diff options
Diffstat (limited to 'Documentation')
-rw-r--r-- | Documentation/pps/pps.txt | 46 |
1 files changed, 46 insertions, 0 deletions
diff --git a/Documentation/pps/pps.txt b/Documentation/pps/pps.txt index 125f4ab48998..d35dcdd82ff6 100644 --- a/Documentation/pps/pps.txt +++ b/Documentation/pps/pps.txt @@ -170,3 +170,49 @@ and the run ppstest as follow: Please, note that to compile userland programs you need the file timepps.h (see Documentation/pps/). + + +Generators +---------- + +Sometimes one needs to be able not only to catch PPS signals but to produce +them also. For example, running a distributed simulation, which requires +computers' clock to be synchronized very tightly. One way to do this is to +invent some complicated hardware solutions but it may be neither necessary +nor affordable. The cheap way is to load a PPS generator on one of the +computers (master) and PPS clients on others (slaves), and use very simple +cables to deliver signals using parallel ports, for example. + +Parallel port cable pinout: +pin name master slave +1 STROBE *------ * +2 D0 * | * +3 D1 * | * +4 D2 * | * +5 D3 * | * +6 D4 * | * +7 D5 * | * +8 D6 * | * +9 D7 * | * +10 ACK * ------* +11 BUSY * * +12 PE * * +13 SEL * * +14 AUTOFD * * +15 ERROR * * +16 INIT * * +17 SELIN * * +18-25 GND *-----------* + +Please note that parallel port interrupt occurs only on high->low transition, +so it is used for PPS assert edge. PPS clear edge can be determined only +using polling in the interrupt handler which actually can be done way more +precisely because interrupt handling delays can be quite big and random. So +current parport PPS generator implementation (pps_gen_parport module) is +geared towards using the clear edge for time synchronization. + +Clear edge polling is done with disabled interrupts so it's better to select +delay between assert and clear edge as small as possible to reduce system +latencies. But if it is too small slave won't be able to capture clear edge +transition. The default of 30us should be good enough in most situations. +The delay can be selected using 'delay' pps_gen_parport module parameter. |