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Re: [gpsd-dev] PPS over USB

From: Ed W
Subject: Re: [gpsd-dev] PPS over USB
Date: Mon, 07 May 2012 16:08:15 +0100
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On 07/05/2012 14:29, Eric S. Raymond wrote:
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.

Yes it's a venus 6.  Resend of the link:

They cost around $15 or so. Perhaps they can be had for less in quantity ($10 might be a reasonable target)

A little further research suggests that the CP210x is a "full speed 12mbit USB2" device. Google suggests that "full speed" devices are still polled at 1ms resolution and that you need a "High speed" device to achieve 125us resolution. As such the expected jitter is at least 500us, ie the normal 1ms jitter.

I see chrony state a 1ms jitter from the NMEA time, which is a bit higher than the expected 0.5ms jitter, but I haven't looked into how chrony computes it's calculation. Also it's possible that there is some gremlin due to the way GPSD computes the NMEA time? (eg I thought the algorithm might kick in once a whole sentence was read and reversed out the length of the sentence to get the start timestamp - if the sentence was read over two 1ms interrupts then we might acquire an additional 1ms jitter that could be eliminated if we used the sentence start timestamp, not the end timestamp? - ready to be told this is wrong...

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.

I guess I need to go find those results - however, might Gary mind resending his results and how to compute them here? For reference I have been feeding my results to chrony and examining the statistics from that. Probably ntp computes it's statistics differently?


Ed W

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