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--- technical_informations [2018/08/29 16:07]
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-====== RFC considerations ======
-While implementing a PPS API as RFC 2783 defines and using an embedded
-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
-   ''time_pps_create()''.
-This implies that the source has a ''/dev/...'' entry. This assumption is
-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
-not always connected with a GPS data source.
-So your programs should check if the GPS data source (the serial port
-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
-into files ''/dev/pps0'', ''/dev/pps1'', etc.
-====== PPS with USB to serial devices ======
-It is possible to grab the PPS from an USB to serial device. However,
-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
-of its undersampling and algorithms.
-If your device doesn't report PPS, you can check that the feature is
-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 ======
-To register a PPS source into the kernel you should define a ''struct
-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
-       STROBE    *------     *
-       D0        *     |     *
-       D1        *     |     *
-       D2        *     |     *
-       D3        *     |     *
-       D4        *     |     *
-       D5        *     |     *
-       D6        *     |     *
-       D7        *     |     *
-      ACK       *     ------*
-      BUSY      *           *
-      PE        *           *
-      SEL       *           *
-      AUTOFD    *           *
-      ERROR     *           *
-      INIT      *           *
-      SELIN     *           *
--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.

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