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Re: [gpsd-dev] [Thumbgps-devel] Progress on Plain Jane

From: tz
Subject: Re: [gpsd-dev] [Thumbgps-devel] Progress on Plain Jane
Date: Tue, 1 May 2012 22:13:13 -0400

If you start to go too exotic, then an Arduino with ethernet shield emitting a packet right at the pps starts to look good, and the parts are at Radio Shack.  But any such not in quantity would be more expensive.  There are plenty of generic usb 2.0 chips and acm or cdc is just a set of bit and endpoint specs.  And any 2.0 chip would need to use an existing linux driver unless someone plans to do a lot of libusb coding.

The base problem is hardware lameness.  Polling instead of an interrupt pin that can immediately respond, much less something like an input capture.  I could probably sync a 2ppm dallas semiconductor i2c real time clock's 1Hz output, but even then there is nothing to sync that with the internal clocks.  (I was already inspired to use pps to create an accurate synced timestamp for j1850 data off my motorcycle - oh, and can do small quantities of small boards themselves cheap, but you still need to build them).

If you have a 2nd usb port and want to get tricky, the pnp/insert/remove event is likely higher priority, so with a FET...

On May 1, 2012 9:32 PM, "Eric S. Raymond" <address@hidden> wrote:
Gary E. Miller <address@hidden>:
> > That's excellent news.  0.5msec jitter is as good as we could have
> > expected and exactly what one would predict from a max error of 1msec
> > due to polling interval. The long-term NMEA/USB drift is consistent
> > with previous observations by Hal Murray and myself.
> Nice when theory and practice agree.  I wonder if there is a USB 2.0
> equivalent of the PL2303 which would give us a 125 microSec poll?
> Might be worth asking navisys about that.

Maybe.  But that would require that they qualify a new and probably more
expensive adapter part. We should get this puppy shipping and let them
see some ROI before we push that.
               <a href="" href="" target="_blank">">Eric S. Raymond</a>

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