(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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