ItsyBitsy M4 Express w/ CP5.3.0: bignum(?) code prevents USB from functioning (mount and REPL access)

[derhexenmeister]

I posted about this here CircuitPython program prevents drive from mounting and mentioned on discord, but am not sure that the right developers have seen it.

Warning- before anyone tries this please make sure that you have a backup of your CIRCUITPY drive, and you know how to run a flash eraser specific to your CircuitPython board. e.g. https://learn.adafruit.com/adafruit-feather-m4-express-atsamd51/troubleshooting

I am using a Mac running 10.14.6 connected to an ItsyBitsy M4 Express running "Adafruit CircuitPython 5.3.0 on 2020-04-29; Adafruit ItsyBitsy M4 Express with samd51g19"

I found that it I put this simple program into code.py, then my ItsyBitsy will not mount as a drive. To recover I had to enter bootloader mode, erase the drive, and then reload CircuitPython. Maybe there's a better way to recover, or there's something unique about my setup which causes this. I'm suspicious that the bignum code is not giving time to other code involved in handling the USB subsystem. (If I had a hacked version of CircuitPython which didn't load code.py, then that could be loaded to enable a mount and cleanup.)

print("The waiting game")
i = 0
while True:
    print("Looping {}".format(i))
    i = i + 1
    a = 2**(65536*2)

[dhalbert]

Try doing a slow double-click. There is a 700ms interval when the on-board DotStar is yellow. If you click during that time, CircuitPython will restart in safe mode and will not run code.py. 700ms is too short for you to see it and react in time, but try a slow double-click and you may hit it.

Of course, we need to figure out what's wrong here in any case.

[derhexenmeister]

That's a great tip - I had no idea that there was a safe mode, but do see it on Troubleshooting now that you mention it. It did allow me to mount CIRCUITPY and recover.

[DavePutz]

Just as a point of info; I tested the sample script on a PyPortal (also an SAMD51; I don't have a ItsyBitsy M4 Express) running CP 5.3.0 on a Windows 10 machine; and saw no issues with accessing CIRCUITPY while the loop was running.

[derhexenmeister]

@DavePutz - can you try that, but reboot the board so that the code runs during startup?

[DavePutz]

Yes, that is how I ran my test. I see the 'looping' messages on the screen, and CIRCUITPY is mounted and accessible.

[derhexenmeister]

@DavePutz - ok thanks, just wanted to make sure. On 5/20 "geekguy" tried this on an ItsyBitsy M4, and said "I just tried your code on my ItsyBitsy M4 and it froze the board up. I can not connect to the REPL." Sounds like good news that the PyPortal doesn't have this issue, and it also uses an M4.

[dhalbert]

I would check whether it has to do with the operating system, since @DavePutz 's test was on Windows, but @derhexenmeister found the problem on MacOS.

[derhexenmeister]

I just tried on a Dell Precision 5530 running Windows 10 Pro (10.0.18363) and it complained "USB device not recognized". CIRCUITPY did not mount, and the virtual com port (USB) was not available.

[dhalbert]

@derhexenmeister I think it's unlikely this has anything to do with it, but what is the version of the bootlloader? (See INFO_UF2.TXT after double-clicking.)

[derhexenmeister]

I believe that I have the latest version (there was a recent recommendation on the forum):

UF2 Bootloader v3.9.0 SFHWRO
Model: ItsyBitsy M4 Express
Board-ID: SAMD51G19A-Itsy-v0

[dhalbert]

I found an easier way to break USB (at least CDC), from the REPL:

Adafruit CircuitPython 5.4.0-beta.1-36-g25d5f2cfc on 2020-06-19; Adafruit Metro M4 Express with samd51j19
>>> 10**40000

FATAL: read zero bytes from port
term_exitfunc: reset failed for dev UNKNOWN: Input/output error