Re: [gpsd-dev] RFC 2783

[Gary E. Miller]

Yo Frank!

On Tue, 10 Mar 2015 14:57:04 -0400
Frank Nicholas <address@hidden> wrote:

> I have this adafruit GPS (https://www.adafruit.com/product/746), and
> had it connected to a RaspberryPi for a year or two.  I used NTPd
> directly.  Nifty piece of hardware and is extremely reliable.  It has
> an external antenna connector that I'm using.  It came with a battery
> holder that I never used.  It locks quick, even with a long off, cold
> start.

I went with the 2324 since it plugs directly into the RasPi.  Otherwise 
they look similar.

How did you get the GPIO on pin 4 to drive your /dev/pps0?

> The adafruit GPS is currently attached to my BSD based pfSense router
> as an NTP server (no GPSd).  I'll probably move it back to a Pi and
> put one of the USB GPS's I just received on the pfSense box (less
> wires in that area).  I'll then have 2 x PPS enabled GPS NTP servers
> at home.  Any advantages to 2 x on the same LAN?  What should I look
> for to dial them in, or should I just let NTP on the clients figure
> it out?

Well, you do reboot things now and again, so you'll always have good time.

Having two PPS lets you measure network latency, bufferbloat and 
asymmetry.

I have been experimenting with linuxptp lately.  By having a PPS
at both ends I have noticed some serious (> 1 Sec!) offsets in the master
and slave PTP.  Users without the dual PPS have been optimizing jitter
aond let offset go totally whacky.

Plus you can brag to all your friends!

RGDS
GARY
---------------------------------------------------------------------------
Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97701
        address@hidden  Tel:+1(541)382-8588
> 
> On Tue, Mar 10, 2015 at 2:45 PM, Gary E. Miller <address@hidden>
> wrote:
> 
> > Yo Eric!
> >
> > I have been looking at the ntpd RFC 2783 enabled drivers hoping to
> > learn something from them.  I found the opposite.  gpsd does a much
> > better job with RFC 2783 than ntpd.  Except for one thing ntpd does
> > better.
> >
> > Gpsd auomatically configures the GPS serial port for PPS, then
> > automatically detects the resulting /dev/ppsX device. gpsd then
> > passes PPS data to ntpd for all offsets from system clock to PPS
> > clock.
> >
> > ntpd does none of that.  In the configuration file you tell ntpd
> > which of /dev/pps[0-3] to use.  Before ntpd will use the PPS input
> > the ntpd time and system time must be within +/- 400 mSec.  That
> > may be hard to do for a GPS and gpsd does not require that.
> >
> > We are getting a lot of pull from the embedded community to support
> > manually configuring the /dev/ppsX.  For example, there are many
> > GPS for RasPi that put PPS on GPIO pin 4.  Adafruit claims to have
> > sold 10,000 plus GPS for RasPi configure this way.  There is no way
> > for the kernel or for gpsd to know the association between GPIO 4
> > and the serial GPS.
> >
> > For those Adafruit, and other, users we need a way to specify a
> > /dev/ppsX manually.  This also solves the problem on other OS that
> > do not allow Linux like determination of serial device to PPS device
> > mapping.
> >
> > How about something like this:
> >
> >         gpsd -n /dev/ttyUSB0 -p /dev/pps0
> >
> > Ideas?
> >
> > I have an adafruit on order so I can test:
> >
> > http://www.adafruit.com/products/2324
> >
> > RGDS
> > GARY
> > ---------------------------------------------------------------------------
> > Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR
> > 97701 address@hidden  Tel:+1(541)382-8588
> >
> >