Custom CircuitPython

Published on 2017-09-29 in PewPew FeatherWing.

Having played a little bit with the prototype of the standalone version, I realized that I really need better button handling. The PewPew Lite did handling on the LED driver chip — the chip would scan the whole matrix every 25ms, and if a button came up as pressed in at least two consecutive scans, would set the corresponding bit in its registers. Reading the registers resets them, so you get a simple “what buttons has been pressed since the last time I checked” behavior, which works quite well for most games.

But the IS31FL3733 chip I’m using the PewPew can’t do that, so I had to connect the buttons to the GPIO pins of the microcontroller, and poll them myself from CircuitPython. But since the games have pretty slow frame rate (about 6FPS), and they only poll once per frame, it’s very easy to skip a fast key press. So I need a better way to read them.

An immediate answer that comes to mind is external interrupts. Problem is, CircuitPython doesn’t support them — they are an advanced feature, and since the callback code has its own limitations, they were skipped in the current implementation. But I can always add my own C code there, and implement a button-handling module. Since I need to compile my own CircuitPython firmware anyways (because I’m using more pins than the Trinket M0 has), this should be doable. So I started reading about the interrupts on the SAMD21 and looking at the CircuitPython code for the best place to fit it.

But then I thought about de-bouncing. Remember that “two consecutive scans” part in the PewPew Lite? That’s the de-bouncing. I should really do it too here, or the users will get too many key presses. To do that, I will need a timer. Timers are also an advanced feature that CircuitPython doesn’t yet implement. So I started reading about timers and looking at CircuitPython code to see which timers are already used and where I could plug in my own code…

And then I realized that if I have a timer that is firing relatively fast (say, every 16ms), I don’t need the interrupts — I can do the same thing that the chip on PewPew Lite did. And I also found a function that CircuitPython calls every millisecond, where I can plug my own code that would poll the keys once every 16 ticks. Then I just need a function to get the current button status, and I’m done.

Now all that is left is to just write the code…