Why is the 32 kHz crystal oscillator used in RTC, instead of the 16 kHz?

Discussion in 'General Electronics Chat' started by Lentol, Jun 19, 2017.

  1. Lentol

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    Jan 21, 2016
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  2. MrChips

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    Oct 2, 2009
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    That's a good question.
    32768Hz can be easily divided by a 15-bit counter.
    So why not use a 16384Hz crystal and divide by 14-bit counter?

    What the heck? Why not go all the way and use a 1Hz crystal so that no division is required?

    Ask the crystal manufacturer.
     
  3. AnalogKid

    AAC Fanatic!

    Aug 1, 2013
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    A 32 kHz "crystal" actually is a type of tuning fork. This is used because at these low frequencies it is much smaller than a standard AT cut crystal would be. Still, size is everything inside a wrist watch. 32 kHz was a tradeoff between size (higher freq = smaller) and circuit complexity (higher freq = more divider stages).

    It has nothing to do with accuracy. A 1% error in a 10 MHz oscillator, a 10 kHz oscillator, and a 10 Hz oscillator will produce the exact same 1% error percentage when they are divided down to 1 Hz.

    ak
     
  4. GopherT

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    He was not asking, he was telling you - he already had the link and the answer - he was asking you to read his link.
     
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  5. Wendy

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    Itwas a component manufactured for old tech color Tvs and repurposed
     
  6. Papabravo

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    Feb 24, 2006
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    I think you are remembering the color burst crystal @ 3.579545 MHz. That was the 1960's TV technology. 32,768 kHz came from digital watches c. 1974
     
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  7. SLK001

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    I need to get me some of them 1Hz crystals! I will make building clocks a piece of cake!
     
  8. GopherT

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    Just grow it by 2^15

     
  9. MrChips

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    You mean an MM5369 60Hz clock generator.
     
  10. AnalogKid

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    Not National's best work.

    ak
     
  11. Picbuster

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  12. DickCappels

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    MrChips, the link appears to be broken.
     
  13. AnalogKid

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  14. EM Fields

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    Try this instead:
    1Hz oscillator.png
     
    Last edited: Jun 20, 2017
  15. Papabravo

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    Sorry -- never heard of that one. Was it used in television sets?
     
  16. MrChips

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    Back in the '70s it was a simple way to get an accurate 60Hz time-base signal for digital clocks using a readily available crystal, the color-burst crystal. I built a communications receiver frequency counter using this part.
     
  17. Papabravo

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    Oh I see, it was a chip used in conjunction with the color burst crystal. I never knew that.
     
  18. DickCappels

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    Edit: The numbers below are incorrect.

    The 3.9545 MHz crystal could be easily divided by a binary divider to obtain nearly 60 Hz (60.6606 Hz).
     
    Last edited: Jun 21, 2017
  19. Papabravo

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    The 3.9545[Correction 3.579545] MHz crystal could be easily divided by a binary divider to obtain nearly 60 Hz (60.6606 Hz).

    3.579545 \times 10^6 / 59659 \;=\;60.00008381
    3.579545 \times 10^6 / 59660 \;=\;59.99907811

    Factor(59660) = 2 x 2 x 5 x 19 x 157 and 59659 is prime. What divisor did you have in mind?
    the closes divisors to 60.6606 Hz are 59009 and 59010.
    Factor(59010) = 2 x 3 x 5 x 7 x 281 and 59009 is prime
     
    Last edited: Jun 20, 2017
  20. AnalogKid

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    The output frequency with a perfect crystal is 60.00008381 Hz. This translates into a steadily accumulating error of 7.241 seconds per day (fast), so the circuit is not nearly as accurate as a crystal oscillator based on something that is a decade or binary multiple of 60 Hz.

    ak
     
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