thanks, thats a really great idea. something I think I will no doubt end
up using. We are in a similar situation with regards to the GPS signal and
will no doubt end up have to lay a special surface cable to recover a
valid signal and repeat it throughout the network.

If anyone has any more info on this technique I would be very interested.
I will have a google around later on for it.

Mat

> However.......
>
> The RTC can be tweaked in software to hold it to a much much better
> long term accuracy. I forget where I read the paper, but its output
> is compared to a GPS and characterized over a period of some number
> of hours (and possible temperature range variations if needed).
>
> Every so often, an extra pulse is added to speed up the clock or the
> clock is inhibited for a number of clock cycles to slow down the clock.
>
> I think someone was using the RTC's for underground earthquake
> sensors in very remote locations where the ground temperate was very
> constant. Complicated and power hungry oven controlled quartz
> oscillators were not practical. By running a wire to the surface,
> they could collect data and tweak the inexpensive RTC as needed, each
> session refined the clocks accuracy until it could eventually run for
> many months without any attention.
>
> The GPS is not needed after the modules smart tweaking procedure is
> completed.
>
> Don't quote me on this, but I think they ended up with less than 5
> seconds of error per year and were working on a similar scheme to
> tweak the RTC's clock to account of aging of the quartz and for the
> change in speed of the oscillator due to voltage drop in the batteries.
>
> Keep up the great work, this is a great list!
>
> Art
>
>
> At 06:06 PM 10/13/2006, you wrote:
>
>> > Last time I checked RTC's weren't accurate to micro seconds,
>> > unless anyone knows of any which are?
>>
>>Maxim DS3231, +/- 2ppm, 0C - 40C (1 minute per 11.57 days)
>>
>>http://www.maxim-ic.com/quick_view2.cfm/qv_pk/4627
>>
>>Taking 50us variation as 1ppm gain/loss and maximum drift,
>>the time would be out by 100us after
>>
>>1,000,000 * 50us * 2 = 100s
>>
>>A minutely refresh would guarantee synchronisation
>>
>>
>>
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