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

Hal Murray hmurray at megapathdsl.net
Wed Nov 1 17:04:20 EDT 2017

>> How are you planning on making ntp use the RTC as a secondary
>> time source?  I don't see that as a supported refclk driver.

> hadn't got there yet likely using NTPsec, as the codebase is available if a
> driver or generic  driver won't work 

The PPS driver expects the pulse to be on the second.  You won't get that 
from a RTC.

If I was doing it, I use the SHM driver.  Then you can do all your work in a 
separate program and feed an offset to ntpd.

I haven't looked at RTC chips recently.  You want a pulse that you can feed 
to the kernel's PPS capture logic.  60Hz works on a PC.  32KHz is probably 
too fast.  (might be worth a try)  You might have to add a divider.

If you use the GPS HAT from Adafruit, it has a prototyping area where you 
could drop in a RTC and wire it up to various GPIO pins.  DIP package would 
be convenient.  The Ethernet on the PI is via USB, so it's crappy for making 
a nutty-good NTP server.  But the PI is convenient and good enough for all 
but nutty uses.

These are my opinions.  I hate spam.

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