20738cb9fa
This not only includes rewording, but also where to put which emphasis on terms in this document. Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com> Reviewed-by: Easwar Hariharan <eahariha@linux.microsoft.com> Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
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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 two-wire protocol with variable
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speed (typically up to 400 kHz, high speed modes up to 5 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/docs/en/user-guide/UM10204.pdf>`_
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published by NXP Semiconductors, version 7 as of this writing.
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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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The I2C bus connects one or more controller chips and one or more target chips.
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.. kernel-figure:: i2c_bus.svg
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:alt: Simple I2C bus with one controller and 3 targets
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Simple I2C bus
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A **controller** chip is a node that starts communications with targets. In the
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Linux kernel implementation it is also called an "adapter" or "bus". Controller
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drivers are usually in the ``drivers/i2c/busses/`` subdirectory.
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An **algorithm** contains general code that can be used to implement a whole
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class of I2C controllers. Each specific controller driver either depends on an
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algorithm driver in the ``drivers/i2c/algos/`` subdirectory, or includes its
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own implementation.
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A **target** chip is a node that responds to communications when addressed by a
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controller. In the Linux kernel implementation it is also called a "client".
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While targets are usually separate external chips, Linux can also act as a
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target (needs hardware support) and respond to another controller on the bus.
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This is then called a **local target**. In contrast, an external chip is called
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a **remote target**.
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Target drivers are kept in a directory specific to the feature they provide,
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for example ``drivers/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 the figure above, you will need one driver for
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the I2C controller, and drivers for your I2C targets. Usually one driver for
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each target.
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Synonyms
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--------
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As mentioned above, the Linux I2C implementation historically uses the terms
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"adapter" for controller and "client" for target. A number of data structures
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have these synonyms in their name. So, when discussing implementation details,
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you should be aware of these terms as well. The official wording is preferred,
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though.
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Outdated terminology
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--------------------
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In earlier I2C specifications, controller was named "master" and target was
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named "slave". These terms have been obsoleted with v7 of the specification and
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their use is also discouraged by the Linux Kernel Code of Conduct. You may
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still find them in references to documentation which has not been updated. The
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general attitude, however, is to use the inclusive terms: controller and
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target. Work to replace the old terminology in the Linux Kernel is on-going.
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