Shawon Shahryiar over at Embedded Lab talks about Atmel XMega in this tutorial:
At present we have different families of XMega devices. Of these many we are focused toward the AU series only throughout this post and its following ones. I’ll explain later why I’m interested in this series. This is the first post of a series of posts that I’ll be publishing time-to-time. Well I should admit that making tutorial notes is not my favourite hobby but it helps me to keep documentations for future uses, reference, etc. and at the same time help others to get an easy access that I have crossed with lot of difficulties.