Merge tag 'v2.6.38' of git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux-2.6

[mv-sheeva.git] / Documentation / pps / pps.txt

diff --git a/Documentation/pps/pps.txt b/Documentation/pps/pps.txt
index 125f4ab4899838d357e4cfb73d27b58397afcd3d..d35dcdd82ff6a6c93b2dae00be717080b9b21cb1 100644 (file)
--- 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.