11.1.1.2 Secondary Oscillator

The Secondary Oscillator (SOSC) is a separate external oscillator block that can be used as an alternate system clock source or as a Timer clock source. The SOSC is optimized for 32.768 kHz, and can be used with either an external quartz crystal connected to the SOSCI and SOSCO pins, or with an external clock source connected to the SOSCI pin as shown in the figures below.

Figure 11-3. SOSC 32.768 kHz Quartz Crystal Oscillator Operation
Figure 11-4. SOSC 32.768 kHz External Clock Operation

The SOSC can be enabled through one of two methods:

  • Programming the RSTOSC Configuration bits to select the SOSC as the system clock
  • Setting the SOSCEN bit to enable the SOSC for use as a peripheral clock source

Two Power modes are available for the secondary oscillator, and are selected using the Secondary Oscillator Power Mode Select (SOSCPWR) bit. When SOSCPWR is clear (SOSCPWR = 0), the oscillator operates in Low Power mode, which is ideal for crystal oscillators with low drive strength. When SOSCPWR is set (SOSCPWR = 1), the oscillator operates in High Power mode, which is ideal for crystal oscillators with high drive strength or high Equivalent Series Resistance (ESR).

Important: The SOSC module must be disabled before changing Power modes. Changes to the Power mode during operation may result in undefined oscillator behavior.