Common Flashing Description for the Adafruit nRF52 Bootloader.
Common Flashing Description for the Adafruit nRF52 Bootloader.
The Adafruit nRF52 Bootloader is used by many Adafruit and Seeedstudio boards that are based on the Nordic Semiconductor nRF52 microcontroller family. It features a Mass Storage Device emulation, so that UF2 files can be uploaded via Drag and Drop. It is also backwards compatible to the BOSSA bootloader. With the Adafruit nRF52 Bootloader, no external debugger such as a J-Link is necessary for the normal usage.
In some cases the bootloader may be too old to even mount on startup. Enter the bootloader mode (usually by double tapping the reset button for nRF52840 based boards) and check the INFO_UF2.TXT for bootloader information. If the version is less than 0.4.0, please refer to Updating Old Bootloaders and Restoring the SoftDevice.
Until and including RIOT Release 2025.01, the SoftDevice on feather-nrf52840 and feather-nrf52840-sense boards got erased when flashing RIOT. To restore the ability of using i.a. Arduino and CircuitPython, the SoftDevice has to be flashed again. Please also refer to Updating Old Bootloaders and Restoring the SoftDevice.
If the bootloader is not present on your board at all, an external debugger such as the J-Link has to be used. Please refer to (Re-)Flashing the UF2 Bootloader with an External Debugger.
The board is flashed using its on-board UF2 bootloader by default. The bootloader will present a mass storage device that has to be mounted so the RIOT buildsystem can find it. If you have an auto-mounter installed this will happen automatically.
The rest of the process is automated in the usual way with e.g. BOARD=feather-nrf52840.
Example with hello-world application:
If RIOT is already running on the board, it will automatically reset the CPU and enter the bootloader. If some other firmware is running or RIOT crashed, you need to enter the bootloader manually by double-tapping the board's reset button while the device is connected.
The board definitions with RIOT retain the SoftDevice firmware blob shipped with the original Adafruit nRF52 Bootloader that is used by i.a. Arduino and CircuitPython but not used by RIOT. If you want to override the SoftDevice to avoid having closed source software on your microcontroller or if you need the additional space, you can specify the UF2_SOFTDEV=DROP option in your make command. Be aware that you have to reflash the SoftDevice if you want to use it in the future!
If your board features a non-standard SoftDevice version, you can specify it with the UF2_SOFTDEV option as well. Currently supported are i.a. SD611 for SoftDevice Version 6.1.1 and SD730 for Version 7.3.0. Setting the wrong SoftDevice version might corrupt the SoftDevice and make a reflash of the SoftDevice necessary. Do not change it unless you know what you're doing!
If the Adafruit nRF52 Bootloader is still present on the board but outdated or the SoftDevice got erased (the INFO_UF2.TXT file will show SoftDevice: not found), you can flash the bootloader via DFU.
As prerequisites, the arm-none-eabi-gcc compiler (which is usually already present if you compiled something for your nRF52 board) and the adafruit-nrfutil have to be installed.
Details on the installation process can be found in Adafruit's GitHub repository.
feather-nrf52840 for example is called feather_nrf52840_express. Be sure to check the boards/ subfolder in the bootloader directory to find out the correct board name.With the following commands, the bootloader with SoftDevice will be compiled and flashed. For MacOS, you might have to select a different serial port.
Sometimes the process has to be done twice until the SoftDevice is successfully restored.
Further information about the update process can be found on the Adafruit Website.
To flash the UF2 bootloader (if it is no longer present on your board), you need to have the original nrfjprog installed and a compatible debugger, such as a J-Link. Make sure to select the right board (see the note in Updating Old Bootloaders and Restoring the SoftDevice).
More information about how to flash nRF52 microcontrollers can be found in the Flashing section in nRF52 common. Some variants of the Adafruit Feather feature an SWD Connector, to which an external debugger can be connected. Other boards such as the Seeedstudio Xiao nRF52840 only have solder pads.
1.12.0