1 ============================= 1 =============================
2 Introduction to I2C and SMBus 2 Introduction to I2C and SMBus
3 ============================= 3 =============================
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 slow two-wire protocol (variable
7 speed (typically up to 400 kHz, high speed mod !! 7 speed, up to 400 kHz), with a high speed extension (3.4 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/webapp/Download?colCode=UM10204>`_
16 published by NXP Semiconductors, version 7 as !! 16 published by NXP Semiconductors. However, you need to log-in to the site to
>> 17 access the PDF. An older version of the specification (revision 6) is archived
>> 18 `here <https://web.archive.org/web/20210813122132/https://www.nxp.com/docs/en/user-guide/UM10204.pdf>`_.
17 19
18 SMBus (System Management Bus) is based on the 20 SMBus (System Management Bus) is based on the I2C protocol, and is mostly
19 a subset of I2C protocols and signaling. Many !! 21 a subset of I2C protocols and signaling. Many I2C devices will work on an
20 SMBus, but some SMBus protocols add semantics 22 SMBus, but some SMBus protocols add semantics beyond what is required to
21 achieve I2C branding. Modern PC mainboards rel !! 23 achieve I2C branding. Modern PC mainboards rely on SMBus. The most common
22 devices connected through SMBus are RAM module 24 devices connected through SMBus are RAM modules configured using I2C EEPROMs,
23 and hardware monitoring chips. 25 and hardware monitoring chips.
24 26
25 Because the SMBus is mostly a subset of the ge 27 Because the SMBus is mostly a subset of the generalized I2C bus, we can
26 use its protocols on many I2C systems. However !! 28 use its protocols on many I2C systems. However, there are systems that don't
27 meet both SMBus and I2C electrical constraints 29 meet both SMBus and I2C electrical constraints; and others which can't
28 implement all the common SMBus protocol semant 30 implement all the common SMBus protocol semantics or messages.
29 31
30 32
31 Terminology 33 Terminology
32 =========== 34 ===========
33 35
34 The I2C bus connects one or more controller ch !! 36 Using the terminology from the official documentation, the I2C bus connects
>> 37 one or more *master* chips and one or more *slave* chips.
35 38
36 .. kernel-figure:: i2c_bus.svg 39 .. kernel-figure:: i2c_bus.svg
37 :alt: Simple I2C bus with one controller !! 40 :alt: Simple I2C bus with one master and 3 slaves
38 41
39 Simple I2C bus 42 Simple I2C bus
40 43
41 A **controller** chip is a node that starts co !! 44 A **master** chip is a node that starts communications with slaves. In the
42 Linux kernel implementation it is also called !! 45 Linux kernel implementation it is called an **adapter** or bus. Adapter
43 drivers are usually in the ``drivers/i2c/busse !! 46 drivers are in the ``drivers/i2c/busses/`` subdirectory.
44 !! 47
45 An **algorithm** contains general code that ca !! 48 An **algorithm** contains general code that can be used to implement a
46 class of I2C controllers. Each specific contro !! 49 whole class of I2C adapters. Each specific adapter driver either depends on
47 algorithm driver in the ``drivers/i2c/algos/`` !! 50 an algorithm driver in the ``drivers/i2c/algos/`` subdirectory, or includes
48 own implementation. !! 51 its own implementation.
49 !! 52
50 A **target** chip is a node that responds to c !! 53 A **slave** chip is a node that responds to communications when addressed
51 controller. In the Linux kernel implementation !! 54 by the master. In Linux it is called a **client**. Client drivers are kept
52 While targets are usually separate external ch !! 55 in a directory specific to the feature they provide, for example
53 target (needs hardware support) and respond to !! 56 ``drivers/media/gpio/`` for GPIO expanders and ``drivers/media/i2c/`` for
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. 57 video-related chips.
60 58
61 For the example configuration in the figure ab !! 59 For the example configuration in figure, you will need a driver for your
62 the I2C controller, and drivers for your I2C t !! 60 I2C adapter, and drivers for your I2C devices (usually one driver for each
63 each target. !! 61 device).
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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