For me, one of the first things I wanted to test is how to setup the system clock. In this tutorial I'll be explaining how to set up external oscillator from 8-16Mhz and how to set up the xmega to use internal 32MHz clock. I think, by providing instructions for these two types of system clocks, anyone should be able to understand the logic behind the datasheet and set up whichever clock he/she wants.