To do PTP "right" requires every switch to support it and a NIC with hardware timestamps. Also, I've seen claims that PTP is no more precise than a good implementation of NTP.
If you look at the white rabbit FPGA PTP updates, its in the ns range.
Any kind of GPS + most intel nics will get you PTP with an accurate clock. If you didn't need to sync too many devices you could use a single system with a bunch of nics as your "switch".
It’s been ten minutes now and chronyc tracking has been marching the offset down. It’s sub 1 us at this point.
System time : 0.000000123 seconds fast of NTP time
Last offset : +0.000000366 seconds
My best lead is to step through the different python timing and scheduler implementations and see which has the lowest jitter relative to the PPS on an oscilloscope.
Even good hardware oscillators can have a wide amount of drift, say 50uS per second, but they tend to be stable over several minutes outside of extreme thermal environments. Therefore, it's pretty easy to estimate and compensate for drift using a PPS signal as a reference. Presumably, that compensation is partially what takes a while for the time daemon to converge on.
Additionally, the clock sync daemon likely takes a while to converge because it isn't directly controlling the system time. Rather, it is sending hints to the kernel for it to adjust the time. The kernel decides how best to do that, and it does it in a way that attempts to avoid breaking other userspace programs that are running. For example, it tries to keep system time monotonically increasing. This means that there's relatively low gain in the feedback loop, and so it takes a while to cancel out error.
It's possible for a userspace program to instead explicitly set system time, but that really isn't intended to be used in Linux unless time is more than 0.5 seconds off. The API call to do that is inherently vulnerable to userspace scheduling jitter, but it's fine since 0.5 seconds is orders of magnitude longer than the expected jitter. You get the system time within the ballpark, and then incrementally adjust it until it's perfect.
If you're not using a kernel driver to capture the PPS edge's timestamp, then you're going to have a rougher time. Either you're just going to have to accept the fact that you can't do better than the scheduling jitter (other than assume it averages out), or you're going to have to do something clever/terrible. One idea would be to have your userspace process go to sleep until, say, 1ms before you expect the next PPS edge to come in. Then, go into a tight polling loop until the edge occurs. As long as reading the PPS pin from userspace is non-blocking and your process doesn't get preempted, you should be able to get at least within microseconds. You can poll system time in the same tight loop, allowing you to fairly reliably detect whether the process got preempted or not.
I have considered adding complexity, such as adding a hardware mux to choose whether to use the GPS PPS signal or the raspberry pi's start-of-second. I should walk before I run though.
This works because a thread of execution generally knows what time it is with higher precision than it can accurately schedule itself.
I'm not sure I understand how you're using a PPS signal to drive a display, though. Is it an LED segment display? I assume you want it to update once a second, precisely on the edge of each second. Displays generally exist for humans, though, and a human isn't going to perceive a few milliseconds of jitter on a 1Hz update.
I know it's absurd to worry about the last few ms, but it's part of what interests me about the project. The goal is to make The Wall Time as accurate as I can. I could go further with a delay locked loop fed from measuring nixie tube current. There is room push down to the dozens of nanoseconds of error relative to the PPS source, but I am content with the 10s of microseconds. I can't imagine ever having access to a camera that could capture that amount of error.
Thanks for the tip. Hardware timers are best. I'll likely have to take some measurements to calibrate the computation time of getting the system time and performing the subtraction.
I'm currently trying to source PPS from software with this repo: https://github.com/twteamware/raspberrypi-ptp/blob/master/pp...
You might find this repo interesting.
I have a 500 MHz oscilloscope on hand so I can decently measure the offset between the raspberry pi PPS and GPS PPS.
As I recall, I had even better performance than that. Around the tens of picoseconds. But I guess the advertised 1 ns is a conservative estimate. The precison is incredible but its not magic, they squeeze the maximum amount of determinism out of custom hardware and fiber optic links. It is a bit of pain too set up, as you need to calibrate each link individually every time you change the fiber or the SFP.
I agree, but PTP will in fact work over regular commercial switches on a LAN. The problem is that it will introduce jitter if there's other traffic on the network, but as long as the paths from master to slave (terms used in the standard) remain symmetric, you can filter this out and achieve performance almost as good as if you were using PTP transparent switches.