XIAO nRF52840 stops on internal flash writes after wrong firmware flash — UF2 works, factory erase hangs

[hryamzik]

Description

I have three identical Seeed Studio XIAO nRF52840 + Wio-SX1262 devices.

Two are working correctly with MeshCore. On the third device I
accidentally selected/flashed the wrong firmware once (meshpocket one). Since then I can
successfully flash the correct MeshCore firmware, but the application
does not work over either USB or Bluetooth.

I'm trying to determine whether this is recoverable without an SWD
debugger, or whether the internal flash may have been damaged or left in
a bad state.

What I have tested

Bootloader

The UF2 bootloader still works.

INFO_UF2.TXT on both a working device and the faulty device is
identical:

UF2 Bootloader 0.6.1 lib/nrfx (v2.0.0) lib/tinyusb (0.10.1-293-gaf8e5a90) lib/uf2 (remotes/origin/configupdate-9-gadbb8c7)
Model: Seeed XIAO nRF52840
Board-ID: Seeed_XIAO_nRF52840_Sense
SoftDevice: S140 version 7.3.0
Date: Nov 12 2021

MeshCore

I tried both:

• Companion Bluetooth
• Companion USB

using the MeshCore web flasher.

The web flasher reports a successful flash, but the faulty XIAO does not
appear over Bluetooth with Companion Bluetooth.

I also tried the web flasher's Erase Flash function.

I swapped the Wio-SX1262 peripheral boards between a working and faulty
XIAO. The working XIAO continued to work, so the problem appears to
follow the XIAO rather than the Wio-SX1262 board.

I also tried:

Xiao_nrf52_QSPIFlash_Format-v1.2.uf2

without success.

Meshtastic test

To eliminate MeshCore and the Wio-SX1262 as variables, I disconnected
the peripheral board and flashed Meshtastic 2.7.26
(seeed_xiao_nrf52840_kit).

Meshtastic boots and USB serial output works.

It can read the existing filesystem successfully, including:

prefs/
  nodes.proto
  config.proto
  channels.proto
  device.proto
adafruit/
bond_prph/
bond_cntr/

However, during startup it reaches:

DEBUG | Save to disk 1

and the device then disappears/reboots.

It also does not become available over Bluetooth.

Factory erase A/B test

I then used the Meshtastic factory erase image matching the SoftDevice:

Meshtastic_nRF52_factory_erase_v3_S140_7.3.0.uf2

On the faulty XIAO, serial output is:

Meshtastic nRF52 Factory Restore v3 27 June 2024 - by Mark Birss

Formating ...

It remains there indefinitely. I waited more than 30 minutes.

For comparison, I ran the exact same UF2 on one of my identical
working XIAOs.

The working device immediately completes:

Meshtastic nRF52 Factory Restore v3 27 June 2024 - by Mark Birss

Formating ... Done, rebooting device into DFU mode

The subsequent serial error is expected because the device deliberately
disconnects and reboots into DFU mode.

So the factory erase application itself runs on both XIAOs, but
formatting completes on the good XIAO and hangs on the faulty one.

Current suspicion

It looks like the faulty XIAO can:

• run the UF2 bootloader
• accept firmware
• boot applications
• use USB serial
• read its internal filesystem

but has a problem when an application tries to write or format its
internal storage.

Since the identical Meshtastic factory-erase UF2 completes on the good
XIAO but hangs on the faulty one, this does not appear to be specific to
MeshCore or the Wio-SX1262.

Questions

1. Is there a known way that flashing incorrect firmware can leave an
   nRF52840/XIAO in this state?
2. Is there anything else I can erase/reset from UF2 without an SWD
   debugger?
3. Could UICR, flash protection, SoftDevice state, or another nRF52
   configuration cause InternalFS.format() to hang like this?
4. Is SWD recover / mass erase the appropriate next step?
5. Is there anything useful I can collect from the device before
   attempting SWD recovery?

I currently have other XIAO nRF52840 boards and jumper wires, but no
dedicated SWD/CMSIS-DAP/J-Link programmer.

[liamcottle]

You probably overwrote some of the soft device when flashing a firmware for a different device. I did this earlier on a Wio Tracker L1 Pro.

Flashing the .uf2 bootloader, erase files or firmware didn't help.
You'll need to flash the .zip version of the bootloader which includes soft device.

Try flashing the OTAFIX bootloader:
https://github.com/oltaco/Adafruit_nRF52_Bootloader_OTAFIX/releases
https://github.com/oltaco/Adafruit_nRF52_Bootloader_OTAFIX/releases/download/0.9.2-OTAFIX2.3-BP1.4/xiao_nrf52840_ble_bootloader-0.9.2-OTAFIX2.3-BP1.4_s140_7.3.0.zip

Not sure if you have the sense or non-sense xiao variant. but check the release link above if you need the sense version instead.

You can use nrfutil with a command similar to the following to flash the bootloader/softdevice zip.

adafruit-nrfutil dfu serial -p /dev/cu.usbmodem14301 -pkg xiao_nrf52840_ble_bootloader-0.9.2-OTAFIX2.3-BP1.4_s140_7.3.0.zip -sb -t 1200

Once done, you should be able to flash normal firmware again.