2 nano 33 iot boards worked for a while and now unresponsive

[Tjpetz]

I've got 2 nano 33 iot boards.  Both boards worked fine for a short period of time.  For the first board I was uploading example sketches, some error occurred (which I forgot to write down  ) during upload.  After that the board would no longer be recognized on the usb.  Tried the double click reset to force the bootloader com port but with no result.  The power light is on but the upload LED will not light.  I figured it was just bad luck.  I ordered another board.

New day, new board, same problem.  The board worked fine to start, I was developing a simple sketch using BLE to communicate with a nano 33 BLE sense.  My sketch was way below the memory limits of the board.  I added a few lines of code, uploaded, there was some error in the upload, I believe related to size, but failing to learn the lesson from the previous day, again failed to save the details of the error message.

At this point neither iot board:

• responds to the double click to reset
• is visible on the USB bus (using various cables) on a MacBook Pro (Cataline 10.15.1), Raspberry Pi 3b, Raspberry Pi 2b.

I've followed the general debugging steps helpfully posted by ballscrewbob.  I've also checked that the CPU 3.3v power out on pin 2 is correct, which suggests the processor is not completely fried.

At this point I'm assuming the bootloader is invalid and needs to be reprogrammed.  I don't have another iot board so I'll be experimenting with using my BLE 33 Sense board and the AdaFruit DAP (https://learn.adafruit.com/programming-an-m0-using-an-arduino) to reprogram.  While the processors are different they're both ARM so I have some hope the code will work.  I'll post an update with results.

Does anyone have any ideas why the bootloader might be getting corrupted when uploading a sketch?

[jojobaa]

Can you please share what is the error message you are getting also check if the board is being detected in a different computer to see if the board behaves the same way. Can you tell us what happens when you press the double-tap of the reset button (the ember light and overall behavior of the board if it is connected to a computer)?

[Tjpetz]

Thanks for responding...

Unfortunately I forgot to save the error message when I lost connectivity.  Is there some log it might be saved in?

When I press the reset button (single, double, multiple times) only the green light is lit.  The amber light never even flickers.

As noted in the original post I've tried the boards on several different computers.  None of the computers even recognized that anything is connected to the usb port.  E.g. on a Raspberry PI an lsusb command shows nothing.  However, when I attach the known good Sense it's visible via the lsusb command.  (Which would suggest the cable is good.)

Re DAP: I did investigate using AdaFruit DAP to reload the bootloader.  However, while I can see what needs to change in the code, the clock signal for the SWD interface needs to be at a set frequency and it's managed in the code via a nop delay.  Unfortunately I'm currently without a scope or frequency counter so have no way to measure what the required delay is to achieve the required frequency.

I have a Segger programmer on the way and will attempt SWD once it arrives.

Thanks in advance for your assistance.

Tom