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![Paul Chavent](/assets/img/avatar_default.png)
The DCD pin of the serial port can receive a PPS signal. By calling the port line discipline dcd handle, this patch allow to monitor PPS through USB serial devices. However the performance aren't as good as the uart drivers, so document this point too. Signed-off-by: Paul Chavent <paul.chavent@onera.fr> Acked-by: Rodolfo Giometti <giometti@enneenne.com> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
233 lines
8.1 KiB
Text
233 lines
8.1 KiB
Text
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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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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 has reported clock instability around +-1ms when
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synchronized with PPS through USB. This isn't suited for time server
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synchronization.
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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_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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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. One way to do this is to
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invent some complicated hardware solutions but it may be neither necessary
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nor affordable. The cheap way is to load a PPS generator on one of the
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computers (master) and PPS clients on others (slaves), and use very simple
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cables to deliver signals using parallel 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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