[time-nuts] New precision watch

Jim Palfreyman jim77742 at gmail.com
Mon Dec 19 21:12:13 EST 2011


Why don't they build a watch that measures the temperature and every time
you accurately set it, it adds to a small database of time change v
temperature and then adjusts itself internally.

Over time it would become quite accurate I would think.

Jim


On 20 December 2011 11:12, Chuck Harris <cfharris at erols.com> wrote:

> Looking at the datasheet for the DS3232, it doesn't appear
> that they mean for it to run off of a small coin cell in a
> watch.  Its battery operating capability is purely to keep
> it running when the main power is turned off.
>
> I would suspect that the DS3232's power consumption is due
> to its being able to drive sizable loads on its various pins.
>
> There is nothing inherent in temperature correcting a clock
> that should take significantly more power than would be used
> in a normal watch chip.  Measuring the temperature would be
> the most power hungry operation, I would suspect... but
> fortunately that doesn't have to be done all that often,
> perhaps once per minute.
>
> After the temperature is measured, a lookup table can be used
> to find a second by second correction value to be added to
> the seconds counter.
>
> -Chuck Harris
>
>
> Dan Rae wrote:
>
>> On 12/19/2011 5:34 AM, Chuck Harris wrote:
>>
>>>
>>>
>>> It is not clear to me that a 32KHz xtal is any less stable
>>> than a 262KHz xtal, though. I would think there would be a lot
>>> more to be gained by using a microprocessor/thermistor to
>>> measure the temperature within the watch, and provide an
>>> adjustment to compensate for the xtal's natural temperature
>>> sensitivity.
>>>
>>>  Chuck, I have recently made a couple of Real Time Clocks for my
>> homebrew radios using
>> the Maxim DS3232 IC which has a built in TCXO at 32+ kHz doing what you
>> describe, the
>> temperature is checked at intervals and the corrections applied. The
>> performance is
>> excellent, once set, getting to what I term "Harrison Level", i.e. less
>> than a second
>> a week error. The downside is that the standby battery demand is pretty
>> high and my
>> first builds using a Lithium button cell ran for only around a month max,
>> so I had to
>> go over to NiMH rechargeables. I suspect using this technology in a watch
>> one would
>> have the same problem.
>>
>> The best crystals for room temperature use are around 4 MHz with
>> temperature
>> inflections around 20C, and this is what was used in an early Braun alarm
>> clock I had
>> which also had this kind of performance. Long gone, alas.
>>
>> Dan
>>
>>
>>
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