Documentation/i2c/summary.rst - linux - Git at Google

 =============================
 Introduction to I2C and SMBus
 =============================

 I²C (pronounce: I squared C and written I2C in the kernel documentation) is
 a protocol developed by Philips. It is a slow two-wire protocol (variable
 speed, up to 400 kHz), with a high speed extension (3.4 MHz).  It provides
 an inexpensive bus for connecting many types of devices with infrequent or
 low bandwidth communications needs.  I2C is widely used with embedded
 systems.  Some systems use variants that don't meet branding requirements,
 and so are not advertised as being I2C but come under different names,
 e.g. TWI (Two Wire Interface), IIC.

 The latest official I2C specification is the `"I2C-bus specification and user
 manual" (UM10204) <https://www.nxp.com/webapp/Download?colCode=UM10204>`_
 published by NXP Semiconductors. However, you need to log-in to the site to
 access the PDF. An older version of the specification (revision 6) is archived
 `here <https://web.archive.org/web/20210813122132/https://www.nxp.com/docs/en/user-guide/UM10204.pdf>`_.

 SMBus (System Management Bus) is based on the I2C protocol, and is mostly
 a subset of I2C protocols and signaling.  Many I2C devices will work on an
 SMBus, but some SMBus protocols add semantics beyond what is required to
 achieve I2C branding.  Modern PC mainboards rely on SMBus.  The most common
 devices connected through SMBus are RAM modules configured using I2C EEPROMs,
 and hardware monitoring chips.

 Because the SMBus is mostly a subset of the generalized I2C bus, we can
 use its protocols on many I2C systems.  However, there are systems that don't
 meet both SMBus and I2C electrical constraints; and others which can't
 implement all the common SMBus protocol semantics or messages.


 Terminology
 ===========

 Using the terminology from the official documentation, the I2C bus connects
 one or more *master* chips and one or more *slave* chips.

 .. kernel-figure::  i2c_bus.svg
    :alt:    Simple I2C bus with one master and 3 slaves

    Simple I2C bus

 A **master** chip is a node that starts communications with slaves. In the
 Linux kernel implementation it is called an **adapter** or bus. Adapter
 drivers are in the ``drivers/i2c/busses/`` subdirectory.

 An **algorithm** contains general code that can be used to implement a
 whole class of I2C adapters. Each specific adapter driver either depends on
 an algorithm driver in the ``drivers/i2c/algos/`` subdirectory, or includes
 its own implementation.

 A **slave** chip is a node that responds to communications when addressed
 by the master. In Linux it is called a **client**. Client drivers are kept
 in a directory specific to the feature they provide, for example
 ``drivers/media/gpio/`` for GPIO expanders and ``drivers/media/i2c/`` for
 video-related chips.

 For the example configuration in figure, you will need a driver for your
 I2C adapter, and drivers for your I2C devices (usually one driver for each
 device).
	=============================
	Introduction to I2C and SMBus
	=============================

	I²C (pronounce: I squared C and written I2C in the kernel documentation) is
	a protocol developed by Philips. It is a slow two-wire protocol (variable
	speed, up to 400 kHz), with a high speed extension (3.4 MHz). It provides
	an inexpensive bus for connecting many types of devices with infrequent or
	low bandwidth communications needs. I2C is widely used with embedded
	systems. Some systems use variants that don't meet branding requirements,
	and so are not advertised as being I2C but come under different names,
	e.g. TWI (Two Wire Interface), IIC.

	The latest official I2C specification is the `"I2C-bus specification and user
	manual" (UM10204) <https://www.nxp.com/webapp/Download?colCode=UM10204>`_
	published by NXP Semiconductors. However, you need to log-in to the site to
	access the PDF. An older version of the specification (revision 6) is archived
	`here <https://web.archive.org/web/20210813122132/https://www.nxp.com/docs/en/user-guide/UM10204.pdf>`_.

	SMBus (System Management Bus) is based on the I2C protocol, and is mostly
	a subset of I2C protocols and signaling. Many I2C devices will work on an
	SMBus, but some SMBus protocols add semantics beyond what is required to
	achieve I2C branding. Modern PC mainboards rely on SMBus. The most common
	devices connected through SMBus are RAM modules configured using I2C EEPROMs,
	and hardware monitoring chips.

	Because the SMBus is mostly a subset of the generalized I2C bus, we can
	use its protocols on many I2C systems. However, there are systems that don't
	meet both SMBus and I2C electrical constraints; and others which can't
	implement all the common SMBus protocol semantics or messages.


	Terminology
	===========

	Using the terminology from the official documentation, the I2C bus connects
	one or more master chips and one or more slave chips.

	.. kernel-figure:: i2c_bus.svg
	:alt: Simple I2C bus with one master and 3 slaves

	Simple I2C bus

	A master chip is a node that starts communications with slaves. In the
	Linux kernel implementation it is called an adapter or bus. Adapter
	drivers are in the ``drivers/i2c/busses/`` subdirectory.

	An algorithm contains general code that can be used to implement a
	whole class of I2C adapters. Each specific adapter driver either depends on
	an algorithm driver in the ``drivers/i2c/algos/`` subdirectory, or includes
	its own implementation.

	A slave chip is a node that responds to communications when addressed
	by the master. In Linux it is called a client. Client drivers are kept
	in a directory specific to the feature they provide, for example
	``drivers/media/gpio/`` for GPIO expanders and ``drivers/media/i2c/`` for
	video-related chips.

	For the example configuration in figure, you will need a driver for your
	I2C adapter, and drivers for your I2C devices (usually one driver for each
	device).