19c2daa00SOllivier Robert<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> 29c2daa00SOllivier Robert<html> 39c2daa00SOllivier Robert<head> 49c2daa00SOllivier Robert<meta http-equiv="content-type" content="text/html;charset=iso-8859-1"> 59c2daa00SOllivier Robert<meta name="generator" content="HTML Tidy, see www.w3.org"> 69c2daa00SOllivier Robert<title>PPS Clock Discipline</title> 79c2daa00SOllivier Robert<!-- Changed by: Harlan &, 31-Mar-2014 --> 89c2daa00SOllivier Robert<link href="scripts/style.css" type="text/css" rel="stylesheet"> 99c2daa00SOllivier Robert</head> 109c2daa00SOllivier Robert<body> 119c2daa00SOllivier Robert<h3>PPS Clock Discipline</h3> 129c2daa00SOllivier Robert<p>Author: David L. Mills (mills@udel.edu)<br> 139c2daa00SOllivier Robert Last change: 149c2daa00SOllivier Robert <!-- #BeginDate format:En2m -->31-Mar-2014 07:46<!-- #EndDate --> 159c2daa00SOllivier Robert UTC</p> 169c2daa00SOllivier Robert <hr> 179c2daa00SOllivier Robert<h4>Synopsis</h4> 189c2daa00SOllivier Robert<p>Address: 127.127.22.<i>u</i><br> 199c2daa00SOllivier Robert Reference ID: <tt>PPS</tt><br> 209c2daa00SOllivier Robert Driver ID: <tt>PPS</tt><br> 219c2daa00SOllivier Robert Serial or Parallel Port: <tt>/dev/pps<i>u</i></tt><br> 229c2daa00SOllivier Robert Requires: PPSAPI signal interface for PPS signal processing.</p> 239c2daa00SOllivier Robert<p>Note: This driver supersedes an older one of the same name. The older driver operated with several somewhat archaic signal interface devices, required intricate configuration and was poorly documented. This driver requires the Pulse per Second API (PPSAPI)<sup>1</sup>. Note also that the <tt>pps</tt> configuration command has been obsoleted by this driver.</p> 249c2daa00SOllivier Robert<h4>Description</h4> 259c2daa00SOllivier Robert<p>This driver furnishes an interface for the pulse-per-second (PPS) signal produced by a cesium clock, radio clock or related devices. It can be used to augment the serial timecode generated by a GPS receiver, for example. It can be used to remove accumulated jitter and re-time a secondary server when synchronized to a primary server over a congested, wide-area network and before redistributing the time to local clients. The driver includes extensive signal sanity checks and grooming algorithms. A range gate and frequency discriminator reject noise and signals with incorrect frequency. A multiple-stage median filter rejects jitter due to hardware interrupt and operating system latencies. A trimmed-mean algorithm determines the best time samples. With typical workstations and processing loads, the incidental jitter can be reduced to a few microseconds.</p> 269c2daa00SOllivier Robert<p>While this driver can discipline the time and frequency relative to the PPS source, it cannot number the seconds. For this purpose an auxiliary source is required, ordinarily a radio clock operated as a primary reference (stratum 1) source; however, another NTP time server can be used as well. For this purpose, the auxiliary source should be specified as the prefer peer, as described in the <a href="../prefer.html">Mitigation Rules and the <tt>prefer</tt> Keyword</a> page.</p> 279c2daa00SOllivier Robert<p>The driver requires the PPSAPI interface<sup>1</sup>, which is a proposed IETF standard. The interface consists of the <tt>timepps.h</tt> header file and associated kernel support. Support for this interface is included in current versions of Solaris, FreeBSD and Linux and proprietary versions of Tru64 (Alpha) and SunOS. See the <a href="../pps.html">Pulse-per-second (PPS) Signal Interfacing</a> page for further information.</p> 289c2daa00SOllivier Robert<p>The PPS source can be connected via a serial or parallel port, depending on the hardware and operating system. A serial port can be dedicated to the PPS source or shared with another device; however, if dedicated the data leads should not be connected, as noise or unexpected signals can cause <tt>ntpd</tt> to exit.</p> 299c2daa00SOllivier Robert<p>A radio clock is usually connected via a serial port and the PPS source 309c2daa00SOllivier Robert connected via a level converter to the data carrier detect (DCD) 319c2daa00SOllivier Robert pin (DB-9 pin 1, DB-25 pin 8) of the same connector. In some systems 329c2daa00SOllivier Robert where a parallel port and driver are available, the PPS signal can 339c2daa00SOllivier Robert be connected directly to the ACK pin (DB25 pin 10) of the connector. 349c2daa00SOllivier Robert Whether the PPS signal is connected via a dedicated port or shared with another 359c2daa00SOllivier Robert device, the driver opens the device <tt>/dev/pps%d</tt>, 369c2daa00SOllivier Robert where <tt>%d</tt> is the unit number. As with other drivers, links can be 379c2daa00SOllivier Robert used to redirect the logical name to the actual physical device.</p> 389c2daa00SOllivier Robert<p>The driver normally operates like any other driver and uses the same mitigation 399c2daa00SOllivier Robert algorithms and PLL/FLL clock discipline incorporated in the daemon. 409c2daa00SOllivier Robert If kernel PLL/FLL support is available, the kernel PLL/FLL clock 419c2daa00SOllivier Robert discipline can be used instead. The default behavior is not to use 429c2daa00SOllivier Robert the kernel PPS clock discipline, even if present. This driver incorporates 439c2daa00SOllivier Robert a good deal of signal processing to reduce jitter using the median 449c2daa00SOllivier Robert filter algorithm in the driver. As the result, performance 459c2daa00SOllivier Robert with <tt>minpoll</tt> configured at 4 (16s) is generally 469c2daa00SOllivier Robert better than the kernel PPS discipline. However, fudge flag 3 can 479c2daa00SOllivier Robert be used to enable the kernel PPS discipline if necessary.</p> 489c2daa00SOllivier Robert<p>This driver 499c2daa00SOllivier Robert is enabled only under one of two conditions (a) a prefer peer other than 509c2daa00SOllivier Robert this driver is among the survivors of the mitigation algorithms or (b) 519c2daa00SOllivier Robert there are no survivors and the <tt>minsane</tt> option 529c2daa00SOllivier Robert of the <tt>tos</tt> command is 0. The prefer peer designates another source 539c2daa00SOllivier Robert that can reliably number the seconds when available . However, if no 549c2daa00SOllivier Robert sources are available, the system clock continues to be disciplined by 559c2daa00SOllivier Robert the PPS driver on an indefinite basis.</p> 569c2daa00SOllivier Robert<p>A scenario where the latter behavior can be most useful is a planetary orbiter 57 fleet, for instance in the vicinity of Mars, where contact between orbiters 58 and Earth only one or two times per Sol (Mars day). These orbiters have a 59 precise timing reference based on an Ultra Stable Oscillator (USO) with accuracy 60 in the order of a Cesium oscillator. A PPS signal is derived from the USO 61 and can be disciplined from Earth on rare occasion or from another orbiter 62 via NTP. In the above scenario the PPS signal disciplines the spacecraft clock 63 between NTP updates.</p> 64<p>In a similar scenario a PPS signal can be used to discipline the clock between 65 updates produced by the modem driver. This would provide precise synchronization 66 without needing the Internet at all.</p> 67<h4>Fudge Factors</h4> 68<dl> 69 <dt><tt>time1 <i>time</i></tt></dt> 70 <dd>Specifies the time offset calibration factor, in seconds and fraction, with default 0.0.</dd> 71 <dt><tt>time2 <i>time</i></tt></dt> 72 <dd>Not used by this driver.</dd> 73 <dt><tt>stratum <i>number</i></tt></dt> 74 <dd>Specifies the driver stratum, in decimal from 0 to 15, with default 0.</dd> 75 <dt><tt>refid <i>string</i></tt></dt> 76 <dd>Specifies the driver reference identifier, an ASCII string from one to four characters, with default <tt>PPS</tt>.</dd> 77 <dt><tt>flag1 0 | 1</tt></dt> 78 <dd>Not used by this driver.</dd> 79 <dt><tt>flag2 0 | 1</tt></dt> 80 <dd>Specifies PPS capture on the rising (assert) pulse edge if 0 (default) or falling 81 (clear) pulse edge if 1. Not used under Windows - if the special <tt>serialpps.sys</tt> serial port driver is installed then the leading edge will <i>always</i> be used.</dd> 82 <dt><tt>flag3 0 | 1</tt></dt> 83 <dd>Controls the kernel PPS discipline: 0 for disable (default), 1 for enable. Not used under Windows - if the special <tt>serialpps.sys</tt> serial port driver is used then kernel PPS will be available and used.</dd> 84 <dt><tt>flag4 0 | 1</tt></dt> 85 <dd>Record a timestamp once for each second if 1. Useful for constructing 86 Allan deviation plots.</dd> 87 . 88</dl> 89<h4>Additional Information</h4> 90<p><a href="../refclock.html">Reference Clock Drivers</a></p> 91<p>Reference</p> 92<ol> 93 <li>Mogul, J., D. Mills, J. Brittenson, J. Stone and U. Windl. Pulse-per-second API for Unix-like operating systems, version 1. Request for Comments RFC-2783, Internet Engineering Task Force, March 2000, 31 pp.</li> 94</ol> 95<hr> 96<script type="text/javascript" language="javascript" src="scripts/footer.txt"></script> 97</body> 98</html> 99