I used:
- ATMEL data sheet: http://www.atmel.com/Images/doc11057.pdf
- API ref: http://asf.atmel.com/docs/latest/api.html
- the code (in the arduino dist)
You have to do some homework as none of the above is a tutorial. Most of the functions/defines were not obvious until I read the code. Had to do the same thing for the DAC interface (e.g., to enable flexible & word mode updates).
Best,
Bruce