Re: [gpsd-dev] SemPiTernal - Bounding PPS uncertainty

[Gary E. Miller]

Yo Phil!

On Fri, 22 Apr 2016 08:29:29 -0400
Phil Salkie <address@hidden> wrote:

> (BTW, if anyone needs to convert NMEA to
> RS-232 with PPS, drop me a line,

Sparkfun has these, I buy them out of Chaina for a few bucks each.

> does everyone who's
> making these hats, coats, shields, etc. pay attention to making sure
> the PPS signal always hits the CPU in such a way that the edge the GPS
> chip thinks is top of second is the edge the kernel thinks is the top
> of second?

Not that we can tell.  I've heard many permutations.


> If PPS signals are always less than 500ms wide,

Uh, no.  gpsd is in use with some GPS models that are exactly 500 mSec wide.
Basically 2Hz.

gpsd has the code to handle that case.  We have also seen 0.5Hz, where
both edges are on the edge of a second.  We have also seen 5Hz and 10Hz.

Best to design knowing that you will hit those odd cases sooner rather
than later.

> it would be pretty
> straightforward for the kernel driver to determine the leading edge by
> looking at the width of a pulse, and inverting its logic if it sees
> the pulse width is >500ms.

1. that is not the kernels's job, that is a job for gpsd or ntpsec.

2. not straightforward when you also handle 0.5Hz, 2Hz, 5Hz and 10Hz.

> (For the curious, the scope is a Hantek DSO-8060, dual channel 60MHz
> storage scope, color 5.7"LCD, arbitrary waveform generator, with full
> independent and isolated DMM functions.  It was a big $408 new on
> ebay, and replaces both the 30 year old Fluke DVM and the Seeed
> Studios DSO-Nano in my field service tools kit.)

My boat anchor Tek 465b still beats that. :-) 100MHz.  Only $1,200, in
1978, after employee discount.  They are cheap on ebay, but you'll get
your exercise moving it.

RGDS
GARY
---------------------------------------------------------------------------
Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
        address@hidden  Tel:+1 541 382 8588

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