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Opinion on - Crystals and external caps :)

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  • Opinion on - Crystals and external caps :)

    Hi all

    Currently I`m designing a board with Atmega32u4 and I came up with question about the external capacitor value for the XTOL, and i`ve wondered if I may get your opinion on that.

    It seems like there is no clear answer for how to calculate the exact value of them, with different formulas are there on the internet and different approaches being taken with different designers.
    I`ve noticed also an 18pF external caps with CL = 18pF were used in the Udemy course.
    - I`d be glad to know how you should choose the external caps.
    Recommended capacitance for the Atmega32u4 [email protected] 12-22pF.

    P.S Thank`s for the helpful course on Udemy, it was very detailed!


    Eli Agronov
    Attached Files

  • #2
    usually it given by the XTAL datasheet, or given in by you as user, (make your own partnumber) there are lots of calaculations.. but in my experience i jus put in a default value, like 12 or 16 pf.. if it doesnt work change it to something else..

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    • #3
      Thanks for the input Paul

      The XTAL I choose is this one:
      https://www.digikey.com/product-deta...6-1-ND/5817640
      The datasheet defines the CL value, not the actual capacitors value to be in the design.

      - So you are saying it doesn't matter what the caps value is, as long as it`s in the required range of the MCU manufacturer?
      Is it safe, and wouldn't it change the resonance frequency, which might stop XTAL from oscillating?

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      • #4
        but in my experience i jus put in a default value, like 12 or 16 pf.. if it doesnt work change it to something else..
        - be careful with choosing a capacitors and crystals which just seems to be working fine. I have seen boards what were NOT booting up for different temperatures if capacitor was not correctly calculated. In most cases and in lab most of the boards worked just fine ... but occasionally and for some conditions some boards were stuck and didn't boot up.

        It seems like there is no clear answer for how to calculate the exact value
        - I do not remember exactly, but I think you are right. I think I had some problems to decide on the exact capacitors and crystal ... I think, I went through some calculations to get approximate values what I need, but in final I found a similar board from ATMEL and that was the capacitor and crystal what I used .... but I do not remember exactly.

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        • #5
          that is what I meant by " if it doesn't work try something else" i have never had these problems since i would order a specific crystal with a manually selected cap value.(no need to calculate ). if it needs to be that critical or temp depended a Xtal is not your best solution. the you have other components to look into.. but in most cases this rarely happens...

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          • #6
            Thanks!

            Originally posted by robertferanec View Post
            I have seen boards what were NOT booting up for different temperatures if capacitor was not correctly calculated. In most cases and in lab most of the boards worked just fine ... but occasionally and for some conditions some boards were stuck and didn't boot up.
            Yes, I would test this to UL and NEMA in temperature chamber and in EMC chamber after initial design.
            Wanted to select correctly on the first attempt, so I think I`ll try your suggestion finding a proved design with schematics.
            - When using from known schematics the xtal-caps combination, Is there a rule of thumb here for verification, just to be on the safe side?

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            • #7
              Generally we do follow the what's been used in reference design given by the Manufacturer.

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              • #8
                ,,I would test this to UL and NEMA in temperature chamber and in EMC chamber after initial design.'' that will cost you a pretty penny...

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                • #9
                  When using from known schematics the xtal-caps combination, Is there a rule of thumb here for verification, just to be on the safe side?
                  - maybe it would be good if the values are in the range what you theoretically calculated - that I would consider to be on the safe side. Also, I consider schematics from bigger companies like ATMEL as a quite reliable source .... but I do not trust schematics which are just downloaded from somewhere created by someone (these schematics can sometimes actually cause more harm than good).

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