Specifications

88 JN-DS-JN5148-001 1v9 © NXP Laboratories UK 2013

Appendix B Development Support

B.1 Crystal Oscillators

This section covers some of the general background to crystal oscillators, to help the user make informed decisions

concerning the choice of crystal and the associated capacitors.

B.1.1 Crystal Equivalent Circuit

C2C1

Where

is the motional capacitance

is the motional inductance. This together with

defines the oscillation frequency (series)

is the equivalent series resistance ( ESR ).

is the shunt or package capacitance and this is a parasitic

B.1.2 Crystal Load Capacitance

The crystal load capacitance is the total capacitance seen at the crystal pins, from all sources. As the load

capacitance (CL) affects the oscillation frequency by a process known as ‘pulling’, crystal manufacturers specify the

frequency for a given load capacitance only. A typical pulling coefficient is 15ppm/pF, to put this into context the

maximum frequency error in the IEEE802.15.4 specification is +/-40ppm for the transmitted signal. Therefore, it is

important for resonance at 32MHz exactly, that the specified load capacitance is provided.

The load capacitance can be calculated using:

Total capacitance

inPT CCCC 1111

++=

Where

is the capacitor component

is the PCB parasitic capacitance. With the recommended layout this is about 1.6pF

inC1

is the on-chip parasitic capacitance and is about 1.4pF typically.

Similarly for

Hence for a 9pF load capacitance, and a tight layout the external capacitors should be 15pF