Hi Iain, Thanks very much for posting, and for sharing the code. I know many of us are interested in how well modern CPU's or SBC's can be used as time interval, time stamping, and frequency counting instruments. I know the BB PRU's have been mentioned before on the list but it's really nice to see actual code and test results.
About the hp 5370 -- realize that these are still 1000x more precise (on the order of tens of ps) than what a BB/PRU is capable of (on the order of tens of ns). But as you observe, they key point is -- for mid- to long-term measurement of free-running time/frequency standards you do not necessarily need ps-level measurement capability. Nanosecond, or even microsecond time resolution is more than enough to create comprehensive plots of time and frequency drift over the long-term. Again, thanks. /tvb ----- Original Message ----- From: "Iain Young" <[email protected]> To: "Discussion of precise time and frequency measurement" <[email protected]> Sent: Sunday, August 31, 2014 1:24 PM Subject: [time-nuts] Poor Mans TIC (Using Beaglebone onboard PRU) > Hi Folks, > > As much as we all love our HP 5370B's, they are a tad expensive if you > want to monitor several PPS sources long term to ensure they are all > closely syncronised. > > In my case, I have three Austron 2100 LORAN receivers and a HP Z3816A > GPS receiver. I wanted to be able to compare each of their PPS outputs > with the PPS output of the Z3816A, as well as each other. > > Clearly, multiple 5370's would have been too expensive, not just for > initial outlay, but also ongoing electrical costs would not be helped! > > > However, the Beaglebone (Both White and Black variants) have two PRUs. > These are real-time units, with clocks that run at 200 MHz, and most > instructions complete in 1 clock cycle (5ns) > > So, I decided to write a TIC in the PRU Assembler to scratch my > particular itch. The current code waits for the "A" clock to go > high, and then counts until "B" goes high, resets it's counters, > and waits for "A" to go high again. > > It also keeps track of a "sequence" number for sanity's sake, and > onward processing. > > Since the Beaglebone's have two PRUs, I have written the code to run > on both at the same time, and use different GPIO pins, so you can > compare up two sets of two clocks, or two clocks with a common > reference. Pins are documented in README.txt > > Now, it's resolution is 20ns. However, it gets confused if the two > pulses are less than around 10-11uS apart. I -think- this is when > it sends the data back to the host processor via shared RAM. > > In my case, this is not an issue, as I can just slew the PPS from > the Austron's (or even use the Fixed PPS), but if you wanted to > compare two GPS receivers, then that would be an issue. > > > I'll have to look if there's a better way to do the shared memory > stuff (interrupts, signaling etc), or store multiple intervals and > send them all at once, although the current code seems pretty > tight. > > I'd like to have tried it with 1MHz, 5MHz, and even 10 MHz clocks, > as 20nS resolution will handle that, but I think I need to fix > the <11uS separation issue first. > > Then again, it was written to compare PPS's from different Austron > 2100's and GPS. It also took less than 24 hours from concept to > running :) > > If anyone wants it, the code is here here: http://hal.g7iii.net/bb_tic/ > > You will need the pasm compiler, and probably the am335x PRU package, > although there are (tiny) binaries there as well > Setup, Compile, and Running instructions are included in README.txt > > Oh, Sample output: > > PRU0: Seq No:848 Interval:11680 ns or 0.000011680 seconds > PRU0: Seq No:849 Interval:11680 ns or 0.000011680 seconds > PRU0: Seq No:850 Interval:11700 ns or 0.000011700 seconds > PRU0: Seq No:851 Interval:11680 ns or 0.000011680 seconds > PRU0: Seq No:852 Interval:11680 ns or 0.000011680 seconds > PRU0: Seq No:853 Interval:11680 ns or 0.000011680 seconds > PRU0: Seq No:854 Interval:11680 ns or 0.000011680 seconds > PRU0: Seq No:855 Interval:11680 ns or 0.000011680 seconds > PRU0: Seq No:856 Interval:11680 ns or 0.000011680 seconds > PRU0: Seq No:857 Interval:11680 ns or 0.000011680 seconds > PRU0: Seq No:858 Interval:11680 ns or 0.000011680 seconds > PRU0: Seq No:859 Interval:11680 ns or 0.000011680 seconds > PRU0: Seq No:860 Interval:11660 ns or 0.000011660 seconds > PRU0: Seq No:861 Interval:11660 ns or 0.000011660 seconds > > You can plainly see the Austron has a jitter of around +/-20 ns from > the GPS PPS (figures confirmed with the 5370). Slew was around 11.5us. > > I must wire up the other two Austron's but will need to build a new BB > image first :) Hope someone else finds the code useful. > > > Iain > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
