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

Hal Murray hmurray at megapathdsl.net
Thu Nov 2 06:19:11 EDT 2017

mlewis000 at rogers.com said:
> - GPS module's secondary PPS disciplining the RTC-counter-divider PPS by
> resetting the RTC's counter/divider (I'm assuming there's one that will
> rest fast enough to sync; I've never looked into these...) 

How many RTCs accept an external PPS?

Your plan sounds complicated.

  If the external PPS comes in roughly the middle of the
  RTC second, how does the RTC decide whether to go
  forward or backwards?

  Does your GPS disable its PPS when it isn't locked?  If not, you need 
additional logic to mask it so the RTC doesn't track a drifting GPS.

I'd let the RTC free run, feed its PPS to a "clock" program, and then feed 
the offset to ntpd via SHM.

These are my opinions.  I hate spam.

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