1030
SAM4CP [DATASHEET]
43051E–ATPL–08/14
45.5.10.6 Crystal Oscillator Design Considerations Information
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 are as follows:
Main Oscillator Input
CLKOUT signal is intended to be used as XIN (system main oscillator) input. When using an external
crystal/oscillator as XIN input instead CLKOUT signal, CLKOUT should be switched off by software to minimize
noise due to coupling in close pins.
Crystal Load Capacitance
The total capacitance loading the crystal, including the oscillator′s internal parasitics and the PCB parasitics, must
match the load capacitance for which the crystal′s frequency is specified. Any mismatch in the load capacitance
with respect to the crystal′s specification will lead to inaccurate oscillation frequency.
Crystal Drive Level
Use only crystals with the specified drive levels greater than the specified MCU oscillator drive level. Applications
that do not respect this criterion may damage the crystal.
Crystal Equivalent Series Resistor (ESR)
Use only crystals with the specified ESR lower than the specified MCU oscillator ESR. In applications where this
criterion is not respected, the crystal oscillator may not start.
Crystal Shunt Capacitance
Use only crystal with the specified shunt capacitance lower than the specified MCU oscillator shunt capacitance. In
applications where this criterion is not respected, the crystal oscillator may not start.
PCB Layout Considerations
To minimize inductive and capacitive parasitics associated with XIN, XOUT, XIN32, XOUT32, CLKEA, CLKEB
nets, it is recommended to route them as short as possible. It is also of prime importance to keep those nets away
from noisy switching signals (clock, data, PWM, etc...). A good practice is to shield them with a quiet ground net to
avoid coupling to neighbour signals.