[nikital]

Awesome post! I'm confused about the schematics of the keyboard and I wonder if anybody here might point me in the right direction: The HWB pin is pulled permanently low, so the MCU runs the default USB DFU bootloader on startup/reset. Then the application firmware can be programmed using dfu-programmer.

But what happens after programming, when you unplug the keyboard and plug it back in? Since HWB is still pulled low, doesn't the MCU go back into the DFU bootloader instead of running the application? How does the keyboard run the actual application firmware rather than the bootloader?

[chadaustin]

Good eyes! That’s a handy feature of this MCU: power-on boots start the application, not the bootloader. External resets switch to the bootloader if HWB is pulled low.