Datasheet

Page 18 of 161
nRF8001 Product Specification
Revision 1.2
6.2.4 32.768 kHz crystal oscillator
The 32.768 kHz crystal oscillator is designed for use with a quartz crystal in parallel resonant mode. To
achieve correct oscillation frequency, the load capacitance must match the specification in the crystal
datasheet. Figure 5. on page 18
shows how the crystal is connected to the 32.768 kHz crystal oscillator.
Figure 5. Circuit diagram of the nRF8001 32.768 kHz crystal oscillator
The load capacitance is the total capacitance seen by the crystal across its terminals and is given by:
C1 and C2 are ceramic SMD capacitors connected between XC1 and XC2 and ground. C_pcb1 and
C_pcb2 are stray capacitances on the PCB. C_pin is the input capacitance on the XC1 and XC2 pins,
typically 1 pF. C1 and C2 should be of the same value.
6.2.5 32.768 kHz RC oscillator
The nRF8001 32.768 kHz RC low frequency oscillator may be used as an alternative to the 32.768 kHz
crystal oscillator. It has a frequency accuracy of ± 250 ppm in a stable temperature environment. The
32.768 kHz RC oscillator does not require external components.
6.2.6 External 32.768 kHz clock
nRF8001 may be used with an external 32.768 kHz clock applied to the XL1 pin. The application controller
sets the reference signal configuration. It can be a rail-to-rail signal or an analog signal. An analog input
signal must have an amplitude of 0.2 V peak-to-peak or greater. Keep the maximum and minimum voltage
levels so that all peak voltages are under recommended maximum operating conditions as specified in
Chapter 11 on page 33
. If the external source is derived from the application controller’s crystal oscillator,
the XL1 pin will load the application’s crystal oscillator with approximately 3 pF in addition to PCB routing.
C1 C2
32.768 kHz
crystal
XL1 XL2
pinCpcbCCC
pinCpcbCCC
CC
CC
load
C
_2_2'2
_1_1'1
)'2'1(
)'2'1(
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


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