[time-nuts] 1 PPS 50-ohm driver

Wojciech Owczarek wojciech at owczarek.co.uk
Mon Apr 18 18:01:39 EDT 2016


Florian, Gerhard, All,

Thank you for the extensive answers. Just to think that at some point I
passed exams where I had to implement truth tables in "raw" NMOS/PMOS -
things evaporate quickly enough if you don't refresh, and then years go by
and you wish you had been paying more attention. The moment you mentioned
that a CMOS NOT gate only requires two transistors, it all came back.

As to Beaglebone Black and "use use whatever you have", fine for 1PPS
input, I'm using an SN74AHCT125 for the moment, I may revisit that. But for
the output, I do want the slope to look decent and the signal to be able to
travel over some length of coax, even if not so precise. I am developing a
PTP slave timing probe based on the BBB - you sync it to PTP and take a
reference 1PPS input to compare the phase. Or alternatively, sync time to
GPS + 1PPS, or just phase to 1PPS, and run PTP read-only and report the
observed offset.

The hardware right now is the bare minimum required to get the signals in.
On the software side, for 1PPS input, I am able to stay within the
resolution of the counter used for event capture, which runs at ~24 MHz, so
about +/-40 ns, I'm getting a pretty clean sawtooth. For the 1PPS output,
I'm using a PWM pulse, which is aligned to top of PTP second using a PI
servo (counter overflow generates a PWM pulse and a timestamp at the same
time). PI servo because it's a standalone device and the real frequency can
be unknown. Well, I can experiment with pure correct-and-rearm next, PI was
a quick and lazy solution. Also the PTP clock resolution is 4 ns, so
quantisation error is a problem, more for the output than for the input. I
can't test the quality of the output until this is hooked up to a proper
counter. Well, in fact I can't compensate for the h/w interface delays
either until I run this against a decent scope.

Yet another embedded project, but I will be happy to share the results when
ready. This is definitely not lab-grade stuff, but it is designed to be
used environments where you're aiming for sub-100 us accuracy, so 100 ns is
well enough, and for cheap. BBB is 100 megabit, but the upcoming BBB
Enhanced (third party fork) is gigabit.

Kind regards,
Wojciech


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