LFCLK controller

Each core has a number of low frequency clock (LFCLK) control instances. Each instance distributes one or more clocks to the core.

The LFCLK control instance in each core distributes the 32.768 kHz PCLK32KI peripheral clock to its corresponding core. The LFCLK clock is sourced from the power and clock subsystem to each LFCLK control instance.

The LFCLK controller and all LFCLK sources are switched off in System OFF mode.

When peripherals require the PCLK32KI clock, such as RTC — Real-time counter and WDT — Watchdog timer, the LFCLK control will automatically request the LFCLK clock to the power and clock subsystem. The default LFCLK source is the LFRC.

When LFCLK control requests are stopped, LFCLK will stop requesting clock from the power and clock subsystem. If there are no LFCLK control requests from other cores, the power and clock subsystem will automatically stop the LFCLK clock and the LFRC source.

The LFCLK source may also be started by triggering the LFCLKSTART task. The LFCLK source is configured by selecting the preferred LFCLK source in register LFCLKSRC. Once selected, the LFCLK source will be started by triggering the LFCLKSTART task.

The LFCLK source can be configured at any time (for instance, when the LFCLK has already been started). The content of the LFCLKSRC register only takes effect when the LFCLKSTART task is triggered.

The LFCLKSTARTED event will be generated after the LFCLKSTART task has been triggered and the LFCLK source has started. Triggering a LFCLKSTART task before the LFCLKSTARTED event from a previous LFCLKSTART task is generated will only generate one LFCLKSTARTED event. Triggering a LFCLKSTART task after a LFCLKSTARTED event is generated will generate a new LFCLKSTARTED event.

The LFCLK clock is stopped when nothing requests it, e.g. RTC — Real-time counter and WDT — Watchdog timer are stopped, and the LFCLKSTOP task is triggered. This must be done for all cores. Triggering the LFCLKSTOP task is required only if the LFCLKSTART task has been triggered before.

When the LFCLKSTART task is triggered, it is possible to trigger a new LFCLKSTART task without triggering a LFCLKSTOP task in between.

If the LFXO is selected as the LFCLK source, the LFCLK clock will initially start running from the LFRC while the LFXO is starting up, and then automatically switch to using the LFXO once this oscillator is running.

Events will be generated in the correct order, even if an LFCLK source that is already started by another LFCLK control instance is requested. The timing of events may differ, depending on whether a LFCLK source is already running or in the process of starting.

If two instances of the LFCLK control system request different LFCLK sources, the power and clock subsystem will secure that the most accurate of the requested LFCLK sources is selected. If one LFCLK control instance requests a particular LFCLK source to stop when another LFCLK control instance (or a peripheral) requests the same source to run, but at a lower accuracy, the power and clock subsystem will switch to the less accurate source. The following table summarizes the priorities of the LFCLK sources.

When switching the LFCLK source, such as from LFRC to LFXO, up to one LFCLK cycle may be lost.