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what size of capacitors should I use with this crystal?

This really requires a book, but here goes:

The shunt capacitance of a crystal is easy -- that's just the capacitance due to the quartz as a dielectric and the electrodes as plates. It's the capacitance of the crystal when measured at low frequencies.

The load capacitance is the capacitance you need to present to the crystal to make it resonate at the desired frequency. If you just had a crystal and no amplifier you'd just put that across the crystal:

+---||---+ | | ---+--|X|---+---

Now if that's 20pF and you split it into two capacitors in series, you'll need 40pF caps:

+---||---||---+ | | ---+-----|X|-----+---

Now if you ground the node between the caps you'll get:

---+----|X|----+--- | | === === | | V V

The crystal doesn't know the difference. Now if you hang an amplifier between the two ends of the crystal you'll get:

| +-| >-+ | |/ | | | +-+-|X|-+-+ | | === === | | V V

If the amplifier is a perfect transconductance amplifier with infinite input and output impedance then the crystal will resonate at the same frequency and the output will grow until it stabilizes when the amplifier output limits.

OK, that's cool -- but a real amplifier has some resistance and capacitance at both input and output. If the amplifier will work it'll have enough gain to overcome the input and output resistances. To make the oscillation frequency right _you_ have to subtract the amplifier's input and output capacitances from your 40pF capacitors -- and that's why you see lower capacitances at those spots in real circuits.

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Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
Reply to
Tim Wescott
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Hi - I posted a few weeks ago regarding a similar question, but I must admit I'm still somewhat confused on this topic. I plan on using this crystal:

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watch the word wrap, and datasheet here:

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With an Atmel ATMEGA16:

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But the only thing I can't figure out is what size of capacitors to use with the crystal. So in the crystal datasheet it says load capicatance

20pf, shunt capicatance 7pf. I'm not sure what to make of either of these numbers. In the ATMEGA16 datasheet it simply reccomends a capacitor of value between 12 adn 22 pf.

Any advice as to what I should use? Thanks!

-Michael Noone

Reply to
Michael Noone

It's not that critical. It will probably oscillate with anything from 12 to

22pF. Choosing the "wrong" value will only put your clock a few parts per million off the nominal frequency which isn't the end of the world.

The crystal oscillates within a specified tolerance of the marked frequency when it "sees" the stated load capacitance which will be the series capacitance of C1 and C2 in parallel with its own parallel shunt capactiance, the input capacitance of the processor clock pin plus circuit strays. The latter are hard to estimate.

If you want accuracy, replace C1 or C2 with a small trimmer and set it using a frequency counter, or test your final circuit layout with pairs of

12pF, 15pF, 18pF and 22pF. Go with the pair that get you closest to the marked crystal frequency.
Reply to
Andrew Holme

I don't understand, what is this amplifier you speak of? Would this be, for my case, inside the Atmel AVR chip? (which the crystal will be attached to) Where would I find information regarding the amplifier's i/o capacitance? I'm not seeing anything in the datasheet, but being that it's 351 pages long I'll allow that I'm not looking in the right place! Thanks,

-Michael

Tim Wescott wrote in news: snipped-for-privacy@corp.supernews.com:

Reply to
Michael Noone

Yes, the amplifier is inside the AVR. I've just done a quick search through that datasheet for "pF" and it doesn't give the input capacitance of the clock pin. They do, however, quote 10pF for the capacitance of the I/O pins. If we assume the clock input plus strays is about 10pF then we need an additional 10pF to make the 20pF load required by the crystal. Since this extra 10pF is supplied by C1 and C2 in series, I would try either C1=C2=22pF or C1=C2=18pF. That should be about right.

Reply to
Andrew Holme

Michael, Fred is right.

Another way to to look at it would be to say the IC pins (including strays) are 10pF each. That's an extra 10pF in parallel with each capacitor. So if we could choose C1=C2=27pF then the crystal would see 37pF in series with

37pF which is 18.5pF. That's close enough to the recommended 20pF.

If you went for C1=C2=22pF (to keep inside the AVR datasheet recommend range) the crystal would see 32pF in series with 32pF = 16pF which is also close enough. A few pF either way doesn't matter.

Reply to
Andrew Holme

"Andrew Holme" a écrit dans le message de news:cq1utu$889$1$ snipped-for-privacy@news.demon.co.uk...

through

No, 10pF strays makes for a total 5pF load for the crystal. Don't forget you have both in series, like the regular additional load caps.

So a required 20pF load will need (20-5)*2 = 30pF capacitors. Try either 27 or 33pF.

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Thanks,
Fred.
Reply to
Fred Bartoli

we use the mega 16 with a pair of 22p cog caps at crystal frequencies from

4MHz to 16MHz- works every time- unless you forget to blow the internal fuse which enables the crystal oscillator when programming -otherwise the shipped default is to use the internal 1MHz RC oscillator This caused major confusion when moving from the mega 161 which defaulted to external xtal Regards Anthony
Reply to
Anthony C Smith

Its all very simple. Just connect a pre-set capacitor in shunt with the crystal. Maximum value a little greater than whatever it says in the specification. And tweak it until the crystal oscillates at exactly the frequency you want.

The various values mentioned in the specification are there just to confuse you and to demonstrate how clever the salesman is.

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Reg.
Reply to
Reg Edwards

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