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Update refclock documentation

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Miroslav Lichvar 2011-04-08 16:53:11 +02:00
parent 20a43409c6
commit bb95c39356
1 changed files with 40 additions and 25 deletions
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@ -2338,30 +2338,33 @@ udp/11123.
@c {{{ refclock
@node refclock directive
@subsection refclock
The @code{refclock} directive allows reference clocks to be specified.
The directive is immediately followed by a refclock driver name and
its parameter.
Reference clocks allows very accurate synchronisation and @code{chronyd}
can function as a stratum 1 server. They are specified by the
@code{refclock} directive. It has two mandatory parameters, a refclock driver
name and a driver specific parameter.
There are currently three drivers implemented:
There are currently three drivers included:
@table @code
@item PPS
Pulse per second (PPS) API driver. The parameter is a path to the PPS
device. Assert events are used by default. Driver option
@code{:clear} can be appended to the path to use clear events instead.
PPSAPI (pulse per second) driver. The parameter is the path to a PPS
device. Assert events are used by default. Driver option @code{:clear}
can be appended to the path if clear events should be used instead.
PPS refclock needs another source (NTP or non-PPS refclock) or local
directive (@pxref{local directive}) enabled to function. For example:
As PPS refclock gets only sub-second time information, it needs another
source (NTP or non-PPS refclock) or local directive (@pxref{local
directive}) enabled to work. For example:
@example
refclock SHM 0 offset 0.5 delay 0.1
refclock PPS /dev/pps0
refclock PPS /dev/pps0 lock NMEA
refclock SHM 0 offset 0.5 delay 0.1 refid NMEA noselect
@end example
@item SHM
NTP shared memory driver. The parameter is the number of the
shared memory segment that should be used for receiving timestamps, usually
0, 1, 2 or 3. For example:
NTP shared memory driver. This driver uses a shared memory segment to
receive data from another daemon which communicates with an actual
reference clock. The parameter is the number of a shared memory segment,
usually 0, 1, 2 or 3. For example:
@example
refclock SHM 1 poll 3 refid GPS1
@ -2371,15 +2374,25 @@ A driver option in form @code{:perm=NNN} can be appended to the
segment number to create the segment with permissions other than the
default @code{0600}.
Software that can be used as a source of reference time includes
@code{gpsd} and @code{shmpps}.
Some examples of applications that can be used as SHM sources are @code{gpsd},
@code{shmpps} and @code{radioclk}.
@item SOCK
Unix domain socket driver. The parameter is a path to the socket
which is used as the source of timestamps. This is as a better
alternative to SHM, it does not require polling, the offset
resolution is not limited to microsecond and it supports PPS.
The format for messages sent over the socket is declared in file
@code{refclock_sock.c}.
Unix domain socket driver. It is similar to the SHM driver, but uses a
different format and uses a socket instead of shared memory. It does not
require polling, the offset resolution is not limited to microseconds and it
supports transmitting of PPS data. The parameter is a path to the socket which
will be created by @code{chronyd} and used to receive the messages. The format
of messages sent over the socket is described in the
@code{refclock_sock.c} file.
Recent versions of the @code{gpsd} daemon include support for the SOCK
protocol. The path where the socket should be created is described in the
@code{gpsd(8)} man page. For example:
@example
refclock SOCK /tmp/chrony.tty0.sock
@end example
@end table
The @code{refclock} command also supports a number of subfields (which
@ -2391,7 +2404,8 @@ Timestamps produced by refclock drivers are not used immediately, but
they are stored and processed by a median filter in intervals
specified by this option. This is defined as a power of 2. The
default is 4 (16 seconds). A shorter interval allows @code{chronyd}
to react faster to frequency changes, but it may increase noise.
to react faster to changes in clock frequency, but it may decrease
the accuracy if the source is too noisy.
@item dpoll
Some drivers are not controlled by external events and thus require
polling. Again this is defined as a power of 2 and can be negative
@ -2400,7 +2414,7 @@ for sub-second intervals. The default is 0 (1 second).
This option is used to specify a reference id of the refclock, as up
to four ASCII characters. By default, first three characters from
driver name and the number of the refclock are used as refid. Each
refclock has to use an unique refid.
refclock must have an unique refid.
@item filter
This option sets the length of the median filter which is used to
reduce noise. With each poll about 40 percent of the stored samples is
@ -2435,7 +2449,8 @@ for SHM refclock, and 1e-9 (1 nanosecond) for SOCK and PPS refclocks.
@item prefer
Prefer this source over sources without prefer option.
@item noselect
Never select this source. This is particularly useful for monitoring.
Never select this source. This is useful for monitoring or with sources
which are not very accurate, but are locked with a PPS refclock.
@end table
@c }}}