[Top][All Lists]

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: [gpsd-dev] PPS over USB

From: Ed W
Subject: Re: [gpsd-dev] PPS over USB
Date: Sun, 06 May 2012 12:54:51 +0100
User-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.6; rv:12.0) Gecko/20120428 Thunderbird/12.0.1


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

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?

   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...


Ed W

reply via email to

[Prev in Thread] Current Thread [Next in Thread]