Messages in this thread | | | Date | Fri, 2 Jan 2015 07:29:20 +0100 | From | Richard Cochran <> | Subject | Re: [PATCH] adjtimex: PPM scaling is by 2^-16 |
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On Thu, Jan 01, 2015 at 08:41:20PM -0600, Jeff Epler wrote: > Like Laurent Georget, I found by reading ntpd source > the scale is 2^16 (i.e., 1 ~= 1.5e-5 ppm, 65536 = 1ppm)
Yep, you wont find this documented in prose anywhere, not even at ntp.org.
> - long freq; /* Frequency offset, as scaled PPM > + long freq; /* Frequency offset, in units of 2^-16 PPM > (parts per million) */
This wording is correct.
IMHO, it is more understandable when you explain that this field is a fixed point number, with a 16 bit binary fraction. For example, see the comment in the function, ppb_to_scaled_ppm(), in Documentation/ptp/testptp.c.
Thanks, Richard
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