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authorMichael Hennerich <michael.hennerich@analog.com>2010-07-06 13:57:12 +0000
committerMike Frysinger <vapier@gentoo.org>2010-08-06 12:55:52 -0400
commit3322c7bbf60801da2a22d7bd88865d2dc3bd73ac (patch)
tree21d2189f14a22c1a5ea2ba0f74b580314949936f /Documentation/blackfin
parent58b0e22ab68d055a9563835daef10fbc9fe8f5ae (diff)
Blackfin: document SPI CS limitations with CPHA=0
With the Blackfin on-chip SPI peripheral, there is some logic tied to the CPHA bit whether the Slave Select Line is controlled by hardware (CPHA=0) or controlled by software (CPHA=1). However, the Linux SPI bus driver assumes that the Slave Select being asserted during the entire SPI transfer. So explain these small details for people who need certain SPI modes with standard CS behavior. Signed-off-by: Michael Hennerich <michael.hennerich@analog.com> Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Diffstat (limited to 'Documentation/blackfin')
-rw-r--r--Documentation/blackfin/00-INDEX5
-rw-r--r--Documentation/blackfin/bfin-spi-notes.txt14
2 files changed, 18 insertions, 1 deletions
diff --git a/Documentation/blackfin/00-INDEX b/Documentation/blackfin/00-INDEX
index 10391d3e9b56..2df0365f2dff 100644
--- a/Documentation/blackfin/00-INDEX
+++ b/Documentation/blackfin/00-INDEX
@@ -1,5 +1,8 @@
00-INDEX
- This file
-bfin-gpio-note.txt
+bfin-gpio-notes.txt
- Notes in developing/using bfin-gpio driver.
+
+bfin-spi-notes.txt
+ - Notes for using bfin spi bus driver.
diff --git a/Documentation/blackfin/bfin-spi-notes.txt b/Documentation/blackfin/bfin-spi-notes.txt
new file mode 100644
index 000000000000..556fa877f2e8
--- /dev/null
+++ b/Documentation/blackfin/bfin-spi-notes.txt
@@ -0,0 +1,14 @@
+SPI Chip Select behavior:
+
+With the Blackfin on-chip SPI peripheral, there is some logic tied to the CPHA
+bit whether the Slave Select Line is controlled by hardware (CPHA=0) or
+controlled by software (CPHA=1). However, the Linux SPI bus driver assumes that
+the Slave Select is always under software control and being asserted during
+the entire SPI transfer. - And not just bits_per_word duration.
+
+In most cases you can utilize SPI MODE_3 instead of MODE_0 to work-around this
+behavior. If your SPI slave device in question requires SPI MODE_0 or MODE_2
+timing, you can utilize the GPIO controlled SPI Slave Select option instead.
+
+You can even use the same pin whose peripheral role is a SSEL,
+but use it as a GPIO instead.

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