[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: [gpsd-users] SemPiTernal - Bounding PPS uncertainty
From: |
Chen Wei |
Subject: |
Re: [gpsd-users] SemPiTernal - Bounding PPS uncertainty |
Date: |
Sat, 23 Apr 2016 16:14:48 +0800 |
User-agent: |
Mutt/1.5.23 (2014-03-12) |
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?
http://www.linuxpps.org/pipermail/discussions/2013-April/004423.html
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.
>
> //TO-DO: Measure the pulse width on the Adafruit and Uputronics hats.
> //Figure out if they make the leading or trailing edge visible, somehow.
--
Chen Wei
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 <=