Recommended crystal type?

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Which type of crystal is best for use with the AVR? A 20pf or a series type? They are often the same price.
What are the advantages and disadvantages of using a ceramic resonator as opposed to a crystal? Resonators tend to be 10-50% cheaper than crystals and include the capacitors, but are they as accurate in regards to frequency? What would be the expected range of frequency variation when using a resonator? Are CRs more temperature sensitive for frequency than crystals?
If using a resistor and capacitor to oscillate the system clock of an AVR, what type of capacitor would be recommended?

I've looked in the data sheets and archives here for insights on these issues but haven't been able to gather much helpful information. Perhaps if there are specific replies to this message it will focus help for AVR users in the future referencing this topic.

Thank you.

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Have you already digested all the information already posted in the Academy and in past threads? For example:

https://www.avrfreaks.net/index.p...

You can put lipstick on a pig, but it is still a pig.

I've never met a pig I didn't like, as long as you have some salt and pepper.

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For the RC oscillator, I would use an NPO or COG type cap.

Ceramic resonators are quite a bit poorer tolerance than a crystal. Sometimes, they are just good enough to time UART-based serial. The resonator data sheet should have tolerances. Ditto, crystal.

Jim

 

Until Black Lives Matter, we do not have "All Lives Matter"!

 

 

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Quote:
Ceramic resonators are quite a bit poorer tolerance than a crystal. Sometimes, they are just good enough to time UART-based serial.

Really?
You do surprise me! I've never yet had a problem with U(S)ART timing caused by using a ceramic resonator instead of a crystal.
Never.
And I'm 52 you know.
I still have some of my own teeth.

Four legs good, two legs bad, three legs stable.

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Some have quite poor tolerance and some are a lot better.

There have been discussions about the required clock frequency tolerance for reliable U(S)ART operation and I don't remember the number that was suggested. Perjaps something in the 1%-ish area? Don't quote me on that. Hope somebody can give a more definitive value.

Still got my teeth, also, and I have a few years on you, hi hi!

Jim

 

Until Black Lives Matter, we do not have "All Lives Matter"!

 

 

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ka7ehk wrote:
There have been discussions about the required clock frequency tolerance for reliable U(S)ART operation and I don't remember the number that was suggested. Perjaps something in the 1%-ish area?

An acceptable baud rate error may be as high as 3% but with worst case conditions, 2% should be okay.

The ceramic resonators I use have a frequency tolerance of +/- 0.5%. I've had no UART problems using ceramic resonators.

The crystals I use have a frequency tolerance of +/- 30 ppm at 25 degrees C. 30 ppm is 0.003%.

Note that the frequency tolerance is only one contributor to baud rate error. For certain frequencies, the baud rate divisor has to be rounded off so an exact match is not possible.

Don

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does atmel specify the xt pin capacitance? i seem to recall running across that in some datasheet or other but i cant find it for the mega16 so as far as calculating the load caps do you just guess?

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Quote:
And I'm 52 you know.

You spring chicken you :-) I have given up using resonators not because of problems but because I have decide that the 10 cents or so extra I pay for a xtal doesn't make any difference in my products as I don't make lots of them. 10-20 is a good run for me! If you make 100s or 1000s then resonators may be a better choice, of course many people get by with just using the internal oscillator.

John Samperi

Ampertronics Pty. Ltd.

https://www.ampertronics.com.au

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