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

[Tom Van Baak]

>> 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