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cap touch works on bare board, does not work on board with springs and plastic overlay

Question asked by John Davis on Aug 2, 2017
Latest reply on Aug 7, 2017 by John Davis

Hello

 

We did a third revision of our board and it came in this week.  This board rev works great with cap touch for two touchkeys and linear sensors.  Without scaling all 5 linear sensors work fine.  However, we place the board in a fixture so that it uses springs and a plastic overlay it has problems.  Both of the two touchkeys no longer work.  If you touch the backside of the board where the spring tabs are exposed it still works.  However if you touch the plastic  panel with the springs in contact with the metal electrode pads it stops working.  

 

As for the 5 linear/rotary touch sensors Four of the five sensors work with the overlay.  One of them does not when pressed on the plastic to springs to the electrode pads.

 

I've tried to adjust various settings in the tsl_conf.h file.  None seem to have any impact.

 

I've tried to use a oscilloscope on the pads but as soon as I touch a pad with the 3.2pf 10MOhm probe it causes the pad to enter the detect state.   If I add a copper or aluminum disk on top, I can probe with the scope and it does not detect.  In this test the waveform has nice share square edges similar to what is shown in the ideal waveform Figure 10 of the DM000088471pdf with title AN4316 Tuning a STMTouch-based application.

 

Here is a picture:

The white signal is the sensor which is detected according to LINEAR_DETECT with pressing the bare pad and the overlay pad.  This picture is taken with the LINEAR_DETECT not activated and the probe resting on a metal disk on top of the overlay.

 

The yellow signal is the sensor which is detected according to LINEAR_DETECT with pressing the bare bad but not when pressed on the overlay pad.  This picture is taken with the LINEAR_DETECT not activated and the probe resting on a metal disk on top of the overlay.

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