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

[Chen Wei]

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