Additional pps-gpio

[Achim Gratz]

Achim Gratz via devel writes:
> Achim Gratz via devel writes:
>> PPS line discipline via USB virtual serial works and produces the
>> expected ~1ms offsets due to the USB poll interval.
>
> Actually, that 1ms offset was due to the fact that the serial interface
> uses negative logic and I really need to align to the clear edge, not
> assert.  Once I set the PPS width from 1ms to 10ms this became really
> obvious.  It seems to converge pretty nicely and with jitter around 40µs
> now (around the same as LAN jitter).

I've put a second one on the rasPi 1B+.  Configuring the PPS for it was
easy enough.  I needed to shift the time1 to 500µs, it would probably
need something closer to 600µs to be completely unbiased vs. the other
stratum-1 servers.  The GPS jitter goes down from ~15ms w/o PPS to ~1ms
w/ PPS (remember that all of this is through USB).  The observable
jitter from the outside actually increases a bit, roughly by a factor of
2, now that it is no longer locking to three servers over network.

That latter part suggests that we'd really would want to get something
functionally similar to peer mode back eventually.


Achim.
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