[ntp:questions] Raspberry Pi error in PPM offset

james.peroulas at gmail.com james.peroulas at gmail.com
Sun Aug 25 14:03:20 UTC 2013

On Saturday, August 24, 2013 9:09:47 AM UTC-5, blu wrote:
> ... 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?

I guess I should have mentioned that the purpose of all this is to actually use the divided frequency output of the RPi as an NTP-disciplined reference oscillator for RF transmissions. So, I'm keenly interested in knowing the actual frequency offset of the crystal inside the Pi, in fact more-so than knowing that the absolute time reported by the Pi is accurate to within 1 ms or so.

Do you have any details or pointers as to how the kernel infers the nominal frequency of the oscillator?

> 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.

So far, in a climate controlled house, the crystal on the Pi itself seems to be quite stable, w.r.t. temperature. Much more stable than 2.5 ppm and hence the reason I'm trying to dig into this more deeply.


More information about the questions mailing list