taken from Hasu codebase (https://github.com/tmk/tmk_keyboard) usb.c/.h
and usb_extra.c/.h, though these files only have the PJRC
copyright/license in the header
(plus a few small aesthetic things in /src/keyboard. i changed some
function like macros to lower-case, because someday they might be
implemented as real functions... and there's no real reason to
distinguish between functions and function like macros in the main() and
other higher level code. at least that's what it seems like to me right
now.)
also, mcp23018_init() needs fixing: `twi_stop()` needs to be at the end
of transmission blocks. i wouldn't think that would be necessary, but
it seems to be the only thing that'll make it work, and it also seems
consistent with the protocol diagram in the datasheet (lol, imagine
that), so i think that's what i'll have to do. not as though it matters
much i guess, since it's a single master / single slave system anyway, i
was just hoping not to release the bus till i was finished..
Stefan Dorn
Jacob McIntosh
Ryan Prince
Ben Blazak