Brad Spencer <b...@anduin.eldar.org> writes: > No, there is no support for the /dev/ttyXX based IOCTLs that glue a PPS > signal to a TTY port [1]. If there is an output on your GPS device for > a pure GPIO style PPS signal, something that is either 5v or 3.3v in > nature and pulses once per second at a digital logic level, you can feed > that into a GPIO pin and use gpiopps(4) to utilize the pulse-per-second > that way [2]. I have been doing that with GPS modules for years with > NetBSD and run 2 GPS Stratum 1 NTP servers and 1 WWVB Stratum 1 NTP > server.
There is actually support for PPS with USB devices that put the pps signal on one of the modem pins, and I'm using it. I think that I added it a few years ago, and it was pretty easy. However, there are two issues: almost no USB GPS receivers bring PPS out on DCD. I have a GR-601W or some model number like that, with a ublox 6, that was a special run that wired PPS to DCD inside. If you have wired up a USB/serial chip and a GPS chip, then you may have this too. But if you just bought a "gps mouse" or dongle, it is highly unlikely. There is 1 ms of fuzz on timing. > As for stability and accuracy... using just the USB data alone will > yield very poor results, as has been mentioned. That can, however, be > used for a quick test with the NEMA driver that ntpd has, just don't be > impressed by it. By adding the digital PPS signal into the mix that > will deal with the USB problems and you will get a good result once the > device and ntpd stabilizes. My modules present their NEMA output as > digital tty (uart) signals that I hook to a FTDI chip and into a USB > port... so the effect is very simular to what you are probably doing. I > also use ntpd which can deal with both a /dev/ttyXX NEMA device and > /dev/gpioppsX PPS device at the same time. In this arrangement, you > won't be using shared memory and your output would look something like > this: It is true that using USB PPS has 1 ms of fuzz. However, people say "stratum 1" and make varying assumptions about what they care about. If the concern is to keep time sync when the Internet is down, 1 ms of fuzz is ok. If you are trying to build something to distribute time to other people, and especially to be a public stratum 1, then it's not ok. (The advice about "use gpio" is good - I am just trying to clarify USB PPS.)
