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Re: [gpsd-dev] Mysteriously vanishing bugs don't make me happy

From: Gary E. Miller
Subject: Re: [gpsd-dev] Mysteriously vanishing bugs don't make me happy
Date: Mon, 4 Nov 2013 15:08:19 -0800

Yo Andy!

On Mon, 04 Nov 2013 17:37:52 -0500
Andy Walls <address@hidden> wrote:

> On Mon, 2013-11-04 at 16:14 -0500, Greg Troxel wrote:
> > Have people been seeing jitter low enough that microsecond
> > quantization looks like a real issue?
> AFAICT gpsd only deals with PPS sources associated with serial port
> control lines.  I'm going to guess that those sources have jitter in
> the +/- 1 microsecond range at best.

I, and others, have seen better.  Remember some serial can do 450 Mbps.

> If both of those are true, then I'd think nanoseconds don't matter
> much.

They do.  Enabling nSec on chronyd shows instant improvement.

> If gpsd would allow "arbitrary" PPS sources to be associated with a
> GPS serial stream, then maybe the precision handled by gpsd would
> need to stretch into the nanoseconds.

Easy to work around if you have a kernel driver available.  Many have
done so.

> >   What kind of clock stability and sync hardware
> > does it take to get to that point?
> A UART watching serial port control line probably isn't the sync
> hardware.

Works for me, and others.  I'm working on some other options as well.

> In measurements, I observe +/- 77 nsecs of jitter in that PPS capture,
> which I will attribute to sample alignment and the (in)stability of
> the 26 MHz crystal.  I just don't believe I'm observing that much of
> the 0.3% of PPS jitter that I could possibly see.


> This example of 77 nanoseconds resolution really doesn't matter to
> gpsd's implementation, unless gpsd lets users associate an "arbitrary"
> PPS device node with a GPS serial stream.

You give me the device, I'll give you the patch.  All you really need
to do is override the computed /dev/ppsX device name with the correct

Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97701
        address@hidden  Tel:+1(541)382-8588

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