108 lines
4.4 KiB
HTML
108 lines
4.4 KiB
HTML
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<!-- $NetBSD: driver22.html,v 1.1 1998/12/30 20:20:34 mcr Exp $ -->
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<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML Strict//EN">
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<html><head><title>
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PPS Clock Discipline
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</title></head><body><h3>
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PPS Clock Discipline
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</h3><hr>
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<p><h4>Synopsis</h4>
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<p>Address: 127.127.22.<var>u</var>
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<br>Reference ID: ATOM
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<br>Driver ID: ATOM-PPS
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<br>Serial Port: <code>/dev/pps<var>u</var></code>; 9600 baud, 8-bits,
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no parity
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<br>Features: <code>tty_clk</code>, <code>ppsclock</code>
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<p><h4>Description</h4>
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<p>This driver furnishes an interface for pulse-per-second (PPS) signals
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produced by a cesium clock, radio clock or related equipment. It can be
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used to remove accumulated jitter and retime a secondary server when
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synchronized to a primary server over a congested, wide-area network and
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before redistributing the time to local clients.
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<p>In order for this driver to work, the local clock must be set to
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within +-500 ms by another means, such as a radio clock or NTP itself.
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The PPS signal is connected via a serial port and <a
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href="gadget.html">gadget box</a> consisting of a one-shot and RS232
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level converter. When operated at 38.4 kbps with a SPARCstation IPC,
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this arrangement has a worst-case jitter less than 26 us.
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<p>There are three ways in which this driver can be used. The first way
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uses the <code>LDISC_CLKPPS</code> line discipline and works only for
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the baseboard serial ports of the Sun SPARCstation running SunOS 4.x. In
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order to use this option, the -DPPS flag must be included in the
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<code>DEFS_LOCAL</code> line of the distribution configuration file
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<code>./Config.local</code>. The PPS signal is connected via the gadget
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box to the carrier detect (DCD) line of a serial port. The signal is
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activated by a <code>fudge flag3 1</code> command following the
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<code>server</code> command in the configuration file. This causes the
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<code>ppsclock</code> streams module to be configured for that port and
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to capture a timestamp at the on-time transition of the PPS signal. This
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driver then reads the timestamp directly by a designated
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<code>ioctl()</code> system call. This provides the most accurate time
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and least jitter of any other scheme. There is no need to configure a
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dedicated device for this purpose, which ordinarily is the device used
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for the associated radio clock.
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<p>The second way uses the <code>LDISC_CLKPPS</code> line discipline and
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works for any architecture supporting a serial port. In order to use
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this option, the -DCLK flag must be included in the
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<code>DEFS_LOCAL</code> line of the distribution configuration file
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<code>./Config.local</code>. If after a few seconds this driver finds no
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<code>ppsclock</code> module configured, it attempts to open a serial
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port device <code>/dev/pps%d</code>, where <code>%d</code> is the unit
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number, and assign the LDISC_CLKPPS line discipline to it. If the line
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discipline fails, no harm is done except the accuracy is reduced
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somewhat. The pulse generator in the gadget box must be adjusted to
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produce a start bit of length 26 usec at 38400 bps. Used with the
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<code>LDISC_CLKPPS</code> line discipline, this produces an ASCII DEL
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character ('\377') followed by a timestamp at the on-time transition of
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the PPS signal.
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<p>The third way involves an auxiliary radio clock driver which calls
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the PPS driver with a timestamp captured by that driver. This use is
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documented in the source code for the driver(s) involved.
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<p><h4>Fudge Factors</h4>
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<dl>
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<dt><code>time1 <i>time</i></code>
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<dd>Specifies the time offset calibration factor, in seconds and
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fraction, with default 0.0. This parameter can be used to compensate for
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the UART and OS delays. Allow about 247 us for UART delays at 38400 bps
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and about 1 ms for SunOS streams nonsense.
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<p><dt><code>time2 <i>time</i></code>
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<dd>Not used by this driver.
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<p><dt><code>stratum <i>number</i></code>
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<dd>Specifies the driver stratum, in decimal from 0 to 15, with default
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0.
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<p><dt><code>refid <i>string</i></code>
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<dd>Specifies the driver reference identifier, an ASCII string from one
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to four characters, with default <code>ATOM</code>.
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<p><dt><code>flag1 0 | 1</code>
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<dd>Not used by this driver.
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<p><dt><code>flag2 0 | 1</code>
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<dd>Not used by this driver.
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<p><dt><code>flag3 0 | 1</code>
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<dd>Enable <code>ppsclock</code> line discipline/streams module if set.
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<p><dt><code>flag4 0 | 1</code>
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<dd>Not used by this driver.
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<p>Additional Information
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<p><a href="refclock.html"> Reference Clock Drivers</a>
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</dl>
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<hr><address>David L. Mills (mills@udel.edu)</address></body></html>
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