Additional pps-gpio

[Hal Murray]

> PPS line discipline via USB virtual serial works and produces the expected
> ~1ms offsets due to the USB poll interval.  It should be possible to remove
> that shift by doing a loopback measurement via RTS/CTS eventually.  Jitter
> is roughly on par with the local stratum-1 over network. 

The USB polling interval is roughly 1 ms so I'd expect 1 ms peak offset, 1/2 
ms if you average over enough samples.

There is at least one USB-serial adapter that uses the USB mode that polls at 
1/4 ms.  It gets the expected better results.  (I forget the terms USB uses 
for the various speeds.)


Do you know about hanging bridges?  It's what you get when your sampling 
clock beats against the clock behind the data source.  Averaging works if the 
clocks are not in sync.  If the clocks are close to in-sync, then the offset 
changes slowly so a batch of samples will get mostly the same offset.

You get the same thing at the ns level if the PPS from your GPS is derived 
from the local oscillator on the GPS receiver.
  http://www.leapsecond.com/pages/m12/sawtooth.htm

I've made similar graphs for PPS over USB but don't have one handy.



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