1 ============================= 1 =============================
2 Introduction to I2C and SMBus 2 Introduction to I2C and SMBus
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4 4
5 I²C (pronounce: I squared C and written I2C i 5 I²C (pronounce: I squared C and written I2C in the kernel documentation) is
6 a protocol developed by Philips. It is a two-w 6 a protocol developed by Philips. It is a two-wire protocol with variable
7 speed (typically up to 400 kHz, high speed mod 7 speed (typically up to 400 kHz, high speed modes up to 5 MHz). It provides
8 an inexpensive bus for connecting many types o 8 an inexpensive bus for connecting many types of devices with infrequent or
9 low bandwidth communications needs. I2C is wid 9 low bandwidth communications needs. I2C is widely used with embedded
10 systems. Some systems use variants that don't 10 systems. Some systems use variants that don't meet branding requirements,
11 and so are not advertised as being I2C but com 11 and so are not advertised as being I2C but come under different names,
12 e.g. TWI (Two Wire Interface), IIC. 12 e.g. TWI (Two Wire Interface), IIC.
13 13
14 The latest official I2C specification is the ` 14 The latest official I2C specification is the `"I²C-bus specification and user
15 manual" (UM10204) <https://www.nxp.com/docs/en 15 manual" (UM10204) <https://www.nxp.com/docs/en/user-guide/UM10204.pdf>`_
16 published by NXP Semiconductors, version 7 as 16 published by NXP Semiconductors, version 7 as of this writing.
17 17
18 SMBus (System Management Bus) is based on the 18 SMBus (System Management Bus) is based on the I2C protocol, and is mostly
19 a subset of I2C protocols and signaling. Many 19 a subset of I2C protocols and signaling. Many I2C devices will work on an
20 SMBus, but some SMBus protocols add semantics 20 SMBus, but some SMBus protocols add semantics beyond what is required to
21 achieve I2C branding. Modern PC mainboards rel 21 achieve I2C branding. Modern PC mainboards rely on SMBus. The most common
22 devices connected through SMBus are RAM module 22 devices connected through SMBus are RAM modules configured using I2C EEPROMs,
23 and hardware monitoring chips. 23 and hardware monitoring chips.
24 24
25 Because the SMBus is mostly a subset of the ge 25 Because the SMBus is mostly a subset of the generalized I2C bus, we can
26 use its protocols on many I2C systems. However 26 use its protocols on many I2C systems. However, there are systems that don't
27 meet both SMBus and I2C electrical constraints 27 meet both SMBus and I2C electrical constraints; and others which can't
28 implement all the common SMBus protocol semant 28 implement all the common SMBus protocol semantics or messages.
29 29
30 30
31 Terminology 31 Terminology
32 =========== 32 ===========
33 33
34 The I2C bus connects one or more controller ch 34 The I2C bus connects one or more controller chips and one or more target chips.
35 35
36 .. kernel-figure:: i2c_bus.svg 36 .. kernel-figure:: i2c_bus.svg
37 :alt: Simple I2C bus with one controller 37 :alt: Simple I2C bus with one controller and 3 targets
38 38
39 Simple I2C bus 39 Simple I2C bus
40 40
41 A **controller** chip is a node that starts co 41 A **controller** chip is a node that starts communications with targets. In the
42 Linux kernel implementation it is also called 42 Linux kernel implementation it is also called an "adapter" or "bus". Controller
43 drivers are usually in the ``drivers/i2c/busse 43 drivers are usually in the ``drivers/i2c/busses/`` subdirectory.
44 44
45 An **algorithm** contains general code that ca 45 An **algorithm** contains general code that can be used to implement a whole
46 class of I2C controllers. Each specific contro 46 class of I2C controllers. Each specific controller driver either depends on an
47 algorithm driver in the ``drivers/i2c/algos/`` 47 algorithm driver in the ``drivers/i2c/algos/`` subdirectory, or includes its
48 own implementation. 48 own implementation.
49 49
50 A **target** chip is a node that responds to c 50 A **target** chip is a node that responds to communications when addressed by a
51 controller. In the Linux kernel implementation 51 controller. In the Linux kernel implementation it is also called a "client".
52 While targets are usually separate external ch 52 While targets are usually separate external chips, Linux can also act as a
53 target (needs hardware support) and respond to 53 target (needs hardware support) and respond to another controller on the bus.
54 This is then called a **local target**. In con 54 This is then called a **local target**. In contrast, an external chip is called
55 a **remote target**. 55 a **remote target**.
56 56
57 Target drivers are kept in a directory specifi 57 Target drivers are kept in a directory specific to the feature they provide,
58 for example ``drivers/gpio/`` for GPIO expande 58 for example ``drivers/gpio/`` for GPIO expanders and ``drivers/media/i2c/`` for
59 video-related chips. 59 video-related chips.
60 60
61 For the example configuration in the figure ab 61 For the example configuration in the figure above, you will need one driver for
62 the I2C controller, and drivers for your I2C t 62 the I2C controller, and drivers for your I2C targets. Usually one driver for
63 each target. 63 each target.
64 64
65 Synonyms 65 Synonyms
66 -------- 66 --------
67 67
68 As mentioned above, the Linux I2C implementati 68 As mentioned above, the Linux I2C implementation historically uses the terms
69 "adapter" for controller and "client" for targ 69 "adapter" for controller and "client" for target. A number of data structures
70 have these synonyms in their name. So, when di 70 have these synonyms in their name. So, when discussing implementation details,
71 you should be aware of these terms as well. Th 71 you should be aware of these terms as well. The official wording is preferred,
72 though. 72 though.
73 73
74 Outdated terminology 74 Outdated terminology
75 -------------------- 75 --------------------
76 76
77 In earlier I2C specifications, controller was 77 In earlier I2C specifications, controller was named "master" and target was
78 named "slave". These terms have been obsoleted 78 named "slave". These terms have been obsoleted with v7 of the specification and
79 their use is also discouraged by the Linux Ker 79 their use is also discouraged by the Linux Kernel Code of Conduct. You may
80 still find them in references to documentation 80 still find them in references to documentation which has not been updated. The
81 general attitude, however, is to use the inclu 81 general attitude, however, is to use the inclusive terms: controller and
82 target. Work to replace the old terminology in 82 target. Work to replace the old terminology in the Linux Kernel is on-going.
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