NRF52811-QCAA-R datasheet(65/444 Pages) NORDIC

65 / 444 page

Power and clock management

SRC

EXTERNAL

BYPASS

Comment

Normal operation, RC 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, synth is source

DO NOT USE

Table 17: LFCLKSRC configuration depending on clock source

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 65 shows the LFXO circuitry.

32.768 kHz

crystal

XL1

XL2

Figure 20: 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

32.768 kHz crystal oscillator (LFXO) on page 73). The load capacitors C1 and C2 should have the same

value.

For more information, see Reference circuitry on page 425.

5.4.2.5 32.768 kHz synthesized from HFCLK (LFSYNT)

LFCLK can also be synthesized from the HFCLK clock source. The accuracy of LFCLK will then be the

accuracy of the HFCLK.

Using the LFSYNT clock avoids the requirement for a 32.768 kHz crystal, but increases average power

consumption as the HFCLK will need to be requested in the system.

4454_140 v1.0

NRF52811-QCAA-R Datasheet(PDF) 65 Page - Nordic Semiconductor