[ntp:questions] Slow convergence of NTP with GPS/PPS

Richard B. Gilbert rgilbert88 at comcast.net
Wed Oct 1 12:50:21 UTC 2008


David Woolley wrote:
> Richard B. Gilbert wrote:
> 
>>
>> I don't recall that +/- 128ms is specified anywhere.  If ntpd gets 
>> it's initial time from a hardware reference clock, the time SHOULD be 
>> very close.  This will, off course, depend on the latencies in the 
>> reference clock's response and the resolution of the time supplied.
>> <snip>
> 
> 
> This is where it is documented in the source code for 4.2.4p4 
> (ntpd/loopfilter.c):
> 
>  *      State   < step          > step          Comments
>  *      ====================================================
>  *      NSET    FREQ            step, FREQ      no ntp.drift
>  *
>  *      FSET    SYNC            step, SYNC      ntp.drift
>  *
> 
> NSET is the initial state for a cold start.  FSET is the initial state 
> for a warm start (ntp.drift) present.  For a fast start you don't want 
> NSET.  Note that FSET goes to fully synchronised on one reading, but 
> only steps the time if the offset is greater than the step limit, which 
> normally 128ms.
> 
> Furthermore, in the branch that is conditioned thus:
> 
>         if (fabs(fp_offset) > clock_max && clock_max > 0) {
> 
> there is this comment:
> 
>                  * In S_FSET state the initial frequency has been set
>                  * from the frequency file. Since the time is outside
>                  * the step threshold, the clock is stepped immediately,
>                  * rather than after the stepout interval. Guys get
>                  * nervous if it takes 17 minutes to set the clock for
>                  * the first time.
>                  *
> 
> immediately followed by:
> 
>                         step_systime(fp_offset);
> 
> There is no step_systime in the else branch.  clock_max is the tinkered 
> value of the parameter that defaults to 128ms.

The above may mean something to YOU!  Sorry but I don't see it!





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