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Building and Flashing

From here on, building and flashing ZMK should all be done from the app/ subdirectory of the ZMK checkout:

cd app
warning

If this is not done, you will encounter errors such as: ERROR: source directory "." does not contain a CMakeLists.txt; is this really what you want to build?

Building​

Building a particular keyboard is done using the west build command. Its usage slightly changes depending on if your build is for a keyboard with an onboard MCU or one that uses an MCU board add-on.

Keyboards with onboard MCU chips are simply treated as the board as far as Zephyr™ is concerned.

Given the following:

  • Keyboard: Planck (rev6)
  • Keymap: default

you can build ZMK with the following:

west build -b planck_rev6

CMake Arguments​

For creating a build system, the west build command uses CMake. This allows for additional arguments to be added to its invocations.

This is done by adding -- after the west build command and listing the CMake arguments afterward. The previous section shows one example of this with -DSHIELD=kyria_left. Another example use case is passing Kconfig flags:

west build -b planck_rev6 -- -DCONFIG_ZMK_SLEEP=y
tip

Once the first generation of the build directory using one-time CMake arguments is done, you can omit the CMake arguments and board selection, instead building with the command:

west build -d <build_dir>

You can also add permanent CMake arguments to west build by using the west config command. These are invoked whenever a new build system is generated. To add permanent arguments, set the build.cmake-args configuration option.

west config build.cmake-args -- -DSHIELD=kyra_left

Multiple arguments are added by assigning a string to the configuration option:

west config build.cmake-args \
  -- "-DSHIELD=kyra_left -DZMK_CONFIG=/absolute/path/to/zmk-config"

Pristine Building​

When building for a new board and/or shield after having built one previously, you may need to enable the pristine build option. This option removes all existing files in the build directory before regenerating them, and can be enabled by adding either --pristine or -p to the command:

west build -p -b nice_nano_v2 -- -DSHIELD=kyria_left

Building For Split Keyboards​

note

For split keyboards, you will have to build and flash each side separately the first time you install ZMK.

By default, the build command outputs a single .uf2 file named zmk.uf2 so building left and then right immediately after will overwrite your left firmware. In addition, you will need to pristine build each side to ensure the correct files are used. To avoid having to pristine build every time and separate the left and right build files, we recommend setting up separate build directories for each half. You can do this by using the -d parameter and first building left into build/left:

west build -d build/left -b nice_nano_v2 -- -DSHIELD=kyria_left

and then building right into build/right:

west build -d build/right -b nice_nano_v2 -- -DSHIELD=kyria_right

This produces left and right subfolders under the build directory and two separate .uf2 files. For future work on a specific half, use the -d parameter again to ensure you are building into the correct location.

tip

Build times can be significantly reduced after the initial build by omitting all build arguments except the build directory, e.g. west build -d build/left. The additional options and intermediate build outputs from your initial build are cached and reused for unchanged files.

Building With External Modules​

ZMK supports loading additional boards, shields, code, etc. from external Zephyr modules, facilitating out-of-tree management and versioning independent of the ZMK repository. To build with any additional modules, use the ZMK_EXTRA_MODULES define added to your west build command.

For instance, building with the my-vendor-keebs-module checked out to your documents directory, you would build like:

west build -b nice_nano_v2 -- -DSHIELD=vendor_shield -DZMK_EXTRA_MODULES="C:/Users/myUser/Documents/my-vendor-keebs-module"

When adding multiple modules, make sure they are separated by a semicolon, e.g.:

west build -b nice_nano_v2 -- -DSHIELD=vendor_shield -DZMK_EXTRA_MODULES="C:/Users/myUser/Documents/my-vendor-keebs-module;C:/Users/myUser/Documents/my-other-keebs-module"

Building from zmk-config Folder​

Instead of building .uf2 files using the default keymap and config files, you can build using files from your zmk-config folder by adding -DZMK_CONFIG="C:/the/absolute/path/config" to your west build command. Notice that this path should point to the folder labeled config within your zmk-config folder.

For instance, building kyria firmware from a user myUser's zmk-config folder on Windows may look something like this:

west build -b nice_nano -- -DSHIELD=kyria_left \
  -DZMK_CONFIG="C:/Users/myUser/Documents/Github/zmk-config/config"
warning

If your config is also a module, then you should also add the root (the folder in which the zephyr folder is found) of your zmk-config as an external module to build with.

Flashing​

The above build commands generate a UF2 file in build/zephyr (or build/left|right/zephyr if you followed the instructions for splits) and is by default named zmk.uf2. If your board supports USB Flashing Format (UF2), copy that file onto the root of the USB mass storage device for your board. The controller should flash your built firmware, unmount the USB storage device and automatically restart once flashing is complete.

Alternatively, if your board supports flashing and you're not developing from within a Dockerized environment, enable Device Firmware Upgrade (DFU) mode on your board and run the following command to flash:

west flash

Multi-CPU and Dual-Chip Bluetooth Boards​

Zephyr supports running the Bluetooth host and controller on separate processors. In such a configuration, ZMK always runs on the host processor, but you may need to build and flash separate firmware for the controller. Zephyr provides sample code which can be used as the controller firmware for Bluetooth HCI over RPMsg, SPI, UART, and USB. See Zephyr's Bluetooth Stack Architecture documentation for more details.

The following documentation shows how to build and flash ZMK for boards that use a dual-chip configuration.

nRF5340​

To build and flash the firmware for the nRF5340 development kit's network core, run the following command from the root of the ZMK repo:

cd zephyr/samples/bluetooth/hci_rpmsg
west build -b nrf5340dk_nrf5340_cpunet
west flash

You can then build and flash ZMK firmware using the normal steps described above. The network core's firmware only needs to be updated whenever ZMK upgrades to a new version of Zephyr.

For a custom nRF5340-based board, you will need to define two Zephyr boards: one for the application core and one for the network core. The nRF5340 DK's board definition can be used as reference. Replace nrf5340dk_nrf5340_cpunet with the name of your network core board.