48 lines
2.1 KiB
Text
48 lines
2.1 KiB
Text
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I2C and SMBus
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=============
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I2C (pronounce: I squared C) is a protocol developed by Philips. It is a
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slow two-wire protocol (variable speed, up to 400 kHz), with a high speed
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extension (3.4 MHz). It provides an inexpensive bus for connecting many
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types of devices with infrequent or low bandwidth communications needs.
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I2C is widely used with embedded systems. Some systems use variants that
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don't meet branding requirements, and so are not advertised as being I2C.
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SMBus (System Management Bus) is based on the I2C protocol, and is mostly
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a subset of I2C protocols and signaling. Many I2C devices will work on an
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SMBus, but some SMBus protocols add semantics beyond what is required to
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achieve I2C branding. Modern PC mainboards rely on SMBus. The most common
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devices connected through SMBus are RAM modules configured using I2C EEPROMs,
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and hardware monitoring chips.
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Because the SMBus is mostly a subset of the generalized I2C bus, we can
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use its protocols on many I2C systems. However, there are systems that don't
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meet both SMBus and I2C electrical constraints; and others which can't
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implement all the common SMBus protocol semantics or messages.
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Terminology
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===========
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When we talk about I2C, we use the following terms:
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Bus -> Algorithm
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Adapter
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Device -> Driver
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Client
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An Algorithm driver contains general code that can be used for a whole class
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of I2C adapters. Each specific adapter driver either depends on one algorithm
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driver, or includes its own implementation.
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A Driver driver (yes, this sounds ridiculous, sorry) contains the general
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code to access some type of device. Each detected device gets its own
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data in the Client structure. Usually, Driver and Client are more closely
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integrated than Algorithm and Adapter.
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For a given configuration, you will need a driver for your I2C bus, and
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drivers for your I2C devices (usually one driver for each device).
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At this time, Linux only operates I2C (or SMBus) in master mode; you can't
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use these APIs to make a Linux system behave as a slave/device, either to
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speak a custom protocol or to emulate some other device.
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