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Programming Nordic MCUs

Authors:
Geoffrey Hunter
Geoffrey Hunter
Published On:
Feb 11, 2025
Last Updated:
Dec 19, 2025

You can program nRF52, nRF54, nRF53, and nRF91 MCUs using the development kits from the same series (or sometimes from a different series). For example, the nRF52-DK can be used to program SoCs from the nRF52 series. I believe the development kits in the nRF91 series can program the nRF91 and nRF52 SoCs (and perhaps the other families too).

To do this, you plug the development kit into the host computer using a USB cable. You then connect the development kit to the target device using the Debug OUT connector, which is a standard 2x5 1.27mm pitch header. It follows the standard ARM SWD pinout. The schematic from the nRF52-DK (it should be either the same or very similar on other development kits) is shown in This is a placeholder for the reference: fig-debug-out-connector-schematic-on-nrf52-dk.

Debug Out connector schematic on the nRF52 DK.

The development kits have a SEGGER J-Link probe built into them (presumably Nordic Semiconductor is licensing the J-Link technology from SEGGER). It’s good marketing for SEGGER too since people become familiar with the J-Link brand and then might buy a dedicated J-Link probe (they are expensive but work well).

Making Sure You Can Only Program External Boards

One annoying issue with using the Nordic development kits for programming is that you can easily accidentally start programming the on-board MCU instead of your external board. All it takes is a loose/unplugged programming cable and the J-Link will automatically switch back to programming the on-board MCU. An easy way to prevent this is to short the SELECT pin on the “Debug Out” connector to GND, as shown in This is a placeholder for the reference: fig-shorting-select-to-gnd-on-nrf52-dk-schematics (the red squiggle is the short). This tricks the J-Link into thinking there is always an external MCU connected and making it impossible to program the on-board MCU.

Shorting select to GND on the nRF52 DK.

Luckily, there is a GND pin adjacent to the SELECT pin on the SMD header, at it is easy to short them together with a small solder blob, as shown in This is a placeholder for the reference: fig-shorting-select-to-gnd-on-nrf52-dk-photo.

Shorting select to GND on the nRF52 DK.

While you are doing this, you may also want to bridge SB32 (which can also be seen in This is a placeholder for the reference: fig-shorting-select-to-gnd-on-nrf52-dk-photo). Bridging this connects the on-board +3.3V to the VDD pin, allowing you to power the target device from the development kit. This can be useful for one cable programming in a production environment. This only works if the current draw of the target device is small.

Also note that some nRF development kits such as the nRF9160-DK have two on-board MCUs that can be programmed (typically a nRF52 and a nRF91). A DIP switch selects the one the J-Link will program (if nothing is connected to the external programming connector).

nRF Connect for Desktop

nRF Connect for Desktop is a suite of software tools for various programming and development tasks for the nRF SoCs. There are two tools in this suite that can be used to program nRF SoCs:

  • Programmer
  • Quick Start

The “Programmer” tool is a GUI tool that can be used to program any nRF SoC using a physically connected programmer such as a nRF development kit or a J-Link probe.

The “Quick Start” tool can also be used to program nRF SoCs, but is only designed to program development kits with example projects.

A screenshot of the nRF Connect for Desktop “Quick Start” tool programming asset tracking example firmware “Asset Tracker v2” onto a nRF9160-DK development kit.

nrfutil

nRF Util (nrfutil) is a Python library and command line tool for programming and debugging Nordic Semiconductor MCUs.

A screenshot of the nRF Util homepage.1

Source Code

It doesn’t appear that the up-to-date source code of nRF Util is available. The older repo (pc-nrfutil) at https://github.com/NordicSemiconductor/pc-nrfutil was deprecated and marked as archived in Jan 2023.2

Installing Sub-Commands

By default, only a basic subset of the total available nRF Util commands are installed. You can install additional commands based on your specific needs via the nrfutil install command. You can use nrfutil search to search for installable commands.

For example:

Terminal window
nrfutil search 91
Command Installed Latest Status
91                0.5.0  Not installed

Then to install the 91 command, run:

Terminal window
nrfutil install 91

Device Sub-Commands

The nRF Util device sub-command can list, program, recover, erase, and do various operations on Nordic devices. It supports MCUboot and J-Link. Install device with:

Terminal window
nrfutil install device

You can list information about a device with nrfutil device device-info:

Terminal window
$ nrfutil device device-info
serial_number: 000960036306
        boardVersion: PCA10090
        deviceFamily: NRF91_FAMILY
        deviceVersion: NRF9151_xxAA_REV3
        jlinkObFirmwareVersion: J-Link OB-K22-NordicSemi compiled Jul  8 2025 10:15:22
        protectionStatus: NRFDL_PROTECTION_STATUS_NONE

This lists things such as the MCU device family (and version, which is like the family, but more specific) and the protection status.

Device recovery can be performed with:

Terminal window
nrfutil device recover
Screenshot showing device recovery (without having to erase the device) using the nrfutil device recover command.

nrfutil device recover can be successfully run on an already recovered device. Once recovered, you should be able to verify it worked by running nrfutil device device-info and checking that the protection status is NRFDL_PROTECTION_STATUS_NONE.

You can reset the MCU by running nrfutil device reset. This command supports a --reset-kind flag which specifies what kind of reset to perform. The following values are supported:

  • RESET_DEFAULT: Selects a predefined reset kind based on the device family. For nRF54H and nRF92 this is RESET_HARD, for all others it is RESET_SYSTEM.
  • RESET_SYSTEM: Trigger an ARM system reset by setting the SYSRESETREQ bit in the AIRCR register of the CPU. On devices with the Secure Domain firmware (SecDom FW), this is supported only on the secure core.
  • RESET_HARD: Trigger a hard reset over the CTRL-AP. This is supported only for devices with the CTRL-AP implementation that has this functionality.
  • RESET_SOFT: Trigger a soft reset over the CTRL-AP. This is supported only for devices with the CTRL-AP implementation that has this functionality.
  • RESET_DEBUG (deprecated): Alias for RESET_SOFT. This option is deprecated and will be removed in a future version.
  • RESET_PIN: Trigger a pin reset by toggling the reset pin.
  • RESET_VIA_SECDOM: Send a reset command over ADAC to the SecDom FW. The SecDom FW handles the reset of other domains and defines how this process is carried out. The SecDom is not reset as part of this process. This option is supported on devices with the SecDom FW.

Devices can be programmed with the nrfutil device program command:

Terminal window
nrfutil device program --firmware <my_firmware_file>

The SWD clock speed can be specified with the --swd-clock-frequency option (in kHz). In my scenario the clock speed was apparently already at the maximum allowed of 2 MHz. Trying to set it higher (e.g. 5 MHz) resulted in an error.

Terminal window
nrfutil.exe device program --firmware C:\mfw_nrf91x1_2.0.2.zip --swd-clock-frequency 5000
[00:00:00] ------   0% [1/3 960036306] Failed, SWD clock speed exceeds maximum of 2000 kHz
Error: One or more program tasks failed:
 * 960036306: SWD clock speed exceeds maximum of 2000 kHz (Generic)

Setting it as --swd-clock-frequency 2000 worked, but the programming was not any faster than it was by default (approx. 30s to program the modem firmware).

pynrfjprog

pynrfjprog is a Python wrapper around the Nordic Semiconductor nrfjprog DLL (dynamic link library). The code is open source and available on GitHub.

Installation

To install pynrfjprog, into your Python environment, run the following command:

Terminal window
pip install pynrfjprog

As always, it is recommended to use a virtual environment to manage your Python packages.

Usage

Use enum_emu_snr() to enumerate the serial numbers of the connected programmers:

from pynrfjprog import LowLevel


with LowLevel.API('NRF52') as api:
    serial_numbers = api.enum_emu_snr()
    if serial_numbers is None:
        print("No programmers found")
    else:
        print(f"Found {len(serial_numbers)} programmers with serial numbers: {serial_numbers}")

enum_emu_snr() returns a list of serial numbers (e.g. [960095105, 1051285627]). If no programmers are found, it returns None.

Footnotes

  1. Nordic Semiconductor. nRF Util [website]. Retrieved 2025-05-15, from https://www.nordicsemi.com/Products/Development-tools/nRF-Util. ↩

  2. Nordic Semiconductor. pc-nrfutil [GitHub repository]. Retrieved 2025-05-15, from https://github.com/NordicSemiconductor/pc-nrfutil. ↩