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doc: update tempcomp description

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Miroslav Lichvar 2014-11-21 13:13:15 +01:00
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chrony.texi.in
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@ -2973,21 +2973,23 @@ distances are in milliseconds.
@c {{{ tempcomp
@node tempcomp directive
@subsection tempcomp
Normally, changes in rate of drift of the system clock are caused mainly by
changes in temperature of the crystal oscillator on the mainboard.
Normally, changes in the rate of drift of the system clock are caused mainly by
changes in the temperature of the crystal oscillator on the mainboard.
If there are available temperature measurements from a sensor close to the
oscillator, @code{tempcomp} directive can be used to compensate for the changes
in rate and possibly improve clock accuracy.
If there are temperature measurements available from a sensor close to the
oscillator, the @code{tempcomp} directive can be used to compensate for the
changes in the temperature and improve the stability and accuracy of the clock.
Whether it will really help depends on many factors, including resolution of
the sensor, noise in measurements, time source polling interval, compensation
update interval, how good are the temperature coefficients, and how close is
the sensor to the oscillator. The frequency reported in tracking.log should
be more stable and the offsets should be smaller.
The result depends on many factors, including the resolution of the sensor,
the amount of noise in the measurements, the polling interval of the time
source, the compensation update interval, how well is the compensation
specified, and how close is the sensor to the oscillator. When it's working
well, the frequency reported in the @file{tracking.log} file is more stable and
the maximum reached offset is smaller.
The directive has six parameters: path to the file which contains current
temperature in text format, update interval (in seconds), and temperature
There are two forms of the directive. The first one has six parameters: a
path to the file containing the current temperature from the sensor (in
text format), the compensation update interval (in seconds), and temperature
coefficients T0, k0, k1, k2.
The frequency compensation is calculated (in ppm) as
@ -2995,23 +2997,48 @@ The frequency compensation is calculated (in ppm) as
@code{k0 + (T - T0) * k1 + (T - T0)^2 * k2}
The result has to be between -10 ppm and 10 ppm, otherwise the measurement is
considered to be faulty and will be ignored. The k0 coefficient can be used to
get the results in that range.
Valid measurements and calculated corrections are logged to tempcomp.log file if
enabled with @code{log tempcomp} directive.
considered invalid and will be ignored. The k0 coefficient can be used to get
the results in that range.
An example of use is
@example
tempcomp /sys/class/hwmon/hwmon1/device/temp2_input 30 26000 0.0 0.000183 0.0
tempcomp /sys/class/hwmon/hwmon0/temp2_input 30 26000 0.0 0.000183 0.0
@end example
The measured temperature will be read from the file in Linux sysfs filesystem
every 30 seconds. When the temperature is 26 degress (26000), the system clock
frequency will not be adjusted. When it is 27 degrees (27000), the clock will
be set to run 0.183ppm faster than it would be without the compensation, etc.
The measured temperature will be read from the file in the Linux sysfs
filesystem every 30 seconds. When the temperature is 26000 (26 degrees
Celsius), the frequency correction will be zero. When it is 27000 (27 degrees
Celsius), the clock will be set to run 0.183ppm faster, etc.
The second form has three parameters, the path to the sensor file, the update
interval and a path to a file containing a list of (temperature, compensation)
points, from which the compensation is linearly interpolated or extrapolated.
An example is
@example
tempcomp /sys/class/hwmon/hwmon0/temp2_input 30 /etc/chrony.tempcomp
@end example
where the @file{chrony.tempcomp} file could have
@example
20000 1.0
21000 0.64
22000 0.36
23000 0.16
24000 0.04
25000 0.0
26000 0.04
27000 0.16
28000 0.36
29000 0.64
30000 1.0
@end example
Valid measurements with corresponding compensations are logged to the
@file{tempcomp.log} file if enabled by the @code{log tempcomp} directive.
@c }}}
@c {{{ user
@node user directive