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/).
