Why 32.768KHz?

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debrajdeb
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2009/03/13 01:50:13 (permalink)
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Why 32.768KHz?

I had been using 32.768KHz XTALs for RTC application, for a long time. And I wonder why this particular value? [8|]

My answer (based on logical guess) -->
This frequency is arrived because of legacy designs, which were based on pure hardware. If we connected the XTAL to a counter with output in binary D15 to D0, then we simply have to monitor the pin D15 and when that pin goes high, we have 1Sec complete. Refer to the following calculation: -

(1/32.768KHz) * 32768 = 1Sec
(1/32768) * 32768 = 1Sec
(1/32768) * 0x8000 = 1Sec  // Hence just monitor the MSB line (D15), to know your 1Sec pulse.

If we use micro controller for RTC function, then it really does not matter if we have a 32.768KHz or any low value, like 10KHz. Thats because, we will anyway compare the entire number and not just 1 bit.

If my above logic is correct, why did NOT we ever used XTALs like 16.384KHz or 8.192KHz or 4.096KHz? Is it something to do with manufacturing constraint?
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    I_am_fubar
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    RE: Why 32.768KHz? 2009/03/13 02:02:40 (permalink)
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    Indeed, this is my understanding. I believe a lot of RTC chips rely on that exact division to stop errors creeping in. As to the speed. Generally, it is taken that the faster something oscillates, the more accurate it is.
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    dwclark
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    RE: Why 32.768KHz? 2009/03/13 02:09:02 (permalink)
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    It's mostly because 32.768kHz crystals are 'cheap as chips' - they are used in nearly every watch, clock and toy ever made. Economies of scale dictate that they will always be cheap - a few pence in most cases. 1kHz crystals will also be physically larger as well as a lot more expensive.
     
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    debrajdeb
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    RE: Why 32.768KHz? 2009/03/13 02:16:35 (permalink)
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    Ok.. So finally, it boils down to what is cheaper and practical to use.
    And because we had been using 32.768KHz for last 20yrs it has become so cheap that we will continue the same value for next 30yrs.. grin

    Otherwise, given an option, I would have liked to use a XTAL with lower frequency so that accumulated error becomes smaller.
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    dwclark
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    RE: Why 32.768KHz? 2009/03/13 03:00:36 (permalink)
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    It doesn't quite work like that. The lower the frequency, the more difficult it is to get crystals and the frequency tolerance is poor. Try getttng a 1kHz crystal... Obviously the ideal is a 1Hz crystal, but it would be quite large and the load capacitors would be large. What is a fact is that the lower the frequency, the lower the power consumption.
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    tunelabguy
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    RE: Why 32.768KHz? 2009/03/13 04:00:50 (permalink)
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    ORIGINAL: debrajdeb
    ...given an option, I would have liked to use a XTAL with lower frequency so that accumulated error becomes smaller.

    The accumulated error in a real-time clock depends only on the accuracy of the crystal frequency in percent or parts per million. There is no reason to believe that a frequency lower than 32.768 kHz would be any more accurate. As crystals go, the 32.768 kHz watch crystals are very good.


    Robert Scott
    Hopkins, MN
     
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    betwixt
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    RE: Why 32.768KHz? 2009/03/13 14:57:57 (permalink)
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    32768 is 2^15  in other words a chain of 15 divide by two stages will give 1Hz out.

    In many applications, particularly digital watches, the current consumption has to be as low as possible to preserve battery life. This frequency was chosen as a best compromise between low frequency (= low power consumption) and convenient manufacture where low frequency generally means the quartz is physically bigger.

    Brian.
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