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Based on kernel version 3.15.4. Page generated on 2014-07-07 09:03 EST.

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