[time-nuts] [Solved] Looking for multiple PPS timestamp logging

Kevin Rosenberg kevin at rosenberg.net
Mon Oct 3 16:19:51 EDT 2011

On Oct 3, 2011, at 12:59 PM, Hal Murray wrote:
> Why do you need microsecond resolution?
> A junk XO will drift ballpark of 1 PPM per C.  That's 86 ms per day or 3.6 ms 
> per hour.

The temperature swings won't be large, just the usual diuneral indoor cycles.
He wants to use compute much much of the frequency change can be explained
by temperature alone, we were going to measure temperature and frequency
as precisely as is reasonable cost. So, it would be nice to measure a
microsecond difference in a second if the temperature very suddenly changed
by 1 degree C.

> What sort of resolution do you think you can get from a parallel port?

I think the point it moot now that tvb has the very useful picPET for
this application. I think resolution will depend upon the load of
the OS. I was going to use an old PC running linux and no graphics
mode. I think sub-millisecond stability is a reasonable expectation.
So, probably need to use fatPPS for the 10 usec PRS10 PPS.

> It might be fun to see how far you can push the parallel port.
> How about a loop like this: [...]

Yes, that about the flow that I was contemplating. I also planned
to use optoisolators on the input pins from the readings I did
on interfacings with a parallel port.

> For extra credit...  Suppose  you are expecting PPS type signals and you know 
> roughly where they are located within a second.  There is no point is 
> spinning when you aren't expecting something to happen.  So you can sleep 
> until a bit before the next event.   That will let the scheduler run other 
> jobs.  Hopefully it will be nicer to you when you do want to run.

Since we'd be measuring 4 PPS signals that won't be in phase, I wasn't planning
on sleeping for long. But, I some 10 usec sleeps with usleep(10000) after
reading a pulse would be reasonable. But, with the system not doing anything 
but polling and writing to the disk, I wasn't expecting much interruptions from
other processes.

But, I really like Tom's picPET, not just for this project, but after my son's
project, I'm going to use 4 picPETs to measure stability across my group of 
Thunderbolts of varying vintage as well as comparing long-term drift across
some Rb oscillators.

Thanks again for the excellent parallel polling idea. I was going to go
with that plan before Tom letting me know about his picPETs.


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