Re: [chrony-users] Setting up Chrony with PPS

[Bill Unruh <unruh@xxxxxxxxxxxxxx>]

On Sat, 20 Feb 2016, Deven Hickingbotham wrote:

> Hi Bill,
>
> On 2/20/2016 5:13 PM, Bill Unruh wrote:
> > I am really worried about your scatter for the PPS, as I was for the
> > scatter
> > in your assert line. You should NOT be getting a 7ms uncertainty in PPS. It
> > should be more like 7us-- ie 1000 times better. There is something wrong.
> > Either your gpio do not raise interrupts and are being read by very bad
> > polling, or there is something very wrong with your GPS. I also cannot
> > understand your HORRIBLE offsets from external sources ( 1.3
> > seconds out) That is absurd.  It looks to me like your NMEA is way out as
> > well. YOur GPS is a piece of junk it looks like.
>
> I have an identical system and will replicate the tests on that unit.  I 
> don't have that system right now, but I'll post results on Monday or Tuesday.
>
> > And destroy your filesystem in the process? The system does not like
> > jumps in
> > time. You can do it at startup of chrony (the -r option) but that should
> > not
> > happen after the system has been running, or things could get upset.
>
> Oh, I don't want to do that.  But I would like to configure chrony so that it 
> quickly adjusts.  Quickly being 20 minutes.

Do you have connection to the net when it boots? 
You could have a small makestep directive in chrony.conf


> The system chrony is running on will not be a time server for any other 
> systems.  The app I'm writing that runs on the system simply needs the most 
> accurate time to within +/- 5ms.  So while I agree with you that PPS should 
> be delivering much better accuracy, 7321us is pretty close to what I need.

It is symptomatic of problems with your system. As I said, look at your clear
and assert results in /sys/ See if the clear gives more reasonable results (ie
consistant nanoseconds after the second). Your results that you reported
looked really screwy. And these results look really screwy. There is no way a
PPS should be that bad.


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