LPC804 evk touch: drift and sudden unexpected jumps

[Egon_Freeman]

I'm using the demo touch shield w/ 5 pads and 5 LEDs.

In order to detect a touch event, I continuously read the touchpad (based on the "capt_basic_continuous" demo from the LPCExpresso SDK). I average over 5 readouts for "current" values, and over 300 for "baseline" values. I then subtract "current" from "baseline" to get relative potential, usually within 0/-1 (idle) to +30 (firm touch).

Once I detect at least one pad going above a predefined threshold (around +10), the one with the highest relative value is registered as being touched. I then stop correcting the baseline until all pads fall below the threshold value, which is my criteria for "back to idle" at - which point I deregister the touch and resume updating baseline values.

For instance, at idle I may see raw values of 644/643, 619/618, 636/636, 651/650, 590/590 - which correspond to -1, -1, 0, -1, 0. On touch, these may rise to e.g. +24, +7, +11, +6, +2 - indicating that pad 0 is being touched. Once finger is removed, these go back to about the same average values (idle).

This works well for short-time runs. It accounts for the slow drift I see in the baseline over time.

However, over longer runs (hours) something happens to the board that I don't understand - if I touch a pad after not touching it for a while (hours), suddenly the readouts change drastically and alter the baseline long-term without giving me a chance to adjust, leading to a "stuck" touchpad.

For example, I'm seeing a readout of say 643 current / 643 baseline on pad 0. The normal behaviour when I touch pad 0 is that it falls to about 620 current, which leads to a diff value of +23 on detection on that pad.

But after a few hours and no touch events, this behaviour changes and when I touch e.g. pad 0, suddenly I see values of -6, +37, +29, +30, +24. The software still manages to catch that I touched pad 0... but once I release it, I then have: -39, +29, +19, +20, +15 - 4/5 of which are now above the predefined threshold of +10.

This indicates that the baseline has shifted quite dramatically when I touched the touchpad, and since 4/5 pads now register as "touching" - I no longer have the chance to readjust. Thus, the keypad is stuck in "being touched" mode, as it will no longer register "idle" (since the new idle is a huge distance away).

What am I doing wrong? What hardware behaviour/phenomenon am I not accounting for? Am I reading too fast? Too slow? Too few samples?

I'm currently using a clock divider of 7 (should be about 2.14 MHz), with a poll count of 9.

[Egon_Freeman]

This is what I'm currently doing:

The device will need to work under a sticker, so that's what I'm simulating. I also have I2C attached.

I also replaced the pins with longer ones. Would that make a significant difference?

You will also notice that I switched one of the lines (YH/ACMP_14). I reassigned them in the mux, of course. I needed P0_7 for a different purpose here (the extra red cable) and it was the most immediately available to me. The capacitor was replaced with an identical value on the ACMP_14 line (where the DNP resistor would have gone).

[carlos_o]

Hi @Egon_Freeman 

The performance of the touch pad is mainly determined by the PCB design and the power supply source. 

So, could you confirm that the power supply is always working at the same value? Any variations on it could affect.

[Egon_Freeman]

I'm currently working off of my PC's USB supply, so the supply is as stable as the USB line.

But would that explain why the floor value drops after I touch it (within a second), and begins slowly climbing again?

I also don't see spontaneous drops. It's always climbing. Shouldn't I expect variable floor (ups and downs) that would follow the voltage changes instead?

[carlos_o]

Hi @Egon_Freeman 

Could you confirm that it is possible replicate the behavior you are experiencing with the example without modification? 

[carlos_o]

Apologies the late reply,

Could you please confirm that the temperature of the device those not increase between your first reading and the last? 

[Egon_Freeman]

This can happen over several hours, but also within an hour even. As far as I can tell, the chip is not warming up at all. The ambient temperature remains the same, give or take normal daily variations. I have it on my table, I'm prototyping. Any increase/decrease would be impossible to spot without measuring equipment.

Also, when I touch the touchpads - they seem to return to their original readings, so it looks like it's drifting until I touch it, at which point it stabilises back to normal. If I were to guess, it kinda looks like it's accumulating charge.

[Egon_Freeman]

Im using SDK version 25.06.