[Chaos in a Can]

You don't need to include "encoder.c", and in fact, you probably shouldn't.

I don't think that's the problem, but if you include a .c file, you're probably going to get an error about something like multiple declarations of something.

The headers tells the file that a function exists somewhere in one of the .c files. The actual code for that function doesn't need to be in the same .c file the function is used in.