path: root/Documentation/pps
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authorAlexander Gordeev <lasaine@lvk.cs.msu.su>2011-01-12 17:00:59 -0800
committerLinus Torvalds <torvalds@linux-foundation.org>2011-01-13 08:03:21 -0800
commit46b402a0e5e4b4d81b11c32dfb2312bf5828ecb5 (patch)
treef04bdc9a375a0f6c7447de58800dd2cd95be9ca5 /Documentation/pps
parenta10203c691eac287664f531b149ddc23056c2f61 (diff)
pps: add parallel port PPS signal generator
Add PPS signal generator which utilizes STROBE pin of a parallel port to send PPS signals. It uses parport abstraction layer and hrtimers to precisely control the signal. [akpm@linux-foundation.org: fix build] Signed-off-by: Alexander Gordeev <lasaine@lvk.cs.msu.su> Acked-by: Rodolfo Giometti <giometti@linux.it> Cc: john stultz <johnstul@us.ibm.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'Documentation/pps')
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/).
+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.

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