Pico W: sleep_us causes execution to hang

Firmware version: MicroPython v1.21.0 on 2023-10-06; Raspberry Pi Pico W with RP2040

On both of my Pico W devices, if I execute the below code on a freshly booted device, it causes execution to hang.

import time

time.sleep_us(5)
time.sleep_us(10)
time.sleep_us(5)
time.sleep_us(10)

time.sleep_ms(1000)
print('its been a second')#doesnt print

The expected output is

its been a second

But the output is

To reproduce this:

create a file called bug.py, insert the above code into it
upload the file to the pico w
do a hard reset
in the REPL terminal execute import bug
execution will hang and will not print the text

see also related issue [Resolved - MicroPython is broken] Timers plus sleep_ms and sleep_us oddness - Raspberry Pi Forums, where poster's code with sleep_ms() works but when replaced with sleep_us() fails.

(OP on Raspberry Pi forum is unable to use github, for some reason.)

I can reproduce the issue. It's apparent only with Pico W firmware and seems to be related to having the lwIP 64ms alarm running.

This code hangs on both Pico W and Pico (both v1.21.0):

# test_us - from https://forums.raspberrypi.com/viewtopic.php?t=312874
from machine import Timer
import time

count = 0


def tick(timer):
    global count
    count += 1


Timer().init(freq=1_000, mode=Timer.PERIODIC, callback=tick)

while True:
    count = 0
    # time.sleep_ms(1_000)
    time.sleep_us(1_000_000)
    n = count
    print("{} ticks per second".format(n))

Substituting the commented-out time.sleep_ms(1_000) runs as expected.

@scruss

Thanks @scruss for that additional test. I can confirm that it displays similar symptoms to the original issue: it seems to hang (no output is printed), but if you press a key at the REPL it does start printing. That's consistent with the RP2040 being blocked in a WFE with no interrupt to wake it, and pressing a key sends a USB packet and wakes it up.

Initial investigation by @projectgus and myself have us suspecting a bug in pico-sdk's alarm pool / pairing heap, when there are multiple timers some of them get dropped. (Note that each machine.Timer allocates a new alarm pool timer, each call to sleep_us and sleep_ms use the alarm pool, and also sitting idle at the REPL uses the alarm pool to wait efficiently.)

I reported this issue to pico-sdk: raspberrypi/pico-sdk#1552

This should now be fixed in MicroPython as #13329 was cherry-picked in 74fb42a.

(The root cause in the alarm pool of pico-sdk probably still exists, and MicroPython still uses the alarm pool however now it is only used for longer timeouts that don't seem to trigger this bug. See the linked commit for more details about removing alarm pool from MP entirely.)

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Pico W: sleep_us causes execution to hang #12873

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Pico W: sleep_us causes execution to hang #12873

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scruss commented on Nov 3, 2023

dpgeorge commented on Nov 14, 2023

scruss commented on Nov 14, 2023

dpgeorge commented on Nov 14, 2023

dpgeorge commented on Nov 20, 2023

projectgus commented on Jun 4, 2024

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