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USB Logging

Overview#

If you are developing ZMK on a device that does not have a built in UART for debugging and log/console output, Zephyr can be configured to create a USB CDC ACM device and the direct all printk, console output, and log messages to that device instead.

Battery Life Impact

Enabling logging increases the power usage of your keyboard, and can have a non-trivial impact to your time on battery. It is recommended to only enable logging when needed, and not leaving it on by default.

Kconfig#

The CONFIG_ZMK_USB_LOGGING KConfig value needs to be set, either by copy and pasting into the app/prj.conf file, or by running west build -t menuconfig and manually enabling the setting in that UI at ZMK -> Advanced -> USB Logging.

note

If you are debugging your own keyboard in your user config repository, use config/boards/shields/<your_keyboard>/<your_keyboard>.conf instead of app/prj.conf. In Github Actions, you can search the Kconfig file build log to verify the options above have been enabled for you successfully.

# Turn on logging, and set ZMK logging to debug output
CONFIG_ZMK_USB_LOGGING=y

Viewing Logs#

After flashing the updated ZMK image, the board should expose a USB CDC ACM device that you can connect to and view the logs.

On Linux, this should be a device like /dev/ttyACM0 and you can connect with minicom or tio as usual, e.g.:

sudo tio /dev/ttyACM0

From there, you should see the various log messages from ZMK and Zephyr, depending on which systems you have set to what log levels.