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Based on kernel version 4.16.1. Page generated on 2018-04-09 11:53 EST.

1	
2				PPS - Pulse Per Second
3				----------------------
4	
5	(C) Copyright 2007 Rodolfo Giometti <giometti@enneenne.com>
6	
7	This program is free software; you can redistribute it and/or modify
8	it under the terms of the GNU General Public License as published by
9	the Free Software Foundation; either version 2 of the License, or
10	(at your option) any later version.
11	
12	This program is distributed in the hope that it will be useful,
13	but WITHOUT ANY WARRANTY; without even the implied warranty of
14	MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
15	GNU General Public License for more details.
16	
17	
18	
19	Overview
20	--------
21	
22	LinuxPPS provides a programming interface (API) to define in the
23	system several PPS sources.
24	
25	PPS means "pulse per second" and a PPS source is just a device which
26	provides a high precision signal each second so that an application
27	can use it to adjust system clock time.
28	
29	A PPS source can be connected to a serial port (usually to the Data
30	Carrier Detect pin) or to a parallel port (ACK-pin) or to a special
31	CPU's GPIOs (this is the common case in embedded systems) but in each
32	case when a new pulse arrives the system must apply to it a timestamp
33	and record it for userland.
34	
35	Common use is the combination of the NTPD as userland program, with a
36	GPS receiver as PPS source, to obtain a wallclock-time with
37	sub-millisecond synchronisation to UTC.
38	
39	
40	RFC considerations
41	------------------
42	
43	While implementing a PPS API as RFC 2783 defines and using an embedded
44	CPU GPIO-Pin as physical link to the signal, I encountered a deeper
45	problem:
46	
47	   At startup it needs a file descriptor as argument for the function
48	   time_pps_create().
49	
50	This implies that the source has a /dev/... entry. This assumption is
51	OK for the serial and parallel port, where you can do something
52	useful besides(!) the gathering of timestamps as it is the central
53	task for a PPS API. But this assumption does not work for a single
54	purpose GPIO line. In this case even basic file-related functionality
55	(like read() and write()) makes no sense at all and should not be a
56	precondition for the use of a PPS API.
57	
58	The problem can be simply solved if you consider that a PPS source is
59	not always connected with a GPS data source.
60	
61	So your programs should check if the GPS data source (the serial port
62	for instance) is a PPS source too, and if not they should provide the
63	possibility to open another device as PPS source.
64	
65	In LinuxPPS the PPS sources are simply char devices usually mapped
66	into files /dev/pps0, /dev/pps1, etc.
67	
68	
69	PPS with USB to serial devices
70	------------------------------
71	
72	It is possible to grab the PPS from an USB to serial device. However,
73	you should take into account the latencies and jitter introduced by
74	the USB stack. Users have reported clock instability around +-1ms when
75	synchronized with PPS through USB. With USB 2.0, jitter may decrease
76	down to the order of 125 microseconds.
77	
78	This may be suitable for time server synchronization with NTP because
79	of its undersampling and algorithms.
80	
81	If your device doesn't report PPS, you can check that the feature is
82	supported by its driver. Most of the time, you only need to add a call
83	to usb_serial_handle_dcd_change after checking the DCD status (see
84	ch341 and pl2303 examples).
85	
86	
87	Coding example
88	--------------
89	
90	To register a PPS source into the kernel you should define a struct
91	pps_source_info as follows:
92	
93	    static struct pps_source_info pps_ktimer_info = {
94		    .name         = "ktimer",
95		    .path         = "",
96		    .mode         = PPS_CAPTUREASSERT | PPS_OFFSETASSERT |
97				    PPS_ECHOASSERT |
98				    PPS_CANWAIT | PPS_TSFMT_TSPEC,
99		    .echo         = pps_ktimer_echo,
100		    .owner        = THIS_MODULE,
101	    };
102	
103	and then calling the function pps_register_source() in your
104	initialization routine as follows:
105	
106	    source = pps_register_source(&pps_ktimer_info,
107				PPS_CAPTUREASSERT | PPS_OFFSETASSERT);
108	
109	The pps_register_source() prototype is:
110	
111	  int pps_register_source(struct pps_source_info *info, int default_params)
112	
113	where "info" is a pointer to a structure that describes a particular
114	PPS source, "default_params" tells the system what the initial default
115	parameters for the device should be (it is obvious that these parameters
116	must be a subset of ones defined in the struct
117	pps_source_info which describe the capabilities of the driver).
118	
119	Once you have registered a new PPS source into the system you can
120	signal an assert event (for example in the interrupt handler routine)
121	just using:
122	
123	    pps_event(source, &ts, PPS_CAPTUREASSERT, ptr)
124	
125	where "ts" is the event's timestamp.
126	
127	The same function may also run the defined echo function
128	(pps_ktimer_echo(), passing to it the "ptr" pointer) if the user
129	asked for that... etc..
130	
131	Please see the file drivers/pps/clients/pps-ktimer.c for example code.
132	
133	
134	SYSFS support
135	-------------
136	
137	If the SYSFS filesystem is enabled in the kernel it provides a new class:
138	
139	   $ ls /sys/class/pps/
140	   pps0/  pps1/  pps2/
141	
142	Every directory is the ID of a PPS sources defined in the system and
143	inside you find several files:
144	
145	   $ ls -F /sys/class/pps/pps0/
146	   assert     dev        mode       path       subsystem@
147	   clear      echo       name       power/     uevent
148	
149	
150	Inside each "assert" and "clear" file you can find the timestamp and a
151	sequence number:
152	
153	   $ cat /sys/class/pps/pps0/assert
154	   1170026870.983207967#8
155	
156	Where before the "#" is the timestamp in seconds; after it is the
157	sequence number. Other files are:
158	
159	 * echo: reports if the PPS source has an echo function or not;
160	
161	 * mode: reports available PPS functioning modes;
162	
163	 * name: reports the PPS source's name;
164	
165	 * path: reports the PPS source's device path, that is the device the
166	   PPS source is connected to (if it exists).
167	
168	
169	Testing the PPS support
170	-----------------------
171	
172	In order to test the PPS support even without specific hardware you can use
173	the pps-ktimer driver (see the client subsection in the PPS configuration menu)
174	and the userland tools available in your distribution's pps-tools package,
175	http://linuxpps.org , or https://github.com/redlab-i/pps-tools.
176	
177	Once you have enabled the compilation of pps-ktimer just modprobe it (if
178	not statically compiled):
179	
180	   # modprobe pps-ktimer
181	
182	and the run ppstest as follow:
183	
184	   $ ./ppstest /dev/pps1
185	   trying PPS source "/dev/pps1"
186	   found PPS source "/dev/pps1"
187	   ok, found 1 source(s), now start fetching data...
188	   source 0 - assert 1186592699.388832443, sequence: 364 - clear  0.000000000, sequence: 0
189	   source 0 - assert 1186592700.388931295, sequence: 365 - clear  0.000000000, sequence: 0
190	   source 0 - assert 1186592701.389032765, sequence: 366 - clear  0.000000000, sequence: 0
191	
192	Please note that to compile userland programs, you need the file timepps.h.
193	This is available in the pps-tools repository mentioned above.
194	
195	
196	Generators
197	----------
198	
199	Sometimes one needs to be able not only to catch PPS signals but to produce
200	them also. For example, running a distributed simulation, which requires
201	computers' clock to be synchronized very tightly. One way to do this is to
202	invent some complicated hardware solutions but it may be neither necessary
203	nor affordable. The cheap way is to load a PPS generator on one of the
204	computers (master) and PPS clients on others (slaves), and use very simple
205	cables to deliver signals using parallel ports, for example.
206	
207	Parallel port cable pinout:
208	pin	name	master      slave
209	1	STROBE	  *------     *
210	2	D0	  *     |     *
211	3	D1	  *     |     *
212	4	D2	  *     |     *
213	5	D3	  *     |     *
214	6	D4	  *     |     *
215	7	D5	  *     |     *
216	8	D6	  *     |     *
217	9	D7	  *     |     *
218	10	ACK	  *     ------*
219	11	BUSY	  *           *
220	12	PE	  *           *
221	13	SEL	  *           *
222	14	AUTOFD	  *           *
223	15	ERROR	  *           *
224	16	INIT	  *           *
225	17	SELIN	  *           *
226	18-25	GND	  *-----------*
227	
228	Please note that parallel port interrupt occurs only on high->low transition,
229	so it is used for PPS assert edge. PPS clear edge can be determined only
230	using polling in the interrupt handler which actually can be done way more
231	precisely because interrupt handling delays can be quite big and random. So
232	current parport PPS generator implementation (pps_gen_parport module) is
233	geared towards using the clear edge for time synchronization.
234	
235	Clear edge polling is done with disabled interrupts so it's better to select
236	delay between assert and clear edge as small as possible to reduce system
237	latencies. But if it is too small slave won't be able to capture clear edge
238	transition. The default of 30us should be good enough in most situations.
239	The delay can be selected using 'delay' pps_gen_parport module parameter.
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