i2c.rst 2.3 KB

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  1. I\ :sup:`2`\ C and SMBus Subsystem
  2. ==================================
  3. I\ :sup:`2`\ C (or without fancy typography, "I2C") is an acronym for
  4. the "Inter-IC" bus, a simple bus protocol which is widely used where low
  5. data rate communications suffice. Since it's also a licensed trademark,
  6. some vendors use another name (such as "Two-Wire Interface", TWI) for
  7. the same bus. I2C only needs two signals (SCL for clock, SDA for data),
  8. conserving board real estate and minimizing signal quality issues. Most
  9. I2C devices use seven bit addresses, and bus speeds of up to 400 kHz;
  10. there's a high speed extension (3.4 MHz) that's not yet found wide use.
  11. I2C is a multi-master bus; open drain signaling is used to arbitrate
  12. between masters, as well as to handshake and to synchronize clocks from
  13. slower clients.
  14. The Linux I2C programming interfaces support the master side of bus
  15. interactions and the slave side. The programming interface is
  16. structured around two kinds of driver, and two kinds of device. An I2C
  17. "Adapter Driver" abstracts the controller hardware; it binds to a
  18. physical device (perhaps a PCI device or platform_device) and exposes a
  19. :c:type:`struct i2c_adapter <i2c_adapter>` representing each
  20. I2C bus segment it manages. On each I2C bus segment will be I2C devices
  21. represented by a :c:type:`struct i2c_client <i2c_client>`.
  22. Those devices will be bound to a :c:type:`struct i2c_driver
  23. <i2c_driver>`, which should follow the standard Linux driver model. There
  24. are functions to perform various I2C protocol operations; at this writing
  25. all such functions are usable only from task context.
  26. The System Management Bus (SMBus) is a sibling protocol. Most SMBus
  27. systems are also I2C conformant. The electrical constraints are tighter
  28. for SMBus, and it standardizes particular protocol messages and idioms.
  29. Controllers that support I2C can also support most SMBus operations, but
  30. SMBus controllers don't support all the protocol options that an I2C
  31. controller will. There are functions to perform various SMBus protocol
  32. operations, either using I2C primitives or by issuing SMBus commands to
  33. i2c_adapter devices which don't support those I2C operations.
  34. .. kernel-doc:: include/linux/i2c.h
  35. :internal:
  36. .. kernel-doc:: drivers/i2c/i2c-boardinfo.c
  37. :functions: i2c_register_board_info
  38. .. kernel-doc:: drivers/i2c/i2c-core-base.c
  39. :export:
  40. .. kernel-doc:: drivers/i2c/i2c-core-smbus.c
  41. :export: