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.