On 7/22/19 09:46, Achim Gratz wrote:
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.
I've also found that my Raspi's give the best results with board temp
right around 60°C. However, I found that the 'CPU as heater', while it
does help dramatically compared to nothing at all, it introduces a lot
of jitter on its own. Much better results by keeping them in an
insulated enclosure in a relatively stable location (for me, a closet!),
and 'tuning' the insulation as needed to balance venting against retention.
I've never tried to hold over since it's really hard to figure out when
to start holding and when to switch back. A 24 hour holdover is pretty
severe, but I guess you could keep it within double-digit ms territory
without getting too fancy if you've got that right.
Instead, by having enough independent NTP servers (all with their own
antenna) the clients figure out by themselves when one is off by more
than about a millisecond and switch to another one. Each server
monitors the other ones and if it finds itself off from that bunch too
much will drop down from stratum-1 to stratum-2. When exactly that
happens depends a bit on how fast the drift rate is, but usually between
3ms and 5ms.
Something that just occurred to me, and I'm curious if I'm off in the
weeds here - while a GPS failure could take many forms, I wonder if
there's a good argument to be made to having at least one or two peers
who are geographically quite *far* from one's location - e.g. the other
side of the world - to at least ensure that some number of satellites
might be available when others might not. Thinking in terms of a
Carrington event scenario. Wouldn't the GNSS on the far side of the
earth possibly fare better in such an event - all other things being
equal? I guess it would depend on how long it took for the mass ejection
to pass the earth.
Yeah, I think I'm off in the weeds.
While I support that sentiment, after initial problems with off-brand
cards the ones I have now are all getting into their third year with no
signs of problems (knocks on wood). In a datacenter you'd almost
certainly net boot the rasPi instead of giving them a microSD card.
I can echo that sentiment re SD cards. I'm running Samsung EVO Plus and
Sandisk Ultra cards in my devices, never a problem. Considering that the
'sweet spot' for pricing on SD cards is now the 32gb ones, it provides a
great buffer for the controller to handle internal block allocation for
longer life.
--
Paul Theodoropoulos
www.anastrophe.com
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