Exploring HDMI CEC

Consumer Electronics Control (CEC) allows control of AV devices that are connected via HDMI. This is the feature of HDMI that enables your TV to automatically turn on and switch to the correct input when you switch on your set-top box, for example. It also allows you to control your set-top box using the TV remote (in some cases).

Electrically, the CEC bus is a single-wire bus that is shared between all HDMI devices, thus any CEC message can be received by all connected devices. Each device then claims one or more logical addresses on which it will receive direct CEC commands.

LG remote

One interesting feature in the HDMI CEC specifications is Remote Control Pass Through, which allows button presses on the remote control to be passed through to HDMI-connected devices. I thought this feature could be used to unify the various remotes in my living room.

However, not all CEC devices are created equal. As usual, some manufacturers will deviate from the specifications, and/or introduce some quirks in their implementation (as you will see later). They also love to brand CEC with their own funky name, such as SimpLink or Anynet+.

Raspberry Pi as a CEC Bridge

As a quick and dirty way to check out the capabilities of my TV, I used a Raspberry Pi which has a HDMI connection that can be software-controlled. This also meant that I didn’t have to build my own CEC transceiver circuit.

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