Editing Flashing bootloaders on AVR (section)

=== What bootloader to write? ===

There's usually quite a few bootloaders that you could use; but this would be for another document. So just briefly - atmega32u4-based keyboards generally seem to use the DFU bootloader, and that one's possible compile yourself either from ATMEL's sources, or from the LUFA library.

For the atmega32a-based keyboards, you should first try to get the one that your PCB came with - and this generally means asking the person who made it for one. They are all based on the same code [https://www.obdev.at/products/vusb/bootloadhid.html bootloadHID], but the point is that keyboard makers seems to do some modifications to it, namely the condition when to enter the bootloader, and maybe add some visual signal that the bootloader is running.

(To fill in some general info on AVR bootloaders, here's how things work: The upper part of the flash memory on AVR chips (&quot;the end&quot;) can contain some bootloader code. When an ATMEL chip powers up or resets, it checks its fuses, and if they say that it should start with a bootloader (rather than normal firmware at the beginning of the flash), it starts executing the code near the end of the flash memory - where the bootloader should reside. Now the bootloader somehow determines whether it should jump back to the &quot;application code&quot; at the beginning of the flash, or whether it should continue running.)