[time-nuts] NTP to discipline Raspberry Pi
Hal Murray
hmurray at megapathdsl.net
Thu Jul 25 03:46:51 UTC 2013
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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