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 _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
