[ntp:questions] how do I lock in average frequency correction

Ron Frazier (NTP) timekeepingntplist at c3energy.com
Sun Feb 12 19:12:47 UTC 2012


On 02/12/2012 05:45 AM, David Woolley wrote:
> David Lord wrote:
>
>>
>> Ntpd does not interact with my systems RTCs. There are
>> probably some utilities that can set the RTC to current
>> time or set the time from the RTC but I don't know of
>> any for my systems. Chrony on Linux is able to use the
>> RTC and compensate for periods when the system is not
>> running.
>>
>>
> I don't think he wants to do this. He wants a time discipline system, 
> that cannot be ntpd, that has a hard wired (pre-calculated) frequency 
> correction that it never updates. Most of the time only the frequency 
> correction should be applied. If the offset becomes too high, it 
> should be rapidly cleared by something like the ntpd stepout 
> mechanism, not the normal PLL.
>

Hi all. I want to say thanks to all who responded to the threads I've 
posted, including this one. There were several good replies that I want 
to respond to later. I just had a couple of minutes right now. My 
thinking on this was as follows. I don't mind if NTP is sitting in the 
system running all the time and tweaking the clock. However, on my 
Windows machines, I notice the frequency wandering all over the place 
and responding to short term variances in the computer's usage. So, by 
polling my GPS every 8 seconds, I can maintain a few ms of offset 
without using PPS. I may use PPS later. I'm fine with that. However, if 
I stop NTP for any reason, to reboot, to configure, or for testing of 
other time related things, or for programming the GPS, based on my 
original post, the system clock drifted 1/6 sec in about a minute of NTP 
downtime. (Chris Albertson said initial status reports from NTP may be 
invalid.) So, assuming the numbers were even valid, if you multiply that 
out, that's about 240 seconds / day of drift if NTP is not running. I 
was just thinking that, if the drift correction is set properly after a 
long period of testing, and if it persists when NTP is not running, even 
if NTP is not running for a while that it shouldn't drift more than a 
few seconds / day or a very few ms in a minute. So, when NTP is started 
again, there should be very little error that needs correcting. I don't 
know if that clarifies or clouds the water.

In terms of the RTC, it would be nice if the computer keeps as good time 
as a $ 10 wristwatch when the computer is shut down, that being, about 
1/2 sec / day of drift. I wasn't sure if there was any way to discipline 
the RTC, but it sounds like from some of the replies, that there is not. 
I wasn't sure when the RTC is set or read by the system or by NTP. I 
have to review all the replies on this thread when I have a bit more time.

Sincerely,

Ron

-- 

(PS - If you email me and don't get a quick response, don't be concerned.
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Ron Frazier
timekeepingdude AT c3energy.com



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