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Question on crystal load capacitance

Anonymous
Anonymous

Hi All,

 

I would like to ask a question on load capacitance for DM6437.

 

From various sources I read that the load capacitance can be calculated as:

 

{(C1+Cin)(C2+Cout) / (C1+Cin+C2+Cout)}  +  PCB strays (2~3PF)

 

http://en.wikipedia.org/wiki/Pierce_oscillator#Load_capacitance

http://www.crystekcrystals.com/crystal/appnotes/PierceGateLoadCap.pdf

 

And guess value of Cin and Cout of 5pf are often used.

 

 

 

 

But in EVM6437, for crystal Y1 which is used to clock the DSP, it has C1 and C2 of 18pf. The BOM indicate that it is of Citizen CS10 type, whose data sheet in turn show CL of 18.0pf (is it only coincidently that CL= (C1, C2)?). Applying the formula above I get the actual CL to be around 13.5pf, significant smaller than the data sheet’s required value.

 

Is there any problem with my calculation or with EVM6437 C1 , C2 selection? Could anyone advise me on this?

 

 

 

 

Thanks,

Zheng

  • 0
    TI__Intellectual 1670 points

    Most likely there is nothing wrong with your calculation.  In fact, typical Cin and Cout values are closer to 2-3pF.  5pF would only be found on larger chips like older DIP parts.  The CL value is only an approximation.  Often crystals will run at much higher and much lower loads than what's stated. 

    Often it matters what type of crystal it is more than other factors.  In some situations we've had, the tiny "can" type crystals ran better than some of the SMT plastic ones.  Choosing crystals with a CL close to the actual is a great place to start, but the only way to know for sure how it will perform is to try it over temperature, voltage, and with a few different lots (from the crystal manufacturer.  In most situations though, a crystal that runs at room temp will perform reliably.  Crystals are rarely a source of problems.

    I hope this helps!

  • Anonymous
    0 Anonymous in reply to KevenC

    Dear Keven,

    I got it, thanks very much for the answer.

     

    Zheng

  • 0
    TI__Intellectual 1670 points

    There's nothing special about the "can" crystals, the point I was making was that there are a few different types of crystals you can get, and some seem to work better than others.  Maybe you can tell from the specs, I didn't get into it that far (we just tried what we had on hand).

     

    Among the types are:

    Plastic SMT

    Ceramic SMT

    Can (can be through-hole or SMT with bent leads)

    The "Can" types have been around the longest, with the metal cylindrical "can" form factor.  They're often the cheapest as well.  I'm talking about something like:

    http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=535-9033-2-ND

    Search for the "cylindrical can" type at Digi-key.

     

    I'm not saying these crystals are better than others, I'm just saying we've had good luck with the ones that come in a can for some reason.

  • Anonymous
    0 Anonymous in reply to KevenC

    Dear Keven,

    I got it, thanks very much for explanation and the link.

     

    Zheng