[ntp:questions] Assistance with PPS on Windows

unruh unruh at wormhole.physics.ubc.ca
Fri May 6 01:16:46 UTC 2011

On 2011-05-05, Chris Albertson <albertson.chris at gmail.com> wrote:
> On Thu, May 5, 2011 at 1:13 PM, Terje Mathisen <"terje.mathisen at
> tmsw.no"@ntp.org> wrote:
>> unruh wrote:
>> Add some extra hw, in the form of a very good crystal (surplus Rb) and a hw
>> counter which starts to run on the PPS signal, so that the sw can query it
>> to determine how much interrupt latency we got this time, and you get a
>> FreeBSD box down in the dual-digit nanoseconds.
> yes, FreeBSD and now Linux do run on a nanosecond clock and it's not
> at all hard to measure interrupt latency once per second.
> If you don't like designing you own electronics use the "pictic".
> (Google pictic) it can measure the time from PPS to the interrupt
> being serviced  to much better then a nanosecond.  Costs about $50 to

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

> build and a PCBs are available.    But this level of precision is not
> needed by most users.  Some do.  My goal is to to learn about the
> ionosphere by measuring how radio waves reflect off of it.
> Even though I have some of this equipment I'm still astonished that
> measurement at the sub-nanosecond level is both possible and
> affordable on a hobbyist budget.  Not to long ago this was only
> possible in large university or government labs.

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