So I'm looking at an AVR Programmers and setup and I see this schematic: enter image description here

How exactly does this work? (Keep in mind I'm a newbie at schematics), but the way the schematic looks makes it seem like that a separate 22pF capacitor should go on each side. Is it because ceramic capacitors have no polarity, you can just put them in between? Am I missing something here or just being stupid? The schematic makes it seem like there should be 3 things where the one blue (what seems to be a capacitor is).


I agree with what Steven said, but want to add a few things.

Ceramic resonators look electrically very similar to crystals. The main differences are:

  1. Resonators are cheaper than crystals. This is the primary reason they are used over crystals, especially in high volume applications. For one offs, the difference is small and therefore irrelevant to hobbyists and anyone doing small scale builds.

  2. Crystals are more accurate, which is the primary reason they are used over resonators. Resonators may be accurate to 1% or 1/2%, but that's still way more slop than the 50 ppm even a cheap crystal can do. Note that 1/2% is 5000 ppm. Crystals good to 20 ppm are readily available and not much more cost than 50 ppm. 10 ppm is available, usually at a premium. Temperature controlled crystals can be good to around 1 ppm.

  3. Resonators are mechanically more robust. They can take more shock and vibration than a crystal can. Resonators are preferred in automotive applications, for example, for this reason.

  4. Resonators usually require a little higher drive to operate than a crystal. Either is still well within what CMOS circuits can do, so this is not a reason to chose one over the other except in very low power applications. But, it is something to consider when using a resonator. Some microcontrollers, for example, have several different drive levels the crystal circuit can be set to. A resonator may need one level higher than a crystal of the same frequency.


I meant to say this before but got distracted. The schematic above is missing the bypass capacitor. This may seem unimportant, but it's not. You should solder a 100nF to 1µF ceramic cap accross the power and ground pins of the chip right under the socket. The loop from chip thru cap and back to chip should be as small as possible. Various flaky things may happen without this capacitor, even if it appears to be working.

  • \$\begingroup\$ Thanks for the explanation, im glad it was at least "slightly" confusing.....lest I look like a dummy again ahah! Speaking of your bypass caps, what is the standard? Should I use a 100nf or 1uf? I know some chips you want to put a smallish cap near the VCC inputs of them, and ground....but im never sure which to use. and you saying you a seperate Capacitor for the Ground and Power pins right? (Im assuming if so they should both be the same capacitance) \$\endgroup\$
    – user3073
    Oct 15 '11 at 20:55
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    \$\begingroup\$ @Sauron: As I said, you should put a single bypass cap accross the power and ground pins. Any ceramic in the 100nF to 1uF range should be fine. It looks like the power and ground leads are next to each other, so soldering a 0805 or 0603 ceramic cap right accross those pins would be best. \$\endgroup\$ Oct 15 '11 at 21:21
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    \$\begingroup\$ In fact, Atmel's datasheets strongly recommend two decoupling caps. The other one is between AVCC and GND on the analogue side of the chip. They claim it's a good idea to do this even if you're not using the ADCs and I believe them. It's a cheap, easy, standard way to avoid trouble down the road. \$\endgroup\$
    – Alexios
    Mar 4 '12 at 0:51

AngryEE is almost right: most ceramic resonators have their capacitors integrated.
What appear to be three separate components in the schematic is one ceramic resonator. You can tell if they have their capacitors integrated by the number of pins. If the caps are integrated the resonator will have 3 pins: one connecting to pin 9 of the controller, one connecting to pin 10, and one (the middle pin) connecting the 2 capacitors to ground.
A resonator without the caps will have only 2 pins.

enter image description here

A ceramic resonator is often used instead of a crystal to save cost. It's less precise and less frequency stable than a crystal, though.
Like Russell says resonators are more shock and vibration resistant than the more fragile crystals.

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    \$\begingroup\$ Resonators also have the advantage of being substantially less fragile than quartz crystals. Crystals do not survive well in very high vibration of high impact situations. \$\endgroup\$
    – Russell McMahon
    Oct 14 '11 at 19:08

You're right to be wary—very confusing situation here. What you might have there in that blue blob is not just a crystal but a 'resonator'. A resonator is a crystal with the load capacitors built in. So the two caps and the crystal are all contained in the blob and the blob has three pins—two clock and one ground.

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    \$\begingroup\$ No, a resonator is not a crystal, it's a piece of tuned ceramic which is less accurate than crystal, but some times lower cost. \$\endgroup\$
    – dren.dk
    Oct 15 '11 at 14:21

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