NRF51822-CFAB-R datasheet(29/130 Pages) NORDIC

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Page 29

nRF51822 Product Specification v3.3

3.6.1

16/32 MHz crystal oscillator

The crystal oscillator can be controlled either by a 16 MHz or a 32 MHz external crystal. However, the system

clock is always 16 MHz, see the nRF51 Series Reference Manual for more details. 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. Figure 7 shows how

the crystal is connected to the 16/32 MHz crystal oscillator.

Figure 7 Circuit diagram of the 16/32 MHz crystal oscillator

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

C1’ = C1 + C_pcb1 + C_pin

C2’ = C2 + C_pcb2 + C_pin

C1 and C2 are ceramic SMD capacitors connected between each crystal terminal and ground. C_pcb1 and

C_pcb2 are stray capacitances on the PCB. C_pin is the pin input capacitance on the XC1 and XC2 pins, see

Table 22 on page 41 (16 MHz) and Table 23 on page 42 (32 MHz). The load capacitors C1 and C2 should have

the same value. See Chapter 11 “Reference circuitry” on page 79 for the capacitance value used for C_pcb1

and C_pcb2 in reference circuitry.

For reliable operation, the crystal load capacitance, shunt capacitance, equivalent series resistance (RS,X16M/

RS,X32M), and drive level must comply with the specifications in Table 22 on page 41 (16 MHz) and Table 23

on page 42 (32 MHz). It is recommended to use a crystal with lower than maximum RS,X16M/RS,X32M if the

load capacitance and/or shunt capacitance is high. This will give faster startup and lower current

consumption. A low load capacitance will reduce both startup time and current consumption.

16/32 MHz

crystal

XC1

XC2

C1' C2'





C1' C2'



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