One-shot timer not calling callback

[johnp789]

I'm working on a method which gets called periodically by a timer. The function starts a check, and inside of the check, I'm trying to use a second timer to detect a timeout condition. There are better ways to implement this, but with this code, the timeout callback never gets called, and the script fails to exit the loop of time.sleep_ms calls. I'd like to understand why self._timeout_timer never calls self._timeout_handler here.

I'm running "MicroPython v1.22.1 on 2024-01-05" (from official downloads page) on a Raspberry Pi Pico with RP2040.

from machine import Timer
import micropython
import time


class Mini:
    def __init__(self):
        self._check_is_running = False
        self._timeoutFlag = False
        self._check_timer = Timer(
            mode=Timer.PERIODIC, period=5000, callback=self._check_callback
        )
        self._timeout_timer = Timer()

    def _check_callback(self, timer):
        if self._check_is_running:
            print(f"{time.ticks_ms()}: check overrun !!!!")
            return
        self._check_is_running = True
        micropython.schedule(self._do_check, None)

    def _timeout_handler(self, timer):
        # This never appears.
        print(f"{time.ticks_ms()}: timeout !!!!")
        self._timeoutFlag = False

    def _do_check(self, _):
        print(f"{time.ticks_ms()}: checking")
        self._timeoutFlag = True
        self._timeout_timer.init(
            mode=Timer.ONE_SHOT, period=2000, callback=self._timeout_handler
        )
        # Do something that will time out in 2000ms.
        while self._timeoutFlag:
            # This prints forever.
            print(f"{time.ticks_ms()}: sleeping")
            time.sleep_ms(500)
        timeout_timer.deinit()
        self._check_is_running = False

m = Mini()

[projectgus]

Hi @johnp789,

The issue here is that on RP2040 the timers are so-called "soft timers", which means the timer callbacks run via the same mechanism as micropython.schedule(). While _do_check() is running in a scheduled callback, the _timeout_handler callback will be queued but it won't be executed.

The best way around this might depend on the details of the work you're doing in _do_check(). Depending, you could perhaps look at running the _do_check function on the other CPU using the _thread module. Or, depending on what action the timeout needs to take, you could possibly use machine.WDT - this would reset the system on timeout though, which maybe isn't what you need.

[johnp789]

Now I get it, _do_check() is itself a scheduled callback, which won't be interrupted by _timeout_handler. Thanks!

I'll probably go to an asyncio approach, but another alternative might be to replace the _timeout_timer stuff with checks of ticks_diff at the top of the loop.

[projectgus]

Both of those sound like good approaches!