Have you seen any projects/code that actually use the BBB PRU's to feed NTP? I have a Pi that can get pretty good stats (Chrony reports sub-microsecond) from GPS PPS with only one kernel flag change but I've always wanted to use the spare BBB I have laying around because it seems like it'd be superior. I've seen people talk about using the PRUs for it, but never actually seen anybody document how to do it and I don't have the time nor expertise to hack on it myself, at least not without some hand holding.