External load capacitors on 32.768 kHz crystal oscillators

Question

I am trying to design a circuit with the MCP79402 RTC. However, I do not understand why the crystal oscillators require external capacitors, and I am confused on how to calculate values for said capacitors. Below is the page in the datasheet related to the calculation of the load capacitors. Here is the crystal I want to use. I am using the 6pF version as the RTC requires a crystal with 6-9pF capacitance.

Andy aka · Answer 1 · 2022-07-30T19:01:03.260

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I do not understand why the crystal oscillators require external capacitors

Without capacitors, the crystal will oscillate at an inaccurate frequency or may not oscillate at all. With too much capacitance, the crystal may exceed is maximum power and become damaged.

If the load capacitor stated in the data sheet is 6 pF then, in total you should initially choose 2 x 12 pF capacitors then, with a little bit of thought about parasitic circuit board capacitance and gate input capacitance, you would might choose 2 x 10 pF capacitors. The two series 10 pF capacitors form a load of 5 pF but, with parasites etc. it'll be closer to 6 pf.

edited Jul 30 '22 at 19:01

answered Jul 30 '22 at 18:45

Andy aka

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Why does the IC datasheet specify crystals that requires a specific load capacitance? In this case it says 6-9 pF CL crystals, and specifically dont recommend a 12.5pF CL crystal. – Linkyyy Jul 30 '22 at 19:28
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@Linkyyy two reasons: (a) if you operate the crystal in an oscillator circuit of a different type to the very commonly used pierce oscillator, you might use one capacitor to parallel tune it and (b), you may decide, in different pierce oscillator configurations to make the two capacitors asymmetrical in value and, what you then need to know is the true effective combined net capacitance. – Andy aka Jul 30 '22 at 19:43
Makes sense. Thanks. – Linkyyy Jul 30 '22 at 21:44

glen_geek · Accepted Answer · 2022-07-30T20:25:49.020

The crystals offered by the manufacturer mentioned can be specified with different \$C_L\$ - if you want the "6 pf" version, the part # would start:
ABS05 – 32.768 kHz-6...

Very often, the two added capacitors have equal value. To oscillate at the desired frequency of 32768 Hz., you'd want \$C_L=6pf\$.
You might estimate stray capacitance (\$C_{STRAY}\$) to be a few picofarads. That would require \$C_{x1}=C_{x2} = 10 pf\$ or perhaps 9 pf as starting values.

If oscillating frequency comes out below 32768 Hz., then you might try reducing values of \$C_{x1}, C_{x2}\$, perhaps 8.2 pf. You may find that some crystals fail to oscillate. \$C_{stray}\$ depends on layout, so hunting for good values of \$C_{x1}, C_{x2}\$ requires testing.

External load capacitors on 32.768 kHz crystal oscillators

2 Answers2