Data Sheet

Page 28
nRF51822 Product Specification v3.1
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 40 (16 MHz) and Table 23 on page 41 (32 MHz). The load capacitors C1 and C2 should have
the same value. See Chapter 11 “Reference circuitry” on page 76 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 (R
S,X16M
/
R
S,X32M
), and drive level must comply with the specifications in Table 22 on page 40 (16 MHz) and Table 23
on page 41 (32 MHz). It is recommended to use a crystal with lower than maximum R
S,X16M
/R
S,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.
C1 C2
16/32 MHz
crystal
XC1 XC2
CL
C1' C2'
C1' C2'+
----------------------------
=