JN5148 NXP Semiconductors, JN5148 Datasheet - Page 88

• Manufacturer Part Number: JN5148
• Description: The JN5148 is an ultra low power, high performance MCU combined with an IEEE802
• Manufacturer: NXP Semiconductors

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
Where
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
Where
Similarly for
Hence for a 9pF load capacitance, and a tight layout the external capacitors should be 15pF
88
C
C
C
C
C
L
R
m
m
m
S
1
1
1
in
P
C
is the motional inductance. This together with
is the motional capacitance
is the equivalent series resistance ( ESR ).
is the capacitor component
is the shunt or package capacitance and this is a parasitic
is the PCB parasitic capacitance. With the recommended layout this is about 1.6pF
T
is the on-chip parasitic capacitance and is about 1.4pF typically.
2
C1
Lm
JN-DS-JN5148-001 1v7
CL
C
T
1
=
=
Cs
C
Rm
C
C
1
C
+
T
T
1
1
m
C
×
+
defines the oscillation frequency (series)
1
C
C
P
T
T
+
2
2
C
Cm
C2
1
in
© NXP Laboratories UK 2011