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Adafruit nRF52 Bootloader

Build Status

A CDC/DFU/UF2 bootloader for Nordic nRF52 microcontroller. UF2 is an easy-to-use bootloader that appears as a flash drive. You can just copy .uf2-format application images to the flash drive to load new firmware. See for more information.

DFU via serial/CDC requires adafruit-nrfutil, a modified version of Nordic nrfutil. Install python3 if it is not installed already and run this command to install adafruit-nrfutil from PyPi:

$ pip3 install --user adafruit-nrfutil

Supported Boards

Officially supported boards are:

In addition, there is also lots of other 3rd-party boards which are added by other makers, users and community. Check out the complete list of all boards here.


  • DFU over Serial and OTA ( application, Bootloader+SD )
  • Self-upgradable via Serial and OTA
  • DFU using UF2 ( (application only)
  • Auto-enter DFU briefly on startup for DTR auto-reset trick (832 only)

How to use

There are two pins, DFU and FRST that bootloader will check upon reset/power:

  • Double Reset Reset twice within 500 ms will enter DFU with UF2 and CDC support (only works with nRF52840)
  • DFU = LOW and FRST = HIGH: Enter bootloader with UF2 and CDC support
  • DFU = LOW and FRST = LOW: Enter bootloader with OTA, to upgrade with a mobile application such as Nordic nrfConnect/Toolbox
  • DFU = HIGH and FRST = LOW: Factory Reset mode: erase firmware application and its data
  • DFU = HIGH and FRST = HIGH: Go to application code if it is present, otherwise enter DFU with UF2
  • The GPREGRET register can also be set to force the bootloader can enter any of above modes (plus a CDC-only mode for Arduino). GPREGRET is set by the application before performing a soft reset.
#include "nrf_nvic.h"
void reset_to_uf2(void) {
  NRF_POWER->GPREGRET = 0x57; // 0xA8 OTA, 0x4e Serial
  NVIC_SystemReset();         // or sd_nvic_SystemReset();

On the Nordic PCA10056 DK board, DFU is connected to Button1, and FRST is connected to Button2. So holding down Button1 while clicking RESET will put the board into USB bootloader mode, with UF2 and CDC support. Holding down Button2 while clicking RESET will put the board into OTA (over-the-air) bootloader mode.

On the Nordic PCA10059 Dongle board, DFU is connected to the white button. FRST is connected to pin 1.10. Ground it to pull FRST low, as if you had pushed an FRST button. There is an adjacent ground pad.

For other boards, please check the board definition for details.

Making your own UF2

To create your own UF2 DFU update image, simply use the Python conversion script on a .bin file or .hex file, specifying the family as 0xADA52840 (nRF52840) or 0x621E937A (nRF52833).

nRF52840 firmware.hex -c -f 0xADA52840

nRF52833 firmware.hex -c -f 0x621E937A

If using a .bin file with the conversion script you must specify application address with the -b switch, this address depend on the SoftDevice size/version e.g S140 v6 is 0x26000, v7 is 0x27000

nRF52840 firmware.bin -c -b 0x26000 -f 0xADA52840

nRF52833 firmware.bin -c -b 0x27000 -f 0x621E937A

To create a UF2 image for bootloader from a .hex file using separated family of 0xd663823c bootloader.hex -c -f 0xd663823c

Burn & Upgrade with pre-built binaries

You can burn and/or upgrade the bootloader with either a J-link or DFU (serial) to a specific pre-built binary version without the hassle of installing a toolchain and compiling the code. This is preferred if you are not developing/customizing the bootloader. Pre-builtin binaries are available on GitHub releases

Note: The bootloader can be downgraded. Since the binary release is a merged version of both bootloader and the Nordic SoftDevice, you can freely upgrade/downgrade to any version you like.

How to compile and build

You should only continue if you are looking to develop bootloader for your own. You must have have a J-Link available to "unbrick" your device.



Firstly clone this repo with following commands

git clone
cd Adafruit_nRF52_Bootloader
git submodule update --init

Then build it with make BOARD={board} all, for example:

make BOARD=feather_nrf52840_express all

For the list of supported boards, run make without BOARD= :

$ make
You must provide a BOARD parameter with 'BOARD='
Supported boards are: feather_nrf52840_express feather_nrf52840_express pca10056
Makefile:90: *** BOARD not defined.  Stop


To flash the bootloader (without softdevice/mbr) using JLink:

make BOARD=feather_nrf52840_express flash

If you are using pyocd as debugger, add FLASHER=pyocd to make command:

make BOARD=feather_nrf52840_express FLASHER=pyocd flash

To upgrade the bootloader using DFU Serial via port /dev/ttyACM0

make BOARD=feather_nrf52840_express SERIAL=/dev/ttyACM0 flash-dfu

To flash SoftDevice (will also erase chip):

make BOARD=feather_nrf52840_express flash-sd

To flash MBR only

make BOARD=feather_nrf52840_express flash-mbr

Common makefile problems

arm-none-eabi-gcc: No such file or directory

If you get the following error ...

$ make BOARD=feather_nrf52840_express all
Compiling file: main.c
/bin/sh: /usr/bin/arm-none-eabi-gcc: No such file or directory
make: *** [_build/main.o] Error 127

... you may need to pass the location of the GCC ARM toolchain binaries to make using the variable CROSS_COMPILE as below:

$ make CROSS_COMPILE=/opt/gcc-arm-none-eabi-9-2019-q4-major/bin/arm-none-eabi- BOARD=feather_nrf52832 all

For other compile errors, check the gcc version with arm-none-eabi-gcc --version to insure it is at least 9.x.

ModuleNotFoundError: No module named 'intelhex'

Install python-intelhex with

pip install intelhex

make: nrfjprog: No such file or directory

Make sure that nrfjprog is available from the command-line. This binary is part of Nordic's nRF5x Command Line Tools.