[time-nuts] PPS for NTP Server - How Close Is "Good Enough"?

Pete Stephenson pete at heypete.com
Wed Jun 10 04:55:09 EDT 2015

On Wed, Jun 10, 2015 at 6:30 AM, Ed Armstrong <eds_equipment at verizon.net> wrote:
> Hi, this is my first post ever to a mailing list, so if I'm doing anything
> wrong please be gentle with your corrections :-)



> My next step was to find out where the even second pulse entered the output
> circuitry. I then broke the trace taking the even second into the output
> circuitry, and ran a piece of 30gauge wire wrapping wire from the via at
> test point 33 to the via at the input to the output circuitry. The wire fit
> so perfectly it felt like the vias were made for just this purpose :-) Now
> I've got a very nice PPS signal available both at the front jack and at the
> backplane connector in the rear of the unit.

Very clever.

> OK, here is the actual question. Do you think it is OK to consider a pulse
> which arise 250 ns early to be close enough? And no, I am not forgetting
> about that 3 ns, there is about 3 ns of delay added by the output circuitry.

Depends on your needs, of course. For NTP, 250ns will be essentially
unnoticeable. For other purposes, it might be an issue, but not for

NTP typically works on the micro- and millisecond level. Any offset or
jitter on the nanosecond level will be lost in the noise, so to speak:
the interrupt latency of the Pi will far exceed any nanosecond-level


Pete Stephenson

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