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Based on kernel version 4.13.3. Page generated on 2017-09-23 13:55 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_s 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_s *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_s 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 /sys/class/pps/pps0/
146	   assert	clear  echo  mode  name  path  subsystem@  uevent
147	
148	Inside each "assert" and "clear" file you can find the timestamp and a
149	sequence number:
150	
151	   $ cat /sys/class/pps/pps0/assert
152	   1170026870.983207967#8
153	
154	Where before the "#" is the timestamp in seconds; after it is the
155	sequence number. Other files are:
156	
157	* echo: reports if the PPS source has an echo function or not;
158	
159	* mode: reports available PPS functioning modes;
160	
161	* name: reports the PPS source's name;
162	
163	* path: reports the PPS source's device path, that is the device the
164	  PPS source is connected to (if it exists).
165	
166	
167	Testing the PPS support
168	-----------------------
169	
170	In order to test the PPS support even without specific hardware you can use
171	the ktimer driver (see the client subsection in the PPS configuration menu)
172	and the userland tools available in your distribution's pps-tools package,
173	http://linuxpps.org , or https://github.com/ago/pps-tools .
174	
175	Once you have enabled the compilation of ktimer just modprobe it (if
176	not statically compiled):
177	
178	   # modprobe ktimer
179	
180	and the run ppstest as follow:
181	
182	   $ ./ppstest /dev/pps0
183	   trying PPS source "/dev/pps1"
184	   found PPS source "/dev/pps1"
185	   ok, found 1 source(s), now start fetching data...
186	   source 0 - assert 1186592699.388832443, sequence: 364 - clear  0.000000000, sequence: 0
187	   source 0 - assert 1186592700.388931295, sequence: 365 - clear  0.000000000, sequence: 0
188	   source 0 - assert 1186592701.389032765, sequence: 366 - clear  0.000000000, sequence: 0
189	
190	Please, note that to compile userland programs you need the file timepps.h .
191	This is available in the pps-tools repository mentioned above.
192	
193	
194	Generators
195	----------
196	
197	Sometimes one needs to be able not only to catch PPS signals but to produce
198	them also. For example, running a distributed simulation, which requires
199	computers' clock to be synchronized very tightly. One way to do this is to
200	invent some complicated hardware solutions but it may be neither necessary
201	nor affordable. The cheap way is to load a PPS generator on one of the
202	computers (master) and PPS clients on others (slaves), and use very simple
203	cables to deliver signals using parallel ports, for example.
204	
205	Parallel port cable pinout:
206	pin	name	master      slave
207	1	STROBE	  *------     *
208	2	D0	  *     |     *
209	3	D1	  *     |     *
210	4	D2	  *     |     *
211	5	D3	  *     |     *
212	6	D4	  *     |     *
213	7	D5	  *     |     *
214	8	D6	  *     |     *
215	9	D7	  *     |     *
216	10	ACK	  *     ------*
217	11	BUSY	  *           *
218	12	PE	  *           *
219	13	SEL	  *           *
220	14	AUTOFD	  *           *
221	15	ERROR	  *           *
222	16	INIT	  *           *
223	17	SELIN	  *           *
224	18-25	GND	  *-----------*
225	
226	Please note that parallel port interrupt occurs only on high->low transition,
227	so it is used for PPS assert edge. PPS clear edge can be determined only
228	using polling in the interrupt handler which actually can be done way more
229	precisely because interrupt handling delays can be quite big and random. So
230	current parport PPS generator implementation (pps_gen_parport module) is
231	geared towards using the clear edge for time synchronization.
232	
233	Clear edge polling is done with disabled interrupts so it's better to select
234	delay between assert and clear edge as small as possible to reduce system
235	latencies. But if it is too small slave won't be able to capture clear edge
236	transition. The default of 30us should be good enough in most situations.
237	The delay can be selected using 'delay' pps_gen_parport module parameter.
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