The system supports the following low frequency clock sources, as described in Clock control.
- 32.768 kHz RC oscillator (LFRC)
- 32.768 kHz crystal oscillator (LFXO)
- 32.768 kHz synthesized from HFCLK (LFSYNT)
LFXO can run in System OFF mode. The other clock sources only run in System ON mode.
The following LF clocks are available in the system.
| Clock | Description |
|---|---|
| PCLK32KI | 32.768 kHz peripheral low-frequency clock. |
| GRTC.LFLPRC | Direct path from 32.768 kHz internal oscillator (LFRC) to GRTC peripheral. Available only in System ON mode. If GRTC.LFLPRC is used, stop GRTC before entering System OFF mode. |
| GRTC.LFXO | Direct path from 32.768 kHz crystal oscillator (LFXO) to GRTC peripheral. Available in System ON or System OFF mode. |
When a peripheral requires the PCLK32KI clock, the LFCLK control automatically requests the LFCLK clock for the power and clock subsystem. The default LFCLK source is the LFRC.
To use a different LFCLK source, select the preferred clock source in register LFCLK.SRC and then trigger the LFCLKSTART task. If LFXO is selected as the clock source, LFCLK initially starts running from the 32.768 kHz LFRC then automatically switches to the crystal once available. The LFCLKSTARTED event is then generated.
When switching the LFCLK source, such as from LFRC to LFXO , up to one LFCLK cycle may be lost.
The LFCLKSTART task will request the clock to keep running until triggering the LFCLKSTOP task to stop the clock.
The LFCLK clock is stopped when there are no requests. For example, WDT is stopped, and the LFCLKSTOP task is triggered. Triggering the LFCLKSTOP task is required after the LFCLKSTART task has been triggered.