1 2 PPS - Pulse Per Second 3 ---------------------- 4 5(C) Copyright 2007 Rodolfo Giometti <giometti@enneenne.com> 6 7This program is free software; you can redistribute it and/or modify 8it under the terms of the GNU General Public License as published by 9the Free Software Foundation; either version 2 of the License, or 10(at your option) any later version. 11 12This program is distributed in the hope that it will be useful, 13but WITHOUT ANY WARRANTY; without even the implied warranty of 14MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the 15GNU General Public License for more details. 16 17 18 19Overview 20-------- 21 22LinuxPPS provides a programming interface (API) to define in the 23system several PPS sources. 24 25PPS means "pulse per second" and a PPS source is just a device which 26provides a high precision signal each second so that an application 27can use it to adjust system clock time. 28 29A PPS source can be connected to a serial port (usually to the Data 30Carrier Detect pin) or to a parallel port (ACK-pin) or to a special 31CPU's GPIOs (this is the common case in embedded systems) but in each 32case when a new pulse arrives the system must apply to it a timestamp 33and record it for userland. 34 35Common use is the combination of the NTPD as userland program, with a 36GPS receiver as PPS source, to obtain a wallclock-time with 37sub-millisecond synchronisation to UTC. 38 39 40RFC considerations 41------------------ 42 43While implementing a PPS API as RFC 2783 defines and using an embedded 44CPU GPIO-Pin as physical link to the signal, I encountered a deeper 45problem: 46 47 At startup it needs a file descriptor as argument for the function 48 time_pps_create(). 49 50This implies that the source has a /dev/... entry. This assumption is 51ok for the serial and parallel port, where you can do something 52useful besides(!) the gathering of timestamps as it is the central 53task for a PPS-API. But this assumption does not work for a single 54purpose 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 56precondition for the use of a PPS-API. 57 58The problem can be simply solved if you consider that a PPS source is 59not always connected with a GPS data source. 60 61So your programs should check if the GPS data source (the serial port 62for instance) is a PPS source too, and if not they should provide the 63possibility to open another device as PPS source. 64 65In LinuxPPS the PPS sources are simply char devices usually mapped 66into files /dev/pps0, /dev/pps1, etc.. 67 68 69Coding example 70-------------- 71 72To register a PPS source into the kernel you should define a struct 73pps_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 85and then calling the function pps_register_source() in your 86intialization routine as follows: 87 88 source = pps_register_source(&pps_ktimer_info, 89 PPS_CAPTUREASSERT | PPS_OFFSETASSERT); 90 91The pps_register_source() prototype is: 92 93 int pps_register_source(struct pps_source_info_s *info, int default_params) 94 95where "info" is a pointer to a structure that describes a particular 96PPS source, "default_params" tells the system what the initial default 97parameters for the device should be (it is obvious that these parameters 98must be a subset of ones defined in the struct 99pps_source_info_s which describe the capabilities of the driver). 100 101Once you have registered a new PPS source into the system you can 102signal an assert event (for example in the interrupt handler routine) 103just using: 104 105 pps_event(source, &ts, PPS_CAPTUREASSERT, ptr) 106 107where "ts" is the event's timestamp. 108 109The same function may also run the defined echo function 110(pps_ktimer_echo(), passing to it the "ptr" pointer) if the user 111asked for that... etc.. 112 113Please see the file drivers/pps/clients/ktimer.c for example code. 114 115 116SYSFS support 117------------- 118 119If the SYSFS filesystem is enabled in the kernel it provides a new class: 120 121 $ ls /sys/class/pps/ 122 pps0/ pps1/ pps2/ 123 124Every directory is the ID of a PPS sources defined in the system and 125inside you find several files: 126 127 $ ls /sys/class/pps/pps0/ 128 assert clear echo mode name path subsystem@ uevent 129 130Inside each "assert" and "clear" file you can find the timestamp and a 131sequence number: 132 133 $ cat /sys/class/pps/pps0/assert 134 1170026870.983207967#8 135 136Where before the "#" is the timestamp in seconds; after it is the 137sequence 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 149Testing the PPS support 150----------------------- 151 152In order to test the PPS support even without specific hardware you can use 153the ktimer driver (see the client subsection in the PPS configuration menu) 154and the userland tools provided into Documentaion/pps/ directory. 155 156Once you have enabled the compilation of ktimer just modprobe it (if 157not statically compiled): 158 159 # modprobe ktimer 160 161and 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 171Please, note that to compile userland programs you need the file timepps.h 172(see Documentation/pps/). 173 174 175Generators 176---------- 177 178Sometimes one needs to be able not only to catch PPS signals but to produce 179them also. For example, running a distributed simulation, which requires 180computers' clock to be synchronized very tightly. One way to do this is to 181invent some complicated hardware solutions but it may be neither necessary 182nor affordable. The cheap way is to load a PPS generator on one of the 183computers (master) and PPS clients on others (slaves), and use very simple 184cables to deliver signals using parallel ports, for example. 185 186Parallel port cable pinout: 187pin name master slave 1881 STROBE *------ * 1892 D0 * | * 1903 D1 * | * 1914 D2 * | * 1925 D3 * | * 1936 D4 * | * 1947 D5 * | * 1958 D6 * | * 1969 D7 * | * 19710 ACK * ------* 19811 BUSY * * 19912 PE * * 20013 SEL * * 20114 AUTOFD * * 20215 ERROR * * 20316 INIT * * 20417 SELIN * * 20518-25 GND *-----------* 206 207Please note that parallel port interrupt occurs only on high->low transition, 208so it is used for PPS assert edge. PPS clear edge can be determined only 209using polling in the interrupt handler which actually can be done way more 210precisely because interrupt handling delays can be quite big and random. So 211current parport PPS generator implementation (pps_gen_parport module) is 212geared towards using the clear edge for time synchronization. 213 214Clear edge polling is done with disabled interrupts so it's better to select 215delay between assert and clear edge as small as possible to reduce system 216latencies. But if it is too small slave won't be able to capture clear edge 217transition. The default of 30us should be good enough in most situations. 218The delay can be selected using 'delay' pps_gen_parport module parameter. 219