Re: [gpsd-users] 1PPS works on CH340G based usb-serial converter

[Gary E. Miller]

Yo Chen!

On Wed, 25 Mar 2015 12:25:01 +0800
Chen Wei <address@hidden> wrote:

> Turns out the problem is my ntp.conf. After "prefer" a peer other than
> the clock from Type 22 driver:

I'm working on a patch set for gpsd to read the /dev/pps0 directly.

> > > probably because the 1PPS from this
> > > USB-GPS combination is not good enough, which indicated by the
> > > kernel error message "hardpps: PPSJITTER: bad pulse".
> > Garbage in, garbage out.  Any idea why?  Have you put a scope on it?
> 
> No scope.

Pity.

> The rising/falling puzzles me. This ublox-5 modules is configured to
> "synchronized at rising edge, pulse length 100ms", but if the ntp
> capture on rising by set Type 22 clock flag2 to 0, the time is 100ms
> off as compare to time from ntp poll servers.

100 mSec is your PPS pulse width.  There ia a bug in the pps-gpio driver
that only lookat at one edge.  I'm ratning at the maintainers to fix that.

For now you only solution is a 100 mSec fudge, or an inverter for the
PPS line.

> > > The jitter of this USB-GPS is around 300-700 micro-seconds.
> > Which jitter, the serial time jitter or the PPS jitter?  I assume
> > you
> > 
> 
> The jitter from ntpq output.

Which should have both the serial tie and the PPS time.  So which?
 
> > How long did you let the system run?  I find it takes days for a
> > RasPi clock to settle down.
> 
> It is connected to a laptop, which runs less than 12 hours a day. The
> PPS jitter reaches 300-700us very quickly, less than 30min I think. In
> the end of the day, the jitter doesn't improve much.  The jitter is
> lower when this GPS module connect to RasPi's GPIO, I don't recall how
> long it takes to settle down, but my impression is it doesn't need
> days.

If you get PPS on USB 1.1 the 300 - 700 uSec is what you should expect.

I expect to do better on GPIO pin, but my test setup it not quite there
yet.  Maybe by the start of next week.  

RGDS
GARY
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Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97701
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