| 1 | I\ :sup:`2`\ C and SMBus Subsystem
|
| 2 | ==================================
|
| 3 |
|
| 4 | I\ :sup:`2`\ C (or without fancy typography, "I2C") is an acronym for
|
| 5 | the "Inter-IC" bus, a simple bus protocol which is widely used where low
|
| 6 | data rate communications suffice. Since it's also a licensed trademark,
|
| 7 | some vendors use another name (such as "Two-Wire Interface", TWI) for
|
| 8 | the same bus. I2C only needs two signals (SCL for clock, SDA for data),
|
| 9 | conserving board real estate and minimizing signal quality issues. Most
|
| 10 | I2C devices use seven bit addresses, and bus speeds of up to 400 kHz;
|
| 11 | there's a high speed extension (3.4 MHz) that's not yet found wide use.
|
| 12 | I2C is a multi-master bus; open drain signaling is used to arbitrate
|
| 13 | between masters, as well as to handshake and to synchronize clocks from
|
| 14 | slower clients.
|
| 15 |
|
| 16 | The Linux I2C programming interfaces support the master side of bus
|
| 17 | interactions and the slave side. The programming interface is
|
| 18 | structured around two kinds of driver, and two kinds of device. An I2C
|
| 19 | "Adapter Driver" abstracts the controller hardware; it binds to a
|
| 20 | physical device (perhaps a PCI device or platform_device) and exposes a
|
| 21 | :c:type:`struct i2c_adapter <i2c_adapter>` representing each
|
| 22 | I2C bus segment it manages. On each I2C bus segment will be I2C devices
|
| 23 | represented by a :c:type:`struct i2c_client <i2c_client>`.
|
| 24 | Those devices will be bound to a :c:type:`struct i2c_driver
|
| 25 | <i2c_driver>`, which should follow the standard Linux driver model. There
|
| 26 | are functions to perform various I2C protocol operations; at this writing
|
| 27 | all such functions are usable only from task context.
|
| 28 |
|
| 29 | The System Management Bus (SMBus) is a sibling protocol. Most SMBus
|
| 30 | systems are also I2C conformant. The electrical constraints are tighter
|
| 31 | for SMBus, and it standardizes particular protocol messages and idioms.
|
| 32 | Controllers that support I2C can also support most SMBus operations, but
|
| 33 | SMBus controllers don't support all the protocol options that an I2C
|
| 34 | controller will. There are functions to perform various SMBus protocol
|
| 35 | operations, either using I2C primitives or by issuing SMBus commands to
|
| 36 | i2c_adapter devices which don't support those I2C operations.
|
| 37 |
|
| 38 | .. kernel-doc:: include/linux/i2c.h
|
| 39 | :internal:
|
| 40 |
|
| 41 | .. kernel-doc:: drivers/i2c/i2c-boardinfo.c
|
| 42 | :functions: i2c_register_board_info
|
| 43 |
|
| 44 | .. kernel-doc:: drivers/i2c/i2c-core-base.c
|
| 45 | :export:
|
| 46 |
|
| 47 | .. kernel-doc:: drivers/i2c/i2c-core-smbus.c
|
| 48 | :export:
|