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 Thread Starter New Member

Jan 21, 2016
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Last edited by a moderator: Jun 20, 2017
2. MrChips Moderator

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?

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

Nov 23, 2012
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5. Wendy Moderator

Mar 24, 2008
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Itwas a component manufactured for old tech color Tvs and repurposed

6. Papabravo Expert

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

7. SLK001 Senior Member

Nov 29, 2011
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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 AAC Fanatic!

Nov 23, 2012
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Just grow it by 2^15

9. MrChips Moderator

Oct 2, 2009
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You mean an MM5369 60Hz clock generator.

10. AnalogKid AAC Fanatic!

Aug 1, 2013
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Not National's best work.

ak

Dec 2, 2013
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12. DickCappels Moderator

Aug 21, 2008
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MrChips, the link appears to be broken.

Aug 1, 2013
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14. EM Fields Active Member

Jun 8, 2016
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Last edited: Jun 20, 2017
15. Papabravo Expert

Feb 24, 2006
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Sorry -- never heard of that one. Was it used in television sets?

16. MrChips Moderator

Oct 2, 2009
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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 Expert

Feb 24, 2006
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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 Moderator

Aug 21, 2008
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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 Expert

Feb 24, 2006
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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 AAC Fanatic!

Aug 1, 2013
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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