[ntp:questions] Assistance with PPS on Windows

Terje Mathisen "terje.mathisen at tmsw.no" at ntp.org
Thu May 5 20:13:22 UTC 2011

unruh wrote:
> Uh, no. There is no way of getting the signal into the computer with

No pure sw way, no.

> nanosecond accuracy. 1usec is about it. The interrupt service routines,
> if you use a specialised driver for some interrupt driven card, will
> take about 1usec to service the interrupt. Thus while the card may
> deliver a pulse whose leading edge is within say 50ns of the correct
> time that is irrelevant if the computer itself cannot respond fast
> enough.
> But I agree, with a GPS you can get usec accuracy in the time delivered
> for less than $100.

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.

Google phk!


- <Terje.Mathisen at tmsw.no>
"almost all programming can be viewed as an exercise in caching"

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