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 69PPS with USB to serial devices 70------------------------------ 71 72It is possible to grab the PPS from an USB to serial device. However, 73you should take into account the latencies and jitter introduced by 74the USB stack. Users has reported clock instability around +-1ms when 75synchronized with PPS through USB. This isn't suited for time server 76synchronization. 77 78If your device doesn't report PPS, you can check that the feature is 79supported by its driver. Most of the time, you only need to add a call 80to usb_serial_handle_dcd_change after checking the DCD status (see 81ch341 and pl2303 examples). 82 83 84Coding example 85-------------- 86 87To register a PPS source into the kernel you should define a struct 88pps_source_info_s as follows: 89 90 static struct pps_source_info pps_ktimer_info = { 91 .name = "ktimer", 92 .path = "", 93 .mode = PPS_CAPTUREASSERT | PPS_OFFSETASSERT | \ 94 PPS_ECHOASSERT | \ 95 PPS_CANWAIT | PPS_TSFMT_TSPEC, 96 .echo = pps_ktimer_echo, 97 .owner = THIS_MODULE, 98 }; 99 100and then calling the function pps_register_source() in your 101intialization routine as follows: 102 103 source = pps_register_source(&pps_ktimer_info, 104 PPS_CAPTUREASSERT | PPS_OFFSETASSERT); 105 106The pps_register_source() prototype is: 107 108 int pps_register_source(struct pps_source_info_s *info, int default_params) 109 110where "info" is a pointer to a structure that describes a particular 111PPS source, "default_params" tells the system what the initial default 112parameters for the device should be (it is obvious that these parameters 113must be a subset of ones defined in the struct 114pps_source_info_s which describe the capabilities of the driver). 115 116Once you have registered a new PPS source into the system you can 117signal an assert event (for example in the interrupt handler routine) 118just using: 119 120 pps_event(source, &ts, PPS_CAPTUREASSERT, ptr) 121 122where "ts" is the event's timestamp. 123 124The same function may also run the defined echo function 125(pps_ktimer_echo(), passing to it the "ptr" pointer) if the user 126asked for that... etc.. 127 128Please see the file drivers/pps/clients/pps-ktimer.c for example code. 129 130 131SYSFS support 132------------- 133 134If the SYSFS filesystem is enabled in the kernel it provides a new class: 135 136 $ ls /sys/class/pps/ 137 pps0/ pps1/ pps2/ 138 139Every directory is the ID of a PPS sources defined in the system and 140inside you find several files: 141 142 $ ls /sys/class/pps/pps0/ 143 assert clear echo mode name path subsystem@ uevent 144 145Inside each "assert" and "clear" file you can find the timestamp and a 146sequence number: 147 148 $ cat /sys/class/pps/pps0/assert 149 1170026870.983207967#8 150 151Where before the "#" is the timestamp in seconds; after it is the 152sequence number. Other files are: 153 154* echo: reports if the PPS source has an echo function or not; 155 156* mode: reports available PPS functioning modes; 157 158* name: reports the PPS source's name; 159 160* path: reports the PPS source's device path, that is the device the 161 PPS source is connected to (if it exists). 162 163 164Testing the PPS support 165----------------------- 166 167In order to test the PPS support even without specific hardware you can use 168the ktimer driver (see the client subsection in the PPS configuration menu) 169and the userland tools provided in the Documentation/pps/ directory. 170 171Once you have enabled the compilation of ktimer just modprobe it (if 172not statically compiled): 173 174 # modprobe ktimer 175 176and the run ppstest as follow: 177 178 $ ./ppstest /dev/pps0 179 trying PPS source "/dev/pps1" 180 found PPS source "/dev/pps1" 181 ok, found 1 source(s), now start fetching data... 182 source 0 - assert 1186592699.388832443, sequence: 364 - clear 0.000000000, sequence: 0 183 source 0 - assert 1186592700.388931295, sequence: 365 - clear 0.000000000, sequence: 0 184 source 0 - assert 1186592701.389032765, sequence: 366 - clear 0.000000000, sequence: 0 185 186Please, note that to compile userland programs you need the file timepps.h 187(see Documentation/pps/). 188 189 190Generators 191---------- 192 193Sometimes one needs to be able not only to catch PPS signals but to produce 194them also. For example, running a distributed simulation, which requires 195computers' clock to be synchronized very tightly. One way to do this is to 196invent some complicated hardware solutions but it may be neither necessary 197nor affordable. The cheap way is to load a PPS generator on one of the 198computers (master) and PPS clients on others (slaves), and use very simple 199cables to deliver signals using parallel ports, for example. 200 201Parallel port cable pinout: 202pin name master slave 2031 STROBE *------ * 2042 D0 * | * 2053 D1 * | * 2064 D2 * | * 2075 D3 * | * 2086 D4 * | * 2097 D5 * | * 2108 D6 * | * 2119 D7 * | * 21210 ACK * ------* 21311 BUSY * * 21412 PE * * 21513 SEL * * 21614 AUTOFD * * 21715 ERROR * * 21816 INIT * * 21917 SELIN * * 22018-25 GND *-----------* 221 222Please note that parallel port interrupt occurs only on high->low transition, 223so it is used for PPS assert edge. PPS clear edge can be determined only 224using polling in the interrupt handler which actually can be done way more 225precisely because interrupt handling delays can be quite big and random. So 226current parport PPS generator implementation (pps_gen_parport module) is 227geared towards using the clear edge for time synchronization. 228 229Clear edge polling is done with disabled interrupts so it's better to select 230delay between assert and clear edge as small as possible to reduce system 231latencies. But if it is too small slave won't be able to capture clear edge 232transition. The default of 30us should be good enough in most situations. 233The delay can be selected using 'delay' pps_gen_parport module parameter. 234