linux/Documentation/i2c/summary
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   1I2C and SMBus
   2=============
   3
   4I2C (pronounce: I squared C) is a protocol developed by Philips. It is a 
   5slow two-wire protocol (variable speed, up to 400 kHz), with a high speed
   6extension (3.4 MHz).  It provides an inexpensive bus for connecting many
   7types of devices with infrequent or low bandwidth communications needs.
   8I2C is widely used with embedded systems.  Some systems use variants that
   9don't meet branding requirements, and so are not advertised as being I2C.
  10
  11SMBus (System Management Bus) is based on the I2C protocol, and is mostly
  12a subset of I2C protocols and signaling.  Many I2C devices will work on an
  13SMBus, but some SMBus protocols add semantics beyond what is required to
  14achieve I2C branding.  Modern PC mainboards rely on SMBus.  The most common
  15devices connected through SMBus are RAM modules configured using I2C EEPROMs,
  16and hardware monitoring chips.
  17
  18Because the SMBus is mostly a subset of the generalized I2C bus, we can
  19use its protocols on many I2C systems.  However, there are systems that don't
  20meet both SMBus and I2C electrical constraints; and others which can't
  21implement all the common SMBus protocol semantics or messages.
  22
  23
  24Terminology
  25===========
  26
  27When we talk about I2C, we use the following terms:
  28  Bus    -> Algorithm
  29            Adapter
  30  Device -> Driver
  31            Client
  32
  33An Algorithm driver contains general code that can be used for a whole class
  34of I2C adapters. Each specific adapter driver either depends on one algorithm
  35driver, or includes its own implementation.
  36
  37A Driver driver (yes, this sounds ridiculous, sorry) contains the general
  38code to access some type of device. Each detected device gets its own
  39data in the Client structure. Usually, Driver and Client are more closely
  40integrated than Algorithm and Adapter.
  41
  42For a given configuration, you will need a driver for your I2C bus, and
  43drivers for your I2C devices (usually one driver for each device).
  44
  45At this time, Linux only operates I2C (or SMBus) in master mode; you can't
  46use these APIs to make a Linux system behave as a slave/device, either to
  47speak a custom protocol or to emulate some other device.
  48
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