Charles Wyble wrote:
I built a dedicated server room in my house, with it's own air
conditioner. I've been working on overall instrumentation , especially
temperature.
If the rasPi is a dedicated system and does not serve extra tasks, just
record its CPU temperature, no extra sensor needed. The absolute
temperature will be an almost constant offset from the ambient, so the
changes are well preserved (except for short periods, e.g. when a cron
job runs).
Any temperature transient will show up in the timing error, however
small. The FLL/PLL code in NTP will need to chase the changing crystal
frequency and then eliminate the already accrued timing error. The
faster the transient, the larger the error. So on-off aircon with
forced convection is pretty close to worst case.
Could you share the snippets of the
PPS logging? I'm not 100% sure what you mean by the PPS timestamps.
Just reading from /dev/pps0 (system time and capture timestamp in my
case), really; additionally system load and CPU temperature and logging
the everything into a file.
Interesting. My ultimate application of this high precision timing is
driving TDMA wifi links as low cost as possible.
I'm not familiar with the requirements for that. I suspect that the
absolute timing between stations is actually pretty unimportant. So the
only thing that matters is that the relative timing error between
beacons or syncs must keep below some threshold, which means the
frequency offset of the system must be kept within (probably not too
tight) bounds. The latter is much easier to achieve and maintain.
Over wired Ethernet you can expect to synchronize a bunch of systems to
within a ~200µs envelope of absolute time and maybe a factor of 2x-3x
lower if you can control certain things more tightly than usual if you
run those system on a single hop switched LAN that have a GPS-based
stratum-1 NTP server. For anything better than that you'd need PTP,
which unfortunately the rasPi is incapable of. Or do you plan to use
the rasPi itself as the RX/TX? The few papers I've just looked up all
use Atheros 9k hardware.
--
Achim.
(on the road :-)
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