RP2: PIO interrupt API for Raspberry Pi Pico
In the Micropython port RP2 for the Raspberry Pi Pico it looks like there are some APIs missing.
Especially it seems it is not possible to enable a PIO FIFO interrupt. (Or I did miss something)
In the Raspberry Pico C API there is the function pio_set_irq0_source_enabled that lets you configure FIFO interrupts:
https://raspberrypi.github.io/pico-sdk-doxygen/group__hardware__pio.html
I would like to see support for that in Micropython. Without that API you have to poll the FIFO which is in my case bad for performance.
rp2: Add execctrl option for the PIO decorator.
Summary
The rp2 PIO assembler includes support for instructions like
mov(y, status)
which can be used to detect when the TX and RX FIFOs pass a configurable threshold, for example to trigger an interrupt handler to fill/empty them.
However, to actually use these instructions, the relevant threshold needs to be configured in the execctrl register, and at present, only the side_pindir bit can be set.
Expose an execctrl= keyword argument in the PIO decorator to do this, together with constants in rp2.PIO which can be used like
@rp2.asm_pio(execctrl = rp2.PIO.STATUS_TXLEVEL + n)
def program():
[...]
mov(y, status)
jmp(not_y, "skip")
irq(rel(0))
label("skip")
[...]
to generate an interrupt if the TX FIFO has fewer than n words left.
This also fixes a bug with the existing side_pindir option, which is meant to be boolean. Previously, if it was passed an integer other than 0 or 1, unintended high bits in the execctrl value would be set, not just the intended bit 29.
Testing
I've tested (and used) this functionality on a real rp2350 board.
Unfortunately there are no in-tree tests at all for the rp2 PIO assembler yet, so there isn't a natural place to add a simple test for this new option. I propose to write a full set of tests to cover as much as I can of that decorator, but will submit in a separate PR.
Trade-offs and Alternatives
It is possible to work around the lack of assembler execctrl support by patching the word of the program array that corresponds to the execctrl register, for example
program[4] = program[4] & ~127 | 1 # set status if txlevel < 1
but this assumes undocumented internal details of the rp2 pio module.