On 22/07/2019 18.46, Achim Gratz wrote: > Tim Shoppa writes: >> The Raspberry Pi processor clock, like any motherboards', will often be off >> by off anywhere from +/-200ppm but the good news is that it usually varies >> by less than +/-10ppm over a day and ntpd does a good job tracking this and >> using the drift correction during no-signal periods. > > All rasPi I've bought so far (5 of them, all different models from three > different sources) were ~10ppm slow at RT and within -6ppm and 0ppm at > the apex temperature point of the crystal (around 60°C). It is possible > to keep the temperature within about 0.2K of that point by using the CPU > itself as a heater and thermometer and the resulting frequency within > +-20ppb over a day when disciplined via pps-gpio. The residual drift is > initially positive and gets smaller over time, mine are down to about > 20~30ppb per week.
Regarding the massive offsets that sometimes come up, it is worth asking when that 'truth' was learned, and with how old a kernel, if Linux was involved. Pre-2011, and if Debian was involved probably pre-2018, there was a rounding error in the kernel, causing the clock code to miscount by a lot (127 ppm) on most motherboards, and worse on boards with better timer resolution. https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?h=v5.3-rc1&id=a386b5af8edda1c742ce9f77891e112eefffc005 After this, the offsets on all the machines I had became a lot more realistic*. Putting together a crystal oscillator and pulling it so that it is 200ppm off, and reliable for manufacturing, is actually hard. As to reliable NTP: Over the past ~17 years I have learned that every receiver will fail at some point, all receivers of a brand will fail at the same time, and you need to plan for that. /Kasper Pedersen *There remains an error of typically 2 ppb after the patch, but I was short on time. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
