There is an article in October's 'Silicon Chip' magazine (www.siliconchip.com.au) that does something similar to what you want to do. It uses an atmel AVR micro and I don't know if they publish the source code. You may be able to disassemble the AVR binary to figure out how it does it's work. Careful- you might get converted to AVR! I know I did!!!!!

I just checked the website -seems they haven't updated it to the latest edition - wait a few days.