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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 | |
a2d81803 | 51 | OK for the serial and parallel port, where you can do something |
eae9d2ba | 52 | useful besides(!) the gathering of timestamps as it is the central |
a2d81803 | 53 | task for a PPS API. But this assumption does not work for a single |
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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 | |
a2d81803 | 56 | precondition for the use of a PPS API. |
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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 | |
fe4c56c9 | 66 | into files /dev/pps0, /dev/pps1, etc. |
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67 | |
68 | ||
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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 | |
fe4c56c9 | 74 | the USB stack. Users have reported clock instability around +-1ms when |
f2c1a053 S |
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. | |
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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 | ||
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87 | Coding example |
88 | -------------- | |
89 | ||
90 | To register a PPS source into the kernel you should define a struct | |
a2d81803 | 91 | pps_source_info as follows: |
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92 | |
93 | static struct pps_source_info pps_ktimer_info = { | |
94 | .name = "ktimer", | |
95 | .path = "", | |
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96 | .mode = PPS_CAPTUREASSERT | PPS_OFFSETASSERT | |
97 | PPS_ECHOASSERT | | |
eae9d2ba RG |
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 | |
c0270efc | 104 | initialization routine as follows: |
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105 | |
106 | source = pps_register_source(&pps_ktimer_info, | |
107 | PPS_CAPTUREASSERT | PPS_OFFSETASSERT); | |
108 | ||
109 | The pps_register_source() prototype is: | |
110 | ||
a2d81803 | 111 | int pps_register_source(struct pps_source_info *info, int default_params) |
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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 | |
a2d81803 | 117 | pps_source_info which describe the capabilities of the driver). |
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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 | ||
5d250eeb | 131 | Please see the file drivers/pps/clients/pps-ktimer.c for example code. |
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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 | ||
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145 | $ ls -F /sys/class/pps/pps0/ |
146 | assert dev mode path subsystem@ | |
147 | clear echo name power/ uevent | |
148 | ||
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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 | ||
a2d81803 | 159 | * echo: reports if the PPS source has an echo function or not; |
eae9d2ba | 160 | |
a2d81803 | 161 | * mode: reports available PPS functioning modes; |
eae9d2ba | 162 | |
a2d81803 | 163 | * name: reports the PPS source's name; |
eae9d2ba | 164 | |
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165 | * path: reports the PPS source's device path, that is the device the |
166 | PPS source is connected to (if it exists). | |
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167 | |
168 | ||
169 | Testing the PPS support | |
170 | ----------------------- | |
171 | ||
172 | In order to test the PPS support even without specific hardware you can use | |
a2d81803 | 173 | the pps-ktimer driver (see the client subsection in the PPS configuration menu) |
e1235e18 | 174 | and the userland tools available in your distribution's pps-tools package, |
a2d81803 | 175 | http://linuxpps.org , or https://github.com/redlab-i/pps-tools. |
eae9d2ba | 176 | |
a2d81803 | 177 | Once you have enabled the compilation of pps-ktimer just modprobe it (if |
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178 | not statically compiled): |
179 | ||
a2d81803 | 180 | # modprobe pps-ktimer |
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181 | |
182 | and the run ppstest as follow: | |
183 | ||
a2d81803 | 184 | $ ./ppstest /dev/pps1 |
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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 | ||
a2d81803 | 192 | Please note that to compile userland programs, you need the file timepps.h. |
e1235e18 | 193 | This is available in the pps-tools repository mentioned above. |
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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. |