Raspberry Pico W MicroPython execution freezes a few seconds after disconnecting screen from UART

[cirosantilli]

I got my program running fine as explained at: https://stackoverflow.com/questions/66183596/how-can-you-make-a-micropython-program-on-a-raspberry-pi-pico-autorun/74078142#74078142

I'm installing a main.py that does:

import machine
import time

led = machine.Pin('LED', machine.Pin.OUT)
# For Rpi Pico (non-W) it was like this instead apparently.
# led = Pin(25, Pin.OUT)

i = 0
while (True):
    led.toggle()
    print(i)
    time.sleep(.5)
    i += 1

When I power the device on by plugging the USB to my laptop, it seems to run fine, with the LED blinking.

Then, if I connect from my laptop to the UART with:

screen /dev/ttyACM0 115200

I can see the numbers coming out on my host terminal correctly, and the LED still blinks, all as expected.

However, when I disconnect from screen with Ctrl-A K, after a few seconds, the LED stops blinking! It takes something around 15 seconds for it to stop, but it does so every time I tested.

If I reconnect the UART again with:

screen /dev/ttyACM0 115200

it starts blinking again.

Also also noticed that after I reconnect the UART and execution resumes, the count has increased much less than the actual time passed, so one possibility is that the Pico is going into some slow low power mode?

If I remove the print() from the program, I noticed that it does not freeze anymore after disconnecting the UART (which is of course shows no data in this case).

Micropython firmware: rp2-pico-w-20221014-unstable-v1.19.1-544-g89b320737.uf2, Ubuntu 22.04 host.

Some variants follow.

picocom /dev/ttyACM0 instead of screen and disconnect with Ctrl-A Ctrl-Q: still freezes like with screen.

The following C analog of the MicroPython hacked from:

• https://github.com/raspberrypi/pico-examples/blob/a7ad17156bf60842ee55c8f86cd39e9cd7427c1d/pico_w/blink
• https://github.com/raspberrypi/pico-examples/blob/a7ad17156bf60842ee55c8f86cd39e9cd7427c1d/hello_world/usb

did not show the same problem, tested on https://github.com/raspberrypi/pico-sdk/tree/2e6142b15b8a75c1227dd3edbe839193b2bf9041

#include <stdio.h>
#include "pico/stdlib.h"
#include "pico/cyw43_arch.h"

int main() {
    stdio_init_all();
    if (cyw43_arch_init()) {
        printf("WiFi init failed");
        return -1;
    }
    int i = 0;
    while (true) {
        printf("%i\n", i);
        cyw43_arch_gpio_put(CYW43_WL_GPIO_LED_PIN, i % 2);
        i++;
        sleep_ms(500);
    }
    return 0;
}

Also asked at:

• https://stackoverflow.com/questions/74081960/raspberry-pico-w-micropython-execution-freezes-a-few-seconds-after-disconnecting

[robert-hh]

The code does not print anything. Why do you see numbers coming out?

[cirosantilli]

Thanks for reproducing and for the picocom suggestion 👍

[cirosantilli]

picocom /dev/ttyACM0 did freeze for me :-( I also did an analog C example, and that didn't freeze.

[robert-hh]

That's bad. So there are dependencies on the OS USB stack as well. But may I confirm that it works, if you just power up the board and do not connect?
Edit: Did you close picocom with Ctrl-A Ctrl-X?

[robert-hh]

I also did an analog C example, and that didn't freeze.

The C-Driver may ignore the fact, that it cannot send data and just continue. See also the analysis here: #9634

[cirosantilli]

That's bad. So there are dependencies on the OS USB stack as well. But may I confirm that it works, if you just power up the board and do not connect? Edit: Did you close picocom with Ctrl-A Ctrl-X?

Without connecting, it does not block.

I was quitting with Ctrl-A Ctrl-Q. Without Ctral-A Ctrl-X it actually does not block, so it is a valid workaround as you suggested.

Ctrl-Q does a Skipping tty reset... while Ctrl-A doesn't.