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 two-wire protocol with variable 7speed (typically up to 400 kHz, high speed modes up to 5 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/docs/en/user-guide/UM10204.pdf>`_ 16published by NXP Semiconductors, version 7 as of this writing. 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 34The I2C bus connects one or more controller chips and one or more target chips. 35 36.. kernel-figure:: i2c_bus.svg 37 :alt: Simple I2C bus with one controller and 3 targets 38 39 Simple I2C bus 40 41A **controller** chip is a node that starts communications with targets. In the 42Linux kernel implementation it is also called an "adapter" or "bus". Controller 43drivers are usually in the ``drivers/i2c/busses/`` subdirectory. 44 45An **algorithm** contains general code that can be used to implement a whole 46class of I2C controllers. Each specific controller driver either depends on an 47algorithm driver in the ``drivers/i2c/algos/`` subdirectory, or includes its 48own implementation. 49 50A **target** chip is a node that responds to communications when addressed by a 51controller. In the Linux kernel implementation it is also called a "client". 52While targets are usually separate external chips, Linux can also act as a 53target (needs hardware support) and respond to another controller on the bus. 54This is then called a **local target**. In contrast, an external chip is called 55a **remote target**. 56 57Target drivers are kept in a directory specific to the feature they provide, 58for example ``drivers/gpio/`` for GPIO expanders and ``drivers/media/i2c/`` for 59video-related chips. 60 61For the example configuration in the figure above, you will need one driver for 62the I2C controller, and drivers for your I2C targets. Usually one driver for 63each target. 64 65Synonyms 66-------- 67 68As mentioned above, the Linux I2C implementation historically uses the terms 69"adapter" for controller and "client" for target. A number of data structures 70have these synonyms in their name. So, when discussing implementation details, 71you should be aware of these terms as well. The official wording is preferred, 72though. 73 74Outdated terminology 75-------------------- 76 77In earlier I2C specifications, controller was named "master" and target was 78named "slave". These terms have been obsoleted with v7 of the specification and 79their use is also discouraged by the Linux Kernel Code of Conduct. You may 80still find them in references to documentation which has not been updated. The 81general attitude, however, is to use the inclusive terms: controller and 82target. Work to replace the old terminology in the Linux Kernel is on-going. 83
