[time-nuts] GPS DO Alternatives

Bob Camp lists at rtty.us
Sun Dec 9 22:56:05 UTC 2012


Without a specific goal, there's no real way to pick a set of hardware. The Freescale board I mentioned earlier is a reasonably cheap (sub $15) ARM board. The only way to decide which / what to use is to have an objective. Without agreement on that we'll each just go on and on tossing up names of boards we happen to have noticed or liked …

Be careful with PWM outputs for 1 pps generation. Most (but not all) PWM's have one or another gotcha when you go to use them for that task. 


On Dec 9, 2012, at 5:43 PM, Dennis Ferguson <dennis.c.ferguson at gmail.com> wrote:

> On 9 Dec, 2012, at 02:14 , Hal Murray <hmurray at megapathdsl.net> wrote:
>> Which brings up another worm for the can.  How are you going to get a PPS 
>> from the OCXO?  That's going to be hard without some soldering.  My straw man 
>> would be to use the OCXO to clock whatever uP you end up using and generate 
>> the PPS with a counter/timer.
> I think I'd do that too, and pick a processor with enough of the right
> peripherals to avoid having to fart around with an external phase comparison
> or interrupt timestamping.
> The TI Stellaris LaunchPad, mentioned earlier, is pretty cheap, the
> processor can generate its internal clocks directly from a 10 MHz
> reference input, and it has a set of 32 bit timer/counters that can either
> capture a timestamp for an input edge event or generate a timed outgoing
> pulse, both with a resolution of 12.5 ns.  The capture could be used to
> time the GPS PPS (in hardware, no external comparator, no interrupt
> timestamps) while I think the PWM generation mode could be used to generate
> a PPS output synchronous with the 10 MHz oscillator.  The only thing that
> is missing for a GPSDO is an internal DAC (it has a bunch of ADCs) so the
> 10 MHz oscillator would need a digital frequency adjustment for the
> processor to be sufficient for the job without any external peripherals.
> The processor core is a fully functional ARM, including floating point,
> which might make it easier to, say, do a Kalman filter implementation
> that wasn't write-only.
> The thing is, I still don't find this all that compelling.  Personally
> I think it might be better to maybe aim higher on the software value
> chain, find a processor board with the same on-chip processor peripherals
> as above along with an on-chip Ethernet MAC and enough external peripherals
> to run a real operating system and use that to build a combination GPSDO
> and very accurate NTP/PTP server.  You generally can't just buy one of
> those on eBay (at least they are much rarer than a GPSDO alone), and it
> is a lot easier to do something which is attractive both in price and in
> quality compared to what you can buy.
> Dennis Ferguson
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