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-
- PPS - Pulse Per Second
- ----------------------
-
-(C) Copyright 2007 Rodolfo Giometti <giometti@enneenne.com>
-
-This program is free software; you can redistribute it and/or modify
-it under the terms of the GNU General Public License as published by
-the Free Software Foundation; either version 2 of the License, or
-(at your option) any later version.
-
-This program is distributed in the hope that it will be useful,
-but WITHOUT ANY WARRANTY; without even the implied warranty of
-MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
-GNU General Public License for more details.
-
-
-
-Overview
---------
-
-LinuxPPS provides a programming interface (API) to define in the
-system several PPS sources.
-
-PPS means "pulse per second" and a PPS source is just a device which
-provides a high precision signal each second so that an application
-can use it to adjust system clock time.
-
-A PPS source can be connected to a serial port (usually to the Data
-Carrier Detect pin) or to a parallel port (ACK-pin) or to a special
-CPU's GPIOs (this is the common case in embedded systems) but in each
-case when a new pulse arrives the system must apply to it a timestamp
-and record it for userland.
-
-Common use is the combination of the NTPD as userland program, with a
-GPS receiver as PPS source, to obtain a wallclock-time with
-sub-millisecond synchronisation to UTC.
-
-
-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..
-
-
-Coding example
---------------
-
-To register a PPS source into the kernel you should define a struct
-pps_source_info_s 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
-intialization 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_s *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_s 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/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 /sys/class/pps/pps0/
- assert clear echo mode name path subsystem@ 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 ktimer driver (see the client subsection in the PPS configuration menu)
-and the userland tools provided into Documentaion/pps/ directory.
-
-Once you have enabled the compilation of ktimer just modprobe it (if
-not statically compiled):
-
- # modprobe ktimer
-
-and the run ppstest as follow:
-
- $ ./ppstest /dev/pps0
- 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
-(see Documentation/pps/).
-
-
-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.