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start [2018/08/29 10:56] – [Contributors] Add lost contributors from web cache giomettistart [2020/11/26 15:06] (current) – [LinuxPPS based projects] New project added giometti
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 sub-millisecond synchronisation to UTC. sub-millisecond synchronisation to UTC.
  
-===== RFC considerations =====+More detailed information regarding PPS and LinuxPPS are available at [[Technical information]] page.
  
-While implementing a PPS API as RFC 2783 defines and using an embedded +===== Resources =====
-CPU GPIO-Pin as physical link to the signal, I encountered a deeper +
-problem:+
  
-   At startup it needs a file descriptor as argument for the function +A LinuxPPS's mail list is available at [[http://www.linuxpps.org/cgi-bin/mailman/listinfo/discussions]] where you can send to your LinuxPPS related questions (please avoid asking about userland problems nor ''ntpd'' or ''chrony'' servers, etc.).
-   ''time_pps_create()''.+
  
-This implies that the source has a ''/dev/...'' entry. This assumption is +However **before** sending your questions to this list, please, **take a look** to the [[LinuxPPS FAQ]] page!
-OK for the serial and parallel port, where you can do something +
-useful besides(!) the gathering of timestamps as it is the central +
-task for a PPS API. But this assumption does not work for a single +
-purpose GPIO line. In this case even basic file-related functionality +
-(like ''read()'' and ''write()'') makes no sense at all and should not be a +
-precondition for the use of a PPS API.+
  
-The problem can be simply solved if you consider that a PPS source is +===== LinuxPPS based projects =====
-not always connected with a GPS data source.+
  
-So your programs should check if the GPS data source (the serial port +Projects using LinuxPPS:
-for instance) is a PPS source too, and if not they should provide the +
-possibility to open another device as PPS source.+
  
-In LinuxPPS the PPS sources are simply char devices usually mapped +  * Dedicated Short Range Communications (DSRC) radio synchronization used by US-DOT's VII [[http://www.its.dot.gov/vii/]] [[http://en.wikipedia.org/wiki/Vehicle_infrastructure_integration]].
-into files ''/dev/pps0'', ''/dev/pps1'', etc.+
  
-===== PPS with USB to serial devices =====+  * [[http://www.mmarray.org/|Owens Valley Radio Observatory]] (OVRO / CARMA) correlator system.
  
-It is possible to grab the PPS from an USB to serial device. However, +  * [[http://ntpi.openchaos.org/pps_pi/|Raspberry Pi Stratum 1 NTP server with PPS]]. Accurate timekeeping at low cost.
-you should take into account the latencies and jitter introduced by +
-the USB stack. Users have reported clock instability around +-1ms when +
-synchronized with PPS through USB. With USB 2.0, jitter may decrease +
-down to the order of 125 microseconds.+
  
-This may be suitable for time server synchronization with NTP because +  * [[https://github.com/rascol/PPS-Client|PPS-Client daemon for Raspberry Pi 2, 3 or 4 B or B+ or an AMD/Intel desktop system]]. Accurate time keeping precision of a timeline with a standard deviation of 0.5 microsecond and a typical time keeping accuracy of 2 microseconds.
-of its undersampling and algorithms.+
  
-If your device doesn't report PPS, you can check that the feature is +If you have a project that is using LinuxPPS and you would like it to be shown here, please email [[giometti@ennenne.com]].
-supported by its driver. Most of the time, you only need to add a call +
-to usb_serial_handle_dcd_change after checking the DCD status (see +
-ch341 and pl2303 examples).+
  
-===== Coding example =====+===== Press =====
  
-To register a PPS source into the kernel you should define a ''struct +Press talked about the LinuxPPS project on:
-pps_source_info'' as follows:+
  
-    static struct pps_source_info pps_ktimer_info = { +  * Linux&C. issue 68 (year 11) at page 5 (Italian magazine).
-            .name         = "ktimer", +
-            .path         = "", +
-            .mode         = PPS_CAPTUREASSERT | PPS_OFFSETASSERT | +
-                            PPS_ECHOASSERT | +
-                            PPS_CANWAIT | PPS_TSFMT_TSPEC, +
-            .echo         = pps_ktimer_echo, +
-            .owner        = THIS_MODULE, +
-    };+
  
-and then calling the function ''pps_register_source()'' in your +  * Linux Journal issue 185 (September 2009) at page 14. 
-initialization routine as follows: +
- +
-    source = pps_register_source(&pps_ktimer_info, +
-                        PPS_CAPTUREASSERT | PPS_OFFSETASSERT); +
- +
-The ''pps_register_source()'' prototype is: +
- +
-  int pps_register_source(struct pps_source_info *info, int default_params) +
- +
-where ''info'' is a pointer to a structure that describes a particular +
-PPS source, ''default_params'' tells the system what the initial default +
-parameters for the device should be (it is obvious that these parameters +
-must be a subset of ones defined in the ''struct +
-pps_source_info'' which describe the capabilities of the driver). +
- +
-Once you have registered a new PPS source into the system you can +
-signal an assert event (for example in the interrupt handler routine) +
-just using: +
- +
-    pps_event(source, &ts, PPS_CAPTUREASSERT, ptr) +
- +
-where ''ts'' is the event's timestamp. +
- +
-The same function may also run the defined echo function +
-(''pps_ktimer_echo()'', passing to it the ''ptr'' pointer) if the user +
-asked for that... etc.. +
- +
-Please see the file ''drivers/pps/clients/pps-ktimer.c'' for example code. +
- +
-===== SYSFS support ===== +
- +
-If the SYSFS filesystem is enabled in the kernel it provides a new class: +
- +
-   $ ls /sys/class/pps/ +
-   pps0/  pps1/  pps2/ +
- +
-Every directory is the ID of a PPS sources defined in the system and +
-inside you find several files: +
- +
-   $ ls -F /sys/class/pps/pps0/ +
-   assert     dev        mode       path       subsystem@ +
-   clear      echo       name       power/     uevent +
- +
- +
-Inside each ''assert'' and ''clear'' file you can find the timestamp and a +
-sequence number: +
- +
-   $ cat /sys/class/pps/pps0/assert +
-   1170026870.983207967#8 +
- +
-Where before the ''#'' is the timestamp in seconds; after it is the +
-sequence number. Other files are: +
- +
- * ''echo'': reports if the PPS source has an echo function or not; +
- * ''mode'': reports available PPS functioning modes; +
- * ''name'': reports the PPS source's name; +
- * ''path'': reports the PPS source's device path, that is the device the +
-   PPS source is connected to (if it exists). +
- +
-===== Testing the PPS support ===== +
- +
-In order to test the PPS support even without specific hardware you can use +
-the ''pps-ktimer'' driver (see the client subsection in the PPS configuration menu) +
-and the userland tools available in your distribution's **pps-tools** package, +
-http://linuxpps.org , or [[https://github.com/redlab-i/pps-tools]]. +
- +
-Once you have enabled the compilation of ''pps-ktimer'' just ''modprobe'' it (if +
-not statically compiled): +
- +
-   # modprobe pps-ktimer +
- +
-and the run ''ppstest'' as follow: +
- +
-   $ ./ppstest /dev/pps1 +
-   trying PPS source "/dev/pps1" +
-   found PPS source "/dev/pps1" +
-   ok, found 1 source(s), now start fetching data... +
-   source 0 - assert 1186592699.388832443, sequence: 364 - clear  0.000000000, sequence: 0 +
-   source 0 - assert 1186592700.388931295, sequence: 365 - clear  0.000000000, sequence: 0 +
-   source 0 - assert 1186592701.389032765, sequence: 366 - clear  0.000000000, sequence: 0 +
- +
-Please note that to compile userland programs, you need the file ''timepps.h''. +
-This is available in the **pps-tools** repository mentioned above. +
- +
-===== Generators ===== +
- +
-Sometimes one needs to be able not only to catch PPS signals but to produce +
-them also. For example, running a distributed simulation, which requires +
-computers' clock to be synchronized very tightly. One way to do this is to +
-invent some complicated hardware solutions but it may be neither necessary +
-nor affordable. The cheap way is to load a PPS generator on one of the +
-computers (master) and PPS clients on others (slaves), and use very simple +
-cables to deliver signals using parallel ports, for example. +
- +
-Parallel port cable pinout: +
-pin     name    master      slave +
-1       STROBE    *------     * +
-2       D0        *     |     * +
-3       D1        *     |     * +
-4       D2        *     |     * +
-5       D3        *     |     * +
-6       D4        *     |     * +
-7       D5        *     |     * +
-8       D6        *     |     * +
-9       D7        *     |     * +
-10      ACK       *     ------* +
-11      BUSY      *           * +
-12      PE        *           * +
-13      SEL       *           * +
-14      AUTOFD    *           * +
-15      ERROR     *           * +
-16      INIT      *           * +
-17      SELIN     *           * +
-18-25   GND       *-----------* +
- +
-Please note that parallel port interrupt occurs only on high->low transition, +
-so it is used for PPS assert edge. PPS clear edge can be determined only +
-using polling in the interrupt handler which actually can be done way more +
-precisely because interrupt handling delays can be quite big and random. So +
-current parport PPS generator implementation (''pps_gen_parport'' module) is +
-geared towards using the clear edge for time synchronization. +
- +
-Clear edge polling is done with disabled interrupts so it's better to select +
-delay between assert and clear edge as small as possible to reduce system +
-latencies. But if it is too small slave won't be able to capture clear edge +
-transition. The default of 30us should be good enough in most situations. +
-The delay can be selected using ''delay'' ''pps_gen_parport'' module parameter. +
- +
-===== Resources ===== +
- +
-A LinuxPPS's mail list is available at [[http://www.linuxpps.org/cgi-bin/mailman/listinfo/discussions]] where you can send to your LinuxPPS related questions (please avoid asking about userland problems nor ''ntpd'' or ''chrony'' servers, etc.).+
  
 ===== Contributors ===== ===== Contributors =====
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 ==== PayPal ==== ==== PayPal ====
  
-* Kenji Hiranuma+  * Kenji Hiranuma
  
-* [[http://www.vanheusden.com/|Folkert van Heusden]]+  * [[http://www.vanheusden.com/|Folkert van Heusden]]
  
 Other contributions can be done via [[http://paypal.me/RodolfoGiometti|PayPal]]. Other contributions can be done via [[http://paypal.me/RodolfoGiometti|PayPal]].
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 ==== Hardware ==== ==== Hardware ====
  
-* [[http://www.meinberg.de/english/products/lantime-m200-gps.htm|LANTIME M200/GPS]] from [[http://www.meinberg.de/Meinberg | Meinberg]].+  * [[http://www.meinberg.de/english/products/lantime-m200-gps.htm|LANTIME M200/GPS]] from [[http://www.meinberg.de/Meinberg | Meinberg]].
  
 ==== Misc ==== ==== Misc ====
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 [[https://www.meinbergglobal.com/|{{:meinberg_logo.png?nolink&100 | Meinberg}}]] [[https://www.meinbergglobal.com/|{{:meinberg_logo.png?nolink&100 | Meinberg}}]]
  
-* Many thanks to [[https://www.meinbergglobal.com/|Meinberg]] for donating this new WEB space.+  * Many thanks to [[https://www.meinbergglobal.com/|Meinberg]] for donating this new WEB space.
  
-* Thanks to [[mailto:andreas.puls@meinberg.de|Andreas Puls]] for the LinuxPPS's logo.+  * Thanks to [[andreas.puls@meinberg.de|Andreas Puls]] for the LinuxPPS's logo.
  
-* Thanks to [[http://www.worldtimesolutions.com/|World Time Solution]] for asking to me to do some improvements on LinuxPPS code.+  * Thanks to [[http://www.worldtimesolutions.com/|World Time Solution]] for asking to me to do some improvements on LinuxPPS code.
  
            
start.1535540175.txt.gz · Last modified: by giometti