NRF52811-QCAA-R7 datasheet(64/444 Pages) NORDIC

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Power and clock management

The LFCLK clock is stopped by triggering the LFCLKSTOP task.

It is not allowed to write to register LFCLKSRC on page 71 when the LFCLK is running.

A LFCLKSTOP task will stop the LFCLK oscillator. However, the LFCLKSTOP task can only be triggered after

the STATE field in register LFCLKSTAT on page 71 indicates a 'LFCLK running' state.

The LFCLK clock controller and all of the LFCLK clock sources are always switched off when in OFF mode.

5.4.2.1 32.768 kHz RC oscillator (LFRC)

The default source of the low frequency clock (LFCLK) is the 32.768 kHz RC oscillator (LFRC).

The LFRC frequency will be affected by variation in temperature. The LFRC oscillator can be calibrated to

improve accuracy by using the HFXO as a reference oscillator during calibration. See Table 32.768 kHz RC

oscillator (LFRC) on page 73 for details on the default and calibrated accuracy of the LFRC oscillator.

The LFRC oscillator does not require additional external components.

5.4.2.2 Calibrating the 32.768 kHz RC oscillator

After the 32.768 kHz RC oscillator is started and running, it can be calibrated by triggering the CAL task. In

this case, the HFCLK will be temporarily switched on and used as a reference.

A DONE event will be generated when calibration has finished. The calibration mechanism will only work

as long as HFCLK is generated from the HFCLK crystal oscillator, it is therefore necessary to explicitly start

this crystal oscillator before calibration can be started, see HFCLKSTART task.

It is not allowed to stop the LFRC during an ongoing calibration.

5.4.2.3 Calibration timer

The calibration timer can be used to time the calibration interval of the 32.768 kHz RC oscillator.

The calibration timer is started by triggering the CTSTART task and stopped by triggering the CTSTOP task.

The calibration timer will always start counting down from the value specified in CTIV and generate a CTTO

timeout event when it reaches 0. The Calibration timer will stop by itself when it reaches 0.

Calibration

timer

CTSTART

CTSTOP

CTTO

CTIV

Figure 19: Calibration timer

Due to limitations in the calibration timer, only one task related to calibration, that is, CAL, CTSTART and

CTSTOP, can be triggered for every period of LFCLK.

5.4.2.4 32.768 kHz crystal oscillator (LFXO)

For higher LFCLK accuracy the low frequency crystal oscillator (LFXO) must be used.

The following external clock sources are supported:

• Low swing clock signal applied to the XL1 pin. The XL2 pin shall then be grounded.

• Rail-to-rail clock signal applied to the XL1 pin. The XL2 pin shall then be grounded or left unconnected.

The LFCLKSRC on page 71 register controls the clock source, and its allowed swing. The truth table for

various situations is as follows:

4454_140 v1.0

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