qmk_firmware/docs/feature_console_in.md

Console In

The "Console" in QMK refers to PJRC's vendor page of 0xFF31 and usage page of 0x74 - this is mainly used for sending debug statements to the computer that can be read via the QMK Toolbox (recommended), or HID listen.

When #define CONSOLE_IN_ENABLE is placed in the config.h file, the keyboard will also listen to messages sent to the keyboard, will be able to respond in a custom way, defined at the quantum, kb, or user level. A keymap's example might look like this:

void process_console_data_user(uint8_t * data, uint8_t length) {
  switch (data[0]) {
    case 0x05:
      print("Sending back a message to the computer\n");
      break;
  }
}

Which will execute when a report is sent to the keyboard when the first byte is 0x05.

Default actions (defined at the quantum level)

When the first byte is:

• 0x01: Print "Saying hello\n" via the console, and play the audio on song
• 0xFE: Enter the bootloader - only works when CONSOLE_IN_BOOTLOADER is defined in the config.h

This needs to be fleshed out a bit more with some sort of versioning.