1*4882a593Smuzhiyun============================= 2*4882a593SmuzhiyunIntroduction to I2C and SMBus 3*4882a593Smuzhiyun============================= 4*4882a593Smuzhiyun 5*4882a593SmuzhiyunI²C (pronounce: I squared C and written I2C in the kernel documentation) is 6*4882a593Smuzhiyuna protocol developed by Philips. It is a slow two-wire protocol (variable 7*4882a593Smuzhiyunspeed, up to 400 kHz), with a high speed extension (3.4 MHz). It provides 8*4882a593Smuzhiyunan inexpensive bus for connecting many types of devices with infrequent or 9*4882a593Smuzhiyunlow bandwidth communications needs. I2C is widely used with embedded 10*4882a593Smuzhiyunsystems. Some systems use variants that don't meet branding requirements, 11*4882a593Smuzhiyunand so are not advertised as being I2C but come under different names, 12*4882a593Smuzhiyune.g. TWI (Two Wire Interface), IIC. 13*4882a593Smuzhiyun 14*4882a593SmuzhiyunThe official I2C specification is the `"I2C-bus specification and user 15*4882a593Smuzhiyunmanual" (UM10204) <https://www.nxp.com/docs/en/user-guide/UM10204.pdf>`_ 16*4882a593Smuzhiyunpublished by NXP Semiconductors. 17*4882a593Smuzhiyun 18*4882a593SmuzhiyunSMBus (System Management Bus) is based on the I2C protocol, and is mostly 19*4882a593Smuzhiyuna subset of I2C protocols and signaling. Many I2C devices will work on an 20*4882a593SmuzhiyunSMBus, but some SMBus protocols add semantics beyond what is required to 21*4882a593Smuzhiyunachieve I2C branding. Modern PC mainboards rely on SMBus. The most common 22*4882a593Smuzhiyundevices connected through SMBus are RAM modules configured using I2C EEPROMs, 23*4882a593Smuzhiyunand hardware monitoring chips. 24*4882a593Smuzhiyun 25*4882a593SmuzhiyunBecause the SMBus is mostly a subset of the generalized I2C bus, we can 26*4882a593Smuzhiyunuse its protocols on many I2C systems. However, there are systems that don't 27*4882a593Smuzhiyunmeet both SMBus and I2C electrical constraints; and others which can't 28*4882a593Smuzhiyunimplement all the common SMBus protocol semantics or messages. 29*4882a593Smuzhiyun 30*4882a593Smuzhiyun 31*4882a593SmuzhiyunTerminology 32*4882a593Smuzhiyun=========== 33*4882a593Smuzhiyun 34*4882a593SmuzhiyunUsing the terminology from the official documentation, the I2C bus connects 35*4882a593Smuzhiyunone or more *master* chips and one or more *slave* chips. 36*4882a593Smuzhiyun 37*4882a593Smuzhiyun.. kernel-figure:: i2c_bus.svg 38*4882a593Smuzhiyun :alt: Simple I2C bus with one master and 3 slaves 39*4882a593Smuzhiyun 40*4882a593Smuzhiyun Simple I2C bus 41*4882a593Smuzhiyun 42*4882a593SmuzhiyunA **master** chip is a node that starts communications with slaves. In the 43*4882a593SmuzhiyunLinux kernel implementation it is called an **adapter** or bus. Adapter 44*4882a593Smuzhiyundrivers are in the ``drivers/i2c/busses/`` subdirectory. 45*4882a593Smuzhiyun 46*4882a593SmuzhiyunAn **algorithm** contains general code that can be used to implement a 47*4882a593Smuzhiyunwhole class of I2C adapters. Each specific adapter driver either depends on 48*4882a593Smuzhiyunan algorithm driver in the ``drivers/i2c/algos/`` subdirectory, or includes 49*4882a593Smuzhiyunits own implementation. 50*4882a593Smuzhiyun 51*4882a593SmuzhiyunA **slave** chip is a node that responds to communications when addressed 52*4882a593Smuzhiyunby the master. In Linux it is called a **client**. Client drivers are kept 53*4882a593Smuzhiyunin a directory specific to the feature they provide, for example 54*4882a593Smuzhiyun``drivers/media/gpio/`` for GPIO expanders and ``drivers/media/i2c/`` for 55*4882a593Smuzhiyunvideo-related chips. 56*4882a593Smuzhiyun 57*4882a593SmuzhiyunFor the example configuration in figure, you will need a driver for your 58*4882a593SmuzhiyunI2C adapter, and drivers for your I2C devices (usually one driver for each 59*4882a593Smuzhiyundevice). 60
