Re: [gpsd-dev] PPS over USB

[Ed W]

Hi

> Navisys has sold us samples of a proposed GR-601W which seem to be
> suitable.  Unit cost should be under $40 and time accuracy better than 1
> milliSec.  Unless we find a show stopper bug, or a better solution, real
> soon, the project will likely go with the GR-601W soon.

Cool - interesting to note


> Ugh.  The Garmin 18s are unsuitable for indoor use.  The uBlox6 in the
> GR-601W works well in my basement.

OK, so you could use some other serial device, my point was whether the 
jointing together needed to be better than DIY?  That said, I'm kinda 
curious that the ublox6 does so much better than a Garmin18, the 
theoretical specs aren't so different?  I have never found any device to 
work inside to be honest?  (at least not for initial acquisition)


> > I got part way through the implementation of using the GPIO inputs as
> > PPS on the PC Engines Alix boards.  The goal was to enable a fairly
> > cheap machine that could connect pretty much directly to something
> > like a Garmin 18 (or the rather nice looking Skytraq modules, some of
> > which seem to offer seriously accurate PPS options) and give you an
> > inexpensive Stratum 1 box. (PC Engines boards are circa $110 for
> > basically an entire 500Mhz i586 including ram, processor, USB,
> > mini-PCI and up to 3 network ports)
> That would be fairly similar to what the bufferbloat project is
> looking to do, except they want it also to work as a standard
> router.

Which bit of the above makes it unsuitable as a standard router?  Too 
expensive?

Just to be clear - the above is a waaay more powerful board than most 
routers, it's i586 compatible (rather than ARM).  However, it is 
probably more expensive than an old Linksys router, and it's switching 
capacity is almost certainly lower than a cutting edge Linksys job.

I'm building a commercial router using these.  Being intel based has 
vastly simplified the process of building a distro.  However, off the 
shelf distros of either xxWrt or Voyager seem to work well with the board.

The bit I was trying to square was that the GPIOs don't currently have a 
linux driver which is interrupt triggered.  However, the boards are 
extremely similar to the OLPC hardware and so I just need time to port 
across their drivers to read the slightly different gpio arrangement on 
the alix.  At that point any recent kernel would get GPIO interrupt 
support and hopefully (untested) the GPIO-PPS driver should just 
work...  Well you get the idea anyway...


> That said, I would personnally love to find a USB 2.0 equivent to the
> USB1.1 PL2303.

Just clicking on the first hit in google takes me to here.  Is this what 
you want?
    http://www.tronisoft.com/prod.php?id=2327

>    Doing so would increase the time resolution from about 1
> milliSec to 125 microSec.  An 8x improvement.

It's not bad, but I think the issue is still that the USB bus is non 
deterministic, so it might still have a good bunch of jitter around 
that.  Over time of course that should average out, but still not perfect

Incidently the Alix has two serial ports, however, for various odd 
reasons they only have the data lines connected, so if you wanted to use 
them then you would need to sync off the data lines, not the CTS or 
whatever - I never investigated if this was even possible (buffering of 
input chars?).  However, interesting



Anyway, I got stalled, but my goal was quite similar.  I wanted to try 
and offer a cheap solution to Stratum 1 servers.  The idea of the Alix 
plus a cheapo serial GPS seemed very attractive.  They can be powered 
via ethernet which potentially makes them easy to put into a hobby box 
and put the whole box on the roof...


Cheers

Ed W