8.2 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 include: oscillator modules (ECH, ECM, ECL mode) and quartz crystal resonators or ceramic resonators (LP, XT and HS modes).

There is also a secondary oscillator block which is optimized for a 32.768 kHz external clock source, which can be used as an alternate clock source.

The HFINTOSC can produce clock frequencies from 1-32 MHz, and is responsible for generating the two MFINTOSC frequencies (500 kHz and 32 kHz) that can be used by some peripherals. The LFINTOSC generates a 31 kHz clock frequency.

There is a 4x PLL that can be used by the external oscillator. Additionally, there is a 2x PLL that can be used by the HFINTOSC at certain frequencies.