62 lines
2.9 KiB
ReStructuredText
62 lines
2.9 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 latest official I2C specification is the `"I2C-bus specification and user
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manual" (UM10204) <https://www.nxp.com/webapp/Download?colCode=UM10204>`_
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published by NXP Semiconductors. However, you need to log-in to the site to
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access the PDF. An older version of the specification (revision 6) is archived
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`here <https://web.archive.org/web/20210813122132/https://www.nxp.com/docs/en/user-guide/UM10204.pdf>`_.
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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_bus.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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