3.3.1 Starting of Clocks

After power-up, the device is set to its initial state and kept in reset until the power has stabilized throughout the device. Once the power has stabilized, the Boot ROM will initialize the device and select the internal OSCHF divided to a 4 MHz clock signal as the default clock for the device. This is also used as the clock source for Generic Clock Generator 0 (GCLK0). In turn, GCLK0 provides the main clock, GCLK_MAIN, which is used by the Main Clock (MCLK) peripheral. Any further configuration of the clock system is done by the application.

Some synchronous system clocks are active after startup, allowing software execution.

Refer to the Clock Mask Register in the MCLK - Main Clock section for the list of default peripheral clocks that are running.