[time-nuts] NTP to discipline Raspberry Pi

[Hal Murray]

james at peroulas.com said:
> Any ideas on where I can look to track down the discrepancy?

Dig out the kernel sources.

There are 2 sources of error.  One is the calibration routine.  It's 
comparing the CPU cycle counter with another counter that runs at a specified 
frequency.  I think recent kernels have cleaned up this area but it used to 
generate a lot of noise.  Boot a system several times and you will get 
several different answers, off by up to 100 ppm.

Grep your log file looking for things like:
  Detected 430.931 MHz processor.
See if you get the same answer each time you boot.

The other possible error is just the main timekeeping routines.  Maybe some 
constant is off a bit.  Unless somebody does your type of measurement, nobody 
would notice as long as it was close enough that NTP could fix it.  (Who but 
a time-nut would care if the crystal was off by 33 ppm or 43 ppm?)


> but it seldom departs from -33ppm by more than 1 or 2 ppm... 

What do you expect?  Ballpark is 1 ppm/C.  How stable is your temperature?




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