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

MLewis mlewis000 at rogers.com
Wed Nov 1 00:14:54 EDT 2017

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

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.


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.

More information about the time-nuts mailing list