173 lines
5.7 KiB
Plaintext
173 lines
5.7 KiB
Plaintext
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PPS - Pulse Per Second
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----------------------
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(C) Copyright 2007 Rodolfo Giometti <giometti@enneenne.com>
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This program is free software; you can redistribute it and/or modify
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it under the terms of the GNU General Public License as published by
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the Free Software Foundation; either version 2 of the License, or
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(at your option) any later version.
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This program is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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GNU General Public License for more details.
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Overview
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--------
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LinuxPPS provides a programming interface (API) to define in the
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system several PPS sources.
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PPS means "pulse per second" and a PPS source is just a device which
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provides a high precision signal each second so that an application
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can use it to adjust system clock time.
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A PPS source can be connected to a serial port (usually to the Data
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Carrier Detect pin) or to a parallel port (ACK-pin) or to a special
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CPU's GPIOs (this is the common case in embedded systems) but in each
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case when a new pulse arrives the system must apply to it a timestamp
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and record it for userland.
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Common use is the combination of the NTPD as userland program, with a
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GPS receiver as PPS source, to obtain a wallclock-time with
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sub-millisecond synchronisation to UTC.
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RFC considerations
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------------------
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While implementing a PPS API as RFC 2783 defines and using an embedded
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CPU GPIO-Pin as physical link to the signal, I encountered a deeper
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problem:
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At startup it needs a file descriptor as argument for the function
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time_pps_create().
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This implies that the source has a /dev/... entry. This assumption is
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ok for the serial and parallel port, where you can do something
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useful besides(!) the gathering of timestamps as it is the central
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task for a PPS-API. But this assumption does not work for a single
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purpose GPIO line. In this case even basic file-related functionality
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(like read() and write()) makes no sense at all and should not be a
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precondition for the use of a PPS-API.
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The problem can be simply solved if you consider that a PPS source is
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not always connected with a GPS data source.
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So your programs should check if the GPS data source (the serial port
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for instance) is a PPS source too, and if not they should provide the
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possibility to open another device as PPS source.
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In LinuxPPS the PPS sources are simply char devices usually mapped
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into files /dev/pps0, /dev/pps1, etc..
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Coding example
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--------------
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To register a PPS source into the kernel you should define a struct
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pps_source_info_s as follows:
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static struct pps_source_info pps_ktimer_info = {
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.name = "ktimer",
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.path = "",
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.mode = PPS_CAPTUREASSERT | PPS_OFFSETASSERT | \
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PPS_ECHOASSERT | \
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PPS_CANWAIT | PPS_TSFMT_TSPEC,
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.echo = pps_ktimer_echo,
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.owner = THIS_MODULE,
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};
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and then calling the function pps_register_source() in your
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intialization routine as follows:
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source = pps_register_source(&pps_ktimer_info,
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PPS_CAPTUREASSERT | PPS_OFFSETASSERT);
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The pps_register_source() prototype is:
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int pps_register_source(struct pps_source_info_s *info, int default_params)
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where "info" is a pointer to a structure that describes a particular
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PPS source, "default_params" tells the system what the initial default
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parameters for the device should be (it is obvious that these parameters
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must be a subset of ones defined in the struct
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pps_source_info_s which describe the capabilities of the driver).
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Once you have registered a new PPS source into the system you can
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signal an assert event (for example in the interrupt handler routine)
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just using:
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pps_event(source, &ts, PPS_CAPTUREASSERT, ptr)
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where "ts" is the event's timestamp.
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The same function may also run the defined echo function
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(pps_ktimer_echo(), passing to it the "ptr" pointer) if the user
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asked for that... etc..
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Please see the file drivers/pps/clients/ktimer.c for example code.
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SYSFS support
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-------------
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If the SYSFS filesystem is enabled in the kernel it provides a new class:
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$ ls /sys/class/pps/
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pps0/ pps1/ pps2/
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Every directory is the ID of a PPS sources defined in the system and
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inside you find several files:
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$ ls /sys/class/pps/pps0/
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assert clear echo mode name path subsystem@ uevent
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Inside each "assert" and "clear" file you can find the timestamp and a
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sequence number:
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$ cat /sys/class/pps/pps0/assert
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1170026870.983207967#8
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Where before the "#" is the timestamp in seconds; after it is the
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sequence number. Other files are:
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* echo: reports if the PPS source has an echo function or not;
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* mode: reports available PPS functioning modes;
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* name: reports the PPS source's name;
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* path: reports the PPS source's device path, that is the device the
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PPS source is connected to (if it exists).
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Testing the PPS support
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-----------------------
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In order to test the PPS support even without specific hardware you can use
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the ktimer driver (see the client subsection in the PPS configuration menu)
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and the userland tools provided into Documentaion/pps/ directory.
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Once you have enabled the compilation of ktimer just modprobe it (if
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not statically compiled):
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# modprobe ktimer
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and the run ppstest as follow:
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$ ./ppstest /dev/pps0
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trying PPS source "/dev/pps1"
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found PPS source "/dev/pps1"
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ok, found 1 source(s), now start fetching data...
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source 0 - assert 1186592699.388832443, sequence: 364 - clear 0.000000000, sequence: 0
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source 0 - assert 1186592700.388931295, sequence: 365 - clear 0.000000000, sequence: 0
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source 0 - assert 1186592701.389032765, sequence: 366 - clear 0.000000000, sequence: 0
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Please, note that to compile userland programs you need the file timepps.h
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(see Documentation/pps/).
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