radioclkd(1) decode time from radio clock(s) attached to serial port


radioclkd [ -tphv ] device


radioclkd is a simple daemon that decodes the time from a radio clock device attached to the DCD and/or CTS and/or DSR status lines of serial port of a computer. It is able to decode the DCF77, MSF and WWVB time signals. The received time is then sent to ntpd using the shared memory reference clock driver. The type of time signal being received is automatically determined. If you have problems getting the program to work using interrupts, the following command is known to help in many instances. If this fails you can always fall back to the polling method.
stty crtscts < /dev/ttyS0

Details on a cheap and easy to make device for receiving these time signals can be found at


-p, --poll
Poll the serial port for changes of status in the DCD, CTS and DSR lines rather than use interrupts
-t, --test
Enter test mode printing the length of each pulse and the decoded time at the end of each minute on stdout. The time is not sent to ntpd using the shared memory reference clock driver in this mode.
-h, --help
Print a short synopsis of the command line arguments.
-v, --version
Print the version number and then exit.


Configuration is very simple. Use server in your ntp.conf file for a clock attached to the DCD line, server for a clock attached to the CTS line, and server for a clock attached to the DSR line. You will also want to use a fudge line on the server to change the displayed refid.


Due to delays in the propogation of the radio signal, it's processing by the receiver board and the latency of the operating system the time decoded by the receiver will be slightly offset from actual UTC. Typically this delay will be less than 20ms, so unless you are very fussy about the time, or are using more than one time source, such as a GPS unit, other radio clock or NTP server on the internet you can ignore this section.

The basics of the calibration procedure is to determine the average offset of the radio receiver, and use the time1 fudge factor in ntp.conf to bring the receiver as close as possible to the real time. The easiest way of determining the offset of the radio receivers time is to run it against a reference clock that does not suffer from these problems. The best reference clock would be a GPS unit. This might be a GPS unit that you don't wish to dedicate to time keeping, or a borrowed unit. If this is not possible you could use a stratum 1 server on the internet.

The method of calibration is quite simple. We attach the calibration reference clock to the computer and fudge the stratum of our radio receiver up to say 5. This way we can be sure that ntpd will lock onto the calibration reference clock. We need to make sure that ntpd is configured to collect peer statistics so make sure we have some lines similar to these in ntp.conf

    statsdir /var/log/ntpstats/
    statistics loopstats peerstats clockstats
    filegen peerstats file peerstats type day enable

After that we restart ntpd and leave it running for several hours. We can then make a copy the peerstats file. The trick is to remove all the entries before ntpd has come into close aggrement with the calibration reference clock and then run the peer.awk script in the scripts/stats directory of the ntp distribution. This will give us a mean offset of our radio receivers in milliseconds. This can them be converted into seconds and added to the fudge line in ntp.conf for our receiver.

The final step is to remove the change in stratum level for our reference clock and restart ntpd. If you move the receiver any significant distance then you will need to repeat this calibration step. Across the room or around the current building will be fine, but if you move it to the next town/city then you will need to recalibrate.


The version of ntpd that comes with most Linux distributions does not have the shared memory reference clock driver compiled in by default. This can be identified by checking the logs after ntpd is started. If the shared memory reference clock driver is not compiled in then the logs will contain warnings about the reference clock driver not being recognized. To compile ntpd with the shared memory reference clock driver you must specify the --enable-SHM option when running configure.

Neither radioclkd or ntpd ever mark the shared memory segment for deletion. If you stop using the shared memory reference clock driver therefore any shared memory segments will persist until you reboot or manually delete the segment using ipcrm. The segments can be identified as the one with key 0x4e545030, 0x4e545031 or 0x4e545032 using the ipcs command.


If you are running a kernel with the PPS kit and have a clock attached to the DCD line you may experience lockups. If you encounter this problem the currently recomended solution is to move the clock to either the CTS or DSR lines.


This program was written by Jonathan Buzzard <[email protected]> and may be freely distributed under the terms of the GNU General Public License. There is ABSOLUTELY NO WARRANTY for this program.