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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 | Coding example | |
70 | -------------- | |
71 | ||
72 | To register a PPS source into the kernel you should define a struct | |
73 | pps_source_info_s as follows: | |
74 | ||
75 | static struct pps_source_info pps_ktimer_info = { | |
76 | .name = "ktimer", | |
77 | .path = "", | |
78 | .mode = PPS_CAPTUREASSERT | PPS_OFFSETASSERT | \ | |
79 | PPS_ECHOASSERT | \ | |
80 | PPS_CANWAIT | PPS_TSFMT_TSPEC, | |
81 | .echo = pps_ktimer_echo, | |
82 | .owner = THIS_MODULE, | |
83 | }; | |
84 | ||
85 | and then calling the function pps_register_source() in your | |
86 | intialization routine as follows: | |
87 | ||
88 | source = pps_register_source(&pps_ktimer_info, | |
89 | PPS_CAPTUREASSERT | PPS_OFFSETASSERT); | |
90 | ||
91 | The pps_register_source() prototype is: | |
92 | ||
93 | int pps_register_source(struct pps_source_info_s *info, int default_params) | |
94 | ||
95 | where "info" is a pointer to a structure that describes a particular | |
96 | PPS source, "default_params" tells the system what the initial default | |
97 | parameters for the device should be (it is obvious that these parameters | |
98 | must be a subset of ones defined in the struct | |
99 | pps_source_info_s which describe the capabilities of the driver). | |
100 | ||
101 | Once you have registered a new PPS source into the system you can | |
102 | signal an assert event (for example in the interrupt handler routine) | |
103 | just using: | |
104 | ||
105 | pps_event(source, &ts, PPS_CAPTUREASSERT, ptr) | |
106 | ||
107 | where "ts" is the event's timestamp. | |
108 | ||
109 | The same function may also run the defined echo function | |
110 | (pps_ktimer_echo(), passing to it the "ptr" pointer) if the user | |
111 | asked for that... etc.. | |
112 | ||
113 | Please see the file drivers/pps/clients/ktimer.c for example code. | |
114 | ||
115 | ||
116 | SYSFS support | |
117 | ------------- | |
118 | ||
119 | If the SYSFS filesystem is enabled in the kernel it provides a new class: | |
120 | ||
121 | $ ls /sys/class/pps/ | |
122 | pps0/ pps1/ pps2/ | |
123 | ||
124 | Every directory is the ID of a PPS sources defined in the system and | |
125 | inside you find several files: | |
126 | ||
127 | $ ls /sys/class/pps/pps0/ | |
128 | assert clear echo mode name path subsystem@ uevent | |
129 | ||
130 | Inside each "assert" and "clear" file you can find the timestamp and a | |
131 | sequence number: | |
132 | ||
133 | $ cat /sys/class/pps/pps0/assert | |
134 | 1170026870.983207967#8 | |
135 | ||
136 | Where before the "#" is the timestamp in seconds; after it is the | |
137 | sequence number. Other files are: | |
138 | ||
139 | * echo: reports if the PPS source has an echo function or not; | |
140 | ||
141 | * mode: reports available PPS functioning modes; | |
142 | ||
143 | * name: reports the PPS source's name; | |
144 | ||
145 | * path: reports the PPS source's device path, that is the device the | |
146 | PPS source is connected to (if it exists). | |
147 | ||
148 | ||
149 | Testing the PPS support | |
150 | ----------------------- | |
151 | ||
152 | In order to test the PPS support even without specific hardware you can use | |
153 | the ktimer driver (see the client subsection in the PPS configuration menu) | |
154 | and the userland tools provided into Documentaion/pps/ directory. | |
155 | ||
156 | Once you have enabled the compilation of ktimer just modprobe it (if | |
157 | not statically compiled): | |
158 | ||
159 | # modprobe ktimer | |
160 | ||
161 | and the run ppstest as follow: | |
162 | ||
163 | $ ./ppstest /dev/pps0 | |
164 | trying PPS source "/dev/pps1" | |
165 | found PPS source "/dev/pps1" | |
166 | ok, found 1 source(s), now start fetching data... | |
167 | source 0 - assert 1186592699.388832443, sequence: 364 - clear 0.000000000, sequence: 0 | |
168 | source 0 - assert 1186592700.388931295, sequence: 365 - clear 0.000000000, sequence: 0 | |
169 | source 0 - assert 1186592701.389032765, sequence: 366 - clear 0.000000000, sequence: 0 | |
170 | ||
171 | Please, note that to compile userland programs you need the file timepps.h | |
172 | (see Documentation/pps/). | |
46b402a0 AG |
173 | |
174 | ||
175 | Generators | |
176 | ---------- | |
177 | ||
178 | Sometimes one needs to be able not only to catch PPS signals but to produce | |
179 | them also. For example, running a distributed simulation, which requires | |
180 | computers' clock to be synchronized very tightly. One way to do this is to | |
181 | invent some complicated hardware solutions but it may be neither necessary | |
182 | nor affordable. The cheap way is to load a PPS generator on one of the | |
183 | computers (master) and PPS clients on others (slaves), and use very simple | |
184 | cables to deliver signals using parallel ports, for example. | |
185 | ||
186 | Parallel port cable pinout: | |
187 | pin name master slave | |
188 | 1 STROBE *------ * | |
189 | 2 D0 * | * | |
190 | 3 D1 * | * | |
191 | 4 D2 * | * | |
192 | 5 D3 * | * | |
193 | 6 D4 * | * | |
194 | 7 D5 * | * | |
195 | 8 D6 * | * | |
196 | 9 D7 * | * | |
197 | 10 ACK * ------* | |
198 | 11 BUSY * * | |
199 | 12 PE * * | |
200 | 13 SEL * * | |
201 | 14 AUTOFD * * | |
202 | 15 ERROR * * | |
203 | 16 INIT * * | |
204 | 17 SELIN * * | |
205 | 18-25 GND *-----------* | |
206 | ||
207 | Please note that parallel port interrupt occurs only on high->low transition, | |
208 | so it is used for PPS assert edge. PPS clear edge can be determined only | |
209 | using polling in the interrupt handler which actually can be done way more | |
210 | precisely because interrupt handling delays can be quite big and random. So | |
211 | current parport PPS generator implementation (pps_gen_parport module) is | |
212 | geared towards using the clear edge for time synchronization. | |
213 | ||
214 | Clear edge polling is done with disabled interrupts so it's better to select | |
215 | delay between assert and clear edge as small as possible to reduce system | |
216 | latencies. But if it is too small slave won't be able to capture clear edge | |
217 | transition. The default of 30us should be good enough in most situations. | |
218 | The delay can be selected using 'delay' pps_gen_parport module parameter. |