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54 lines
2.7 KiB
54 lines
2.7 KiB
8 years ago
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Serial Peripheral Interface (SPI)
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=================================
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SPI is the "Serial Peripheral Interface", widely used with embedded
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systems because it is a simple and efficient interface: basically a
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multiplexed shift register. Its three signal wires hold a clock (SCK,
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often in the range of 1-20 MHz), a "Master Out, Slave In" (MOSI) data
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line, and a "Master In, Slave Out" (MISO) data line. SPI is a full
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duplex protocol; for each bit shifted out the MOSI line (one per clock)
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another is shifted in on the MISO line. Those bits are assembled into
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words of various sizes on the way to and from system memory. An
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additional chipselect line is usually active-low (nCS); four signals are
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normally used for each peripheral, plus sometimes an interrupt.
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The SPI bus facilities listed here provide a generalized interface to
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declare SPI busses and devices, manage them according to the standard
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Linux driver model, and perform input/output operations. At this time,
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only "master" side interfaces are supported, where Linux talks to SPI
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peripherals and does not implement such a peripheral itself. (Interfaces
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to support implementing SPI slaves would necessarily look different.)
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The programming interface is structured around two kinds of driver, and
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two kinds of device. A "Controller Driver" abstracts the controller
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hardware, which may be as simple as a set of GPIO pins or as complex as
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a pair of FIFOs connected to dual DMA engines on the other side of the
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SPI shift register (maximizing throughput). Such drivers bridge between
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whatever bus they sit on (often the platform bus) and SPI, and expose
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the SPI side of their device as a :c:type:`struct spi_master
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<spi_master>`. SPI devices are children of that master,
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represented as a :c:type:`struct spi_device <spi_device>` and
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manufactured from :c:type:`struct spi_board_info
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<spi_board_info>` descriptors which are usually provided by
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board-specific initialization code. A :c:type:`struct spi_driver
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<spi_driver>` is called a "Protocol Driver", and is bound to a
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spi_device using normal driver model calls.
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The I/O model is a set of queued messages. Protocol drivers submit one
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or more :c:type:`struct spi_message <spi_message>` objects,
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which are processed and completed asynchronously. (There are synchronous
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wrappers, however.) Messages are built from one or more
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:c:type:`struct spi_transfer <spi_transfer>` objects, each of
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which wraps a full duplex SPI transfer. A variety of protocol tweaking
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options are needed, because different chips adopt very different
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policies for how they use the bits transferred with SPI.
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.. kernel-doc:: include/linux/spi/spi.h
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:internal:
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.. kernel-doc:: drivers/spi/spi.c
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:functions: spi_register_board_info
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.. kernel-doc:: drivers/spi/spi.c
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:export:
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