249 lines
8.5 KiB
ReStructuredText
249 lines
8.5 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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======================
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PPS - Pulse Per Second
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======================
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Copyright (C) 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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PPS with USB to serial devices
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------------------------------
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It is possible to grab the PPS from an USB to serial device. However,
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you should take into account the latencies and jitter introduced by
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the USB stack. Users have reported clock instability around +-1ms when
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synchronized with PPS through USB. With USB 2.0, jitter may decrease
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down to the order of 125 microseconds.
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This may be suitable for time server synchronization with NTP because
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of its undersampling and algorithms.
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If your device doesn't report PPS, you can check that the feature is
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supported by its driver. Most of the time, you only need to add a call
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to usb_serial_handle_dcd_change after checking the DCD status (see
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ch341 and pl2303 examples).
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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 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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initialization 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 *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 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/pps-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 -F /sys/class/pps/pps0/
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assert dev mode path subsystem@
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clear echo name power/ 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 pps-ktimer driver (see the client subsection in the PPS configuration menu)
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and the userland tools available in your distribution's pps-tools package,
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http://linuxpps.org , or https://github.com/redlab-i/pps-tools.
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Once you have enabled the compilation of pps-ktimer just modprobe it (if
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not statically compiled)::
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# modprobe pps-ktimer
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and the run ppstest as follow::
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$ ./ppstest /dev/pps1
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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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This is available in the pps-tools repository mentioned above.
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Generators
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----------
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Sometimes one needs to be able not only to catch PPS signals but to produce
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them also. For example, running a distributed simulation, which requires
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computers' clock to be synchronized very tightly.
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Parallel port generator
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------------------------
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One way to do this is to invent some complicated hardware solutions but it
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may be neither necessary nor affordable. The cheap way is to load a PPS
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generator on one of the computers (master) and PPS clients on others
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(slaves), and use very simple cables to deliver signals using parallel
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ports, for example.
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Parallel port cable pinout::
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pin name master slave
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1 STROBE *------ *
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2 D0 * | *
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3 D1 * | *
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4 D2 * | *
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5 D3 * | *
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6 D4 * | *
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7 D5 * | *
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8 D6 * | *
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9 D7 * | *
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10 ACK * ------*
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11 BUSY * *
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12 PE * *
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13 SEL * *
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14 AUTOFD * *
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15 ERROR * *
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16 INIT * *
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17 SELIN * *
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18-25 GND *-----------*
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Please note that parallel port interrupt occurs only on high->low transition,
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so it is used for PPS assert edge. PPS clear edge can be determined only
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using polling in the interrupt handler which actually can be done way more
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precisely because interrupt handling delays can be quite big and random. So
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current parport PPS generator implementation (pps_gen_parport module) is
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geared towards using the clear edge for time synchronization.
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Clear edge polling is done with disabled interrupts so it's better to select
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delay between assert and clear edge as small as possible to reduce system
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latencies. But if it is too small slave won't be able to capture clear edge
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transition. The default of 30us should be good enough in most situations.
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The delay can be selected using 'delay' pps_gen_parport module parameter.
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