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- 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 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).
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