[time-nuts] Linux PPS clues?
Gary E. Miller
gem at rellim.com
Fri Oct 21 16:18:31 EDT 2016
Yo Chris!
On Thu, 20 Oct 2016 23:37:23 -0700
Chris Albertson <albertson.chris at gmail.com> wrote:
> I think your graph only shows 1/2 of the problem. It is the easy part
> because all that code is written and likely already installed on the
> OP's computer.
Yup, of course. But my actual results do not match people's
expectations as recently stated on this list. To ground the data I am
merely providing an actual baseline for the quality of the system clock
on a RasPi 3 when GPS stabilized using the techniques documented in the
gpsd and ntpsec documentation.
> The other half of the problem is responding to events and getting
> them time stamped with very low latency and jitter.
Yup, of course. I have not commented on the second part of the problem.
I am enjoying the many and varied solutions being presented here. I look
forward to actual implementations and hard performance data on them.
> I think the best way is to copy the design of the Linux PPS
> system.
I'll reserve my judgement on 'best'. Since my data says the Linux
kernel PPS system can reach about 1 µs standard deviation on a
RasPi3 using the GPIO lines, I would expect a solution based on that
would behave similarly.
> Then assume the error in time stamp is a little larger
> than double your graph.
Ah, double which error? I give error calculations of one standard
deviation, 90% percentile range, and 98% percentile range. My guess
would be that any technique using the same method on the same hardware
would have a similar 98% range.
My 98% range on this RasPi3 is under 5 µs. I'll leave it to the
OP to decide if that is good enough, easy enough, etc.
You can see the live numbers here: https://pi4.rellim.com/day/
> But if the event time stamp code is not so well designed the results
> might to 10X worse not just 2x worse.
Once can always do things badly. :-)
RGDS
GARY
---------------------------------------------------------------------------
Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
gem at rellim.com Tel:+1 541 382 8588
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