[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.  :-)

Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
	gem at rellim.com  Tel:+1 541 382 8588
-------------- next part --------------
A non-text attachment was scrubbed...
Name: not available
Type: application/pgp-signature
Size: 455 bytes
Desc: OpenPGP digital signature
URL: <http://www.febo.com/pipermail/time-nuts/attachments/20161021/1b0a797a/attachment.sig>

More information about the time-nuts mailing list