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Topic: Why are real-time clocks always 32.768khz? (Read 3 times) previous topic - next topic

bobthebanana

I once heard somewhere that a specific natural occuring shape of crystal would oscillate at exactly 32.768 khz. Don't know if this holds any truth though

BillO

Or even 8,192?

Well, I think the answer is that even the 32768Hz is a pretty delicate and susceptible crystal as it is.  For 16384, they'd have to make them twice as long or half as thick and this would make them weaker (will tend to age quicker) and more susceptible to interference.  As it is, for maximum stability the 32768Hz crystals need special guard planes and such to keep out EMI.  Although lower frequency crystals are available, their stability/susceptibility characteristics don't make them ideal for clocks.  32.768kHz seems to be the sweet spot.
Facts just don't care if you ignore them.

BillO


I once heard somewhere that a specific natural occuring shape of crystal would oscillate at exactly 32.768 khz. Don't know if this holds any truth though


Clock crystals are usually artificially grown and cut in a tuning fork shape.  Tuning forks give pretty pure signals with less dominant overtones near the fundamental frequency.  I think their 2nd overtone is about 6 times fo.  This makes it easy to filter out without diminishing the fundamental too much.
Facts just don't care if you ignore them.

dc42

The 32768Hz frequency was chosen many years ago, in the early days of digital watches. The need was for a crystal that was small and tough enough to use in a watch, but the IC used to divide it down to 1 second pulses had to draw very little current. So the frequency chosen was a power of 2 to keep the divider IC as simple as possible, and 32768Hz was a good compromise. Lower would have required a larger, more fragile crystal. Higher would have increased the current consumption of the divider chip (current consumption of CMOS chips mostly scales with frequency). These days, chips take less current, and I believe modern watches use smaller, higher frequency crystals. But clocks and RTCs typically still use 32768Hz.
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afremont

First quartz controlled wrist watch at 8192Hz in 1969:
http://en.wikipedia.org/wiki/Astron_%28wristwatch%29
Analog dial and hands.

The Bulova Accutron (ca. 1960) watches used an actual mechanical tuning fork that oscillated at 360Hz.

Experience, it's what you get when you were expecting something else.

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