[linux-2.6-block.git] / Documentation / i2c / summary.rst

=============
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 official I2C specification is the `"I2C-bus specification and user
manual" (UM10204) <https://www.nxp.com/docs/en/user-guide/UM10204.pdf>`_
published by NXP Semiconductors.

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
===========

When we talk about I2C, we use the following terms::

  Bus    -> Algorithm
            Adapter
  Device -> Driver
            Client

An Algorithm driver contains general code that can be used for a whole class
of I2C adapters. Each specific adapter driver either depends on one algorithm
driver, or includes its own implementation.

A Driver driver (yes, this sounds ridiculous, sorry) contains the general
code to access some type of device. Each detected device gets its own
data in the Client structure. Usually, Driver and Client are more closely
integrated than Algorithm and Adapter.

For a given configuration, you will need a driver for your I2C bus, and
drivers for your I2C devices (usually one driver for each device).
Commit	Line	Data
ccf988b6	1	=============
1da177e4 LT	2	I2C and SMBus
	3	=============
	4
096c22f8 LC	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.
1da177e4	17
4298cfc3 DB	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.
1da177e4	24
4298cfc3 DB	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.
1da177e4 LT	29
	30
	31	Terminology
	32	===========
	33
ccf988b6 MCC	34	When we talk about I2C, we use the following terms::
ccf988b6 MCC	35
1da177e4 LT	36	Bus -> Algorithm
	37	Adapter
	38	Device -> Driver
	39	Client
	40
	41	An Algorithm driver contains general code that can be used for a whole class
45ccc6c5 JD	42	of I2C adapters. Each specific adapter driver either depends on one algorithm
45ccc6c5 JD	43	driver, or includes its own implementation.
4298cfc3	44
1da177e4 LT	45	A Driver driver (yes, this sounds ridiculous, sorry) contains the general
	46	code to access some type of device. Each detected device gets its own
	47	data in the Client structure. Usually, Driver and Client are more closely
	48	integrated than Algorithm and Adapter.
	49
45ccc6c5 JD	50	For a given configuration, you will need a driver for your I2C bus, and
45ccc6c5 JD	51	drivers for your I2C devices (usually one driver for each device).