Building and using MCUboot with Zephyr

MCUboot began its life as the bootloader for Mynewt. It has since acquired the ability to be used as a bootloader for Zephyr as well. There are some pretty significant differences in how apps are built for Zephyr, and these are documented here.

Please see the design document for documentation on the design and operation of the bootloader itself. This functionality should be the same on all supported RTOSs.

The first step required for Zephyr is making sure your board has flash partitions defined in its device tree. These partitions are:

  • boot_partition: for MCUboot itself

  • image_0_primary_partition: the primary slot of Image 0

  • image_0_secondary_partition: the secondary slot of Image 0

  • scratch_partition: the scratch slot

Currently, the two image slots must be contiguous. If you are running MCUboot as your stage 1 bootloader, boot_partition must be configured so your SoC runs it out of reset. If there are multiple updateable images then the corresponding primary and secondary partitions must be defined for the rest of the images too (e.g. image_1_primary_partition and image_1_secondary_partition for Image 1).

The flash partitions are typically defined in the Zephyr boards folder, in a file named boards/<arch>/<board>/<board>.dts. An example .dts file with flash partitions defined is the frdm_k64f’s in boards/arm/frdm_k64f/frdm_k64f.dts. Make sure the labels in your board’s .dts file match the ones used there.

Installing requirements and dependencies

Install additional packages required for development with MCUboot:

  cd ~/mcuboot  # or to your directory where MCUboot is cloned
  pip3 install --user -r scripts/requirements.txt

Building the bootloader itself

The bootloader is an ordinary Zephyr application, at least from Zephyr’s point of view. There is a bit of configuration that needs to be made before building it. Most of this can be done as documented in the CMakeLists.txt file in boot/zephyr. There are comments there for guidance. It is important to select a signature algorithm, and decide if the primary slot should be validated on every boot.

To build MCUboot, create a build directory in boot/zephyr, and build it as usual:

  cd boot/zephyr
  mkdir build && cd build
  cmake -GNinja -DBOARD=<board> ..

In addition to the partitions defined in DTS, some additional information about the flash layout is currently required to build MCUboot itself. All the needed configuration is collected in boot/zephyr/include/target.h. Depending on the board, this information may come from board-specific headers, Device Tree, or be configured by MCUboot on a per-SoC family basis.

After building the bootloader, the binaries should reside in build/zephyr/zephyr.{bin,hex,elf}, where build is the build directory you chose when running cmake. Use the Zephyr build system flash target to flash these binaries, usually by running make flash (or ninja flash, etc.) from the build directory. Depending on the target and flash tool used, this might erase the whole of the flash memory (mass erase) or only the sectors where the bootloader resides prior to programming the bootloader image itself.

Building applications for the bootloader

In addition to flash partitions in DTS, some additional configuration is required to build applications for MCUboot.

This is handled internally by the Zephyr configuration system and is wrapped in the CONFIG_BOOTLOADER_MCUBOOT Kconfig variable, which must be enabled in the application’s prj.conf file.

The directory samples/zephyr/hello-world in the MCUboot tree contains a simple application with everything you need. You can try it on your board and then just make a copy of it to get started on your own application; see samples/zephyr/ for a tutorial.

The Zephyr CONFIG_BOOTLOADER_MCUBOOT configuration option documentation provides additional details regarding the changes it makes to the image placement and generation in order for an application to be bootable by MCUboot.

With this, build the application as your normally would.

Signing the application

In order to upgrade to an image (or even boot it, if MCUBOOT_VALIDATE_PRIMARY_SLOT is enabled), the images must be signed. To make development easier, MCUboot is distributed with some example keys. It is important to stress that these should never be used for production, since the private key is publicly available in this repository. See below on how to make your own signatures.

Images can be signed with the scripts/ script. It is best to look at samples/zephyr/Makefile for examples on how to use this.

Flashing the application

The application itself can flashed with regular flash tools, but will need to be programmed at the offset of the primary slot for this particular target. Depending on the platform and flash tool you might need to manually specify a flash offset corresponding to the primary slot starting address. This is usually not relevant for flash tools that use Intel Hex images (.hex) instead of raw binary images (.bin) since the former include destination address information. Additionally you will need to make sure that the flash tool does not perform a mass erase (erasing the whole of the flash) or else you would be deleting MCUboot. These images can also be marked for upgrade, and loaded into the secondary slot, at which point the bootloader should perform an upgrade. It is up to the image to mark the primary slot as “image ok” before the next reboot, otherwise the bootloader will revert the application.

Managing signing keys

The signing keys used by MCUboot are represented in standard formats, and can be generated and processed using conventional tools. However, scripts/ is able to generate key pairs in all of the supported formats. See the docs for more details on this tool.

Generating a new keypair

Generating a keypair with imgtool is a matter of running the keygen subcommand:

    $ ./scripts/ keygen -k mykey.pem -t rsa-2048

The argument to -t should be the desired key type. See the the docs for more details on the possible key types.

Extracting the public key

The generated keypair above contains both the public and the private key. It is necessary to extract the public key and insert it into the bootloader. The keys live in boot/zephyr/keys.c, and can be extracted using imgtool:

    $ ./scripts/ getpub -k mykey.pem

This will output the public key as a C array that can be dropped directly into the keys.c file.

Once this is done, this new keypair file (mykey.pem in this example) can be used to sign images.