| rjw | 1f88458 | 2022-01-06 17:20:42 +0800 | [diff] [blame] | 1 | I2C and SMBus |
| 2 | ============= |
| 3 | |
| 4 | I2C (pronounce: I squared C) is a protocol developed by Philips. It is a |
| 5 | slow two-wire protocol (variable speed, up to 400 kHz), with a high speed |
| 6 | extension (3.4 MHz). It provides an inexpensive bus for connecting many |
| 7 | types of devices with infrequent or low bandwidth communications needs. |
| 8 | I2C is widely used with embedded systems. Some systems use variants that |
| 9 | don't meet branding requirements, and so are not advertised as being I2C. |
| 10 | |
| 11 | SMBus (System Management Bus) is based on the I2C protocol, and is mostly |
| 12 | a subset of I2C protocols and signaling. Many I2C devices will work on an |
| 13 | SMBus, but some SMBus protocols add semantics beyond what is required to |
| 14 | achieve I2C branding. Modern PC mainboards rely on SMBus. The most common |
| 15 | devices connected through SMBus are RAM modules configured using I2C EEPROMs, |
| 16 | and hardware monitoring chips. |
| 17 | |
| 18 | Because the SMBus is mostly a subset of the generalized I2C bus, we can |
| 19 | use its protocols on many I2C systems. However, there are systems that don't |
| 20 | meet both SMBus and I2C electrical constraints; and others which can't |
| 21 | implement all the common SMBus protocol semantics or messages. |
| 22 | |
| 23 | |
| 24 | Terminology |
| 25 | =========== |
| 26 | |
| 27 | When we talk about I2C, we use the following terms: |
| 28 | Bus -> Algorithm |
| 29 | Adapter |
| 30 | Device -> Driver |
| 31 | Client |
| 32 | |
| 33 | An Algorithm driver contains general code that can be used for a whole class |
| 34 | of I2C adapters. Each specific adapter driver either depends on one algorithm |
| 35 | driver, or includes its own implementation. |
| 36 | |
| 37 | A Driver driver (yes, this sounds ridiculous, sorry) contains the general |
| 38 | code to access some type of device. Each detected device gets its own |
| 39 | data in the Client structure. Usually, Driver and Client are more closely |
| 40 | integrated than Algorithm and Adapter. |
| 41 | |
| 42 | For a given configuration, you will need a driver for your I2C bus, and |
| 43 | drivers for your I2C devices (usually one driver for each device). |