| 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). |
| |
| 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. |
