phc2sys(8) synchronize two or more clocks


phc2sys -a [ -r ] [ -r ] [ options ]
phc2sys [ -d pps-device ] [ -s device ] [ -c device ] [ -O offset ] [ -w ] [ options ]


phc2sys is a program which synchronizes two or more clocks in the system. Typically, it is used to synchronize the system clock to a PTP hardware clock (PHC), which itself is synchronized by the ptp4l(8) program.

With the -a option, the clocks to synchronize are fetched from the running ptp4l daemon and the direction of synchronization automatically follows changes of the PTP port states.

Manual configuration is also possible. When using manual configuration, two synchronization modes are supported, one uses a pulse per second (PPS) signal provided by the source clock and the other mode reads time from the source clock directly. Some clocks can be used in both modes, the mode which will synchronize the slave clock with better accuracy depends on hardware and driver implementation.


Read the clocks to synchronize from running ptp4l and follow changes in the port states, adjusting the synchronization direction automatically. The system clock (CLOCK_REALTIME) is not synchronized, unless the -r option is also specified.
Only valid together with the -a option. Instructs phc2sys to also synchronize the system clock (CLOCK_REALTIME). By default, the system clock is not considered as a possible time source. If you want the system clock to be eligible to become a time source, specify the -r option twice.
-d pps-device
Specify the PPS device of the master clock (e.g. /dev/pps0). With this option the PPS synchronization mode is used instead of the direct mode. As the PPS signal does not specify time and only marks start of a second, the slave clock should be already close to the correct time before phc2sys is started or the -s option should be used too. With the -s option the PPS signal of the master clock is enabled automatically, otherwise it has to be enabled before phc2sys is started (e.g. by running echo 1 > /sys/class/ptp/ptp0/pps_enable). This option can be used only with the system clock as the slave clock. Not compatible with the -a option.
-s device
Specify the master clock by device (e.g. /dev/ptp0) or interface (e.g. eth0) or by name (e.g. CLOCK_REALTIME for the system clock). When this option is used together with the -d option, the master clock is used only to correct the offset by whole number of seconds, which cannot be fixed with PPS alone. Not compatible with the -a option.
-i interface
Performs the exact same function as -s for compatibility reasons. Previously enabled specifying master clock by network interface. However, this can now be done using -s and this option is no longer necessary. As such it has been deprecated, and should no longer be used.
-c device
Specify the slave clock by device (e.g. /dev/ptp1) or interface (e.g. eth1) or by name. The default is CLOCK_REALTIME (the system clock). Not compatible with the -a option.
-E servo
Specify which clock servo should be used. Valid values are pi for a PI controller, linreg for an adaptive controller using linear regression, and ntpshm for the NTP SHM reference clock to allow another process to synchronize the local clock. The default is pi.
-P kp
Specify the proportional constant of the PI controller. The default is 0.7.
-I ki
Specify the integral constant of the PI controller. The default is 0.3.
-S step
Specify the step threshold of the servo. It is the maximum offset that the servo corrects by changing the clock frequency instead of stepping the clock. The clock is stepped on start regardless of the option if the offset is larger than 20 microseconds (unless the -F option is used). It's specified in seconds. The value of 0.0 disables stepping after the start. The default is 0.0.
-F step
Specify the step threshold applied only on the first update. It is the maximum offset that is corrected by adjusting clock. It's specified in seconds. The value of 0.0 disables stepping on start. The default is 0.00002 (20 microseconds).
-R update-rate
Specify the slave clock update rate when running in the direct synchronization mode. The default is 1 per second.
-N phc-num
Specify the number of master clock readings per one slave clock update. Only the fastest reading is used to update the slave clock, this is useful to minimize the error caused by random delays in scheduling and bus utilization. The default is 5.
-O offset
Specify the offset between the slave and master times in seconds. Not compatible with the -a option. See TIME SCALE USAGE below.
-L freq-limit
The maximum allowed frequency offset between uncorrected clock and the system monotonic clock in parts per billion (ppb). This is used as a sanity check of the synchronized clock. When a larger offset is measured, a warning message will be printed and the servo will be reset. When set to 0, the sanity check is disabled. The default is 200000000 (20%).
-M segment
The number of the SHM segment used by ntpshm servo. The default is 0.
-u summary-updates
Specify the number of clock updates included in summary statistics. The statistics include offset root mean square (RMS), maximum absolute offset, frequency offset mean and standard deviation, and mean of the delay in clock readings and standard deviation. The units are nanoseconds and parts per billion (ppb). If zero, the individual samples are printed instead of the statistics. The messages are printed at the LOG_INFO level. The default is 0 (disabled).
Wait until ptp4l is in a synchronized state. If the -O option is not used, also keep the offset between the slave and master times updated according to the currentUtcOffset value obtained from ptp4l and the direction of the clock synchronization. Not compatible with the -a option.
-n domain-number
Specify the domain number used by ptp4l. The default is 0.
When a leap second is announced, don't apply it in the kernel by stepping the clock, but let the servo correct the one-second offset slowly by changing the clock frequency (unless the -S option is used).
-z uds-address
Specifies the address of the server's UNIX domain socket. The default is /var/run/ptp4l.
-l print-level
Set the maximum syslog level of messages which should be printed or sent to the system logger. The default is 6 (LOG_INFO).
Print messages to the standard output.
Don't send messages to the system logger.
Display a help message.
Prints the software version and exits.


Ptp4l uses either PTP time scale or UTC (Coordinated Universal Time) time scale. PTP time scale is continuous and shifted against UTC by a few tens of seconds as PTP time scale does not apply leap seconds.

In hardware time stamping mode, ptp4l announces use of PTP time scale and PHC is used for the stamps. That means PHC must follow PTP time scale while system clock follows UTC. Time offset between these two is maintained by phc2sys.

Phc2sys acquires the offset value either by reading it from ptp4l when -a or -w is in effect or from command line when -O is supplied. Failure to maintain the correct offset can result in local system clock being off some seconds to domain master system clock when in slave mode, or incorect PTP time announced to the network in case the host is the domain master.


Synchronize time automatically according to the current ptp4l state, synchronize the system clock to the remote master.

phc2sys -a -r

Same as above, but when the host becomes the domain master, synchronize time in the domain to its system clock.

phc2sys -a -rr

The host is a domain master, PTP clock is synchronized to system clock and the time offset is obtained from ptp4l. Phc2sys waits for ptp4l to get at least one port in master or slave mode before starting the synchronization.

phc2sys -c /dev/ptp0 -s CLOCK_REALTIME -w

Same as above, time offset is provided on command line and phc2sys does not wait for ptp4l.

phc2sys -c /dev/ptp0 -s CLOCK_REALTIME -O 35

The host is in slave mode, system clock is synchronized from PTP clock, phc2sys waits for ptp4l and the offset is set automatically.

phc2sys -s /dev/ptp0 -w

Same as above, PTP clock id is read from the network interface, the offset is provided on command line phc2sys does not wait.

phc2sys -s eth0 -O -35