I see that I should check my posts for repeating figures-of-speech... .But let me try a second (and last time) to persuade you of my arguments! :-) But I can a

I see that I should check my posts for repeating figures-of-speech... .But let me try a second (and last time) to persuade you of my arguments! :-) But I can assure you that it's ok to have a timing GPS connected to a USB serial port, as long as you feed the timing signal to an input that supports proper PPS timestamping. Here's a graph of a Trimble Thunderbolt PPS connected to a PC's parallel port. The serial port the device connects to is a USB dongle, but that's completely irrelevant for the PPS precision. http://vogelchr.blogspot.de/2011/10/thunderbolt-gpsdo-pps-signal.html http://vogelchr.blogspot.de/2011/10/thunderbolt-gpsdo-pps-si... (it's a pretty busy old Pentium) So, up to this point, there's no speculation about using a USB serial device, while the actual timing signal is connected elsewhere, and here's where I speculate only a little: I have no reason to assume that the generic PPS on GPIO support would be any worse than the parport- solution, because the implementation is pretty much identical (recording a timestamp in a minimal interrupt routine), especially as it's been shown to work for a conceptually similar CPU.