driver28.html revision 1.1
1 1.1 kardel <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> 2 1.1 kardel 3 1.1 kardel <html> 4 1.1 kardel 5 1.1 kardel <head> 6 1.1 kardel <meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1"> 7 1.1 kardel <meta name="GENERATOR" content="Mozilla/4.01 [en] (Win95; I) [Netscape]"> 8 1.1 kardel <title>Shared memoy Driver</title> 9 1.1 kardel <link href="scripts/style.css" type="text/css" rel="stylesheet"> 10 1.1 kardel </head> 11 1.1 kardel 12 1.1 kardel <body> 13 1.1 kardel <h3>Shared Memory Driver</h3> 14 1.1 kardel <hr> 15 1.1 kardel <h4>Synopsis</h4> 16 1.1 kardel <p>Address: 127.127.28.<i>u</i><br> 17 1.1 kardel Reference ID: <tt>SHM</tt><br> 18 1.1 kardel Driver ID: <tt>SHM</tt></p> 19 1.1 kardel 20 1.1 kardel <h4>Description</h4> 21 1.1 kardel <p>This driver receives its reference clock info from a shared memory-segment. The shared memory-segment is created with owner-only access for unit 0 and 1, and world access for unit 2 and 3</p> 22 1.1 kardel 23 1.1 kardel <h4>Structure of shared memory-segment</h4> 24 1.1 kardel <pre>struct shmTime { 25 1.1 kardel int mode; /* 0 - if valid set 26 1.1 kardel * use values, 27 1.1 kardel * clear valid 28 1.1 kardel * 1 - if valid set 29 1.1 kardel * if count before and after read of 30 1.1 kardel * values is equal, 31 1.1 kardel * use values 32 1.1 kardel * clear valid 33 1.1 kardel */ 34 1.1 kardel int count; 35 1.1 kardel time_t clockTimeStampSec; /* external clock */ 36 1.1 kardel int clockTimeStampUSec; /* external clock */ 37 1.1 kardel time_t receiveTimeStampSec; /* internal clock, when external value was received */ 38 1.1 kardel int receiveTimeStampUSec; /* internal clock, when external value was received */ 39 1.1 kardel int leap; 40 1.1 kardel int precision; 41 1.1 kardel int nsamples; 42 1.1 kardel int valid; 43 1.1 kardel int dummy[10]; 44 1.1 kardel };</pre> 45 1.1 kardel 46 1.1 kardel <h4>Operation mode=0</h4> 47 1.1 kardel <p>Each second, the valid-flag of the shared memory-segment is checked:</p> 48 1.1 kardel <p>If set, the values in the record (clockTimeStampSec, clockTimeStampUSec, receiveTimeStampSec, receiveTimeStampUSec, leap, precision) are passed to ntp, and the valid-flag is cleared and a counter is bumped.</p> 49 1.1 kardel <p>If not set, a counter is bumped</p> 50 1.1 kardel <h4>Operation mode=1</h4> 51 1.1 kardel <p>Each second, the valid-flag of the shared memory-segment is checked:</p> 52 1.1 kardel <p>If set, the count-field of the record is remembered, and the values in the record (clockTimeStampSec, clockTimeStampUSec, receiveTimeStampSec, receiveTimeStampUSec, leap, precision) are read. Then, the remembered count is compared to the count now in the record. If both are equal, the values read from the record are passed to ntp. If they differ, another process has modified the record while it was read out (was not able to produce this case), and failure is reported to ntp. The valid flag is cleared and a counter is bumped.</p> 53 1.1 kardel <p>If not set, a counter is bumped</p> 54 1.1 kardel 55 1.1 kardel 56 1.1 kardel <h4>gpsd</h4> 57 1.1 kardel 58 1.1 kardel <a href="http://gpsd.berlios.de/"><i>gpsd</i></a> 59 1.1 kardel knows how to talk to many GPS devices. 60 1.1 kardel It works with <i>ntpd</i> through the SHM driver. 61 1.1 kardel <P> 62 1.1 kardel The <i>gpsd</i> man page suggests setting minpoll and maxpoll to 4. 63 1.1 kardel That was an attempt to reduce jitter. 64 1.1 kardel The SHM driver was fixed (ntp-4.2.5p138) to collect data each second rather than 65 1.1 kardel once per polling interval so that suggestion is no longer reasonable. 66 1.1 kardel <P> 67 1.1 kardel 68 1.1 kardel 69 1.1 kardel <h4>Clockstats</h4> 70 1.1 kardel If flag4 is set when the driver is polled, a clockstats record is written. 71 1.1 kardel The first 3 fields are the normal date, time, and IP address common to all clockstats records. 72 1.1 kardel <P> 73 1.1 kardel The 4th field is the number of second ticks since the last poll. 74 1.1 kardel The 5th field is the number of good data samples found. The last 64 will be used by ntpd. 75 1.1 kardel The 6th field is the number of sample that didn't have valid data ready. 76 1.1 kardel The 7th field is the number of bad samples. 77 1.1 kardel The 8th field is the number of times the the mode 1 info was update while nptd was trying to grab a sample. 78 1.1 kardel <P> 79 1.1 kardel 80 1.1 kardel Here is a sample showing the GPS reception fading out: 81 1.1 kardel <pre> 82 1.1 kardel 54364 84927.157 127.127.28.0 66 65 1 0 0 83 1.1 kardel 54364 84990.161 127.127.28.0 63 63 0 0 0 84 1.1 kardel 54364 85053.160 127.127.28.0 63 63 0 0 0 85 1.1 kardel 54364 85116.159 127.127.28.0 63 62 1 0 0 86 1.1 kardel 54364 85180.158 127.127.28.0 64 63 1 0 0 87 1.1 kardel 54364 85246.161 127.127.28.0 66 66 0 0 0 88 1.1 kardel 54364 85312.157 127.127.28.0 66 50 16 0 0 89 1.1 kardel 54364 85375.160 127.127.28.0 63 41 22 0 0 90 1.1 kardel 54364 85439.155 127.127.28.0 64 64 0 0 0 91 1.1 kardel 54364 85505.158 127.127.28.0 66 36 30 0 0 92 1.1 kardel 54364 85569.157 127.127.28.0 64 0 64 0 0 93 1.1 kardel 54364 85635.157 127.127.28.0 66 0 66 0 0 94 1.1 kardel 54364 85700.160 127.127.28.0 65 0 65 0 0 95 1.1 kardel </pre> 96 1.1 kardel 97 1.1 kardel <h4>Fudge Factors</h4> 98 1.1 kardel <dl> 99 1.1 kardel <dt><tt>time1 <i>time</i></tt> 100 1.1 kardel <dd>Specifies the time offset calibration factor, in seconds and fraction, with default 0.0. 101 1.1 kardel <dt><tt>time2 <i>time</i></tt> 102 1.1 kardel <dd>Not used by this driver. 103 1.1 kardel <dt><tt>stratum <i>number</i></tt> 104 1.1 kardel <dd>Specifies the driver stratum, in decimal from 0 to 15, with default 0. 105 1.1 kardel <dt><tt>refid <i>string</i></tt> 106 1.1 kardel <dd>Specifies the driver reference identifier, an ASCII string from one to four characters, with default <tt>SHM</tt>. 107 1.1 kardel <dt><tt>flag1 0 | 1</tt> 108 1.1 kardel <dd>Not used by this driver. 109 1.1 kardel <dt><tt>flag2 0 | 1</tt> 110 1.1 kardel <dd>Not used by this driver. 111 1.1 kardel <dt><tt>flag3 0 | 1</tt> 112 1.1 kardel <dd>Not used by this driver. 113 1.1 kardel <dt><tt>flag4 0 | 1</tt> 114 1.1 kardel <dd>If flag4 is set, clockstats records will be written when the driver is polled. 115 1.1 kardel <h4>Additional Information</h4> 116 1.1 kardel <p><a href="../refclock.html">Reference Clock Drivers</a></p> 117 1.1 kardel </dl> 118 1.1 kardel <hr> 119 1.1 kardel <script type="text/javascript" language="javascript" src="scripts/footer.txt"></script> 120 1.1 kardel </body> 121 1.1 kardel 122 1.1 kardel </html> 123 1.1 kardel 124
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