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 latest official I2C specification is the `"I2C-bus specification and user 15manual" (UM10204) <https://www.nxp.com/webapp/Download?colCode=UM10204>`_ 16published by NXP Semiconductors. However, you need to log-in to the site to 17access 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>`_. 19 20SMBus (System Management Bus) is based on the I2C protocol, and is mostly 21a subset of I2C protocols and signaling. Many I2C devices will work on an 22SMBus, but some SMBus protocols add semantics beyond what is required to 23achieve I2C branding. Modern PC mainboards rely on SMBus. The most common 24devices connected through SMBus are RAM modules configured using I2C EEPROMs, 25and hardware monitoring chips. 26 27Because the SMBus is mostly a subset of the generalized I2C bus, we can 28use its protocols on many I2C systems. However, there are systems that don't 29meet both SMBus and I2C electrical constraints; and others which can't 30implement all the common SMBus protocol semantics or messages. 31 32 33Terminology 34=========== 35 36Using the terminology from the official documentation, the I2C bus connects 37one or more *master* chips and one or more *slave* chips. 38 39.. kernel-figure:: i2c_bus.svg 40 :alt: Simple I2C bus with one master and 3 slaves 41 42 Simple I2C bus 43 44A **master** chip is a node that starts communications with slaves. In the 45Linux kernel implementation it is called an **adapter** or bus. Adapter 46drivers are in the ``drivers/i2c/busses/`` subdirectory. 47 48An **algorithm** contains general code that can be used to implement a 49whole class of I2C adapters. Each specific adapter driver either depends on 50an algorithm driver in the ``drivers/i2c/algos/`` subdirectory, or includes 51its own implementation. 52 53A **slave** chip is a node that responds to communications when addressed 54by the master. In Linux it is called a **client**. Client drivers are kept 55in a directory specific to the feature they provide, for example 56``drivers/media/gpio/`` for GPIO expanders and ``drivers/media/i2c/`` for 57video-related chips. 58 59For the example configuration in figure, you will need a driver for your 60I2C adapter, and drivers for your I2C devices (usually one driver for each 61device). 62
