Re: [chrony-users] gpsd, pps and chrony

[Ed Wildgoose <ed@xxxxxxxxxxxxxx>]

Hi

> The problem is not the serial port interrupt. It is as fast as any
> other I
> believe. The problem is the time between the interrupt being serviced
> by the
> kernel, and the program (gpsd) being notified that an interrupt has
> occured
> and then timestamped.

Forgive my ignorance, but doesn't kernel PPS buy us 90% of that?

> If you rewrite the serial interrupt  service routine to
> timestamp the interrupt as soon as it gets it, it should be much much
> better. I wrote a parallel port interrupt service routine to do that.
> You still get
> usec level wander (ie it takes about 1-2 usec for the system to
> service the
> interrupt and do the time stamping

Sure - I will need to go recheck the datasheet, but I think my
interrupts are generated by a 40Mhz clock taking something like 3 ticks
to notice the edge trigger, so my accuracy is going to be bounded by
this.  However, what accuracy does one expect a statum 1 clock to be
tracking?  +/- 20 uS?  Less?  (Note I do get that less is possible in
theory, just wondering what accuracy, real, public, Stratum 1 servers
are likely to be hitting?)

> A
> Garmin 18 is not capable of better than that anyway.
>
> For a cheaper version, get the sure GPS board. It is supposed to give
> about
> 20ns timing on the PPS line, rather than garmin's 500-1000ns

Just intuitively it seems likely that the accuracy ought to be better
than 1uS for a PPS line since that would translate into fairly poor
accuracy on the ground?  What kind of reasons cause the PPS to be "so poor"?

Incidently, while we are geeking out on chipsets, I saw Skytraq
mentioned on the gpsd mailing list and they do all kinds of fun stuff,
some with fairly inexpensive eval boards, this has 30ns PPS pecision:
    http://www.skytraq.com.tw/news20090701.htm

(They also have a 20hz GPS with decent sensitivity and a GPS/GLONASS
hybrid board...)




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