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authorDavid Brownell <david-b@pacbell.net>2008-04-09 00:41:58 (GMT)
committerLinus Torvalds <torvalds@linux-foundation.org>2008-04-09 01:25:53 (GMT)
commit6395bee7e92bf34e95dc67c1da5acc30e8b98244 (patch)
tree98a5f30911f1b28f1b9c921b9112f199fb044c43 /Documentation/spi
parentf9e522caece074b9a985436d611127e8e96ad446 (diff)
spi: documentation tweaks
Update SPI documentation to clarify some areas of recent confusion: clock polarity takes effect when chipselect goes active; and zero length buffers are OK in certain cases. Signed-off-by: David Brownell <dbrownell@users.sourceforge.net> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'Documentation/spi')
-rw-r--r--Documentation/spi/spi-summary15
1 files changed, 14 insertions, 1 deletions
diff --git a/Documentation/spi/spi-summary b/Documentation/spi/spi-summary
index 8861e47..6d5f1814 100644
--- a/Documentation/spi/spi-summary
+++ b/Documentation/spi/spi-summary
@@ -116,6 +116,13 @@ low order bit. So when a chip's timing diagram shows the clock
starting low (CPOL=0) and data stabilized for sampling during the
trailing clock edge (CPHA=1), that's SPI mode 1.
+Note that the clock mode is relevant as soon as the chipselect goes
+active. So the master must set the clock to inactive before selecting
+a slave, and the slave can tell the chosen polarity by sampling the
+clock level when its select line goes active. That's why many devices
+support for example both modes 0 and 3: they don't care about polarity,
+and alway clock data in/out on rising clock edges.
+
How do these driver programming interfaces work?
------------------------------------------------
@@ -379,8 +386,14 @@ any more such messages.
+ when bidirectional reads and writes start ... by how its
sequence of spi_transfer requests is arranged;
+ + which I/O buffers are used ... each spi_transfer wraps a
+ buffer for each transfer direction, supporting full duplex
+ (two pointers, maybe the same one in both cases) and half
+ duplex (one pointer is NULL) transfers;
+
+ optionally defining short delays after transfers ... using
- the spi_transfer.delay_usecs setting;
+ the spi_transfer.delay_usecs setting (this delay can be the
+ only protocol effect, if the buffer length is zero);
+ whether the chipselect becomes inactive after a transfer and
any delay ... by using the spi_transfer.cs_change flag;

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