BeagleBoard/SPI

Background
Serial Perhiperal Interface (also known under the names of Microwire or four-wire) is a general-purpose digital I/O interface used by many ICs including sensors, converters, audio codecs, and various types of memory. An SPI bus consists of at least three pins: a clock, a slave-input/master-output (SIMO) pin, a slave-output/master-input (SOMI) pin, and zero or more chip-select(CS) pins.

The OMAP3 has four McSPI controllers, each capable of driving an SPI interface at up to 48 MHz. Each of these controllers has a finite number of chip select lines. Of these four, only SPI3 and SPI4 are brought out on the BeagleBoard.
 * McSPI1: 4 channels
 * McSPI2: 2 channels
 * McSPI3: 3 channels (2 CS brought out)
 * McSPI4: 1 channel (1 CS brought out)

Below is a description of the steps necessary to use SPI on BeagleBoard. If you are particularly impatient, you can start with one of the patches found in the Kernel Patches section.

Configuring Pinmux
These devices are fully supported by the Linux kernel's mcspi driver. That being said, one will need to make some minor modifications to the BeagleBoard's board file (arch/arm/mach-omap2/board-omap3beagle.c) in order to use them.

First, the pin multiplexer needs to be configured to expose the McSPI signals. As of BeagleBoard rev.C4, the following SPI signals are available on the Beagle's 28-pin expansion header; the SPI signals are available in pinmux mode 1, while mode 0 names are also given for reference.

(*)  Important:  The CLK pins must be put in input mode for the chip to correctly register input on SOMI. Failure to set CLK as an input will result in all reads producing zeros.

Configuring spi_board_info
In addition to the pin multiplexer, one must also tell the kernel a bit about the SPI controller itself. This is done in the board file using an spi_board_info struct. For example, to configure McSPI3.0, one might use the following, static struct spi_board_info beagle_mcspi_board_info = { .modalias	= "spidev", .max_speed_hz	= 48000000, //48 Mbps .bus_num	= 3, .chip_select	= 0, .mode = SPI_MODE_1, };

One would then register this with the SPI subsystem during board initialization (i.e. omap3_beagle_init), with the following, spi_register_board_info(beagle_mcspi_board_info,	ARRAY_SIZE(beagle_mcspi_board_info));

Patches
This patch was developed against 2.6.32 and exposes through spidev all of the McSPI controllers and chip selects brought out on the BeagleBoard.

Discussion
Here are some mailing list threads of various peoples' experiences getting SPI running on the Beagle:


 * |MCSPI3 working, both ways
 * |MCSPI TX works fine, but RX all zeros
 * |SPI testing
 * |SPI Troubles
 * |SPI with Beagleboard (for a newbie in Linux)
 * |McSPI with REV C