Re: [gpsd-dev] RFC 2783

[Gary E. Miller]

Yo Frank!

On Tue, 10 Mar 2015 15:22:46 -0400
Frank Nicholas <address@hidden> wrote:

> > I went with the 2324 since it plugs directly into the RasPi.
> > Otherwise they look similar.
> I think they have the same GPS module.  The one you purchased is a
> newer version and has a RTC built-in.

Cool.

> There was a custom kernel module called "pps-gpio" that did the magic.
> Note that this dates back to mid 2012, and now I think this stuff is
> in current kernels, when configured for RaspBerry Pi.  From kernel
> 3.18.7:

Ah, thanks, I'll check it out.

> I still have "/etc" tar'd up if you're interested in a copy of my old
> system for the NTP config.  I use the config files for reference...

I think I have what I need.  I'll ask when I get the unit if I get lost.

> In Eric's writeup "Introduction to Time Service", he lists the 1PPS
> via USB as having a deviation of up to 1ms vs. 1PPS via Serial at
> only up to 5uSec.

That is about right.  I have a few serial PPS that chronyd thinks
have a jitter of 200 nSec.  No way to independently test that.

> The USB GPS's I have will have THAT much latency
> on the PPS? That's a factor of 200, USB vs. Serial!

Yes.  USB 1.1 uses 1024 assigned timme slots.  So that limits the
measurements to once about every 1 mSec.

I am beating my head on a USB 2.0 GPS solution.  Will report when I have
results.

> If so, I may shy
> away from using the PPS on the USB GPS and get another unit from
> adafruit...

Yes, if 1 mSec is not good enough you need the GPIO PPS.

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