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Regular Contributor
Posts: 27
Registered: ‎04-14-2009

crystal capacitors for an Atmel AVR32

How do I select the crystal capacitors for an Atmel AVR32?
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Contributor
Posts: 26
Registered: ‎04-16-2009

Re: crystal capacitors for an Atmel AVR32

In a commonly used approximation, the crystal capacitors rely on three parameters:
1. The Cl which is the crystal equivalent capacitance the crystal must see to operate at its typical frequency.
2. The Ci which is the internal capacitance of the MCU oscillator (see datasheet).
3. The Cs which is the stray capacitance of the circuit board. The Cs is negligible as crystal is usually connected very close to the MCU.
The calculation formula is (C1 and C2 being the capacitance on XIN and XOUT pins)
Cl = (C1 * C2) / (C1 + C2) + Ci + Cs (1)
If we consider C1 = C2 = C and Cs negligible:
(1) => Cl = C / 2 + Ci (2)
(2) => C = 2(Cl - Ci)