[time-nuts] Holdover, RTC for Pi as NTP GPS source

Bob kb8tq kb8tq at n1k.org
Thu Nov 2 16:06:49 EDT 2017


There’s no particular reason to stop at the 100:1 point. You can run multiple 
loops at the same time and get out to essentially any level of precision. The
only question is over what averaging interval the precision applies. In some 
cases this elastic definition does just fine. 


> On Nov 2, 2017, at 3:32 PM, Tom Van Baak <tvb at LeapSecond.com> wrote:
>>> The DS3231 has an 8 bit register that will change its frequency in
>>> increments of about 0.1ppm. Thus you could discipline it to get its pps
>>> aligned with your reference.
>> That sounds like you just designed the worst GPSDO ever.
> You could argue that the worst GPSDO ever is an operating system running NTP ;-) A PC running NTP at +/- 10 us is a thousand times worse in time accuracy and a million times worse in frequency stability than a TBolt GPSDO.
> But back to the DS3231. If the 0.1 ppm increment sounds too coarse to you, then just step between N-1 / N / N+1, similar to how PWM works. Don't laugh. Some microcontrollers also have programmable oscillator tuning and I tested this on a PIC -- microstepping 100 times a second -- as part of my "best worst GPSDO" project.
> http://leapsecond.com/pic/src/pg41.asm
> /tvb
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