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      • USB Logging

      RMK uses defmt as the default logger, which works great if you have a debug probe. If you don’t have a debug probe, you can still view logs over USB by configuring USB as a serial port.

      Usage

      To enable USB logging, disable the default features and then enable the usb_log feature in Cargo.toml:

      rmk = { version = "0.9", default-features = false, features = [
    "storage",
    "usb_log", # Enable USB logging
    "..",
] }

      The log level is fixed at debug: the usb_log feature enables log/max_level_debug, and the log crate rejects a second max_level_* feature at compile time.

      Tip

      Don't forget to re-enable the other default features you need (such as storage, vial, host_lock, and watchdog) — but not defmt: usb_log is based on the log crate, which cannot be enabled together with the defmt feature.

      To view the logs, you'll need to install a serial port monitor. Open your serial monitor, select the port corresponding to your keyboard, and connect. The logs will be displayed in the monitor window. Note that logs from the boot stage cannot be captured by the USB logger. You will only be able to see logs after the serial port connection is established.

      Some microcontrollers (like ESP32S3) don't have enough USB endpoints, so USB logging cannot be enabled for those microcontrollers. To enable USB logging, make sure that your microcontroller has at least 5 In + 4 OUT endpoints available (except the control endpoint, EP0).

      usb_log fails to compile on two groups of chips:

      • Chips without USB, such as nrf52832_ble, nrf52810_ble, nrf52811_ble, nrf54l15_ble, esp32c3_ble, esp32c6_ble and esp32h2_ble.
      • High-speed USB chips, currently nrf54lm20_ble: embassy-usb-logger only handles 64-byte packets, which high-speed bulk endpoints can't use. Use defmt on these chips.