david-tay...@blueyonder.co.uk said: > But having the PPS on the DCD over USB is not as useless as you might first > think, because in tests here using the DCD/PPS over USB produced better > results with NTP than an internet connection alone. It is worth checking - > your results may differ.
The Ethernet is also on USB so it will have the USB jitter as well as any jitter from the network. Even if the remote NTP system is perfect (or at least very good relative to the R-PI), I'd expect a local PPS via USB to be slightly better than internet time. tic-...@bodosom.net said: > I'll admit, to my shame, that I have yet to deduce how to use USB provided > DCD for PPS. I've looked, really I have but to no avail. Linux has two APIs to PPS. gpsd uses TIOCMIWAIT, an ioctl that lets a userland program wait for the PPS/DCD change. You can feed that to ntpd via SHM. The ATOM and NMEA drivers in ntpd use the API described in RFC 2783. It's in sys/timepps.h On Fedora, it comes from the ps-tools-devel package. This needs a running ldattach 18 /dev/xxx for each PPS source. The interrupt driver grabs a timestamp so the timing accuracy should avoid most of the jitter associated with getting to userland. I think most real serial ports have support for both. Support for USB serial devices is not so good. I haven't checked recently. I think TIOCMIWAIT support is generally better. -- These are my opinions. I hate spam. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.