Nano 33 BLE: SoftDevice Relocation Timeout After Re-Flashing

[HeinerTholen]

Hello there,

I am encountering a strange issue where the MicroPython firmware on my Arduino Nano 33 BLE fails to start up after a re-flash (using both the MicroPython installer and bossac).

MicroPython was installed and worked fine. Problems started when trying to re-flash microPython. Normal arduino scripts can be flashed and run fine.

I tried 3 different ways:
a) The MicroPython Installer always times out with the message: "Waiting for the device to relocate the SoftDevice."
b) After flashing "Nano33_updateBLandSoftDevice", relocating the SoftDevice and flashing with bossac the device doesn't appear for a usb connection.
c) After flashing with the openMV IDE, the device also doesn't appear for a usb connection.

System & Setup Details:

• Boards (both nRF52840, tried all steps with both boards):
  • Arduino Nano 33 BLE (older, bought in '22 or '23, I thought it might be bricked, so I got an ...)
  • Arduino Nano 33 BLE rev2 (one week old)
• Firmwares tried:
  • Auto-downloaded from MicroPython installer, I suppose 1.26.1
  • Manually downloaded 1.26.1, 1.26.0 and 1.25.0 from here
  • openMV 4.7.0 downloaded from github release page.
• Flashing Tools:
  • MicroPython Installer App
  • arduino IDE + bossac
  • openMV IDE

Operating System:

• maxOS 26.1 for
  • "MicroPython Installer 1.4.0",
  • "Arduino IDE 2.3.6" with Arduino Mbed OS Nano Boards 4.4.1 installed
  • and also "Arduino Lab for MicroPython" for the REPL interface.
• Raspberry Pi OS Lite (headless) 6.12 for
  • bossac, used latest Arduino BOSSA with nRF support downloaded here and
  • mpremote for REPL access.

More info and possible clues:

• I find it so odd that I had MicroPython properly running on both boards, but can't re-flash. It was 1.26.1 on the rev1 and openMV 4.7.0 on the rev2. On both boards all the sensors worked, ADCs, Timers, everything worked. (I wanted to go from 1.26.1 to openMV for ulab and other libs, but then found that disable_irq/enable_irq weren't working on openMV (fixed on main now), resulting in problems with an HX711 driver, so I wanted to go back to 1.26.1 on the rev2.)
• Full Factory Reset done: Uploaded the basic "Blink" sketch using the Arduino IDE to perform a complete memory erase and factory firmware restoration.
• Full File System Wipe Attempt: Flashed the MicroPython .bin to offset 0x0 instead of 0x16000 to attempt to erase any file system interference. This did not resolve the subsequent connection failure after a correct SoftDevice installation and re-flash.
• SoftDevice/Bootloader Update: Successfully ran the Nano33_updateBLandSoftDevice example sketch. Replied "Y" so the SoftDevice is installed. Example output:

Would you like to install the Soft Device (required for OpenMV)? Y/N
Storing SoftDevice length info at 0xFF000...
Flashing SoftDevice...
Flash Address = A0000
Flashed 2%
Flash Address = A1000
Flashed 5%
[... removed for brevity ...]
Flash Address = C3000
Flashed 97%
Flash Address = C4000
Flashed 100%


SoftDevice update complete! The board is restarting...
Would you like to install the Soft Device (required for OpenMV)? Y/N

• bossac command taken from microPython download page. Always had to put the board into bootloader mode with double-tapping reset. Example command and the output:

./bin/bossac -e -w --offset=0x16000 --port=ttyACM0 -i -d -U -R ARDUINO_NANO_33_BLE_SENSE-20250911-v1.26.1.bin
Set binary mode
version()=Arduino Bootloader (SAM-BA extended) 2.0 [Arduino:IKXYZ]
Connected at 921600 baud
identifyChip()=nRF52840-QIAA
write(addr=0,size=0x34)
writeWord(addr=0x30,value=0x400)
writeWord(addr=0x20,value=0)
version()=Arduino Bootloader (SAM-BA extended) 2.0 [Arduino:IKXYZ]
Device       : nRF52840-QIAA
Version      : Arduino Bootloader (SAM-BA extended) 2.0 [Arduino:IKXYZ]
Address      : 0x0
Pages        : 256
Page Size    : 4096 bytes
Total Size   : 1024KB
Planes       : 1
Lock Regions : 0
Locked       : none
Security     : false
Erase flash
chipErase(addr=0x16000)

Done in 0.002 seconds
Write 279220 bytes to flash (69 pages)
[                              ] 0% (0/69 pages)write(addr=0x34,size=0x1000)
writeBuffer(scr_addr=0x34, dst_addr=0x16000, size=0x1000)
write(addr=0x34,size=0x1000)
writeBuffer(scr_addr=0x34, dst_addr=0x17000, size=0x1000)
write(addr=0x34,size=0x1000)
writeBuffer(scr_addr=0x34, dst_addr=0x18000, size=0x1000)
[=                             ] 4% (3/69 pages)write(addr=0x34,size=0x1000)
writeBuffer(scr_addr=0x34, dst_addr=0x19000, size=0x1000)
[=                             ] 5% (4/69 pages)write(addr=0x34,size=0x1000)
[... removed for brevity ...]
[============================= ] 97% (67/69 pages)write(addr=0x34,size=0x1000)
writeBuffer(scr_addr=0x34, dst_addr=0x59000, size=0x1000)
[============================= ] 98% (68/69 pages)write(addr=0x34,size=0x1000)
writeBuffer(scr_addr=0x34, dst_addr=0x5a000, size=0x1000)
[==============================] 100% (69/69 pages)
Done in 15.104 seconds
reset()

• Serial Connection Test: After successful flashing via bossac, the device did actually reset, i.e. the yellow led stopped flashing (bootloader mode). I also tried to immediately unplug and replug the devices. Connection attempts via generic serial terminals (115200 baud) and mpremote both failed to establish a REPL connection (the devices don't even seem to appear on usb at all).
• I compared the MBR.h, bootloader.h and SoftDevice.h files that came bundled with the micropython installer app and the ones I flashed using the Arduino IDE using diff. The corresponding files are identical.

Conclusion/Question:

The boards were freshly flashed microPython and openMV, but can't be reflashed. SoftDevice/Bootloader and a clean flash of MicroPython all done, the persistent "SoftDevice Relocation" timeout may suggest an issue during the MicroPython boot sequence when it attempts to initialize the SoftDevice?

Could this be related to a specific configuration register or a known timing issue when starting the BLE stack on the nRF52840 after the bossac reset?

Do I have to take the "nuclear option", i.e. SWD and nrfjprog? Would that even help?

Thank you in advance for any insight and/or guidance!