Problem with GPSD refclock

Gary E. Miller gem at
Thu Jan 26 19:06:23 UTC 2017

Yo Hal!

On Thu, 26 Jan 2017 10:46:20 -0800
Hal Murray <hmurray at> wrote:

> richw at said:
> > When I first wired up a GPS in 2009 for use as an NTP refclock, my
> > understanding was that USB-based devices (including "serial port"
> > PCIe cards which really used USB technology under the hood) were
> > simply not capable of the exact timing necessary to handle a PPS
> > signal from a GPS unit.  
> I've never seen a PCI serial card that used USB.  (But there is a lot
> of the world I haven't seen.)

Check out the M.2 standard.  PCIe + USB on one connector:

There are some GPS that use that connector.

But that is side-by-side PCIe and USB, not consecutive.  So not what
OP talked about.

> PPS over USB adds roughly 1 ms of noise/jitter.  It works, just not
> as well as a real serial port.

Careful there.  PPS over USB 2.0 adds about 250 micro Seconds of noise
and jitter.  USB 1.0 is worse.  USB 2.0 graphs here:

I rebooted yesterday, give it a day or two to reconverge, then you'll see
250 micro Seconds.

Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
	gem at  Tel:+1 541 382 8588

	    Veritas liberabit vos. -- Quid est veritas?
    "If you can’t measure it, you can’t improve it." - Lord Kelvin
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