Question: GPS disciplined stratum 1 NTP service with PPS?

[Matthew Dillon]

I don't think anyone has for DFly, but perhaps someone has done a GPS
interface on FreeBSD.   parallel/pps is probably your best bet but the
time-sync signal can be tricky without an input-capture timer.  You might
be able to use BRK handling in the serial driver to detect the time-sync.
Polling won't work very well in terms of accuracy.

Also look into RS485 to RS232 converters, possibly.  You can the route GPIO
to the RS485 side and if you need to invert it you can do that by selecting
which of the two RS485 polarity data pins to shove the signal into.  You
would need to bias the other polarity pin to roughly 1/2 the voltage to
make it work reliably.

A lot of people push those GPS receivers into raspberry PI's, Raspberry Pi
Pico's, or Arduino Nano boards as an intermediary.  Depends how interested
you are in learning basic micro-controller interfacing but these days it is
fairly easy to get started... you can just do it in C and learn the
hardware (which is usually fairly straight-forward).   Other popular
microcontroller chips are the PIC series of chips, typically the PIC16
series (the PIC8 series doesn't have enough program space for anything
serious).   All of those I think have PIO edge capture interrupts and some
(like the PIC16) can even do automatic timer-capture-on-edge for fairly
precise event timestamps.

-Matt
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