summary.rst 2.6 KB

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  1. =============================
  2. Introduction to I2C and SMBus
  3. =============================
  4. I²C (pronounce: I squared C and written I2C in the kernel documentation) is
  5. a protocol developed by Philips. It is a slow two-wire protocol (variable
  6. speed, up to 400 kHz), with a high speed extension (3.4 MHz). It provides
  7. an inexpensive bus for connecting many types of devices with infrequent or
  8. low bandwidth communications needs. I2C is widely used with embedded
  9. systems. Some systems use variants that don't meet branding requirements,
  10. and so are not advertised as being I2C but come under different names,
  11. e.g. TWI (Two Wire Interface), IIC.
  12. The official I2C specification is the `"I2C-bus specification and user
  13. manual" (UM10204) <https://www.nxp.com/docs/en/user-guide/UM10204.pdf>`_
  14. published by NXP Semiconductors.
  15. SMBus (System Management Bus) is based on the I2C protocol, and is mostly
  16. a subset of I2C protocols and signaling. Many I2C devices will work on an
  17. SMBus, but some SMBus protocols add semantics beyond what is required to
  18. achieve I2C branding. Modern PC mainboards rely on SMBus. The most common
  19. devices connected through SMBus are RAM modules configured using I2C EEPROMs,
  20. and hardware monitoring chips.
  21. Because the SMBus is mostly a subset of the generalized I2C bus, we can
  22. use its protocols on many I2C systems. However, there are systems that don't
  23. meet both SMBus and I2C electrical constraints; and others which can't
  24. implement all the common SMBus protocol semantics or messages.
  25. Terminology
  26. ===========
  27. Using the terminology from the official documentation, the I2C bus connects
  28. one or more *master* chips and one or more *slave* chips.
  29. .. kernel-figure:: i2c_bus.svg
  30. :alt: Simple I2C bus with one master and 3 slaves
  31. Simple I2C bus
  32. A **master** chip is a node that starts communications with slaves. In the
  33. Linux kernel implementation it is called an **adapter** or bus. Adapter
  34. drivers are in the ``drivers/i2c/busses/`` subdirectory.
  35. An **algorithm** contains general code that can be used to implement a
  36. whole class of I2C adapters. Each specific adapter driver either depends on
  37. an algorithm driver in the ``drivers/i2c/algos/`` subdirectory, or includes
  38. its own implementation.
  39. A **slave** chip is a node that responds to communications when addressed
  40. by the master. In Linux it is called a **client**. Client drivers are kept
  41. in a directory specific to the feature they provide, for example
  42. ``drivers/media/gpio/`` for GPIO expanders and ``drivers/media/i2c/`` for
  43. video-related chips.
  44. For the example configuration in figure, you will need a driver for your
  45. I2C adapter, and drivers for your I2C devices (usually one driver for each
  46. device).