MSP430FR2533: Countermeasures against water drops and metal effects on Captivate

Hi Ito,

Unfortunately, anything that touches the sensor that is capacitivley coupled to ground or ground potential, will be detected as a touch.  The methods to prevent this, as much as possible, is to have proper sensor PCB design and set the touch sensor thresholds properly.  This information is located in the Captivate Technology Design guide.

For example, a person touching a capacitive sensor changes the sensor's capacitance because the person is coupled to ground and is detected as a touch.

A small amount of water touching a capacitive sensor by itself may not change the sensor's capacitance, but if the amount of water is large enough to couple to any nearby ground potential on the PCB, like ground plane, the sensor's capacitance changes and it will be detected as a touch.

If a metal object at ground potential touches a sensor, the sensor's capacitance changes and will be detected as a touch.

You can see in these three cases, any conductive object can be troublesome, and from the perspective of Captivate, they all look a touch.

From your description, it sounds like a person is wearing some clothing with a zipper, correct?  And when that person positioned their body such that the zipper came in contact with the sensor (switch), a touch was detected, correct?  The zipper couples to the body and the body to ground so the zipper looks just like a finger.

To Captivate, it only measures a change in capacitance, and based on the amount of change and the touch threshold it will determine it to be a touch.  In this case there isn't any design method that would prevent this. However, if the difference between finger touch and zipper touch measured conversion counts is large enough, raising the conversion count touch threshold higher than the zipper touch can help.  When the zipper touches, the change is below the touch threshold and not detected.

For water specific designs, see TIDM-1021 application note.

Does this help, or do you need more specific information?

Hi Lehman,

I have understood how the mechanism to detect touch works.
However, I am unable to open the link for 'Captivate Technology Design guide'.
Could you please provide other links or documents?

'Metal zipper touched the sensor and it was detected as a touch.'

My customer collect these events seen in private lives.
If you know of any similar incidents, please let me know.

Best Regards,l

Ito