kmk_firmware/docs/en/boot.md

boot.py

boot.py lives in the root of your keyboard when mounted as a storage device. There is a more detailed explanation in the circuit python docs, however there are some common use cases for your keyboard listed here.

Hiding device storage

You can hide your device from showing up as a USB storage by default (this can be overridden at startup if desired, per the example code further down this page).

storage.disable_usb_drive()

Using your keyboard before the OS loads

You can make your keyboard work in your computer's BIOS and bootloader (useful if you dual-boot).

usb_hid.enable(boot_device=1)

Disabling serial communication

By default, you can connect to your board's serial console, which can be useful for debugging, but may not be desired for everyday use.

# Equivalent to usb_cdc.enable(console=False, data=False)
usb_cdc.disable()

Example code

Below is a fully working example, which disables USB storage, CDC and enables BIOS mode.

import supervisor
import board
import digitalio
import storage
import usb_cdc
import usb_hid

from kb import KMKKeyboard
from kmk.scanners import DiodeOrientation


# If this key is held during boot, don't run the code which hides the storage and disables serial
# This will use the first row/col pin. Feel free to change it if you want it to be another pin
col = digitalio.DigitalInOut(KMKKeyboard.col_pins[0])
row = digitalio.DigitalInOut(KMKKeyboard.row_pins[0])

if KMKKeyboard.diode_orientation == DiodeOrientation.COLUMNS:
    col.switch_to_output(value=True)
    row.switch_to_input(pull=digitalio.Pull.DOWN)
else:
    col.switch_to_input(pull=digitalio.Pull.DOWN)
    row.switch_to_output(value=True)

if not row.value:
    storage.disable_usb_drive()
    # Equivalent to usb_cdc.enable(console=False, data=False)
    usb_cdc.disable()
    usb_hid.enable(boot_device=1)

row.deinit()
col.deinit()