[ntp:questions] Assistance with PPS on Windows

Chris Albertson albertson.chris at gmail.com
Fri May 6 04:43:37 UTC 2011

On Thu, May 5, 2011 at 6:16 PM, unruh <unruh at wormhole.physics.ubc.ca> wrote:

> We are talking about synchronizing the PC clock to GPS to a certain
> accuracy. Not measuring time intervals between inputs to a certain accuracy.
> The "interupt being serviced" is the problem. The time it takes for an
> interrupt service routine to get the interrupt, and to read the system
> clock is certainly longer that 1ns as as far as I can see is closer to
> 1us.

We don't really care how long it take to process the pulse with an
interrupt.  what we care about is the uncertaintain in the llength of
this process.  It is ALWAY took EXACTLY 10 us then we's have error
much under a few ns.   But not even this maters, see below

> But if it is possible, I would love to hear about it.

The entire process is moved outside the computer.  Use a good qualty
hardware counter and the GPS' pulse cause the count to be transferred
to a hardware buffer.  the computer only needs to poll the buffer a
few times per second and the result is sub-nanosecond timing.  Yes it
might take time even for hardware gates to switch but as long as this
remains constant (clocked logic) we are OK.

Chris Albertson
Redondo Beach, California

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