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 outputCONFIG_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.

Linux

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

Windows

On Windows, you can use PuTTY. Once installed, use Device Manager to figure out which COM port your controller is communicating on (listed under 'Ports (COM & LPT)') and specify that as the 'Serial line' in PuTTY.

If you already have the Ardunio IDE installed you can also use its built-in Serial Monitor.

MacOS

On MacOS, the device name is something like /dev/tty.usbmodemXXXXX where XXXXX is some numerical ID. You can connect to the device with tio (can be installed via Homebrew):

sudo tio /dev/tty.usbmodem14401

You should see tio printing Disconnected or Connected when you disconnect or reconnect the USB cable.

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