There are quite a few different types of bootloaders that keyboards use, and just about all of the use a different flashing method. Luckily, projects like the QMK Toolbox aim to be compatible with all the different types without having to think about it much, but this article will describe the different types of bootloaders, and available methods for flashing them.
Atmel's DFU bootloader comes on all atmega32u4 chips by default, and is used by many keyboards that have their own ICs on their PCBs (Older OLKB boards, Clueboards). Some keyboards may also use LUFA's DFU bootloader (or QMK's fork) (Newer OLKB boards) that adds in additional features specific to that hardware.
These bootloaders are usually 4096 bytes for the atmega32u4 chip.
Compatible flashers:
:dfu
in QMK (recommended commandline)Flashing sequence:
RESET
keycode, or tap the RESET button (or short RST to GND).or:
make <keyboard>:<keymap>:dfu
Arduino boards and their clones use the Caterina bootloader (any keyboard built with a Pro Micro, or clone), and uses the avr109 protocol to communicate through virtual serial. Bootloaders like A-Star are based on Caterina.
This block of code allows for Caterina compatibility in QMK:
#define CATERINA_BOOTLOADER
These bootloaders are usually 4096 bytes for the atmega32u4 chip.
Compatible flashers:
:avrdude
(recommended commandline)Flashing sequence:
RESET
keycode, or short RST to GND quickly (you only have 7 seconds to flash once it enters)or
make <keyboard>:<keymap>:avrdude
Halfkay is a super-slim protocol developed by PJRC that uses HID, and come on all Teensys (namely the 2.0).
This bootloader is 512 bytes.
Compatible flashers:
Flashing sequence:
RESET
keycode, or short RST to GND quickly (you only have 7 seconds to flash once it enters)