Datasheet

716
11011B–ATARM–21-Feb-12
SAM3N
35.4.9 Crystal Oscillators Design Consideration Information
35.4.9.1 Choosing a Crystal
When choosing a crystal for the 32768 Hz Slow Clock Oscillator or for the 3-20 MHz Oscillator,
several parameters must be taken into account. Important parameters between crystal and
SAM3N specifications are as follows:
• Load Capacitance
–C
crystal
is the equivalent capacitor value the oscillator must “show” to the crystal in
order to oscillate at the target frequency. The crystal must be chosen according to
the internal load capacitance (C
L
) of the on-chip oscillator. Having a mismatch for the
load capacitance will result in a frequency drift.
• Drive Level
– Crystal drive level >= Oscillator Drive Level. Having a crystal drive level number
lower than the oscillator specification may damage the crystal.
• Equivalent Series Resistor (ESR)
– Crystal ESR <= Oscillator ESR Max. Having a crystal with ESR value higher than
the oscillator may cause the oscillator to not start.
• Shunt Capacitance
– Max. crystal Shunt capacitance <= Oscillator Shunt Capacitance (
C
SHUNT
). Having a
crystal with ESR value higher than the oscillator may cause the oscillator to not start.
35.4.9.2 Printed Circuit Board (PCB)
SAM3N Oscillators are low power oscillators requiring particular attention when designing PCB
systems.