[time-nuts] Noob question, NTP stratum 1.

[Kasper Pedersen]

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.