To get started on hacking the firmware, you will need some software first. See FirmWareDevelopment for install instructions.
See the MicroProcessor page for a list of inputs/output ports.
Here is a list of important functions and macros that you need to know to understand what is happening with the firmware:
|cbi||Clear Bit at Port||port #, bit #|
|sbi||Set Bit at Port||port #, bit #|
|cli||Turn Off Interupts|
|sei||Turn On Interupts|