[time-nuts] Small NTP appliance

[Eamonn Nugent]

Hi, all

This is my first major post on the list. I've been really enjoying the back
and forth, and especially loved reading about some of the Cesium clocks,
their limitations, and their hardware (especially the thought of trying to
repair a Cesium lamp on a 5061A - or, more accurately, how it would be
impossible).

I've been toying around with the idea of building a GPS-synchronized NTP
appliance for myself based off a microcontroller with Ethernet. This is
purely for fun at the moment, I want to learn the theory and design, rather
than just buying one off the shelf. I don't need to bore with the details,
but essentially, I'm going to take a GPS receiver chip (NEO-M8P), hook it
up to a small microcontroller (STM32F767ZI, if anyone's asking), and use
the GPS unit's 1PPS signal to synchronize an internal clock, which then can
be read for NTP-related purposes.

I have one general question. I don't believe that an internal crystal in
the microcontroller will have the accuracy or precision required to have
better than a few milliseconds of accuracy (whereas NTP likes to live in
the microsecond realm), though I very well could be wrong on that one. My
general thought was to find an accurate enough external crystal, or perhaps
OCXO, to synchronize with GPS, and then read from that unit. Are there any
"best practices" I should be following, with regards to this? Or any
suggestions from the Old Guard as to the most "proper" way to accomplish my
goal? I can wire in an external resonator to drive the processor, at up to
26MHz, so I was thinking I could use that to build a relatively simple
counter of some sort to be read from, or perhaps grab an RTC chip, but my
skills and knowledge fall off in this realm.

For context, I'm a software engineer with no fear of C, and have worked
with microcontrollers before, along with writing a small OS for fun, so I'm
not worried about any of the software side. Just want to get some input on
the best method of "timekeeping."

Can't wait to hear feedback!

P.S. If anybody has recs for a Rubidium clock to pick up, just for fun,
feel free to send them my way.

Thanks,

Eamonn