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1 | ============================= |
2 | Introduction to I2C and SMBus | |
3 | ============================= | |
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5 | I²C (pronounce: I squared C and written I2C in the kernel documentation) is |
6 | a protocol developed by Philips. It is a slow two-wire protocol (variable | |
7 | speed, up to 400 kHz), with a high speed extension (3.4 MHz). It provides | |
8 | an inexpensive bus for connecting many types of devices with infrequent or | |
9 | low bandwidth communications needs. I2C is widely used with embedded | |
10 | systems. Some systems use variants that don't meet branding requirements, | |
11 | and so are not advertised as being I2C but come under different names, | |
12 | e.g. TWI (Two Wire Interface), IIC. | |
13 | ||
14 | The official I2C specification is the `"I2C-bus specification and user | |
15 | manual" (UM10204) <https://www.nxp.com/docs/en/user-guide/UM10204.pdf>`_ | |
16 | published by NXP Semiconductors. | |
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18 | SMBus (System Management Bus) is based on the I2C protocol, and is mostly |
19 | a subset of I2C protocols and signaling. Many I2C devices will work on an | |
20 | SMBus, but some SMBus protocols add semantics beyond what is required to | |
21 | achieve I2C branding. Modern PC mainboards rely on SMBus. The most common | |
22 | devices connected through SMBus are RAM modules configured using I2C EEPROMs, | |
23 | and hardware monitoring chips. | |
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25 | Because the SMBus is mostly a subset of the generalized I2C bus, we can |
26 | use its protocols on many I2C systems. However, there are systems that don't | |
27 | meet both SMBus and I2C electrical constraints; and others which can't | |
28 | implement all the common SMBus protocol semantics or messages. | |
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29 | |
30 | ||
31 | Terminology | |
32 | =========== | |
33 | ||
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34 | Using the terminology from the official documentation, the I2C bus connects |
35 | one or more *master* chips and one or more *slave* chips. | |
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36536a02 | 37 | .. kernel-figure:: i2c_bus.svg |
020bc5b9 | 38 | :alt: Simple I2C bus with one master and 3 slaves |
1da177e4 | 39 | |
020bc5b9 | 40 | Simple I2C bus |
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42 | A **master** chip is a node that starts communications with slaves. In the |
43 | Linux kernel implementation it is called an **adapter** or bus. Adapter | |
44 | drivers are in the ``drivers/i2c/busses/`` subdirectory. | |
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46 | An **algorithm** contains general code that can be used to implement a |
47 | whole class of I2C adapters. Each specific adapter driver either depends on | |
48 | an algorithm driver in the ``drivers/i2c/algos/`` subdirectory, or includes | |
49 | its own implementation. | |
50 | ||
51 | A **slave** chip is a node that responds to communications when addressed | |
52 | by the master. In Linux it is called a **client**. Client drivers are kept | |
53 | in a directory specific to the feature they provide, for example | |
54 | ``drivers/media/gpio/`` for GPIO expanders and ``drivers/media/i2c/`` for | |
55 | video-related chips. | |
56 | ||
57 | For the example configuration in figure, you will need a driver for your | |
58 | I2C adapter, and drivers for your I2C devices (usually one driver for each | |
59 | device). |