1============================= 2Introduction to I2C and SMBus 3============================= 4 5I²C (pronounce: I squared C and written I2C in the kernel documentation) is 6a protocol developed by Philips. It is a slow two-wire protocol (variable 7speed, up to 400 kHz), with a high speed extension (3.4 MHz). It provides 8an inexpensive bus for connecting many types of devices with infrequent or 9low bandwidth communications needs. I2C is widely used with embedded 10systems. Some systems use variants that don't meet branding requirements, 11and so are not advertised as being I2C but come under different names, 12e.g. TWI (Two Wire Interface), IIC. 13 14The official I2C specification is the `"I2C-bus specification and user 15manual" (UM10204) <https://www.nxp.com/docs/en/user-guide/UM10204.pdf>`_ 16published by NXP Semiconductors. 17 18SMBus (System Management Bus) is based on the I2C protocol, and is mostly 19a subset of I2C protocols and signaling. Many I2C devices will work on an 20SMBus, but some SMBus protocols add semantics beyond what is required to 21achieve I2C branding. Modern PC mainboards rely on SMBus. The most common 22devices connected through SMBus are RAM modules configured using I2C EEPROMs, 23and hardware monitoring chips. 24 25Because the SMBus is mostly a subset of the generalized I2C bus, we can 26use its protocols on many I2C systems. However, there are systems that don't 27meet both SMBus and I2C electrical constraints; and others which can't 28implement all the common SMBus protocol semantics or messages. 29 30 31Terminology 32=========== 33 34Using the terminology from the official documentation, the I2C bus connects 35one or more *master* chips and one or more *slave* chips. 36 37.. kernel-figure:: i2c_bus.svg 38 :alt: Simple I2C bus with one master and 3 slaves 39 40 Simple I2C bus 41 42A **master** chip is a node that starts communications with slaves. In the 43Linux kernel implementation it is called an **adapter** or bus. Adapter 44drivers are in the ``drivers/i2c/busses/`` subdirectory. 45 46An **algorithm** contains general code that can be used to implement a 47whole class of I2C adapters. Each specific adapter driver either depends on 48an algorithm driver in the ``drivers/i2c/algos/`` subdirectory, or includes 49its own implementation. 50 51A **slave** chip is a node that responds to communications when addressed 52by the master. In Linux it is called a **client**. Client drivers are kept 53in a directory specific to the feature they provide, for example 54``drivers/media/gpio/`` for GPIO expanders and ``drivers/media/i2c/`` for 55video-related chips. 56 57For the example configuration in figure, you will need a driver for your 58I2C adapter, and drivers for your I2C devices (usually one driver for each 59device). 60
