Yo Chris! On Thu, 20 Oct 2016 23:37:23 -0700 Chris Albertson <[email protected]> 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 [email protected] Tel:+1 541 382 8588
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