[willis936]

Thank you for the detailed response! The PPS is currently driving a hardware interrupt on the raspberry pi that is read in by kernel mode software. My project is to drive an external display. Normally I would bypass the raspberry pi altogether and connect the PPS signal to the strobe input of the SIPO shift register. The problem is that the PPS signal cannot be trusted to always exist. Using a raspberry pi has a few benefits. Setting the timezone based on location, leap seconds, and smoothing out inconsistent GPS data. So while opting to use system time to drive the start of second adds error, I think the tradeoff for reliability is worth it.

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.

[sgtnoodle]

If you want to precisely generate a PPS edge in software with less jitter than you can schedule, you can use a PWM peripheral. Wake up a few milliseconds before the PPS edge is due, get the system time, and compute the precise time until the PPS is due. Initialize the PWM peripheral to transition that far into the future, then go back to sleep until a bit after the transition should have happened, and disable the PWM peripheral.

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.

[willis936]

Nixie tubes driven by a pair of cascaded HV5122 (driver + shift register). The strobe input is what updates the output registers with the recently shifted in contents. The driver takes 500 ns to turn on and the nixie tubes take about 10 us to fire once the voltage is applied.

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.

[sgtnoodle]

Sounds like fun! For what it's worth, ublox GPS modules and their clones should be configurable to always produce a PPS signal regardless of whether or not they have a satellite fix. The module would probably do a better job than software on a pi could during transient periods without a fix (due to how accurate the oscillators need to be in a GPS module). So, as long as you can trust the GPS module to exist and be powered, you should be able to reliably clock your display update with it. The only reason really to generate your own PPS would be if you want it to work without a GPS module at all, perhaps by NTP or something; you're then of course again looking at only a millisecond or so of accuracy.

[willis936]

I'm using an uputronics GPS/RTC hat that has a u-blox M8 engine. I set it to stationary mode for extra accuracy. I'll have to look into other configuration options.

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.