Hi
> On Nov 1, 2017, at 12:14 AM, MLewis <[email protected]> wrote: > > (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. The gotcha in a real environment often involves people. They walk by (creating a draft). They turn on all the lights and equipment. They open or close the blinds to let in or block the sun. They tend to do this in an unpredictable / chaotic fashion. All of this makes a correction process based on “normal operation” a bit difficult. Something goes wrong, and the unit goes into holdover. People suddenly start dashing around and the temperature is not what it has been …. Bob > > 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. >> >> >> >> >> > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
