On Sunday, 10 September 2017 16:24:51 BST Ralph Corderoy wrote:
> > this makes us wait for up to 10 minutes before the system syncs, but
> > when it does the two clocks are exactly the same.
>
> That's pretty lousy. It would be nice for the NTP server to know to
> trust the RTC and be happy to start serving it from the off.
Yes. But we can live with this if we have to.
> > The bad news is that I have got very unreliable timekeeping from the
> > realtime clock on the master Pi. Sometimes it appears to be keeping
> > time, sometimes it gains or loses several minutes in an hour.
>
> Start with a freshly downloaded Raspbian image, do the minimum required
> from the instructions to use the RTC, do not add NTP, etc. Just see if
> the RTC, the kernel, and userspace play well together over the hours and
> days. Then you'll know it's your tinkering that's the problem. It
> still smells like multiple parties are trying to adjust for creep.
I'm kind of doing the other thing at the moment; eg disabling NTP and then
using the hwclock-set script from the Bells system if that doesn't work.
There's a fair amount of work gone into configuring the system at the moment
and I'd rather not have to rebuild it if I don't have to.
Having said that, I did upgrade both Pis from Jessie to Stretch, so I may end
up having to start from scratch.
> Did I ask this before? Why do the Pis need to be in sync, and to what
> accuracy? Do they need to know the actual time on the audience's
> wrists, or just be in agreement with each other that it's 15 minutes
> since they last gave us a twirl?
Both Pis are taking readings and logging them to a file. It is really
confusing if the Sump Pi says that there was 700 mm of water in the sump at
10:45 on the 11th and at the same time the Butts Pi is recording 100 mm of
water in the butts at 17:50 on the 10th.
If NTP proves to be the problem and I'm not able to suss out why, then we'll
write a script to run at boot-up on the Butts Pi to read the system clock on
the Sump Pi and then set its own clock accordingly. It's not hard to do, I
suspect, but NTP seemed to be designed to do that, so that's what we did.
--
Terry Coles
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