linux/Documentation/i2c/summary

This is an explanation of what i2c is, and what is supported in this package.

I2C and SMBus
=============

I2C (pronounce: I squared C) 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.

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 depends on one algorithm
driver.

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 (usually
a separate Adapter and Algorithm driver), and drivers for your I2C devices
(usually one driver for each device). There are no I2C device drivers
in this package. See the lm_sensors project http://www.lm-sensors.nu
for device drivers.

At this time, Linux only operates I2C (or SMBus) in master mode; you can't
use these APIs to make a Linux system behave as a slave/device, either to
speak a custom protocol or to emulate some other device.


Included Bus Drivers
====================
Note that only stable drivers are patched into the kernel by 'mkpatch'.


Base modules
------------

i2c-core: The basic I2C code, including the /proc/bus/i2c* interface
i2c-dev:  The /dev/i2c-* interface
i2c-proc: The /proc/sys/dev/sensors interface for device (client) drivers

Algorithm drivers
-----------------

i2c-algo-bit:    A bit-banging algorithm
i2c-algo-pcf:    A PCF 8584 style algorithm
i2c-algo-ibm_ocp: An algorithm for the I2C device in IBM 4xx processors (NOT BUILT BY DEFAULT)

Adapter drivers
---------------

i2c-elektor:     Elektor ISA card (uses i2c-algo-pcf)
i2c-elv:         ELV parallel port adapter (uses i2c-algo-bit)
i2c-pcf-epp:     PCF8584 on a EPP parallel port (uses i2c-algo-pcf) (NOT mkpatched)
i2c-philips-par: Philips style parallel port adapter (uses i2c-algo-bit)
i2c-adap-ibm_ocp: IBM 4xx processor I2C device (uses i2c-algo-ibm_ocp) (NOT BUILT BY DEFAULT)
i2c-pport:       Primitive parallel port adapter (uses i2c-algo-bit)
i2c-velleman:    Velleman K8000 parallel port adapter (uses i2c-algo-bit)