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start [2018/08/29 12:57] giometti [Generators] fix up ASCII art |
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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 | + | ===== LinuxPPS based projects ===== |
- | 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 | + | Projects using LinuxPPS: |
- | not always connected with a GPS data source. | + | |
- | So your programs should check if the GPS data source (the serial port | + | * 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]]. |
- | 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 | + | * [[http://www.mmarray.org/|Owens Valley Radio Observatory]] (OVRO / CARMA) correlator system. |
- | into files ''/dev/pps0'', ''/dev/pps1'', etc. | + | |
- | ===== PPS with USB to serial devices ===== | + | * [[http://ntpi.openchaos.org/pps_pi/|Raspberry Pi Stratum 1 NTP server with PPS]]. Accurate timekeeping at low cost. |
- | It is possible to grab the PPS from an USB to serial device. However, | + | If you have a project that is using LinuxPPS and you would like it to be shown here, please email [[mailto:giometti@ennenne.com]]. |
- | 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 | + | ===== Press ===== |
- | of its undersampling and algorithms. | + | |
- | If your device doesn't report PPS, you can check that the feature is | + | Press talked about the LinuxPPS project on: |
- | 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 ===== | + | * Linux&C. issue 68 (year 11) at page 5 (Italian magazine). |
- | To register a PPS source into the kernel you should define a ''struct | + | * Linux Journal issue 185 (September 2009) at page 14. |
- | pps_source_info'' as follows: | + | |
- | + | ||
- | static struct pps_source_info pps_ktimer_info = { | + | |
- | .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 | + | |
- | 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 [[mailto: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. |