11.2.1 External Clock Sources
An external clock source can be used as the device system clock by performing the
following actions:
- Program the RSTOSC Configuration bits to select an external clock source that will be used as the default system clock upon a device Reset.
- Program the FEXTOSC Configuration bits to select either the ECH (16 MHz and higher) or ECL (below 16 MHz) mode based on the frequency of the external clock.