[time-nuts] Holdover, RTC for Pi as NTP GPS source

[MLewis]

(I suspect this is drifting from the original thread too much, so new 
subject)

Temperature ranges from 65F to 78F, with the potential for drafts, but 
is more typically 76F.

I read about the NTPsec runs with insulating a Pi and running a load 
generating program to better maintain a stable core temperature.
Just today I've put my GPS module inside a case for an RF shield that is 
also semi insulated. It's feeding LH on a PC while I do the next step.
The Pi 3 is going inside a large enough tea tin and that will be lined 
with insulation.
I'm wondering about insulating the RTC...

The low cost for a 'precision' RTC means it is cheap to test.

I'd completely discounted coasting with the system clock, as I have 
fixed in my head the variable loads on my production machine mean that 
Window's time lags variable amounts, as the CPU load is variable with 
variable burst loads every 1/8 of a second.

Michael

On 31/10/2017 11:45 PM, Hal Murray wrote:
>> I'm intending to add a "precision" (well, precision to the Pi world) RTC  to
>> my Pi 3 to use for a holdover source when it hasn't got PPS from the  GPS
>> module.
>> An RTC that +/- 3 PPM over 24 hours would be great for holdovers of one  to
>> 20 minutes.
> Run some experiments to collect some data and play with the numbers.
>
> How stable is the temperature in your environment?
>
> The key to keeping sane time on a PC or Raspberry PI is to calibrate the
> crystal.  Most CPUs have a register that counts at the CPU clock frequency -
> or something in that range.  Most systems smear the clock to keep the FCC
> happy...
>
> Most OSes keep time by watching that register and dividing by the clock rate.
>   The actual clock rate doesn't usually match the number printed on the
> crystal.  It's close, but ntpd can easily measure the error and tell the
> kernel so the kernel can use the right value.  If you turn on loopstats, ntpd
> will log it and you can graph it.
>
> If you are writing an embedded system, you will want that sort of logic too.
>
> My guess is that in the under 30 minute range, you will get better results by
> just coasting with the system clock rather that using a RTC.  It would be an
> interesting experiment.  Implement both clocking schemes and compare them.
>
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