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