Problems with _pew on SAMD21

[deshipu]

I have been receiving some reports that the gamepad library and the _pew library don't work reliable on SAMD21 on 6.x. I suspect it's related to changes around the system tick. I didn't have time to investigate this properly yet, but I'm creating this issue to let everyone know that there is a potential problem.

[deshipu]

I'm looking into it now, and it seems like the timer is getting triggered in a very uneven way, which results in flickering.

https://youtu.be/bLqYwwU1rS0

[deshipu]

I connected one of the row pins to a logic analyzer. It should go high for one cycle, and they stay low for seven cycles, at regular intervals. Instead I got this:

[deshipu]

I'm stupid, this was a wrong column to check, I will check again with the proper measurement. Still, it looks to be more irregular than it should.

[deshipu]

I did this properly now. I made two analog captures from the R5 pin, which is a row pin. The first one is running 5.3:

The second one is with 6.0-beta-1:

You can tell the frequency is much lower and much more uneven.
I can send the captures on request, but they are about 100MB, a bit too big to attach here.

[deshipu]

Could this be caused by changes in interrupt priorities?

[tannewt]

What code is setting the pin? Is the pulse width how long the interrupt is occurring? We have to fake the once a ms timing so our math could be bad.

https://github.com/adafruit/circuitpython/blob/main/ports/atmel-samd/supervisor/port.c#L493

[deshipu]

It's an interrupt driven by the GCLK0 timer, the timer setup is here: https://github.com/adafruit/circuitpython/blob/main/ports/atmel-samd/common-hal/_pew/PewPew.c#L97-L115 and the interrupt code is here: https://github.com/adafruit/circuitpython/blob/main/ports/atmel-samd/common-hal/_pew/__init__.c#L36

The interrupt should be quite fast, all it does is changing values of 10 outputs GPIOs and checking the value of one GPIO.

[deshipu]

Note that the frequency being lower is not a problem, but the fact that it is inconsistent causes flickering.

[tannewt]

@deshipu Do you have anything else running concurrently (maybe gamepad). I think the RTC has a slow re-sync process on the SAMD21 which could be causing trouble.

[deshipu]

No, _pew has its own key scanning built in.

[tannewt]

Hrm, I'm not sure what the issue is then. It'll take more digging. Maybe @DavePutz will have a guess.