How do I deal with different XTAL pin capacitance when selecting XTAL and load capacitors

Question

I'm trying to select a 32kHz crystal and appropriate load capacitors for Atmega48A MCU.

Atmega48A specifies different capacitances for Xtal1 and Xtal2 pins:

18pF for Xtal1 8pF for Xtal2

The datasheet says: "The capacitance (Ce+Ci) needed at each TOSC pin can be calculated by using:C = 2 * CL – Cs" Where CL is load capacitance specified for the crystal and Cs is stray capacitance.

Can I assume that each pin has (18+8)/2=13pF of capacitance? Can I select a 8pF crystal and use it without external load caps assuming ~3pF of stray capacitance?

score 2 · Answer 1 · answered Mar 14 '15 at 21:20

2

The capacitance of the two pins in series is (18*8)/(18+8) = 5.5pF. So with a little stray capacitance (5pF per pin, approx. 2.5pF in series), that should match the 8pF crystal without additional capacitance.

answered Mar 14 '15 at 21:20

rioraxe

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Seems you know your stuff... could you take a peek at http://electronics.stackexchange.com/questions/250608/crystal-oscillator-load-capacitance-again to see if you have any comments? – PkP Aug 04 '16 at 10:38

How do I deal with different XTAL pin capacitance when selecting XTAL and load capacitors

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