[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
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.
Yes.
> 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
one.
RGDS
GARY
---------------------------------------------------------------------------
Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97701
address@hidden Tel:+1(541)382-8588
signature.asc
Description: PGP signature
- [gpsd-dev] Mysteriously vanishing bugs don't make me happy, Eric S. Raymond, 2013/11/04
- Re: [gpsd-dev] Mysteriously vanishing bugs don't make me happy, Gary E. Miller, 2013/11/04
- Re: [gpsd-dev] Mysteriously vanishing bugs don't make me happy, Eric S. Raymond, 2013/11/04
- Re: [gpsd-dev] Mysteriously vanishing bugs don't make me happy, Gary E. Miller, 2013/11/04
- Re: [gpsd-dev] Mysteriously vanishing bugs don't make me happy, Greg Troxel, 2013/11/04
- Re: [gpsd-dev] Mysteriously vanishing bugs don't make me happy, Gary E. Miller, 2013/11/04
- Re: [gpsd-dev] Mysteriously vanishing bugs don't make me happy, Miroslav Lichvar, 2013/11/05
- Re: [gpsd-dev] Mysteriously vanishing bugs don't make me happy, Gary E. Miller, 2013/11/05
- Re: [gpsd-dev] Mysteriously vanishing bugs don't make me happy, Harlan Stenn, 2013/11/04
- Re: [gpsd-dev] Mysteriously vanishing bugs don't make me happy, Eric S. Raymond, 2013/11/04
- Re: [gpsd-dev] Mysteriously vanishing bugs don't make me happy, Gary E. Miller, 2013/11/04
- Re: [gpsd-dev] Mysteriously vanishing bugs don't make me happy, Harlan Stenn, 2013/11/04