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. :) _______________________________________________ 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.
