[ntp:questions] Raspberry Pi error in PPM offset

Brian Utterback brian.utterback at oracle.com
Sat Aug 24 14:09:47 UTC 2013

On 8/24/2013 9:46 AM, james.peroulas at gmail.com wrote:
> So, are you saying that the PPM value returned by NTP is to be 
> ignored, is purely for information purposes, and is not to be 
> interpreted as having any real-world meaning? istinfo/questions 

Yes and no. The value displayed is the adjustment to the frequency that 
NTP had to apply to maintain the time offset at zero. So your 
supposition that the actual system clock frequency is 2.5 ppm different 
than the clock frequency the kernel is using is correct, and indeed if 
you could accurately tell the kernel the correct frequency then you 
could get that number to zero. However, what is the point? Why do you 
care if the value ends up at zero? If you are using the kernel 
discipline (ntp_adjtime call) then the kernel has been told the more 
accurate number anyway and it is using it to update the clock. That is 
the whole point of making that calculation.

Another problem is that the actual frequency usually isn't static. It 
changes with the temperature among other factors. So even if you got the 
kernel to use the correct value, it would still be off some of the time.

Brian Utterback

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