160 lines
5.9 KiB
HTML
160 lines
5.9 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN">
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<meta name="generator" content="HTML Tidy, see www.w3.org">
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<title>PPS Clock Discipline</title>
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</head>
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<body>
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<h3>PPS Clock Discipline</h3>
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<hr>
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<h4>Synopsis</h4>
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Address: 127.127.22.<i>u</i> <br>
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Reference ID: <tt>PPS</tt> <br>
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Driver ID: <tt>PPS</tt> <br>
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Serial or Parallel Port: <tt>/dev/pps<i>u</i></tt> <br>
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Requires: PPSAPI interface
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<p>Note: This driver supersedes an older one of the same name. The
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older driver operated with several somewhat archaic signal
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interface devices, required intricate configuration and was poorly
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documented. This driver operates only with the PPSAPI interface
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proposed as an IETF standard. Note also that the <tt>pps</tt>
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configuration command has been obsoleted by this driver.</p>
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<h4>Description</h4>
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<p>This driver furnishes an interface for the pulse-per-second
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(PPS) produced by a cesium clock, radio clock or related equipment.
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It can be used to augment the serial timecode generated by a GPS
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receiver, for example. It can be used to remove accumulated jitter
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and re-time a secondary server when synchronized to a primary
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server over a congested, wide-area network and before
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redistributing the time to local clients. The driver includes
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extensive signal sanity checks and grooming algorithms. A range
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gate and frequency discriminator reject noise and signals with
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incorrect frequency. A multiple-stage median filter rejects jitter
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due to hardware interrupt and operating system latencies. A
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trimmed-mean algorithm determines the best time samples. With
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typical workstations and processing loads, the incidental jitter
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can be reduced to less than a microsecond.</p>
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<p>While this driver can discipline the time and frequency relative
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to the PPS source, it cannot number the seconds. For this purpose a
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auxiliary source is required, ordinarily a radio clock operated as
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a primary reference (stratum 1) source; however, another NTP time
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server can be used as well. For this purpose, the auxiliary source
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is marked as the prefer peer, as described in the <a href=
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"prefer.htm">Mitigation Rules and the <tt>prefer</tt> Keyword</a>
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page.</p>
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<p>The driver requires the PPSAPI interface<sup>1</sup>, which is a
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proposed IETF standard. The interface consists of the <tt>
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timepps.h</tt> header file and associated kernel support. Support
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for this interface is included in current versions of FreeBSD and
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Linux and proprietary versions for Digital/Compaq Tru64 (Alpha),
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Sun Solaris and Sun SunOS. See the <a href="pps.htm">
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Pulse-per-second (PPS) Signal Interfacing</a> page for further
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information.</p>
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<p>The PPS source can be connected via a serial or parallel port,
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depending on the hardware and operating system. The port can be
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dedicated to the PPS source or shared with another device. A radio
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clock is usually connected via a serial port and the PPS source
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connected via a level converter to the data carrier detect (DCD)
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pin (DB-9 pin 1, DB-25 pin 8) of the same connector. In some
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systems where a parallel port and driver are available, the PPS
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signal can be connected directly to the ACK pin (pin 10) of the
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connector. Whether the PPS signal is connected via a dedicated port
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or shared with another device, the driver opens the device <tt>
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/dev/pps%d</tt>, where <tt>%d</tt> is the unit number. As with
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other drivers, links can be used to redirect the logical name to
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the actual physical device.</p>
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<p>The driver normally operates like any other driver and uses the
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same mitigation algorithms and PLL/FLL clock discipline
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incorporated in the daemon. If kernel PLL/FLL support is available,
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the kernel PLL/FLL clock discipline is used instead. The default
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behavior is not to use the kernel PPS clock discipline, even if
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present. This driver incorporates a good deal of signal processing
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to reduce jitter using the median filter and trimmed average
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algorithms in the driver interface. As the result, performance with
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minpoll and maxpoll configured at the minimum 4 (16s) is generally
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better than the kernel PPS clock discipline. However, fudge flag 3
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can be used to enable this discipline if necessary.</p>
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<p>Note that the PPS source is considered reachable only if the
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auxiliary source is the prefer peer, is reachable and is selected
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to discipline the system clock. The stratum assigned to the PPS
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source is automatically determined. If the auxiliary source is
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unreachable or inoperative, the stratum is set to 16; otherwise it
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is set to match the stratum of the auxiliary source. Since the
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stratum is determined dynamically, it is not possible to assign
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another stratum using the <tt>fudge</tt> command as in other
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drivers.</p>
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<h4>Fudge Factors</h4>
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<dl>
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<dt><tt>time1 <i>time</i></tt></dt>
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<dd>Specifies the time offset calibration factor, in seconds and
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fraction, with default 0.0.dd></dd>
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<dt><tt>time2 <i>time</i></tt></dt>
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<dd>Not used by this driver.</dd>
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<dt><tt>stratum <i>number</i></tt></dt>
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<dd>Specifies the driver stratum, in decimal from 0 to 15, with
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default 0.</dd>
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<dt><tt>refid <i>string</i></tt></dt>
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<dd>Specifies the driver reference identifier, an ASCII string from
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one to four characters, with default <tt>PPS</tt>.</dd>
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<dt><tt>flag1 0 | 1</tt></dt>
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<dd>Not used by this driver.</dd>
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<dt><tt>flag2 0 | 1</tt></dt>
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<dd>Specifies the PPS signal on-time edge: 0 for assert (default),
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1 for clear.</dd>
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<dt><tt>flag3 0 | 1</tt></dt>
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<dd>Controls the kernel PPS discipline: 0 for disable (default), 1
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for enable.</dd>
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<dt><tt>flag4 0 | 1</tt></dt>
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<dd>Not used by this driver.</dd>
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</dl>
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<p>Additional Information</p>
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<p><a href="refclock.htm">Reference Clock Drivers</a></p>
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<p>Reference</p>
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<ol>
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<li>Mogul, J., D. Mills, J. Brittenson, J. Stone and U. Windl.
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Pulse-per-second API for Unix-like operating systems, version 1.
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Request for Comments RFC-2783, Internet Engineering Task Force,
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March 2000, 31 pp.</li>
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</ol>
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<hr>
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<a href="index.htm"><img align="left" src="pic/home.gif" alt=
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"gif"></a>
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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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</body>
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</html>
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