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My understanding of how this works is quite poor, I've tried reading many sources but just got even more confused by the terms crystals, crystal resonators, ceramic resonators, oscillators, crystal oscillators.

What I know is that when you have a simple 2 pin crystal you add 2 caps to it so that it resonates properly. And the AVR chip I am connecting it too (Atmega328p) has some other internal circuitry for it to work.

I am looking at using an smd crystal in my design and my question is, for a crystal like this: enter image description here

Do I still need the external capacitors or not? I originally thought I would but looking at the specs of the parts they all have a label "capacitance". Does that imply it has caps inside? Or is that recommended value for my caps? Or is that some other capacitance?

Sorry if this doesn't make sense I am just really confused. Thanks in advance!

Edit to include specifics: I have these crystals https://lcsc.com/product-detail/SMD-Crystal-Resonators_Seiko-Epson-X1E000021011900_C89371.html But it seems they are quite standard so here is one with English datasheet: https://www.mouser.co.uk/ProductDetail/ECS/ECS-160-12-37B-CTN-TR?qs=sGAEpiMZZMukHu%252BjC5l7Yb4vS8HZNl%252BL%2F78jMNI6Q2E%3D

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  • \$\begingroup\$ I suggest that you study existing "microcontroller with a crystal oscillator" designs and try to find one that does not have the two crystal load capacitors. I bet that you will not find any because they are needed. In some cases the oscillator might work without the capacitors but no one counts on that. You're making too much fuss over two small capacitors. \$\endgroup\$
    – Bimpelrekkie
    Commented Aug 12, 2020 at 12:07
  • \$\begingroup\$ I think the confusion might come from the fact that ceramic resonators can include the internal load capacitors and that is why they have a ground pin too. \$\endgroup\$
    – Justme
    Commented Aug 12, 2020 at 12:10
  • \$\begingroup\$ Ye ik they are required in the cercuit somewhere but I've seen people use parts which contain the capacitors inside them so no additional ones required outside. But don't know what its called. \$\endgroup\$
    – Eugene Levinson
    Commented Aug 12, 2020 at 12:10

1 Answer 1

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These parts are crystals that can be used with typical microcontrollers that have a built-in Pierce oscillator, and thus require the rated external load capacitance in order to oscillate at the rated frequency.

So yes, external capacitors are needed as usual for crystals. The extra two pins are just to ground the metal lid.

And no, the rated load capacitance does not mean the value of the capacitors. It is the total capacitance as seen by the crystal, which includes the capacitors, PCB wire capacitance, MCU IO pin capacitance and other stray capacitances. The ECS part is rated for 12pF load capacitance, so two 24pF capacitors would give it 12pF of load if no other capacitances are considered. But the capacitors must have smaller value due to the other stray capacitances.16pF capacitors would be used to give 8pF load if the stray capacitances are approximated to be 4pF.

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  • \$\begingroup\$ Thanks a lot! So the specified capacitance is the recoomended/required value of the capacitors? \$\endgroup\$
    – Eugene Levinson
    Commented Aug 12, 2020 at 12:08
  • \$\begingroup\$ @JustMe What makes you say they are "meant for parallel resonance circuit"? \$\endgroup\$
    – Andy aka
    Commented Aug 12, 2020 at 12:09
  • \$\begingroup\$ @Andyaka because they have a load capacitance rating it means they are for parallel resonant circuit. Crystals meant for series resonance don't have load capacitance rating, as they read "series" in place of the capacitance. Of course the crystal does not know in which circuit it is used, it will still oscillate but it won't run at the rated frequency. \$\endgroup\$
    – Justme
    Commented Aug 12, 2020 at 12:14
  • \$\begingroup\$ When there are load capacitors used, the actual crystal runs close to its series resonance i.e. it's low impedance point. This is how it produces a phase shift of nearly 180 degrees. It can't achieve that when run in it's parallel resonance point. Maybe you are (we are) confusing the terms here? An xtal for a standard MCU oscillator will run close to its series resonant point. \$\endgroup\$
    – Andy aka
    Commented Aug 12, 2020 at 12:50
  • \$\begingroup\$ @Andyaka You are quite right, the crystal is not run at the parallel resonance point. The actual operating point is between series and parallel resonance points, called the "area of usual parallel resonance", often simplified to as just "parallel resonance". In that area, the reactance is positive, so the crystal looks inductive. The oscillator circuits are also simply classified as series or parallel circuit, and the Pierce type which is typical oscillator type within MCUs is a parallel type. The crystal is just manufactured so that it runs at the labeled frequency with the rated capacitance. \$\endgroup\$
    – Justme
    Commented Aug 12, 2020 at 14:36

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