Additional pps-gpio

[Hal Murray]

> For starters, the two PPS pulses should be close enough together to trigger
> a back-to-back queued interrupt, so the second will have to wait for the
> first handler to complete. 

You might learn something by connecting the same PPS signal to two pins and 
comparing the time stamps.  The difference is a lower bound on the latency.

Most GPSDOs produce a narrow pulse, 10 or 20 microseconds.  You might learn 
something by capturing both edges and looking at the difference.  Maybe most 
of the code will still be in the cache.

With 2 GPS devices, I'd expect the PPS pulses to wander back and forth.  If 
you are locked on to pps0 and the order of  the pulse arrival changes, I'd 
expect a slight jump in timing.  It would be interesting to see if that is 
significant.

I'd expect another step if the timing difference between pulses is such that 
it changes from 1 interrupt to 2.  The two interrupt case will add the time 
to return from an interrupt and take the second one.  Maybe a one-shot with a 
knob so you can adjust the delay and plot the difference.


-- 
These are my opinions.  I hate spam.