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Some of the section names are not very clear. Reading those names in the index.rst page does not help much in grasping what the content is supposed to be. Rename those sections to clarify their content, especially when reading the index page. Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net> Acked-by: Peter Rosin <peda@axentia.se> Reviewed-by: Jean Delvare <jdelvare@suse.de> Signed-off-by: Wolfram Sang <wsa@the-dreams.de>
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60 lines
2.6 KiB
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=============================
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Introduction to I2C and SMBus
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=============================
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I²C (pronounce: I squared C and written I2C in the kernel documentation) is
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a protocol developed by Philips. It is a slow two-wire protocol (variable
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speed, up to 400 kHz), with a high speed extension (3.4 MHz). It provides
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an inexpensive bus for connecting many types of devices with infrequent or
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low bandwidth communications needs. I2C is widely used with embedded
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systems. Some systems use variants that don't meet branding requirements,
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and so are not advertised as being I2C but come under different names,
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e.g. TWI (Two Wire Interface), IIC.
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The official I2C specification is the `"I2C-bus specification and user
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manual" (UM10204) <https://www.nxp.com/docs/en/user-guide/UM10204.pdf>`_
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published by NXP Semiconductors.
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SMBus (System Management Bus) is based on the I2C protocol, and is mostly
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a subset of I2C protocols and signaling. Many I2C devices will work on an
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SMBus, but some SMBus protocols add semantics beyond what is required to
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achieve I2C branding. Modern PC mainboards rely on SMBus. The most common
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devices connected through SMBus are RAM modules configured using I2C EEPROMs,
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and hardware monitoring chips.
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Because the SMBus is mostly a subset of the generalized I2C bus, we can
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use its protocols on many I2C systems. However, there are systems that don't
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meet both SMBus and I2C electrical constraints; and others which can't
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implement all the common SMBus protocol semantics or messages.
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Terminology
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===========
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Using the terminology from the official documentation, the I2C bus connects
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one or more *master* chips and one or more *slave* chips.
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.. kernel-figure:: i2c.svg
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:alt: Simple I2C bus with one master and 3 slaves
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Simple I2C bus
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A **master** chip is a node that starts communications with slaves. In the
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Linux kernel implementation it is called an **adapter** or bus. Adapter
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drivers are in the ``drivers/i2c/busses/`` subdirectory.
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An **algorithm** contains general code that can be used to implement a
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whole class of I2C adapters. Each specific adapter driver either depends on
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an algorithm driver in the ``drivers/i2c/algos/`` subdirectory, or includes
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its own implementation.
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A **slave** chip is a node that responds to communications when addressed
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by the master. In Linux it is called a **client**. Client drivers are kept
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in a directory specific to the feature they provide, for example
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``drivers/media/gpio/`` for GPIO expanders and ``drivers/media/i2c/`` for
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video-related chips.
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For the example configuration in figure, you will need a driver for your
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I2C adapter, and drivers for your I2C devices (usually one driver for each
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device).
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