1.. _cec-intro:
2
3Introduction
4============
5
6HDMI connectors provide a single pin for use by the Consumer Electronics
7Control protocol. This protocol allows different devices connected by an
8HDMI cable to communicate. The protocol for CEC version 1.4 is defined
9in supplements 1 (CEC) and 2 (HEAC or HDMI Ethernet and Audio Return
10Channel) of the HDMI 1.4a (:ref:`hdmi`) specification and the
11extensions added to CEC version 2.0 are defined in chapter 11 of the
12HDMI 2.0 (:ref:`hdmi2`) specification.
13
14The bitrate is very slow (effectively no more than 36 bytes per second)
15and is based on the ancient AV.link protocol used in old SCART
16connectors. The protocol closely resembles a crazy Rube Goldberg
17contraption and is an unholy mix of low and high level messages. Some
18messages, especially those part of the HEAC protocol layered on top of
19CEC, need to be handled by the kernel, others can be handled either by
20the kernel or by userspace.
21
22In addition, CEC can be implemented in HDMI receivers, transmitters and
23in USB devices that have an HDMI input and an HDMI output and that
24control just the CEC pin.
25
26Drivers that support CEC will create a CEC device node (/dev/cecX) to
27give userspace access to the CEC adapter. The
28:ref:`CEC_ADAP_G_CAPS` ioctl will tell userspace what it is allowed to do.
29
30In order to check the support and test it, it is suggested to download
31the `v4l-utils <https://git.linuxtv.org/v4l-utils.git/>`_ package. It
32provides three tools to handle CEC:
33
34- cec-ctl: the Swiss army knife of CEC. Allows you to configure, transmit
35  and monitor CEC messages.
36
37- cec-compliance: does a CEC compliance test of a remote CEC device to
38  determine how compliant the CEC implementation is.
39
40- cec-follower: emulates a CEC follower.
41