NRF52820-QDAA-R datasheet(73/437 Pages) NORDIC

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Power and clock management

82 and generate a CTTO event when it reaches 0. The calibration timer will automatically stop when it

reaches 0.

Calibration

timer

CTSTART

CTSTOP

CTTO

CTIV

CTSTARTED

CTSTOPPED

Figure 22: Calibration timer

After a CTSTART task has been triggered, the calibration timer will ignore further tasks until it has returned

the CTSTARTED event. Likewise, after a CTSTOP task has been triggered, the calibration timer will ignore

further tasks until it has returned a CTSTOPPED event. Triggering CTSTART while the calibration timer

is running will immediately return a CTSTARTED event. Triggering CTSTOP when the calibration timer is

stopped will immediately return a CTSTOPPED event.

5.4.2.4 32.768 kHz crystal oscillator (LFXO)

For higher LFCLK accuracy (when better than +/- 500 ppm accuracy is required), 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.

To achieve correct oscillation frequency, the load capacitance must match the specification in the crystal

data sheet. Circuit diagram of the 32.768 kHz crystal oscillator on page 73 shows the LFXO circuitry.

32.768 kHz

crystal

XL1

XL2

Figure 23: Circuit diagram of the 32.768 kHz crystal oscillator

The load capacitance (CL) is the total capacitance seen by the crystal across its terminals and is given by:

C1 and C2 are ceramic SMD capacitors connected between each crystal terminal and ground. Cpcb1 and

Cpcb2 are stray capacitances on the PCB. Cpin is the pin input capacitance on the XC1 and XC2 pins (see Low

frequency crystal oscillator (LFXO) on page 83). The load capacitors C1 and C2 should have the same

value.

4463_156 v1.0

NRF52820-QDAA-R Datasheet(PDF) 73 Page - Nordic Semiconductor