[ntp:questions] Assistance with PPS on Windows

unruh unruh at wormhole.physics.ubc.ca
Fri May 6 06:14:34 UTC 2011


On 2011-05-06, Chris Albertson <albertson.chris at gmail.com> wrote:
> 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.

The question is whether or not the clock on the computer can be
disciplined by the use of ntp and gps to ns accuracy. Thus, the
interrupt can trigger the counter, the computer can read the counter,
but the computer must also read the computer clock at the same time

The question is whether or not the clock on the computer can be
disciplined by the use of ntp and gps to ns accuracy. Thus, the
interrupt can trigger the counter, the computer can read the counter,
but the computer must also read the computer clock at the same time as
it reads the counter so it can know how long it takes to read the
computer clock. 
And of course the time it takes to read the computer clock does not
necessarily stays the same-- other jobs on the computer can delay those
reads. 
If you mean by the "entire process" also that the clock is moved outside
the computer, then there is still the issue of getting that time into
the computer. 
 

>




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