# Internal Clock Sources

The internal oscillator block contains two independent oscillators that can produce two internal system clock sources:
• High-Frequency Internal Oscillator (HFINTOSC)
• Low-Frequency Internal Oscillator (LFINTOSC)
Internal oscillator selection is performed one of two ways:
1. 1.Program the RSTOSC Configuration bits to select one of the INTOSC sources which will be used upon a device Reset.
2. 2.Write the New Oscillator Source Request (NOSC) bits to select an internal oscillator during run time.

In INTOSC mode, the OSC1/CLKIN and OSC2/CLKOUT pins are available for use as a general purpose I/Os, provided that no external oscillator is connected. The function of the OSC2/CLKOUT pin is determined by the CLKOUTEN Configuration bit. When CLKOUTEN is set (CLKOUTEN = 1), the pin functions as a general-purpose I/O. When CLKOUTEN is clear (CLKOUTEN = 0), the system instruction clock (FOSC/4) is available as an output signal on the pin.