1 ============================= !! 1 =============
2 Introduction to I2C and SMBus !! 2 I2C and SMBus
3 ============================= !! 3 =============
4 !! 4
5 I²C (pronounce: I squared C and written I2C i !! 5 I2C (pronounce: I squared C) is a protocol developed by Philips. It is a
6 a protocol developed by Philips. It is a two-w !! 6 slow two-wire protocol (variable speed, up to 400 kHz), with a high speed
7 speed (typically up to 400 kHz, high speed mod !! 7 extension (3.4 MHz). It provides an inexpensive bus for connecting many
8 an inexpensive bus for connecting many types o !! 8 types of devices with infrequent or low bandwidth communications needs.
9 low bandwidth communications needs. I2C is wid !! 9 I2C is widely used with embedded systems. Some systems use variants that
10 systems. Some systems use variants that don't !! 10 don't meet branding requirements, and so are not advertised as being I2C.
11 and so are not advertised as being I2C but com <<
12 e.g. TWI (Two Wire Interface), IIC. <<
13 <<
14 The latest official I2C specification is the ` <<
15 manual" (UM10204) <https://www.nxp.com/docs/en <<
16 published by NXP Semiconductors, version 7 as <<
17 11
18 SMBus (System Management Bus) is based on the 12 SMBus (System Management Bus) is based on the I2C protocol, and is mostly
19 a subset of I2C protocols and signaling. Many !! 13 a subset of I2C protocols and signaling. Many I2C devices will work on an
20 SMBus, but some SMBus protocols add semantics 14 SMBus, but some SMBus protocols add semantics beyond what is required to
21 achieve I2C branding. Modern PC mainboards rel !! 15 achieve I2C branding. Modern PC mainboards rely on SMBus. The most common
22 devices connected through SMBus are RAM module 16 devices connected through SMBus are RAM modules configured using I2C EEPROMs,
23 and hardware monitoring chips. 17 and hardware monitoring chips.
24 18
25 Because the SMBus is mostly a subset of the ge 19 Because the SMBus is mostly a subset of the generalized I2C bus, we can
26 use its protocols on many I2C systems. However !! 20 use its protocols on many I2C systems. However, there are systems that don't
27 meet both SMBus and I2C electrical constraints 21 meet both SMBus and I2C electrical constraints; and others which can't
28 implement all the common SMBus protocol semant 22 implement all the common SMBus protocol semantics or messages.
29 23
30 24
31 Terminology 25 Terminology
32 =========== 26 ===========
33 27
34 The I2C bus connects one or more controller ch !! 28 When we talk about I2C, we use the following terms::
35 29
36 .. kernel-figure:: i2c_bus.svg !! 30 Bus -> Algorithm
37 :alt: Simple I2C bus with one controller !! 31 Adapter
>> 32 Device -> Driver
>> 33 Client
>> 34
>> 35 An Algorithm driver contains general code that can be used for a whole class
>> 36 of I2C adapters. Each specific adapter driver either depends on one algorithm
>> 37 driver, or includes its own implementation.
>> 38
>> 39 A Driver driver (yes, this sounds ridiculous, sorry) contains the general
>> 40 code to access some type of device. Each detected device gets its own
>> 41 data in the Client structure. Usually, Driver and Client are more closely
>> 42 integrated than Algorithm and Adapter.
38 43
39 Simple I2C bus !! 44 For a given configuration, you will need a driver for your I2C bus, and
40 !! 45 drivers for your I2C devices (usually one driver for each device).
41 A **controller** chip is a node that starts co <<
42 Linux kernel implementation it is also called <<
43 drivers are usually in the ``drivers/i2c/busse <<
44 <<
45 An **algorithm** contains general code that ca <<
46 class of I2C controllers. Each specific contro <<
47 algorithm driver in the ``drivers/i2c/algos/`` <<
48 own implementation. <<
49 <<
50 A **target** chip is a node that responds to c <<
51 controller. In the Linux kernel implementation <<
52 While targets are usually separate external ch <<
53 target (needs hardware support) and respond to <<
54 This is then called a **local target**. In con <<
55 a **remote target**. <<
56 <<
57 Target drivers are kept in a directory specifi <<
58 for example ``drivers/gpio/`` for GPIO expande <<
59 video-related chips. <<
60 <<
61 For the example configuration in the figure ab <<
62 the I2C controller, and drivers for your I2C t <<
63 each target. <<
64 <<
65 Synonyms <<
66 -------- <<
67 <<
68 As mentioned above, the Linux I2C implementati <<
69 "adapter" for controller and "client" for targ <<
70 have these synonyms in their name. So, when di <<
71 you should be aware of these terms as well. Th <<
72 though. <<
73 <<
74 Outdated terminology <<
75 -------------------- <<
76 <<
77 In earlier I2C specifications, controller was <<
78 named "slave". These terms have been obsoleted <<
79 their use is also discouraged by the Linux Ker <<
80 still find them in references to documentation <<
81 general attitude, however, is to use the inclu <<
82 target. Work to replace the old terminology in <<
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