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Re: [gpsd-users] SemPiTernal - Bounding PPS uncertainty
From: |
Eric S. Raymond |
Subject: |
Re: [gpsd-users] SemPiTernal - Bounding PPS uncertainty |
Date: |
Sat, 23 Apr 2016 05:00:01 -0400 |
User-agent: |
Mutt/1.5.23 (2014-03-12) |
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>
Re: [gpsd-users] SemPiTernal - Bounding PPS uncertainty, David J Taylor, 2016/04/22
Re: [gpsd-users] SemPiTernal - Bounding PPS uncertainty, Anthony Stirk, 2016/04/23
Re: [gpsd-users] SemPiTernal - Bounding PPS uncertainty, Chen Wei, 2016/04/23
- Re: [gpsd-users] SemPiTernal - Bounding PPS uncertainty,
Eric S. Raymond <=