In this exercise, we will learn how to perform DFU over UART. First, we will enable Serial Recovery, and then we will add DFU from the application functionality. Both of these can be active at the same time, as the bootloader operates independently from the application.
Serial Recovery
DFU from the application
✅ Simplicity: Easier to implement since it only involves the bootloader. ✅ Single slot support: Can use a single slot for firmware updates, reducing flash memory requirements. ✅ Reliability: The user can always recover if the application fails since the bootloader has DFU capabilities. ❌Limited features: Only supports basic recovery functions, lacking advanced wireless transports such as Bluetooth LE. ❌Manual DFU mode activation: Requires the user to have physical access to the device and manually put into DFU mode (e.g., by pressing a button or using a jumper).
✅ Flexibility: Can support more advanced features, such as wireless transport (e.g., Bluetooth LE, Wi-Fi, cellular IoT), image compression, etc. ✅ Better user experience: Provides a more user-friendly experience and does not require manual DFU mode activation. It also gives possibility to implement remote firmware upgrade. ✅ Seamless updates: Allows for updates to be performed in the background while the application is running, minimizing downtime. ❌Increased complexity: Requires additional development effort to implement robust update mechanisms. ❌Dual slot requirement: A dual-slot setup is necessary to recover from a corrupted DFU attempt.
UART0 on our development kits are routed to the debugger chip and exposed to the connected PC as a VCOM port. Therefore, we do not have to connect anything extra to the DK to test DFU over UART.
Exercise prerequisites
To send firmware updates via UART, we will use a desktop tool called AuTerm. For alternative tools, check out this page.
Install AuTerm by following its downloading and Setup README. Pre-release builds are available for Windows, MacOS, and Linux from the releases page (Expand assets).
In AuTerm, select the Config tab first (1). As the initial configuration step, change the Config Port Settings to the port the DK is connected to (2).
Note that the port number will vary from machine to machine and DK to DK. Use the Connected Devices view in nRF Connect for VS Code to know the port of your DK.
We will extensively use the MCUmgr tab (3) in this exercise.
Prepare the DK
This is not a required but a recommended step. The DK debugger (Interface MCU)has a virtual mass storage disk. This may interfere with UART DFU transfer since the Interface MCU is also used for UART<> USB conversion, so it is recommended to disable it using the J-link Commander(Windows), or JLinkExe(Linux) :
The Development Kit must then be power-cycled for the settings to take effect. Replace SEGGER_ID with the SEGGER ID of your DK. The SEGGER ID is for the on-board debugger on the DK. Each DK has its unique SEGGER ID. The SEGGER ID is displayed in the Connected Devices View in nRF Connect for VS Code, as shown in the screenshot below. It’s also printed on the kit label.
Note that this setting remains valid even if you program another firmware onto the device.
Exercise steps
For this exercise, The base code is simply a copy of the Blinky sample. The goal here is to show you how to add MCUboot to an existing nRF Connect SDK application.
Open the code base of the exercise by navigating to Create a new application in the nRF Connect for VS Code extension, select Copy a sample, and search for Lesson 9 – Exercise 1.
1.2 By default, MCUboot splits the non-volatile memory into two slots. Since we will use Serial Recovery in this exercise, we can use only one slot instead to get more available non-volatile memory. We can do this by setting SB_CONFIG_MCUBOOT_MODE_SINGLE_APP.
Copy
# STEP 1.2 - Configure bootloader to use only one slot. SB_CONFIG_MCUBOOT_MODE_SINGLE_APP=y
Kconfig
2. Enable logging for MCUboot.
2.1 Create a sysbuild/ directory to configure child images.
2.2 Create a file to configure MCUboot in the child image folder and add the following comments inside mcuboot.conf
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# STEP 2.3 - Enable logging for MCUboot# STEP 3.1 - Enable Serial Recovery over UART# STEP 3.2 - Disable UART, since Serial Recovery uses it# STEP 3.3 - Turn on a LED so we can see when Serial Recovery mode is active
Kconfig
The folder structure should now look like this:
2.3 Next, we will enable logging for MCUboot. For this, we both need to enable logs and set the log level:
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CONFIG_LOG=yCONFIG_MCUBOOT_LOG_LEVEL_INF=y
Kconfig
After this, we can build and run the sample to see it run with MCUboot. Observe logs from nRF Serial Terminal:
*** Booting MCUboot v2.1.0-dev-12e5ee106034 ****** Using nRF Connect SDK ****** Using Zephyr OS v3.7.99-1f8f3dc29142 ***I: Starting bootloaderI: Bootloader chainload address offset: 0xc000*** Booting My Application v0.0.0 - unknown commit ****** Using nRF Connect SDK v2.9.0-7787b2649840 ****** Using Zephyr OS v3.7.99-1f8f3dc29142 ***
Terminal
Important
On the nRF7002 DK only, ensure that the SPI NOR driver is disabled by default by adding the following lines in sysbuild/mcuboot.conf
CONFIG_SPI=n
CONFIG_SPI_NOR=n
We can also see the partitioning of the device, with the nRF Connect for VS Code Memory Report
Side note: Without the SB_CONFIG_MCUBOOT_MODE_SINGLE_APP=y, you will have two slots mcuboot_primary and mcuboot_secondary as shown below:
Now, we have MCUboot running before the application. However, we can not yet update the application.
3. Add DFU over UART to MCUboot.
Next, we will add DFU over UART to MCUboot, also known as Serial Recovery. MCUboot configuration options can be found here, and MCUboot Kconfig options specific for Serial Recovery are found here.
3.1 To enable Serial Recovery, we must set CONFIG_MCUBOOT_SERIAL and to enable transport over UART, we must set BOOT_SERIAL_UART. Since these are both for the MCUboot child image, we must set them in mcuboot.conf.
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CONFIG_MCUBOOT_SERIAL=yCONFIG_BOOT_SERIAL_UART=y
Kconfig
Important
If you are developing for the nRF54L Series DK, be sure to increase the space allocated for MCUboot by adding the following line to your mcuboot.conf file: CONFIG_PM_PARTITION_SIZE_MCUBOOT=0xF000. The value 0xF000 was chosen somewhat arbitrarily, so remember to optimize this size for your specific project. Additionally, for the nRF54LM20 DK it is needed to increase the maximum number of image sectors MCUboot can handle by setting: CONFIG_BOOT_MAX_IMG_SECTORS=256
3.2 Since we now use UART for Serial Recovery, we must disable UART from the console from MCUboot, so this does not try to use the UART at the same time.
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CONFIG_UART_CONSOLE=n
Kconfig
3.3 To indicate that MCUboot is in Serial Recovery mode, it is nice to have an indicator LED. We can enable this by setting CONFIG_MCUBOOT_INDICATION_LED.
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CONFIG_MCUBOOT_INDICATION_LED=y
Kconfig
3.4 We will set which button and LED will be used before testing. This is set using a Devicetree overlay. For this, we will create the file sysbuild/mcuboot.overlay. Now, the folder structure will then look like this:
Add the following comment to mcuboot.overlay:
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/* Step 3.4 - Configure button and LED for Serial Recovery */
Note that the alias button1 represents the physical Button 2 on the DK (all Nordic DKs, except nRF54L Series DK). led1 alias represents the physical LED2 on the DK. Aliases and node identifiers in devicetree start from 0, while on the physical board, they start from 1.
Note
On the nRF54L Series DK, the aliases button1 and led1 actually refer to Button 1 and LED1. Since the PCB labels on that DK start from 0.
Test Serial Recovery
4.6 After the successful upload, reset the DK to see the new image in effect. The video above also shows how to reset the DK over SMP, but you can also use the reset button if you want.
DFU over UART from the application
5. Add DFU over UART to the application.
Next up, we will add DFU over UART to the application. This method uses the mcumgr library.
5.1 First, we must go back to dual slots, as DFU from the application must have two slots. Change SB_CONFIG_MCUBOOT_MODE_SINGLE_APP from y to n in sysbuild.conf. Or simply comment out the line, as the default mode is dual slot.
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SB_CONFIG_MCUBOOT_MODE_SINGLE_APP=n
Kconfig
5.2 Then we add mcumgr configurations and dependencies to the application configuration file (prj.conf). We can take inspiration from the SMP Server sample to find which configurations we need.
For this exercise, we can use the following Kconfigs in prj.conf
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# STEP 5.2 - Enable mcumgr DFU in application# Enable MCUMGR CONFIG_MCUMGR=y# Enable MCUMGR management for both OS and ImagesCONFIG_MCUMGR_GRP_OS=yCONFIG_MCUMGR_GRP_IMG=y# Configure MCUMGR transport to UARTCONFIG_MCUMGR_TRANSPORT_UART=y# Dependencies# Configure dependencies for CONFIG_MCUMGR CONFIG_NET_BUF=yCONFIG_ZCBOR=yCONFIG_CRC=y# Configure dependencies for CONFIG_MCUMGR_GRP_IMG CONFIG_FLASH=yCONFIG_IMG_MANAGER=y# Configure dependencies for CONFIG_IMG_MANAGER CONFIG_STREAM_FLASH=yCONFIG_FLASH_MAP=y# Configure dependencies for CONFIG_MCUMGR_TRANSPORT_UART CONFIG_BASE64=y
Kconfig
5.3 Build and flash the application to the DK.
With this, we can do DFU over UART without entering bootloader mode manually.
Build, flash, and start the DK normally (do not hold any buttons). Now, we can use AuTerm to list images, as before:
Then we change the application code again. Also, increment VERSION_MINOR in the VERSION file. We will be able to see the version in AuTerm.
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VERSION_MAJOR=0# STEP 5.3 - Increment minor versionVERSION_MINOR=1PATCHLEVEL=0VERSION_TWEAK=0EXTRAVERSION=
Pristine build the code (Do not flash using the Debugging) and upload the new firmware image using AuTerm.
The “image number” that you can choose here should stay at 0. If you got another pair of primary/secondary slots, the next pair would be image 1, and so on. An example of this will be when we update the network core on the nRF5340, which we will see in a later exercise.
Select “Test” to tag the image as “test” after the upload:
5.4 Since we now use a dual slot configuration, an image tagged as “test” will initiate a swapon next reboot. Check Slot 1 in the image list in AuTerm.
Then reboot the DK.
The images will be swapped. We can check this by listing images again.
Now, we can confirm the image by setting the state. This will prevent the image from swapping back on next reset.
Optional: If you have not confirmed, the image swaps back to the old firmware on reset. DFU again with minor version 2 and do not confirm to test this.
Note
To restore the default DK debugger (Interface MCU) behavior with a virtual mass storage disk enabled, use the following J-Link command: MSDEnable. This command can be executed via J-Link Commander (on Windows) or JLinkExe (on Linux).
nRF5340 update
The application core of the nRF5340 can be updated, as explained above.
When doing DFU from the application, no extra configurations are needed to update the network core. Another DFU package file must be used for the network core. Instead of zephyr.signed.bin, use signed_by_mcuboot_and_b0_ipc_radio.bin.
This course does not cover how to use Serial Recovery to update the network core.
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•Support for nRF54LS05 DK (Available through the early access sampling program) •Support for the nRF54LM20B with Axon NPU for Edge AI applications
Bluetooth LE updates
•Quality of Service module is now production-ready. •New experimental features for RF testing (Direct Test Mode) and low-latency packet handling (LE Flushable ACL).
MCUboot & Partition Manager
•Single-Slot DFU and RAM Load mode are both promoted to fully supported •Partition Manager is officially deprecated in favor of Zephyr's devicetree-based partitioning.
