NRF52820-QDAA-R datasheet(72/437 Pages) NORDIC

72 / 437 page

Power and clock management

The LFCLK clock is started by first selecting the preferred clock source in register LFCLKSRC on page 81

and then triggering the LFCLKSTART task. If the LFXO is selected as the clock source, the LFCLK will initially

start running from the 32.768 kHz LFRC while the LFXO is starting up and automatically switch to using the

LFXO once this oscillator is running. The LFCLKSTARTED event will be generated when the LFXO has been

started.

The register LFXODEBOUNCE on page 81 is used to configure the LFXO debounce time. The

Recommended operating conditions on page 430. The LFXO start up time will be increased as a result.

The LFCLK clock is stopped by triggering the LFCLKSTOP task.

various situations is as follows:

SRC

EXTERNAL

BYPASS

Comment

Normal operation, LFRC is source

DO NOT USE

Normal XTAL operation

Apply external low swing signal to XL1, ground XL2

Apply external full swing signal to XL1, leave XL2 grounded or unconnected

DO NOT USE

Normal operation, LFSYNT is source

DO NOT USE

Table 18: LFCLKSRC configuration depending on clock source

It is not allowed to write to register LFCLKSRC on page 81 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 80 indicates LFCLK running state.

The synthesized 32.768 kHz clock depends on the HFCLK to run. If high accuracy is required for the LFCLK

running off the synthesized 32.768 kHz clock, the HFCLK must running from the HFXO source.

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 oscillator does not require additional external components.

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.

5.4.2.2 Calibrating the 32.768 kHz RC oscillator

After the LFRC oscillator is started and running, it can be calibrated by triggering the CAL task.

The LFRC oscillator will then temporarily request the HFCLK to be used as a reference for the calibration. A

DONE event will be generated when calibration has finished. The HFCLK crystal oscillator has to be started

(by triggering the HFCLKSTART task) in order for the calibration mechanism to work.

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 ( Retained ) on page

4463_156 v1.0

NRF52820-QDAA-R Datasheet(PDF) 72 Page - Nordic Semiconductor

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