# Clock Source Types

Clock sources can be classified as external or internal.

External clock sources rely on external circuitry for the clock source to function. Examples of external clock sources include:
• Digital oscillator modules
• Quartz crystal resonators
• Ceramic resonators

A 4x PLL is provided for use with external clock sources.

Internal clock sources are contained within the oscillator module. The internal oscillator block features two internal oscillators that are used to generate internal system clock sources. The High-Frequency Internal Oscillator (HFINTOSC) can produce a wide range of frequencies which are determined via the HFINTOSC Frequency Selection (OSCFRQ) register. The Low-Frequency Internal Oscillator (LFINTOSC) generates a fixed nominal 31 kHz clock signal. The internal oscillator block also features an RC oscillator which is dedicated to the Analog-to-Digital Converter (ADC).

The oscillator module allows the system clock source or system clock frequency to be changed through clock switching. Clock source selections are made via the New Oscillator Source Request (NOSC) bits. Once the clock source has been selected, the clock source base frequency can be divided (post-scaled) via the New Divider Selection Request (NDIV) bits.

The instruction clock (FOSC/4) can be routed to the OSC2/CLKOUT pin when the pin is not in use. The Clock Out Enable (CLKOUTEN) Configuration bit controls the functionality of the CLKOUT signal. When CLKOUTEN is clear (CLKOUTEN = 0), the CLKOUT signal is routed to the OSC2/CLKOUT pin. When CLKOUTEN is set (CLKOUTEN = 1), the OSC2/CLKOUT pin functions as an I/O pin.