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 the external clock source (RSTOSC = 111)
- Program the FEXTOSC Configuration
                    bits to select the appropriate External Clock (EC) mode:- ECH mode for oscillators operating at or above 16 MHz (FEXTOSC =  11)
- ECL mode for oscillator operating below 16 MHz (FEXTOSC =  01)
 
- ECH mode for oscillators operating at or above 16 MHz (FEXTOSC =  
