9.3.2 Starting of Clocks
Once the power has stabilized and the internal Reset is released, the device will use a 4MHz clock by default. The clock source for this clock signal is OSC16M, which is enabled and configured at 4MHz after a reset by default. This is also the default time base for Generic Clock Generator 0. In turn, Generator 0 provides the main clock GCLK_MAIN which is used by the Power Manager (PM).
Some synchronous system clocks are active after Start-Up, allowing software execution. Synchronous system clocks that are running receive the 4MHz clock from Generic Clock Generator 0. Other generic clocks are disabled.
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