[time-nuts] Raspberry Pi NTP server

[Hal Murray]

tpiepho at gmail.com said:
> It seems like it would not be that hard to get the USB frame sequence phase
> locked to the system clock.  One would need a way to measure the phase offset
> of the USB S-o-F vs the system clock, and then it's a standard process to
> phase lock, with the necessary control to do this described above. 

The catch is that getting the frame lined up doesn't provide you with any info 
about the offset.  (I'm assuming we have something like a RS-232 to USB setup 
with the PPS signal connected to one of the modem control signals - Carrier 
Detect is the most popular.)

If you have a knob to slew the phase of the USB signal, and things are stable, 
you could move the USB signal so that it is exactly lined up.  Then noise in 
the system would make half the samples come up "no" and the other half come up 
"yes".  That probably depends on something like the noise being large relative 
to the stability of the system.  (You could add artificial noise.)  If things 
aren't that stable, you can steer the USB timing back and forth to bracket the 
PPS signal.

It all seems much simpler to put a counter in the USB device to measure the 
time between the PPS and the poll.  That actually sounds like a fun hack.  
Does anybody have a favorite dev board with a CPU and USB that would be 
appropriate for doing that?


-- 
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