I think I managed to brick the bootloader by writing to the flash from my application (should have read this before: XIAO BLE Sense persistent data).
The XIAO BLE sense does not show up anymore as USB device, i. e. there is no serial port or mass storage device showing up, even when the bootloaded is activated via double-pressing the reset button.
How can I flash the original booloader firmware, and where can I find it? (And which hardware is needed for that?)
Ok I think I can write the bootloader with OpenOCD using just a Raspberry PI over the SWD debug interface (via bottom pads TP1, TP2, TP3 and TP5 on the XIAO BLE).
Anyway, where can I find the original bootloader binary of the XIAO BLE sense?
I was able to unbrick my XIAO BLE sense with OpenOCD using a raspberry pi as SWD debugger / programmer.
I just set up a fresh Raspberry PI OS on a Raspberry PI Zero W, and installed openocd (“sudo apt install openocd”), wired the XIAO BLE sense to the Raspberry PI (see below), and executed the following command: