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