There are some constraints to the oscillators on the samd5x.
- DFLL is fixed to run at 48 MHz
- DPLL can run at 96 to 200 MHz
Always use DFLL for frequencies <= 48 MHz.
For frequencies >= 96 MHz, use DPLL directly.
For frequencies < 96 MHz, clock DPLL at twice the desired frequency
and use a divider.
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