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5e995786 JC |
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 | ||
c251e2a8 WS |
16 | The Linux I2C programming interfaces support the master side of bus |
17 | interactions and the slave side. The programming interface is | |
5e995786 JC |
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 | |
c251e2a8 WS |
25 | <i2c_driver>`, which should follow the standard Linux driver model. There |
26 | are functions to perform various I2C protocol operations; at this writing | |
5e995786 JC |
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 | ||
91ed5349 | 44 | .. kernel-doc:: drivers/i2c/i2c-core-base.c |
5e995786 | 45 | :export: |
22c78d1c WS |
46 | |
47 | .. kernel-doc:: drivers/i2c/i2c-core-smbus.c | |
5e995786 | 48 | :export: |