1.. SPDX-License-Identifier: GPL-2.0 2 3====================== 4PPS - Pulse Per Second 5====================== 6 7Copyright (C) 2007 Rodolfo Giometti <giometti@enneenne.com> 8 9This program is free software; you can redistribute it and/or modify 10it under the terms of the GNU General Public License as published by 11the Free Software Foundation; either version 2 of the License, or 12(at your option) any later version. 13 14This program is distributed in the hope that it will be useful, 15but WITHOUT ANY WARRANTY; without even the implied warranty of 16MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the 17GNU General Public License for more details. 18 19 20 21Overview 22-------- 23 24LinuxPPS provides a programming interface (API) to define in the 25system several PPS sources. 26 27PPS means "pulse per second" and a PPS source is just a device which 28provides a high precision signal each second so that an application 29can use it to adjust system clock time. 30 31A PPS source can be connected to a serial port (usually to the Data 32Carrier Detect pin) or to a parallel port (ACK-pin) or to a special 33CPU's GPIOs (this is the common case in embedded systems) but in each 34case when a new pulse arrives the system must apply to it a timestamp 35and record it for userland. 36 37Common use is the combination of the NTPD as userland program, with a 38GPS receiver as PPS source, to obtain a wallclock-time with 39sub-millisecond synchronisation to UTC. 40 41 42RFC considerations 43------------------ 44 45While implementing a PPS API as RFC 2783 defines and using an embedded 46CPU GPIO-Pin as physical link to the signal, I encountered a deeper 47problem: 48 49 At startup it needs a file descriptor as argument for the function 50 time_pps_create(). 51 52This implies that the source has a /dev/... entry. This assumption is 53OK for the serial and parallel port, where you can do something 54useful besides(!) the gathering of timestamps as it is the central 55task for a PPS API. But this assumption does not work for a single 56purpose GPIO line. In this case even basic file-related functionality 57(like read() and write()) makes no sense at all and should not be a 58precondition for the use of a PPS API. 59 60The problem can be simply solved if you consider that a PPS source is 61not always connected with a GPS data source. 62 63So your programs should check if the GPS data source (the serial port 64for instance) is a PPS source too, and if not they should provide the 65possibility to open another device as PPS source. 66 67In LinuxPPS the PPS sources are simply char devices usually mapped 68into files /dev/pps0, /dev/pps1, etc. 69 70 71PPS with USB to serial devices 72------------------------------ 73 74It is possible to grab the PPS from an USB to serial device. However, 75you should take into account the latencies and jitter introduced by 76the USB stack. Users have reported clock instability around +-1ms when 77synchronized with PPS through USB. With USB 2.0, jitter may decrease 78down to the order of 125 microseconds. 79 80This may be suitable for time server synchronization with NTP because 81of its undersampling and algorithms. 82 83If your device doesn't report PPS, you can check that the feature is 84supported by its driver. Most of the time, you only need to add a call 85to usb_serial_handle_dcd_change after checking the DCD status (see 86ch341 and pl2303 examples). 87 88 89Coding example 90-------------- 91 92To register a PPS source into the kernel you should define a struct 93pps_source_info as follows:: 94 95 static struct pps_source_info pps_ktimer_info = { 96 .name = "ktimer", 97 .path = "", 98 .mode = PPS_CAPTUREASSERT | PPS_OFFSETASSERT | 99 PPS_ECHOASSERT | 100 PPS_CANWAIT | PPS_TSFMT_TSPEC, 101 .echo = pps_ktimer_echo, 102 .owner = THIS_MODULE, 103 }; 104 105and then calling the function pps_register_source() in your 106initialization routine as follows:: 107 108 source = pps_register_source(&pps_ktimer_info, 109 PPS_CAPTUREASSERT | PPS_OFFSETASSERT); 110 111The pps_register_source() prototype is:: 112 113 int pps_register_source(struct pps_source_info *info, int default_params) 114 115where "info" is a pointer to a structure that describes a particular 116PPS source, "default_params" tells the system what the initial default 117parameters for the device should be (it is obvious that these parameters 118must be a subset of ones defined in the struct 119pps_source_info which describe the capabilities of the driver). 120 121Once you have registered a new PPS source into the system you can 122signal an assert event (for example in the interrupt handler routine) 123just using:: 124 125 pps_event(source, &ts, PPS_CAPTUREASSERT, ptr) 126 127where "ts" is the event's timestamp. 128 129The same function may also run the defined echo function 130(pps_ktimer_echo(), passing to it the "ptr" pointer) if the user 131asked for that... etc.. 132 133Please see the file drivers/pps/clients/pps-ktimer.c for example code. 134 135 136SYSFS support 137------------- 138 139If the SYSFS filesystem is enabled in the kernel it provides a new class:: 140 141 $ ls /sys/class/pps/ 142 pps0/ pps1/ pps2/ 143 144Every directory is the ID of a PPS sources defined in the system and 145inside you find several files:: 146 147 $ ls -F /sys/class/pps/pps0/ 148 assert dev mode path subsystem@ 149 clear echo name power/ uevent 150 151 152Inside each "assert" and "clear" file you can find the timestamp and a 153sequence number:: 154 155 $ cat /sys/class/pps/pps0/assert 156 1170026870.983207967#8 157 158Where before the "#" is the timestamp in seconds; after it is the 159sequence number. Other files are: 160 161 * echo: reports if the PPS source has an echo function or not; 162 163 * mode: reports available PPS functioning modes; 164 165 * name: reports the PPS source's name; 166 167 * path: reports the PPS source's device path, that is the device the 168 PPS source is connected to (if it exists). 169 170 171Testing the PPS support 172----------------------- 173 174In order to test the PPS support even without specific hardware you can use 175the pps-ktimer driver (see the client subsection in the PPS configuration menu) 176and the userland tools available in your distribution's pps-tools package, 177http://linuxpps.org , or https://github.com/redlab-i/pps-tools. 178 179Once you have enabled the compilation of pps-ktimer just modprobe it (if 180not statically compiled):: 181 182 # modprobe pps-ktimer 183 184and the run ppstest as follow:: 185 186 $ ./ppstest /dev/pps1 187 trying PPS source "/dev/pps1" 188 found PPS source "/dev/pps1" 189 ok, found 1 source(s), now start fetching data... 190 source 0 - assert 1186592699.388832443, sequence: 364 - clear 0.000000000, sequence: 0 191 source 0 - assert 1186592700.388931295, sequence: 365 - clear 0.000000000, sequence: 0 192 source 0 - assert 1186592701.389032765, sequence: 366 - clear 0.000000000, sequence: 0 193 194Please note that to compile userland programs, you need the file timepps.h. 195This is available in the pps-tools repository mentioned above. 196 197 198Generators 199---------- 200 201Sometimes one needs to be able not only to catch PPS signals but to produce 202them also. For example, running a distributed simulation, which requires 203computers' clock to be synchronized very tightly. 204 205To do so the class pps-gen has been added. PPS generators can be 206registered in the kernel by defining a struct pps_gen_source_info as 207follows:: 208 209 static const struct pps_gen_source_info pps_gen_dummy_info = { 210 .use_system_clock = true, 211 .get_time = pps_gen_dummy_get_time, 212 .enable = pps_gen_dummy_enable, 213 }; 214 215Where the use_system_clock states if the generator uses the system 216clock to generate its pulses, or they are from a peripheral device 217clock. Method get_time() is used to query the time stored into the 218generator clock, while the method enable() is used to enable or 219disable the PPS pulse generation. 220 221Then calling the function pps_gen_register_source() in your 222initialization routine as follows creates a new generator in the 223system:: 224 225 pps_gen = pps_gen_register_source(&pps_gen_dummy_info); 226 227Generators SYSFS support 228------------------------ 229 230If the SYSFS filesystem is enabled in the kernel it provides a new class:: 231 232 $ ls /sys/class/pps-gen/ 233 pps-gen0/ pps-gen1/ pps-gen2/ 234 235Every directory is the ID of a PPS generator defined in the system and 236inside of it you find several files:: 237 238 $ ls -F /sys/class/pps-gen/pps-gen0/ 239 dev enable name power/ subsystem@ system time uevent 240 241To enable the PPS signal generation you can use the command below:: 242 243 $ echo 1 > /sys/class/pps-gen/pps-gen0/enable 244 245Parallel port generator 246------------------------ 247 248One way to do this is to invent some complicated hardware solutions but it 249may be neither necessary nor affordable. The cheap way is to load a PPS 250generator on one of the computers (master) and PPS clients on others 251(slaves), and use very simple cables to deliver signals using parallel 252ports, for example. 253 254Parallel port cable pinout:: 255 256 pin name master slave 257 1 STROBE *------ * 258 2 D0 * | * 259 3 D1 * | * 260 4 D2 * | * 261 5 D3 * | * 262 6 D4 * | * 263 7 D5 * | * 264 8 D6 * | * 265 9 D7 * | * 266 10 ACK * ------* 267 11 BUSY * * 268 12 PE * * 269 13 SEL * * 270 14 AUTOFD * * 271 15 ERROR * * 272 16 INIT * * 273 17 SELIN * * 274 18-25 GND *-----------* 275 276Please note that parallel port interrupt occurs only on high->low transition, 277so it is used for PPS assert edge. PPS clear edge can be determined only 278using polling in the interrupt handler which actually can be done way more 279precisely because interrupt handling delays can be quite big and random. So 280current parport PPS generator implementation (pps_gen_parport module) is 281geared towards using the clear edge for time synchronization. 282 283Clear edge polling is done with disabled interrupts so it's better to select 284delay between assert and clear edge as small as possible to reduce system 285latencies. But if it is too small slave won't be able to capture clear edge 286transition. The default of 30us should be good enough in most situations. 287The delay can be selected using 'delay' pps_gen_parport module parameter. 288 289 290Intel Timed I/O PPS signal generator 291------------------------------------ 292 293Intel Timed I/O is a high precision device, present on 2019 and newer Intel 294CPUs, that can generate PPS signals. 295 296Timed I/O and system time are both driven by same hardware clock. The signal 297is generated with a precision of ~20 nanoseconds. The generated PPS signal 298is used to synchronize an external device with system clock. For example, 299it can be used to share your clock with a device that receives PPS signal, 300generated by Timed I/O device. There are dedicated Timed I/O pins to deliver 301the PPS signal to an external device. 302 303Usage of Intel Timed I/O as PPS generator: 304 305Start generating PPS signal:: 306 307 $echo 1 > /sys/class/pps-gen/pps-genx/enable 308 309Stop generating PPS signal:: 310 311 $echo 0 > /sys/class/pps-gen/pps-genx/enable 312