9.3 Clocks After Reset

After a Device Reset, the Boot ROM configures the OSCHF oscillator to 4 MHz (OSCHF divided by 6) and assigns it as the clock source for GCLK Generator 0 (GCLK_GEN0), which is also the clock source for MCLK (GCLK_MAIN). All other GCLK generators and 32K clock sources are disabled by a Device Reset.