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](https://github.com/qmk/qmk_toolbox/releases) 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.
If you have a bootloader selected with the `BOOTLOADER` variable in your `rules.mk`, QMK will automatically calculate if your .hex file is the right size to be flashed to the device, and output the total size in bytes (along with the max). To run this process manually, compile with the target `check-size`, eg `make planck/rev4:default:check-size`.
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.
QMK has a fork of the LUFA DFU bootloader that allows for a simple matrix scan for exiting the bootloader and returning to the application, as well as flashing an LED/making a ticking noise with a speaker when things are happening. To enable these features, use this block in your `config.h` (The key that exits the bootloader needs to be hooked-up to the INPUT and OUTPUT defined here):
#define QMK_ESC_OUTPUT F1 // usually COL
#define QMK_ESC_INPUT D5 // usually ROW
#define QMK_LED E6
#define QMK_SPEAKER C6
The Manufacturer and Product names are automatically pulled from your `config.h`, and "Bootloader" is added to the product.
To generate this bootloader, use the `bootloader` target, eg `make planck/rev4:default:bootloader`.
To generate a production-ready .hex file (containing the application and the bootloader), use the `production` target, eg `make planck/rev4:default:production`.
There are a number of DFU commands that you can use to flash firmware to a DFU device:
*`:dfu` - This is the normal option and waits until a DFU device is available, and then flashes the firmware. This will check every 5 seconds, to see if a DFU device has appeared.
*`:dfu-ee` - This flashes an `eep` file instead of the normal hex. This is uncommon.
*`:dfu-split-left` - This flashes the normal firmware, just like the default option (`:dfu`). However, this also flashes the "Left Side" EEPROM file for split keyboards. _This is ideal for Elite C based split keyboards._
*`:dfu-split-right` - This flashes the normal firmware, just like the default option (`:dfu`). However, this also flashes the "Right Side" EEPROM file for split keyboards. _This is ideal for Elite C based split keyboards._
Arduino boards and their clones use the [Caterina bootloader](https://github.com/arduino/ArduinoCore-avr/tree/master/bootloaders/caterina) (any keyboard built with a Pro Micro, or clone), and uses the avr109 protocol to communicate through virtual serial. Bootloaders like [A-Star](https://www.pololu.com/docs/0J61/9) are based on Caterina.
4. Reset the device into application mode (may be done automatically)
## STM32
All STM32 chips come preloaded with a factory bootloader that cannot be modified nor deleted. Some STM32 chips have bootloaders that do not come with USB programming (e.g. STM32F103) but the process is still the same.
At the moment, no `BOOTLOADER` variable is needed on `rules.mk` for STM32.
1. Enter the bootloader using any of the following methods:
* Tap the `RESET` keycode (may not work on STM32F042 devices)
* If a reset circuit is present, tap the RESET button
* Otherwise, you need to bridge BOOT0 to VCC (via BOOT0 button or bridge), short RESET to GND (via RESET button or bridge), and then let go of the BOOT0 bridge
2. Wait for the OS to detect the device
3. Flash a .bin file
* You will receive a warning about the DFU signature; Just ignore it
4. Reset the device into application mode (may be done automatically)
* If you are building from command line (e.g. `make planck/rev6:default:dfu-util`), make sure that `:leave` is passed to the `DFU_ARGS` variable inside your `rules.mk` (e.g. `DFU_ARGS = -d 0483:df11 -a 0 -s 0x08000000:leave`) so that your device resets after flashing
