Raise exception from timer interrupt routine Micropython RP2040

[John1900]

Is it possible to raise an exception from a timer interrupt?

Having tried variations on this code there is likely something I just don't get! Thanks

    timer = Timer(-1)
    timer.init(period=1000, mode=Timer.ONE_SHOT, callback=lambda t:raise Exception("Error"))  

[jimmo]

Just to clarify, what do you expect to happen with the raised exception? Interrupt the main thread?

That's what will happen... an exception will be scheduled to be raised when the VM resumes operating after the IRQ finishes. (Edit: see later post. Soft interrupts and scheduled tasks have their own catch-all exception handlers)

[John1900]

Yes, raise the exception. The timeout will occur because a series of operations using servo motors will not have completed in a reasonable time (indicated by a switch state). The timer should be easier than checking individually the time taken for each operation. Without checking the system will just hang when a server jams or something else goes wrong.

[robert-hh]

Just for interest I tried that. For some reason I could not put the raise into the lambda function. But in separate small function it worked.

from machine import Timer
import time

def r(t):
    raise Exception("Error")

timer = Timer(-1, period=3000, mode=Timer.ONE_SHOT, callback=r)

while 1:
    try:
        print("Wait for the exception")
        time.sleep(1)
    except Exception as e:
        print("Exception", e)

[John1900]

Thanks but that shows another problem. The "except" code never runs and no way to exit the while loop:

a = True
while a:
    try:
        print("Wait for the exception")
        time.sleep(1)
    except Exception as e:
        print("Exception here", e)
        a = False
        break

[John1900]

from machine import Timer
import time
def r(t):
     raise Exception("Error")

timer = Timer(-1, period=3000, mode=Timer.ONE_SHOT, callback=r)

a = True
while a:
    try:
        print("Wait for the exception")
        time.sleep(1)
    except Exception as e:
        print("Exception here", e)
        a = False
        break

Sorry about the formatting! (Edit by @jimmo to fix formatting)

[robert-hh]

About formatting:
you have to enclose code into three backticks. An easy way of doing that is to highlight the pasted code and push Ctrl-E.

But interestingly, this does not work either. The break does not.

[jimmo]

@John1900 Please see https://github.com/orgs/micropython/discussions/9111 for more info about code formatting.

[robert-hh]

Here, the except code runs, as the print statement is executed. But the loop does not stop, since a is not modified. Putting all into a class does not help either. Hmm...

[John1900]

I also thought the except code ran so try changing to print("Exception here", e) I only noticed cause it prints with a colon which was not in your print. Yes, Hmm...

[jimmo]

Sorry! I said the wrong thing in my first reply.

The timer runs its callback via the scheduler (it's a soft interrupt in the context of https://docs.micropython.org/en/latest/reference/isr_rules.html ). Scheduled tasks have a built-in catch-all exception handler which just prints the exception out directly.

If you want to signal from an interrupt handler, then you should do so with a global variable.

from machine import Timer
import time

timeout = False

def r(t):
    global timeout
    timeout = True

timer = Timer(-1, period=3000, mode=Timer.ONE_SHOT, callback=r)

while True:
    if timeout:
        print("Timeout")
        break
    # otherwise keep working

In general though it's likely easier to implement timeouts by measuring time taken..

import time

t_start = time.ticks_ms()
while True:
    if time.ticks_diff(time.ticks_ms(), t_start) > 3000:
        print("Timeout")
        break
    # otherwise keep working

[John1900]

Thanks, works great, just what I needed.

I agree ticks_diff is much easier but I have several places with; "while something is happening/sleep". Its much easier to have one timer for a group of operations which I can cancel on normal completion - rather than add time code to each operation.