linux/Documentation/i2c/summary.rst
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   1=============================
   2Introduction to I2C and SMBus
   3=============================
   4
   5I\xC2\xB2C (pronounce: I squared C and written I2C in the kernel documentation) is
   6a protocol developed by Philips. It is a slow two-wire protocol (variable
   7speed, up to 400 kHz), with a high speed extension (3.4 MHz).  It provides
   8an inexpensive bus for connecting many types of devices with infrequent or
   9low bandwidth communications needs.  I2C is widely used with embedded
  10systems.  Some systems use variants that don't meet branding requirements,
  11and so are not advertised as being I2C but come under different names,
  12e.g. TWI (Two Wire Interface), IIC.
  13
  14The official I2C specification is the `"I2C-bus specification and user
  15manual" (UM10204) <https://www.nxp.com/docs/en/user-guide/UM10204.pdf>`_
  16published by NXP Semiconductors.
  17
  18SMBus (System Management Bus) is based on the I2C protocol, and is mostly
  19a subset of I2C protocols and signaling.  Many I2C devices will work on an
  20SMBus, but some SMBus protocols add semantics beyond what is required to
  21achieve I2C branding.  Modern PC mainboards rely on SMBus.  The most common
  22devices connected through SMBus are RAM modules configured using I2C EEPROMs,
  23and hardware monitoring chips.
  24
  25Because the SMBus is mostly a subset of the generalized I2C bus, we can
  26use its protocols on many I2C systems.  However, there are systems that don't
  27meet both SMBus and I2C electrical constraints; and others which can't
  28implement all the common SMBus protocol semantics or messages.
  29
  30
  31Terminology
  32===========
  33
  34Using the terminology from the official documentation, the I2C bus connects
  35one or more *master* chips and one or more *slave* chips.
  36
  37.. kernel-figure::  i2c_bus.svg
  38   :alt:    Simple I2C bus with one master and 3 slaves
  39
  40   Simple I2C bus
  41
  42A **master** chip is a node that starts communications with slaves. In the
  43Linux kernel implementation it is called an **adapter** or bus. Adapter
  44drivers are in the ``drivers/i2c/busses/`` subdirectory.
  45
  46An **algorithm** contains general code that can be used to implement a
  47whole class of I2C adapters. Each specific adapter driver either depends on
  48an algorithm driver in the ``drivers/i2c/algos/`` subdirectory, or includes
  49its own implementation.
  50
  51A **slave** chip is a node that responds to communications when addressed
  52by the master. In Linux it is called a **client**. Client drivers are kept
  53in a directory specific to the feature they provide, for example
  54``drivers/media/gpio/`` for GPIO expanders and ``drivers/media/i2c/`` for
  55video-related chips.
  56
  57For the example configuration in figure, you will need a driver for your
  58I2C adapter, and drivers for your I2C devices (usually one driver for each
  59device).
  60