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3.2" TFT LCD with Resistive Touch square-wave pickup
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3.2" TFT LCD with Resistive Touch square-wave pickup

by pearlste on Mon Feb 11, 2019 8:22 pm

Hi - I've spent a bit of time trying to debug a problem found by my students on the resistive touchscreen. On the Y sense lines (Y+ and Y-) we're seeing about a 1 volt p-p square wave (approximately 1.5 volt DC offset), at about 16 KHz. Strangely there are occasional dropouts of positive or negative pulses, which suggests that this isn't some unstable loop oscillating, but pickup of something like a delta-sigma or dithered clock oscillator.

This can't simply be capacitive coupling, because of the DC offset. It seems that there is some leakage getting into the sense lines. We tried a 10K ohm to ground, with the notion that the leakage may have very high impedance, but made absolutely no change -- suggesting that the source is relatively low impedance.

Is this normal? BTW - sense lines function normally when screen is depressed -- this only happens when the panel is untouched.

Thanks!

pearlste
 
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Joined: Sun May 01, 2016 5:28 pm

Re: 3.2" TFT LCD with Resistive Touch square-wave pickup

by vicpashmiscreen on Sat Feb 16, 2019 5:46 am

This looks very troublesome. Maybe you can learn how does resistive touch screen work?

vicpashmiscreen
 
Posts: 2
Joined: Sat Feb 16, 2019 4:11 am

Re: 3.2" TFT LCD with Resistive Touch square-wave pickup

by pearlste on Sat Feb 16, 2019 10:48 am

Hi - thank you for your response, but this does not address our problem. It seems that panel is malfunctioning, but the question is -- might there be a simple remedy (e.g. RC network) that will allow this to work? Has anyone seen this issue before?

Feels to me like a partial short-circuit. I initially thought it could be electromagnetic crosstalk, but there is a DC shift and, when the issue arises, the signal on the sense lines has relatively low source impedance. FWIW - the issue does not arise instantly upon power-up, it takes a few seconds.

The waveform suggests that there is some clock-ish signal (not a perfect square wave, but mostly square wave with some stuttering) used somewhere in the panel that is partially shorted to the sense lines.

Would Adafruit replace this panel with a working one?

Thanks!

Larry

pearlste
 
Posts: 2
Joined: Sun May 01, 2016 5:28 pm

Please be positive and constructive with your questions and comments.