'p' a1-a4 linux/pps.h LinuxPPS

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PPS - Pulse Per Second

----------------------

(C) Copyright 2007 Rodolfo Giometti <[email protected]>

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/).

PPS SUPPORT

P: Rodolfo Giometti

M: [email protected]

W: http://wiki.enneenne.com/index.php/LinuxPPS_support

L: [email protected] (subscribers-only)

S: Maintained

source "drivers/pps/Kconfig"

obj-$(CONFIG_PPS) += pps/

#

# PPS support configuration

#

menu "PPS support"

config PPS

tristate "PPS support"

depends on EXPERIMENTAL

---help---

PPS (Pulse Per Second) is a special pulse provided by some GPS

antennae. Userland can use it to get a high-precision time

reference.

Some antennae's PPS signals are connected with the CD (Carrier

Detect) pin of the serial line they use to communicate with the

host. In this case use the SERIAL_LINE client support.

Some antennae's PPS signals are connected with some special host

inputs so you have to enable the corresponding client support.

To compile this driver as a module, choose M here: the module

will be called pps_core.ko.

config PPS_DEBUG

bool "PPS debugging messages"

depends on PPS

help

Say Y here if you want the PPS support to produce a bunch of debug

messages to the system log. Select this if you are having a

problem with PPS support and want to see more of what is going on.

endmenu

pps_core-y := pps.o kapi.o sysfs.o

obj-$(CONFIG_PPS) := pps_core.o

ccflags-$(CONFIG_PPS_DEBUG) := -DDEBUG