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

[Eric S. Raymond]

Chen Wei <address@hidden>:
> On Fri, Apr 22, 2016 at 01:26:43AM -0400, Eric S. Raymond wrote:
> >    As the pps-gpio module is in April 2016 it has a flaw. It catches only
> >    one edge of the PPS. You have a 50/50 chance you are seeing the
> >    trailing edge rather than the leading edge (which is the actual top of
> >    second).  A patch to fix this has been submitted to the Linux kernel
> >    maintainers but not merged.
> > 
> >    Which edge the kernel will see, and the pulse width, are constant
> >    depending on the GPS type and firmware. If the kernel sees the
> >    trailing edge, the width of the pulse emitted by your GPS will
> >    introduce a fixed lag from top of second to the time when you
> >    actually see the PPS.
> 
> Or worse, because the trailing edge has programmable width therefore
> higher jitter?

Programmable on *some* chips, like the u-blox.  I don't see how this
produces higher jitter, though, unless you're randomly changing the
pulse width during a session.

> http://www.linuxpps.org/pipermail/discussions/2013-April/004423.html

I wish I fully understood that.  Gary, you *certainly* need to read and
understand it, if you hadn't already.

> Beside the capture-clear patch Gary submitted, I think the pps-gpio
> module needs a parameter to better determine assert-rising or falling;
> it also will be nice to specify gpio pin as module parameter.

Are you in a position to push this on LKML? Gary reports not being able to
get them to move on his patch.
-- 
                <a href="http://www.catb.org/~esr/";>Eric S. Raymond</a>