Modulating Capacitor Damage - KBA85698

Version: **

 

Question: Is there a way to detect if Rb or Cmod are damaged?

 

Answer:

You can use supervisory code to detect if Rb or Cmod are damaged by monitoring them for shorts or opens. The four possible failure modes are:

Cmod Shorted: If this is the case, the input of the CapSense module will be connected directly to ground. Whether the sensor is active or not, the raw counts and baseline will snap down to zero and the sensor will be turned OFF. The counts will be zero irrespective of the state of Rb (open/shorted/normal).

Cmod Open: If this is the case, the device will continue to operate with a higher level of noise. If your application uses a thin overlay and has a strong touch signal this may not cause a problem. However, if the touch signal is weak in your application, you could encounter false button activations.

Rb Shorted: If this is the case, the device will continue to operate with a higher level of noise. The raw counts will decrease and the baseline will follow because of its negative baseline reset. If your application uses a thin overlay and has a strong touch signal this may not cause a problem. However, if the touch signal is weak in your application, you could encounter false button activations.

Rb Open: If this is the case, the raw counts will saturate at 2(Resolution -1). For example, if the resolution is 12, raw counts will be 4095 even if Cmod if open.