[time-nuts] Poor Mans TIC (Using Beaglebone onboard PRU)

Tom Van Baak tvb at LeapSecond.com
Sun Aug 31 22:26:56 EDT 2014


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" <iain at g7iii.net>
To: "Discussion of precise time and frequency measurement" <time-nuts at febo.com>
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
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