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Re: [gpsd-dev] PPS over USB

From: Eric S. Raymond
Subject: Re: [gpsd-dev] PPS over USB
Date: Mon, 7 May 2012 09:29:33 -0400
User-agent: Mutt/1.5.21 (2010-09-15)

Ed W <address@hidden>:
> On 07/05/2012 04:20, Eric S. Raymond wrote:
> >Hal Murray<address@hidden>:
> >>Is there a list of serial-USB chips that are known to work and/or not work?
> >You can generate one by grepping for TIOCMIWAIT in the driver tree.  Gary
> >did this as one point.  The PL2303 is defintely good, the CP210x bad.
> Apart from PPS signals, does TIOCMIWAIT improve gpsd accuracy in any
> other way?  Phrased another way are CP210x any worse than a generic
> PL2303 device?

You haven't been following the thumbgps list, I see.  SiRF-III chips
have a 100-170ms long-period wobble in their message timing, observed
by Hal Murray and Gary Miller and myself. That's what PPS fixes,
giving top-of-second to 1ms.
> I couldn't get a definite opinion, but CP210x devices appear to be
> USB2? I currently see around 1ms jitter on my device using just the
> NMEA clock - perhaps I ought to be expecting lower than this?

That's news to us.  What GPS chip is in your devices?  If it's 
Venus 6 that's interesting - suggests they don't have the timing 
wobble that SiRF chips do.

> I think TIOCMIWAIT could be added to the driver. However, does this
> buy me anything on generic non PPS devices?


>                                 Any other changes which
> would improve precision of the NMEA date?

Not that I know of.  The SiRF wobble seems to be something happening
on the chip itself.
> Big picture question - Am I going to get better accuracy using your
> new device than I am from these CP210x Venux6 devices?

Maybe.  Gary is actually measuring 0.5 ms jitter.
                <a href="";>Eric S. Raymond</a>

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