60 lines
2.6 KiB
ReStructuredText
60 lines
2.6 KiB
ReStructuredText
|
=============================
|
||
|
|
Introduction to I2C and SMBus
|
||
|
|
=============================
|
||
|
|
|
||
|
|
I²C (pronounce: I squared C and written I2C in the kernel documentation) is
|
||
|
|
a protocol developed by Philips. It is a slow two-wire protocol (variable
|
||
|
|
speed, up to 400 kHz), with a high speed extension (3.4 MHz). It provides
|
||
|
|
an inexpensive bus for connecting many types of devices with infrequent or
|
||
|
|
low bandwidth communications needs. I2C is widely used with embedded
|
||
|
|
systems. Some systems use variants that don't meet branding requirements,
|
||
|
|
and so are not advertised as being I2C but come under different names,
|
||
|
|
e.g. TWI (Two Wire Interface), IIC.
|
||
|
|
|
||
|
|
The official I2C specification is the `"I2C-bus specification and user
|
||
|
|
manual" (UM10204) <https://www.nxp.com/docs/en/user-guide/UM10204.pdf>`_
|
||
|
|
published by NXP Semiconductors.
|
||
|
|
|
||
|
|
SMBus (System Management Bus) is based on the I2C protocol, and is mostly
|
||
|
|
a subset of I2C protocols and signaling. Many I2C devices will work on an
|
||
|
|
SMBus, but some SMBus protocols add semantics beyond what is required to
|
||
|
|
achieve I2C branding. Modern PC mainboards rely on SMBus. The most common
|
||
|
|
devices connected through SMBus are RAM modules configured using I2C EEPROMs,
|
||
|
|
and hardware monitoring chips.
|
||
|
|
|
||
|
|
Because the SMBus is mostly a subset of the generalized I2C bus, we can
|
||
|
|
use its protocols on many I2C systems. However, there are systems that don't
|
||
|
|
meet both SMBus and I2C electrical constraints; and others which can't
|
||
|
|
implement all the common SMBus protocol semantics or messages.
|
||
|
|
|
||
|
|
|
||
|
|
Terminology
|
||
|
|
===========
|
||
|
|
|
||
|
|
Using the terminology from the official documentation, the I2C bus connects
|
||
|
|
one or more *master* chips and one or more *slave* chips.
|
||
|
|
|
||
|
|
.. kernel-figure:: i2c_bus.svg
|
||
|
|
:alt: Simple I2C bus with one master and 3 slaves
|
||
|
|
|
||
|
|
Simple I2C bus
|
||
|
|
|
||
|
|
A **master** chip is a node that starts communications with slaves. In the
|
||
|
|
Linux kernel implementation it is called an **adapter** or bus. Adapter
|
||
|
|
drivers are in the ``drivers/i2c/busses/`` subdirectory.
|
||
|
|
|
||
|
|
An **algorithm** contains general code that can be used to implement a
|
||
|
|
whole class of I2C adapters. Each specific adapter driver either depends on
|
||
|
|
an algorithm driver in the ``drivers/i2c/algos/`` subdirectory, or includes
|
||
|
|
its own implementation.
|
||
|
|
|
||
|
|
A **slave** chip is a node that responds to communications when addressed
|
||
|
|
by the master. In Linux it is called a **client**. Client drivers are kept
|
||
|
|
in a directory specific to the feature they provide, for example
|
||
|
|
``drivers/media/gpio/`` for GPIO expanders and ``drivers/media/i2c/`` for
|
||
|
|
video-related chips.
|
||
|
|
|
||
|
|
For the example configuration in figure, you will need a driver for your
|
||
|
|
I2C adapter, and drivers for your I2C devices (usually one driver for each
|
||
|
|
device).
|