Re: [chrony-users] Newbie Help Needed

[Stephen Satchell <list@xxxxxxxxxxxx>]

On 01/03/2018 05:48 PM, Stuart Maclean wrote:
> As you can imagine, I want to keep my Linux system time in line with,
> i.e. disciplined from. my reference clock C.
>
> Can chronyc do this?  Do I have the appropriate set up for this to work?
>   I must admit that all the talk of 'ref clocks' and 'locking' confuses
> me no end.  I have NO idea what my .conf file would look like.

I'm somewhat new to the chrony-users list, but let's look at your request.

1)  "I also have a cheap timer crystal, 32Khz, that serves as the 
default 'Linux system clock'.  It is wildly inaccurate..."

There's a major part of your problem.

Chronyd, and ntpd for that matter, assume that the local clock has poor 
absolute regulation, but good relative regulation -- that translates to 
"the local clock is predictably bad."  Yours has drift over temperature, 
so any solution with chronyd or ntpd is bad news.

2)  "Clock C outputs a 'seconds-since-the-epoch string', call it S, 
if/when I send it a 'request for time'...C is temperature compensated."

So I come away with the impression that your Clock C can be depended on 
for hours/days, far more so than your local clock.

Do I have that right, in a nutshell?

So the "correct" answer is to forget about using NTP protocol, and make 
use of the seconds-since-epoch to correct your system clock.  If you do 
this frequently enough, you should avoid step issues.  I would write my 
own daemon that squats on the serial port.  Assuming that the time is 
transmitted once a second, you can then convert that string 
appropriately, and use date(1) to set the system time.  The last digit 
of the time send should be useful as a "downbeat" for setting the local 
clock.  Python or Perl would be up to this task -- you don't have to 
write it in C.

Does your embedded system have any applications that would be affected 
by stepping time?

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