ATmega168 - what capacitors to use with resonator

Hello. I'm making schematics for my project, it is based on Arduino Pro Mini 5V/16MHz, but is using ATmega168 instead of 328. The problem is I don't know what capacitors should I connect to the resonator. Pro Mini has some capacitors inside its resonator, but I have no idea what kind of capacitors these are, I can't read it from the Pro Mini schematics. I was searching on the internet and I found out that most people use 22pF capacitors on the resonator. Are 2x 22pF caps ok for the 16MHz resonator for ATmega168? If they are, another problem is that I can't find resonator with internal 22pF capacitors, I can only find with 18pF. Will 18pF be ok? I would rather use resonator with internal capacitors, instead of using a resonator and two additional capacitors.

Cheers, Daniel

In general crystals require external capacitors (frequently 22pF) and resonators have the required capacitors inside the package. If you use a resonator you do not have to worry about the size of the capacitors since they have been provided by the manufacturer. The presence of the internal capacitors is the reason that the resonators have a third pin which must be connected to GND.


I meant crystal, sorry. Yeah, I think I know the difference between crystal and resonator. Resonator = crystal + capacitors, right? So the last question: can I just use this - and forget about any capacitors? It is not labeled as resonator, but the description says it is 18pF, so I assume it is a resonator... Is 18pF ok? Or do I have to look for 22pF?

Yes, that surface mount part is a crystal and needs capacitors.

Per 8.3 of the datasheet: "Low Power Crystal Oscillator Pins XTAL1 and XTAL2 are input and output, respectively, of an inverting amplifier which can be configured for use as an On-chip Oscillator, as shown in Figure 8-2 on page 29. Either a quartz crystal or a ceramic resonator may be used.

C1 and C2 should always be equal for both crystals and resonators. The optimal value of the capacitors depends on the crystal or resonator in use, the amount of stray capacitance, and the electromagnetic noise of the environment. Some initial guidelines for choosing capacitors for use with crystals are given in Table 8-3 on page 29. For ceramic resonators, the capacitor values given by the manufacturer should be used."

Attached chart shows 12-22pF for 16 MHz operation.

Yeah, I think I know the difference between crystal and resonator. Resonator = crystal + capacitors, right?

No, I think you don’t.

A resonator is not a crystal. It consists of a voltage-variable capacitor that acts in some ways like a quartz crystal. (according to wikipedia)

The Maritex part is a crystal and it needs capacitors. The description designates the recommended load… so you add capacitors… it does not indicate they are included in the “package”

I think that the term 'resonator' is a rather generic term used for components that have a specific frequency response. Assuming that a 'resonator' is a specific type of component can lead to misunderstanding. A quartz crystal is a crystal 'resonator', as are three terminal ceramic 'resonators'.

Using the term resonator by itself can lead to misunderstanding. In the arduino world resonator tends to mean the three terminal ceramic resonators that have built in padding capacitors, but again it's doesn't hurt to be more explicit if possible. A simple L/C network can be considered a 'resonator' in that it will have one specific resonate frequency depending on the value of the L and C components.


And not to be confused with the Dobro Resonator:

Ok, so I don't get it. It's confusing... So if I use this thing I will have to buy two capacitors also. BUT if I use this I won't have to buy any capacitors, right? Please tell me I'm right this time...

Attached chart shows 12-22pF for 16 MHz operation.

So it doesn't matter if it is 12pF or 22pF? So 18pF should be fine, right?

  1. count the pins

  2. if pins = 2 then add two capacitors

  3. if pins = 3 then don't add capacitors