projects / linux-2.6-block.git / blame
| Commit | Line | Data |
|---|---|---|
| ccf988b6 | 1 | ============= |
| 1da177e4 LT |
2 | I2C and SMBus |
| 3 | ============= | |
| 4 | ||
| ccf988b6 | 5 | I2C (pronounce: I squared C) is a protocol developed by Philips. It is a |
| 4298cfc3 DB |
6 | slow two-wire protocol (variable speed, up to 400 kHz), with a high speed |
| 7 | extension (3.4 MHz). It provides an inexpensive bus for connecting many | |
| 8 | types of devices with infrequent or low bandwidth communications needs. | |
| 9 | I2C is widely used with embedded systems. Some systems use variants that | |
| 10 | don't meet branding requirements, and so are not advertised as being I2C. | |
| 1da177e4 | 11 | |
| 4298cfc3 DB |
12 | SMBus (System Management Bus) is based on the I2C protocol, and is mostly |
| 13 | a subset of I2C protocols and signaling. Many I2C devices will work on an | |
| 14 | SMBus, but some SMBus protocols add semantics beyond what is required to | |
| 15 | achieve I2C branding. Modern PC mainboards rely on SMBus. The most common | |
| 16 | devices connected through SMBus are RAM modules configured using I2C EEPROMs, | |
| 17 | and hardware monitoring chips. | |
| 1da177e4 | 18 | |
| 4298cfc3 DB |
19 | Because the SMBus is mostly a subset of the generalized I2C bus, we can |
| 20 | use its protocols on many I2C systems. However, there are systems that don't | |
| 21 | meet both SMBus and I2C electrical constraints; and others which can't | |
| 22 | implement all the common SMBus protocol semantics or messages. | |
| 1da177e4 LT |
23 | |
| 24 | ||
| 25 | Terminology | |
| 26 | =========== | |
| 27 | ||
| ccf988b6 MCC |
28 | When we talk about I2C, we use the following terms:: |
| 29 | ||
| 1da177e4 LT |
30 | Bus -> Algorithm |
| 31 | Adapter | |
| 32 | Device -> Driver | |
| 33 | Client | |
| 34 | ||
| 35 | An Algorithm driver contains general code that can be used for a whole class | |
| 45ccc6c5 JD |
36 | of I2C adapters. Each specific adapter driver either depends on one algorithm |
| 37 | driver, or includes its own implementation. | |
| 4298cfc3 | 38 | |
| 1da177e4 LT |
39 | A Driver driver (yes, this sounds ridiculous, sorry) contains the general |
| 40 | code to access some type of device. Each detected device gets its own | |
| 41 | data in the Client structure. Usually, Driver and Client are more closely | |
| 42 | integrated than Algorithm and Adapter. | |
| 43 | ||
| 45ccc6c5 JD |
44 | For a given configuration, you will need a driver for your I2C bus, and |
| 45 | drivers for your I2C devices (usually one driver for each device). |