[time-nuts] Update on my Arduino GPSDO / NTP server - going atomic

[Ryan Stasel]

Andrew,

My only comment, since I’m working on something very similar myself (just a GPS frequency standard that will be able to have the OCXO shut off, and just output 1PPS for an NTP system), would be to not output any signal/pps/ntp timing unless you have a solid GPS lock, since before that, I would assume your PLL loop is sweeping. But I’m sure that’s already in your code… just didn’t look through it all. =)

Cool idea though. I’ve found very few (none) instances of people actually running NTP servers from arduino hardware… most use Raspi or the like. 

Thanks! 

-Ryan Stasel

On Sep 5, 2014, at 12:07 , Andrew Rodland <andrew at cleverdomain.org> wrote:

> After some productive work, and some frustrating weeks spent fighting
> weird flakiness and needlessly replacing components, only to find that
> the problems went away after I reseated my main power connector, IT
> WORKS!
> 
> Here's where I am now:
> 
> * Main board: Arduino Due (ATSAM3X ARM Cortex-M3 CPU @ 84MHz)
> * Oscillator: Symmetricom X72.
> * GPS: Trimble Resolution T with a cheap Gilsson puck antenna.
> * Ethernet: Wiznet W5100.
> 
> The X72 is used to externally clock one of the ARM's hardware
> timer/counters at 10MHz (I'm not multiplying it up and using it to
> clock the CPU). The same timer timestamps the rising edge of the PPS
> using capture mode, jitter free @ 100ns resolution.
> 
> All the PLL is done digitally using these values and the adjustment is
> sent to the X72 over serial (DDS, 2 ppt resolution).
> 
> After about a day's solid running, the PPS phase stays within +/-
> 100ns as measured on the board itself, even out to a PLL tau of 1
> hour, and the frequency adjust stays within <1 ppt when 5-minute
> averaged. I'm collecting data against an outside reference now (PPS
> generated by the board against the PPS of a Spectracom NetClock with
> OCXO option). Too early for graphs, but it looks good.
> 
> On the Ethernet end, things are less good, but still respectable --
> about 10us RMS added jitter. I think a lot of this is introduced by
> the W5100, and I'm working on getting my hands on a board that uses
> the same chip but actually makes use of the onchip Ethernet MAC that
> the Arduino doesn't bother to route, which should help substantially.
> Already it's better than conventional wisdom says NTP gets :)
> 
> Questions I still have:
> 
> 1. Should I try using the analog EFC to zero out the amount of
> correction I ask the X72's DDS for? Could reduce jitter in the
> timebase, could just add noise. I suppose I can test this one easily
> enough.
> 
> 2. Is there any point in decoding the sawtooth correction from the
> GPS, or in wiring up the PICTIC and using it to measure the GPS offset
> more accurately, when my clock granularity is 100ns anyway? I suppose
> at best I'd be improving my accuracy from +/- 1.5 ticks to +/- 0.5
> ticks.
> 
> 3. Anything else I should consider?
> 
> If anyone is curious, code is at
> https://github.com/arodland/Due-GPS-NTP-Server .
> 
> Thanks for following,
> 
> Andrew
> 
> On Tue, Jul 29, 2014 at 12:42 AM, Andrew Rodland
> <andrew at cleverdomain.org> wrote:
>> Hi all,
>> 
>> After a couple years not doing anything except letting it sit in my
>> den and provide time for my home network, I've decided to start
>> hacking on my hobby project again.
>> 
>> For reference, what I've got right now is a Freetronics EtherMega
>> (ATMega2560-based Arduino clone with integrated W5100 ethernet), wired
>> up to a USGlobalSat ET-318-02 (a pretty cheap consumer SiRF-III
>> module). It runs totally custom timekeeping, PLL, and NTP protocol
>> code. The timebase is the onboard crystal, which I have no way of
>> influencing directly, so it basically does DDS, adding or duplicating
>> the occasional tick to keep lock. For such a ramshackle collection of
>> equipment it does a pretty good job, tracking within around 10us of a
>> Spectracom NetClock (and showing less Ethernet-induced jitter than the
>> NetClock itself)
>> 
>> I've been thinking for years about building a next-gen version, and
>> sketched a few designs, but I could never quite find a board that I
>> wanted to use as the core of it. Well, Freetronics sent me a product
>> announcement for their EtherDue board (built around the ATSAM3X, which
>> is an ARM Cortex-M3 chip from AVR), I read some specs, and decided to
>> dive in.
>> 
>> I've got a working, tuned-up LPRO-101, and I always figured that my
>> next build would desolder the clock crystal and use the Rb as the CPU
>> timebase, like most builds I've seen do. But I realized that the
>> ATSAM3X is happy to run its timer/counters off of an external clock as
>> long as it's less than 1:2.5 the CPU clock. 10MHz fits that bill. I
>> lose a little bit on granularity by not letting the CPU multiply that
>> up 8x for me, but probably no real change in accuracy. Just feed the
>> Rb to a pair of pins and get a register that counts up every 100ns,
>> seems simple!
>> 
>> For locking to the PPS I could do the usual thing and use input
>> capture on the timer clocked by the Rb, which would handily timestamp
>> the rising edge of the PPS. But I have a couple of PICTIC IIs laying
>> around, and I'm a bit tempted to instead use the timer to generate a
>> PPS from my board and let the PICTICs compare. Since START has to come
>> before STOP I figure I need two of them in parallel, only listen to
>> the one that gives a report < 0.5 seconds, and which one gives me the
>> sign. Does that make sense? Or should I just use one and lock to a
>> nonzero offset? I've found surprisingly little material on the tricks
>> of using a TIC in a digital GPSDO.
>> 
>> Finally there's adjusting the Rb. It would be nice to be able to slew
>> nice and gently by actually nudging the clock instead of
>> adding/dropping them, especially if I have the PICTIC to give me
>> precision offsets. I'm not sure whether the 12-bit DAC on the board
>> stands any chance of being clean or accurate enough to drive the
>> LPRO's C-field adjust, or whether I need something external, or
>> whether I should just locate an Rb with digital adjustment (on a
>> related note, has the price of surplus Rbs gone up a *lot* lately?
>> Anyone know why? Can't be hobbyist demand, can it?)
>> 
>> Got a lot of questions to answer, but I'm ready to start building and
>> learning again. :)
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