NRF52811-QCAA-R datasheet(63/444 Pages) NORDIC

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Power and clock management

The crystal oscillator is designed for use with an AT-cut quartz crystal in parallel resonant mode. To achieve

correct oscillation frequency, the load capacitance must match the specification in the crystal data sheet.

Circuit diagram of the 64 MHz crystal oscillator on page 63 shows how the 32 MHz crystal is connected

to the 64 MHz crystal oscillator.

32 MHz

crystal

XC1

XC2

Figure 18: Circuit diagram of the 64 MHz 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. For more

information, see Reference circuitry on page 425. Cpcb1 and Cpcb2 are stray capacitances on the PCB. Cpin

is the pin input capacitance on the

XC1 and XC2 pins. See table 64 MHz crystal oscillator (HFXO) on page

72. The load capacitors C1 and C2 should have the same value.

For reliable operation, the crystal load capacitance, shunt capacitance, equivalent series resistance, and

drive level must comply with the specifications in table 64 MHz crystal oscillator (HFXO) on page 72. It

is recommended to use a crystal with lower than maximum load capacitance and/or shunt capacitance. A

low load capacitance will reduce both start up time and current consumption.

5.4.2 LFCLK clock controller

The system supports several low frequency clock sources.

As illustrated in Clock control on page 62, the system supports the following low frequency clock

sources:

• 32.768 kHz RC oscillator (LFRC)

• 32.768 kHz crystal oscillator (LFXO)

• 32.768 kHz synthesized from HFCLK (LFSYNT)

The LFCLK clock is started by first selecting the preferred clock source in register LFCLKSRC on page 71

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.

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NRF52811-QCAA-R Datasheet(PDF) 63 Page - Nordic Semiconductor