1 Serial Peripheral Interface (SPI) 2 ================================= 3 4 SPI is the "Serial Peripheral Interface", widely used with embedded 5 systems because it is a simple and efficient interface: basically a 6 multiplexed shift register. Its three signal wires hold a clock (SCK, 7 often in the range of 1-20 MHz), a "Master Out, Slave In" (MOSI) data 8 line, and a "Master In, Slave Out" (MISO) data line. SPI is a full 9 duplex protocol; for each bit shifted out the MOSI line (one per clock) 10 another is shifted in on the MISO line. Those bits are assembled into 11 words of various sizes on the way to and from system memory. An 12 additional chipselect line is usually active-low (nCS); four signals are 13 normally used for each peripheral, plus sometimes an interrupt. 14 15 The SPI bus facilities listed here provide a generalized interface to 16 declare SPI busses and devices, manage them according to the standard 17 Linux driver model, and perform input/output operations. At this time, 18 only "master" side interfaces are supported, where Linux talks to SPI 19 peripherals and does not implement such a peripheral itself. (Interfaces 20 to support implementing SPI slaves would necessarily look different.) 21 22 The programming interface is structured around two kinds of driver, and 23 two kinds of device. A "Controller Driver" abstracts the controller 24 hardware, which may be as simple as a set of GPIO pins or as complex as 25 a pair of FIFOs connected to dual DMA engines on the other side of the 26 SPI shift register (maximizing throughput). Such drivers bridge between 27 whatever bus they sit on (often the platform bus) and SPI, and expose 28 the SPI side of their device as a :c:type:`struct spi_controller 29 <spi_controller>`. SPI devices are children of that master, 30 represented as a :c:type:`struct spi_device <spi_device>` and 31 manufactured from :c:type:`struct spi_board_info 32 <spi_board_info>` descriptors which are usually provided by 33 board-specific initialization code. A :c:type:`struct spi_driver 34 <spi_driver>` is called a "Protocol Driver", and is bound to a 35 spi_device using normal driver model calls. 36 37 The I/O model is a set of queued messages. Protocol drivers submit one 38 or more :c:type:`struct spi_message <spi_message>` objects, 39 which are processed and completed asynchronously. (There are synchronous 40 wrappers, however.) Messages are built from one or more 41 :c:type:`struct spi_transfer <spi_transfer>` objects, each of 42 which wraps a full duplex SPI transfer. A variety of protocol tweaking 43 options are needed, because different chips adopt very different 44 policies for how they use the bits transferred with SPI. 45 46 .. kernel-doc:: include/linux/spi/spi.h 47 :internal: 48 49 .. kernel-doc:: drivers/spi/spi.c 50 :functions: spi_register_board_info 51 52 .. kernel-doc:: drivers/spi/spi.c 53 :export:
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