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