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<title>Reference Clock Drivers</title>
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<h3>Reference Clock Drivers</h3>
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<img align="left" src="pic/stack1a.jpg" alt="gif">Master Time
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Facility at the <a href="http://www.eecis.udel.edu/~mills/lab.htm">
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UDel Internet Research Laboratory</a>: <br clear="left">
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<p>Support for most of the commonly available radio and modem
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reference clocks is included in the default configuration of the
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NTP daemon for Unix <tt>ntpd</tt>. Individual clocks can be
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activated by configuration file commands, specifically the <tt>
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server</tt> and <tt>fudge</tt> commands described in the <a href=
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"ntpd.htm"><tt>ntpd</tt> program manual page</a>. The following
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discussion presents Information on how to select and configure the
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device drivers in a running Unix system.</p>
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<p>Many radio reference clocks can be set to display local time as
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adjusted for timezone and daylight saving mode. For use with NTP
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the clock must be set for Coordinated Universal Time (UTC) only.
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Ordinarily, these adjustments are performed by the kernel, so the
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fact that the clock runs on UTC will be transparent to the
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<p>Radio and modem clocks by convention have addresses in the form
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127.127.<i>t.u</i>, where <i>t</i> is the clock type and <i>u</i>
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is a unit number in the range 0-3 used to distinguish multiple
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instances of clocks of the same type. Most of these clocks require
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support in the form of a serial port or special bus peripheral, but
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some can work directly from the audio codec found in some
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workstations. The particular device is normally specified by adding
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a soft link <tt>/dev/device<i>u</i></tt> to the particular hardware
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device involved, where <i><tt>u</tt></i> correspond to the unit
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<p>Most clock drivers communicate with the reference clock using a
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serial port, usually at 9600 bps. There are several application
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program interfaces (API) used in the various Unix and NT systems,
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most of which can be detected at configuration time. Thus, it is
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important that the NTP daemon and utilities be compiled on the
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target system or clone. In some cases special features are
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available, such as timestamping in the kernel or pulse-per-second
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(PPS) interface. In most cases these features can be detected at
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configuration time as well; however, the kernel may have to be
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recompiled in order for them to work.</p>
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<p>The audio drivers are a special case. These include support for
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the NIST time/frequency stations WWV and WWVH, the Canadian
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time/frequency station CHU and generic IRIG signals. Currently,
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support for the Solaris and SunOS audio API is included in the
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distribution. It is left to the volunteer corps to extend this
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support to other systems. Further information on hookup, debugging
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and monitoring is given in the <a href="audio.htm">Audio
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<p>The local clock driver is also a special case. A server
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configured with this driver can operate as a primary server to
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synchronize other clients when no other external synchronization
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sources are available. If the server is connected directly or
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indirectly to the public Internet, there is some danger that it can
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adversely affect the operation of unrelated clients. Carefully read
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the <a href="driver1.htm">Undisciplined Local Clock</a> page and
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respect the stratum limit.</p>
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<p>The local clock driver also supports an external synchronization
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source such as a high resolution counter disciplined by a GPS
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receiver, for example. Further information is on the <a href=
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"extern.htm">External Clock Discipline and the Local Clock
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<h4>Driver Calibration</h4>
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<p>Some drivers depending on longwave and shortwave radio services
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need to know the radio propagation time from the transmitter to the
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receiver, which can amount to some tens of milliseconds. This must
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be calculated for each specific receiver location and requires the
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geographic coordinates of both the transmitter and receiver. The
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transmitter coordinates for various radio services are given in the
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<a href="qth.htm">Stations, Frequencies and Geographic
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Coordinates</a> page. Receiver coordinates can be obtained or
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estimated from various sources. The actual calculations are beyond
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the scope of this document.</p>
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<p>When more than one clock driver is supported, it is often the
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case that each shows small systematic offset differences relative
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to the rest. To reduce the effects of jitter when switching from
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one driver to the another, it is useful to calibrate the drivers to
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a common ensemble offset. The <tt>enable calibrate</tt>
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configuration command in the <a href="miscopt.htm">Miscellaneous
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Options</a> page is useful for this purpose. The calibration
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function can also be enabled and disabled using the <tt>ntpdc</tt>
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<p>Most clock drivers use the <tt>time1</tt> value specified in the
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<tt>fudge</tt> configuration command to provide the calibration
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correction when this cannot be provided by the clock or interface.
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When the calibration function is enabled, the <tt>time1</tt> value
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is automatically adjusted to match the offset of the remote server
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or local clock driver selected for synchronization. Ordinarily, the
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NTP selection algorithm chooses the best from among all sources,
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usually the best radio clock determined on the basis of stratum,
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synchronization distance and jitter. The calibration function
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adjusts the <tt>time1</tt> values for all clock drivers except this
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source so that their indicated offsets tend to zero. If the
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selected source is the kernel PPS discipline, the <tt>fudge
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time1</tt> values for all clock drivers are adjusted.</p>
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<p>The adjustment function is an exponential average designed to
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improve accuracy, so the function takes some time to converge. The
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recommended procedure is to enable the function, let it run for an
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hour or so, then edit the configuration file using the <tt>
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time1</tt> values displayed by the <tt>ntpq</tt> utility and <tt>
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clockvar</tt> command. Finally, disable the calibration function to
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avoid possible future disruptions due to misbehaving clocks or
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<h4>Performance Enhancements</h4>
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<p>In general, performance can be improved, especially when more
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than one clock driver is supported, to use the prefer peer function
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described in the <a href="prefer.htm">Mitigation Rules and the <tt>
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prefer</tt> Keyword</a> page. The prefer peer is ordinarily
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designated the remote peer or local clock driver which provides the
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best quality time. All other things equal, only the prefer peer
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source is used to discipline the system clock and jitter-producing
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"clockhopping" between sources is avoided. This is valuable when
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more than one clock driver is present and especially valuable when
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the PPS clock driver (type 22) is used. Support for PPS signals is
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summarized in the <a href="pps.htm">Pulse-per-second (PPS) Signal
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Interfacing</a> page.</p>
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<p>Where the highest performance is required, generally better than
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one millisecond, additional hardware and/or software functions may
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be required. Kernel modifications for precision time are described
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in the <a href="kern.htm">A Kernel Model for Precision
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Timekeeping</a> page. Special line discipline and streams modules
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for use in capturing precision timestamps are described in the <a
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href="ldisc.htm">Line Disciplines and Streams Drivers</a> page.</p>
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<h4>Comprehensive List of Clock Drivers</h4>
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<p>Following is a list showing the type and title of each driver
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currently implemented. The compile-time identifier for each is
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shown in parentheses. Click on a selected type for specific
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description and configuration documentation, including the clock
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address, reference ID, driver ID, device name and serial line
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speed, and features (line disciplines, etc.). For those drivers
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without specific documentation, please contact the author listed in
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the <a href="copyright.htm">Copyright Notice</a> page.</p>
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<p><a href="driver1.htm">Type 1</a> Undisciplined Local Clock
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<a href="driver2.htm">Type 2</a> Trak 8820 GPS Receiver
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(<tt>GPS_TRAK</tt>)<br>
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<a href="driver3.htm">Type 3</a> PSTI/Traconex 1020 WWV/WWVH
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Receiver (<tt>WWV_PST</tt>)<br>
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<a href="driver4.htm">Type 4</a> Spectracom WWVB and GPS Receivers
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(<tt>WWVB_SPEC</tt>)<br>
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<a href="driver5.htm">Type 5</a> TrueTime GPS/GOES/OMEGA Receivers
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(<tt>TRUETIME</tt>)<br>
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<a href="driver6.htm">Type 6</a> IRIG Audio Decoder
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(<tt>IRIG_AUDIO</tt>)<br>
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<a href="driver7.htm">Type 7</a> Radio CHU Audio
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Demodulator/Decoder (<tt>CHU</tt>)<br>
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<a href="driver8.htm">Type 8</a> Generic Reference Driver
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<a href="driver9.htm">Type 9</a> Magnavox MX4200 GPS Receiver
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(<tt>GPS_MX4200</tt>)<br>
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<a href="driver10.htm">Type 10</a> Austron 2200A/2201A GPS
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Receivers (<tt>GPS_AS2201</tt>)<br>
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<a href="driver11.htm">Type 11</a> Arbiter 1088A/B GPS Receiver
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(<tt>GPS_ARBITER</tt>)<br>
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<a href="driver12.htm">Type 12</a> KSI/Odetics TPRO/S IRIG
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Interface (<tt>IRIG_TPRO</tt>)<br>
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Type 13 Leitch CSD 5300 Master Clock Controller
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(<tt>ATOM_LEITCH</tt>)<br>
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Type 14 EES M201 MSF Receiver (<tt>MSF_EES</tt>)<br>
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<a href="driver5.htm">Type 15</a> * TrueTime generic receivers<br>
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<a href="driver16">Type 16</a> Bancomm GPS/IRIG Receiver
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(<tt>GPS_BANCOMM</tt>)<br>
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Type 17 Datum Precision Time System (<tt>GPS_DATUM</tt>)<br>
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<a href="driver18.htm">Type 18</a> NIST Modem Time Service
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(<tt>ACTS_NIST</tt>)<br>
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<a href="driver19.htm">Type 19</a> Heath WWV/WWVH Receiver
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(<tt>WWV_HEATH</tt>)<br>
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<a href="driver20.htm">Type 20</a> Generic NMEA GPS Receiver
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Type 21 TrueTime GPS-VME Interface (<tt>GPS_VME</tt>)<br>
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<a href="driver22.htm">Type 22</a> PPS Clock Discipline
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<a href="driver23.htm">Type 23</a> PTB Modem Time Service
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(<tt>ACTS_PTB</tt>)<br>
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<a href="driver24.htm">Type 24</a> USNO Modem Time Service
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(<tt>ACTS_USNO</tt>)<br>
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<a href="driver5.htm">Type 25</a> * TrueTime generic receivers<br>
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<a href="driver26.htm">Type 26</a> Hewlett Packard 58503A GPS
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Receiver (<tt>GPS_HP</tt>)<br>
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<a href="driver27.htm">Type 27</a> Arcron MSF Receiver
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(<tt>MSF_ARCRON</tt>)<br>
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<a href="driver28.htm">Type 28</a> Shared Memory Driver
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<a href="driver29.htm">Type 29</a> Trimble Navigation Palisade GPS
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(<tt>GPS_PALISADE</tt>)<br>
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<a href="driver30.htm">Type 30</a> Motorola UT Oncore GPS
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(<tt>GPS_ONCORE</tt>)<br>
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Type 31 Rockwell Jupiter GPS (<tt>GPS_JUPITER</tt>)<br>
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<a href="driver32.htm">Type 32</a> Chrono-log K-series WWVB
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receiver <a href="driver33.htm">Type 33</a> Dumb Clock <a href=
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"driver34.htm">Type 34</a> Ultralink WWVB Receivers<br>
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<a href="driver35.htm">Type 35</a> Conrad Parallel Port Radio Clock
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<a href="driver36.htm">Type 36</a> Radio WWV/H Audio
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Demodulator/Decoder(<tt>WWV</tt>)<br>
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<a href="driver37.htm">Type 37</a> Forum Graphic GPS Dating station
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<a href="driver38.htm">Type 38</a> hopf GPS/DCF77 6021/komp for
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Serial Line (<tt>HOPF_S</tt>)<br>
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<a href="driver39.htm">Type 39</a> hopf GPS/DCF77 6039 for PCI-Bus
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(<tt>HOPF_P</tt>)</p>
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<p>* All TrueTime receivers are now supported by one driver, type
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5. Types 15 and 25 will be retained only for a limited time and may
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be reassigned in future.</p>
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<p>Additional Information</p>
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<p><a href="prefer.htm">Mitigation Rules and the <tt>prefer</tt>
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<a href="rdebug.htm">Debugging Hints for Reference Clock
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<a href="kern.htm">A Kernel Model for Precision Timekeeping</a><br>
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<a href="ldisc.htm">Line Disciplines and Streams Drivers</a><br>
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<a href="audio.htm">Reference Clock Audio Drivers</a><br>
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<a href="pps.htm">Pulse-per-second (PPS) Signal Interfacing</a><br>
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<a href="howto.htm">How To Write a Reference Clock Driver</a></p>
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<a href="index.htm"><img align="left" src="pic/home.gif" alt=
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<address><a href="mailto:mills@udel.edu">David L. Mills
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<mills@udel.edu></a></address>
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