The Atmel® XMEGA® Clock System is a set of highly flexible modules that provides a large portfolio of internal and external clock sources. An internal high-frequency PLL and a flexible prescaler block provide a vast amount of possible clock source configurations, both for the CPU and peripherals. An external oscillator failure detector and optional automatic runtime calibration of internal oscillators reduce external component count and help designing for accuracy and safety.
This application describes the XMEGA Clock System with detailed configuration procedures and a readyto-use software driver.
Read more Here