23.6.4 16MHz Internal Oscillator (OSC16M) Operation
The OSC16M is an internal oscillator operating in open-loop mode and generating 4, 8, 12, or 16MHz frequency. The OSC16M frequency is selected by writing to the Frequency Select field in the OSC16M register (OSC16MCTRL.FSEL). OSC16M is enabled by writing '1' to the Oscillator Enable bit in the OSC16M Control register (OSC16MCTRL.ENABLE), and disabled by writing a '0' to this bit. Frequency selection must be done when OSC16M is disabled.
After enabling OSC16M, the OSC16M clock is output as soon as the oscillator is ready (STATUS.OSC16MRDY=1). User must ensure that the OSC16M is fully disabled before enabling it by reading STATUS.OSC16MRDY=0.
After reset, OSC16M is enabled and serves as the default clock source at 4MHz.
OSC16M will behave differently in different sleep modes based on the settings of OSC16MCTRL.RUNSTDBY, OSC16MCTRL.ONDEMAND, and OSC16MCTRL.ENABLE. If OSC16MCTRL.ENABLE=0, the OSC16M will be always stopped. For OSC16MCTRL.ENABLE=1, this table is valid:
| CPU Mode | OSC16MCTRL.RUNSTDBY | OSC16MCTRL.ONDEMAND | Sleep Behavior |
|---|---|---|---|
| Active or Idle | - | 0 | Always run |
| Active or Idle | - | 1 | Run if requested by peripheral |
| Standby | 1 | 0 | Always run |
| Standby | 1 | 1 | Run if requested by peripheral |
| Standby | 0 | - | Run if requested by peripheral |
OSC16M is used as a clock source for the generic clock generators. This is configured by the Generic Clock Generator Controller.