mirror of
https://github.com/torvalds/linux.git
synced 2024-11-15 00:21:59 +00:00
20738cb9fa
This not only includes rewording, but also where to put which emphasis on terms in this document. Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com> Reviewed-by: Easwar Hariharan <eahariha@linux.microsoft.com> Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
83 lines
3.8 KiB
ReStructuredText
83 lines
3.8 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 two-wire protocol with variable
|
|
speed (typically up to 400 kHz, high speed modes up to 5 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 latest 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, version 7 as of this writing.
|
|
|
|
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
|
|
===========
|
|
|
|
The I2C bus connects one or more controller chips and one or more target chips.
|
|
|
|
.. kernel-figure:: i2c_bus.svg
|
|
:alt: Simple I2C bus with one controller and 3 targets
|
|
|
|
Simple I2C bus
|
|
|
|
A **controller** chip is a node that starts communications with targets. In the
|
|
Linux kernel implementation it is also called an "adapter" or "bus". Controller
|
|
drivers are usually in the ``drivers/i2c/busses/`` subdirectory.
|
|
|
|
An **algorithm** contains general code that can be used to implement a whole
|
|
class of I2C controllers. Each specific controller driver either depends on an
|
|
algorithm driver in the ``drivers/i2c/algos/`` subdirectory, or includes its
|
|
own implementation.
|
|
|
|
A **target** chip is a node that responds to communications when addressed by a
|
|
controller. In the Linux kernel implementation it is also called a "client".
|
|
While targets are usually separate external chips, Linux can also act as a
|
|
target (needs hardware support) and respond to another controller on the bus.
|
|
This is then called a **local target**. In contrast, an external chip is called
|
|
a **remote target**.
|
|
|
|
Target drivers are kept in a directory specific to the feature they provide,
|
|
for example ``drivers/gpio/`` for GPIO expanders and ``drivers/media/i2c/`` for
|
|
video-related chips.
|
|
|
|
For the example configuration in the figure above, you will need one driver for
|
|
the I2C controller, and drivers for your I2C targets. Usually one driver for
|
|
each target.
|
|
|
|
Synonyms
|
|
--------
|
|
|
|
As mentioned above, the Linux I2C implementation historically uses the terms
|
|
"adapter" for controller and "client" for target. A number of data structures
|
|
have these synonyms in their name. So, when discussing implementation details,
|
|
you should be aware of these terms as well. The official wording is preferred,
|
|
though.
|
|
|
|
Outdated terminology
|
|
--------------------
|
|
|
|
In earlier I2C specifications, controller was named "master" and target was
|
|
named "slave". These terms have been obsoleted with v7 of the specification and
|
|
their use is also discouraged by the Linux Kernel Code of Conduct. You may
|
|
still find them in references to documentation which has not been updated. The
|
|
general attitude, however, is to use the inclusive terms: controller and
|
|
target. Work to replace the old terminology in the Linux Kernel is on-going.
|