1132451Sroberto<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> 2132451Sroberto<html> 3290001Sglebius<head> 4290001Sglebius<meta http-equiv="content-type" content="text/html;charset=iso-8859-1"> 5290001Sglebius<meta name="generator" content="HTML Tidy, see www.w3.org"> 6290001Sglebius<title>PPS Clock Discipline</title> 7290001Sglebius<!-- Changed by: Harlan &, 31-Mar-2014 --> 8290001Sglebius<link href="scripts/style.css" type="text/css" rel="stylesheet"> 9290001Sglebius</head> 10290001Sglebius<body> 11290001Sglebius<h3>PPS Clock Discipline</h3> 12290001Sglebius<p>Author: David L. Mills (mills@udel.edu)<br> 13290001Sglebius Last change: 14290001Sglebius <!-- #BeginDate format:En2m -->31-Mar-2014 07:46<!-- #EndDate --> 15290001Sglebius UTC</p> 16290001Sglebius <hr> 17290001Sglebius<h4>Synopsis</h4> 18290001Sglebius<p>Address: 127.127.22.<i>u</i><br> 19290001Sglebius Reference ID: <tt>PPS</tt><br> 20290001Sglebius Driver ID: <tt>PPS</tt><br> 21290001Sglebius Serial or Parallel Port: <tt>/dev/pps<i>u</i></tt><br> 22290001Sglebius Requires: PPSAPI signal interface for PPS signal processing.</p> 23290001Sglebius<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> 24290001Sglebius<h4>Description</h4> 25290001Sglebius<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> 26290001Sglebius<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> 27290001Sglebius<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> 28290001Sglebius<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> 29290001Sglebius<p>A radio clock is usually connected via a serial port and the PPS source 30290001Sglebius connected via a level converter to the data carrier detect (DCD) 31290001Sglebius pin (DB-9 pin 1, DB-25 pin 8) of the same connector. In some systems 32290001Sglebius where a parallel port and driver are available, the PPS signal can 33290001Sglebius be connected directly to the ACK pin (DB25 pin 10) of the connector. 34290001Sglebius Whether the PPS signal is connected via a dedicated port or shared with another 35290001Sglebius device, the driver opens the device <tt>/dev/pps%d</tt>, 36290001Sglebius where <tt>%d</tt> is the unit number. As with other drivers, links can be 37290001Sglebius used to redirect the logical name to the actual physical device.</p> 38290001Sglebius<p>The driver normally operates like any other driver and uses the same mitigation 39290001Sglebius algorithms and PLL/FLL clock discipline incorporated in the daemon. 40290001Sglebius If kernel PLL/FLL support is available, the kernel PLL/FLL clock 41290001Sglebius discipline can be used instead. The default behavior is not to use 42290001Sglebius the kernel PPS clock discipline, even if present. This driver incorporates 43290001Sglebius a good deal of signal processing to reduce jitter using the median 44290001Sglebius filter algorithm in the driver. As the result, performance 45290001Sglebius with <tt>minpoll</tt> configured at 4 (16s) is generally 46290001Sglebius better than the kernel PPS discipline. However, fudge flag 3 can 47290001Sglebius be used to enable the kernel PPS discipline if necessary.</p> 48290001Sglebius<p>This driver 49290001Sglebius is enabled only under one of two conditions (a) a prefer peer other than 50290001Sglebius this driver is among the survivors of the mitigation algorithms or (b) 51290001Sglebius there are no survivors and the <tt>minsane</tt> option 52290001Sglebius of the <tt>tos</tt> command is 0. The prefer peer designates another source 53290001Sglebius that can reliably number the seconds when available . However, if no 54290001Sglebius sources are available, the system clock continues to be disciplined by 55290001Sglebius the PPS driver on an indefinite basis.</p> 56290001Sglebius<p>A scenario where the latter behavior can be most useful is a planetary orbiter 57290001Sglebius fleet, for instance in the vicinity of Mars, where contact between orbiters 58290001Sglebius and Earth only one or two times per Sol (Mars day). These orbiters have a 59290001Sglebius precise timing reference based on an Ultra Stable Oscillator (USO) with accuracy 60290001Sglebius in the order of a Cesium oscillator. A PPS signal is derived from the USO 61290001Sglebius and can be disciplined from Earth on rare occasion or from another orbiter 62290001Sglebius via NTP. In the above scenario the PPS signal disciplines the spacecraft clock 63290001Sglebius between NTP updates.</p> 64290001Sglebius<p>In a similar scenario a PPS signal can be used to discipline the clock between 65290001Sglebius updates produced by the modem driver. This would provide precise synchronization 66290001Sglebius without needing the Internet at all.</p> 67290001Sglebius<h4>Fudge Factors</h4> 68290001Sglebius<dl> 69290001Sglebius <dt><tt>time1 <i>time</i></tt></dt> 70290001Sglebius <dd>Specifies the time offset calibration factor, in seconds and fraction, with default 0.0.</dd> 71290001Sglebius <dt><tt>time2 <i>time</i></tt></dt> 72290001Sglebius <dd>Not used by this driver.</dd> 73290001Sglebius <dt><tt>stratum <i>number</i></tt></dt> 74290001Sglebius <dd>Specifies the driver stratum, in decimal from 0 to 15, with default 0.</dd> 75290001Sglebius <dt><tt>refid <i>string</i></tt></dt> 76290001Sglebius <dd>Specifies the driver reference identifier, an ASCII string from one to four characters, with default <tt>PPS</tt>.</dd> 77290001Sglebius <dt><tt>flag1 0 | 1</tt></dt> 78290001Sglebius <dd>Not used by this driver.</dd> 79290001Sglebius <dt><tt>flag2 0 | 1</tt></dt> 80290001Sglebius <dd>Specifies PPS capture on the rising (assert) pulse edge if 0 (default) or falling 81290001Sglebius (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> 82290001Sglebius <dt><tt>flag3 0 | 1</tt></dt> 83290001Sglebius <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> 84290001Sglebius <dt><tt>flag4 0 | 1</tt></dt> 85290001Sglebius <dd>Record a timestamp once for each second if 1. Useful for constructing 86290001Sglebius Allan deviation plots.</dd> 87290001Sglebius . 88290001Sglebius</dl> 89290001Sglebius<h4>Additional Information</h4> 90290001Sglebius<p><a href="../refclock.html">Reference Clock Drivers</a></p> 91290001Sglebius<p>Reference</p> 92290001Sglebius<ol> 93290001Sglebius <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> 94290001Sglebius</ol> 95290001Sglebius<hr> 96290001Sglebius<script type="text/javascript" language="javascript" src="scripts/footer.txt"></script> 97290001Sglebius</body> 98290001Sglebius</html> 99