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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.