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Update firmware using the UF2 Bootloader


nRF52840 MDK USB Dongle is shipped with the UF2 Bootloader, which is an easy-to-use and self-upgradable bootloader that can be used to update the firmware by just copying the .uf2-format images to the flash drive without using an external programmer.

This section details how to update firmware using the UF2 Bootloader.

UF2 Bootloader Update Notification

The dongle manufactured after July 20 2023 is shipped with the latest UF2 Bootloader 0.7.1. You may get a dongle without the latest bootloader, and if you would like to explore new features of the latest bootloader, just follow the Updating the UF2 Bootloader section.

Flash memory layout

When updating firmware using the UF2 Bootloader, you must be aware of where in the device memory the different firmware components are located.

The following figure shows the default flash memory layout of nRF52840 MDK USB Dongle:

Usage Memory location Size
Bootloader settings 0x000FF000 4 KB
MBR parameter storage 0x000FE000 4 KB
Bootloader config 0x000FD800 2 KB
Bootloader 0x000F4000 38 KB
Application 0x00001000 972 KB
Master Boot Record (MBR) 0x00000000 4 KB

Installing UF2 Converter

uf2conv is an open source Python based tool for packing and unpacking UF2 files.

Before you install uf2conv, make sure that you have Python 3.6.0 or later installed.

Then open up a terminal and run the following command to install the latest prerelease version from the HEAD of the main branch:

py -3 -m pip install --pre -U git+
python3 -m pip install --pre -U git+

You can run uf2conv --help for a list of available options:

$ uf2conv --help

usage: uf2conv [-h] [-b BASE] [-f FAMILY] [-o FILE] [-d DEVICE_PATH] [-l] [-c] [-D] [-w] [-C] [-i] [INPUT]

Convert to UF2 or flash directly.

positional arguments:
  INPUT                 input file (HEX, BIN or UF2)

  -h, --help            show this help message and exit
  -b BASE, --base BASE  set base address of application for BIN format (default: 0x2000)
  -f FAMILY, --family FAMILY
                        specify familyID - number or name (default: 0x0)
  -o FILE, --output FILE
                        write output to named file; defaults to "flash.uf2" or "flash.bin" where sensible
                        select a device path to flash
  -l, --list            list connected devices
  -c, --convert         do not flash, just convert
  -D, --deploy          just flash, do not convert
  -w, --wait            wait for device to flash
  -C, --carray          convert binary file to a C array, not UF2
  -i, --info            display header information from UF2, do not convert

Generating the firmware in UF2

To generate the application firmware in UF2, simply use uf2conv on a .bin file or .hex file, specifying the family as 0xADA52840:

uf2conv -f 0xADA52840 -c -o application.uf2 application.hex
uf2conv -f 0xADA52840 -c -b 0x1000 -o application.uf2 application.bin

To generate a UF2 image for bootloader from a .hex file, specifying the separated family of 0xD663823C:

uf2conv -f 0xD663823C -c -o bootloader.uf2 bootloader.hex

Updating the application firmware

Generate the correct UF2 application firmware for your dongle by following the steps above.

To update the application firmware, complete the following steps:

  1. Push and hold the button and plug your dongle into the USB port of your computer. Release the button after your dongle is connected. The RGB LED turns green.

  2. It will mount as a Mass Storage Device called UF2BOOT.

  3. Drag and drop the UF2 file onto the UF2BOOT volume. The RGB LED blinks red fast during flashing.

  4. Re-plug the dongle and the new application firmware is running.

Double-Reset to enter DFU mode

If the button has been programmed as RESET functionality, reset twice within 500 ms will enter DFU with UF2 and CDC support.

Updating the UF2 Bootloader

The UF2 Bootloader can be upgraded by using a newer UF2 Bootloader with the name update-uf2_bootloader-nrf52840_mdk_usb_dongle-<version>-nosd.uf2. This is the fastest and safest way to update bootloader. However, it requires your existing bootloader is at least 0.4.0. If not, follow Flashing UF2 Bootloader using an external debugger section.

To update the bootloader using UF2, complete the following steps:

  1. Push and hold the button and plug your dongle into the USB port of your computer. Release the button after your dongle is connected. The RGB LED turns green.

  2. Open INFO_UF2.TXT in the UF2BOOT volume with a text editor, and check the current version of bootloader. It should be 0.4.0 or newer. The figure below shows we are running the 0.7.1 version.

  3. Check if newer updates are available in the firmware/uf2_bootloader folder. The update is released with the name update-uf2_bootloader-nrf52840_mdk_usb_dongle-<version>-nosd.uf2.

  4. Drag and drop the update file onto the UF2BOOT volume.

  5. Enter DFU mode again and verify the version printed in INFO_UF2.TXT.

Installing UF2 Bootloader using an external debugger

You can always install a fresh .hex-format bootloader using an external debugger if your existing bootloader is out-of-date or gets corrupted somehow. The .hex-format bootloader is located in the firmware/uf2_bootloader folder. Follow the instructions below for programming: