High-definition multimedia interface, consumer electronics control (HDMI-CEC)
AN3127
Doc ID 16896 Rev 2
Figure 3.
Blocks
All these blocks are made of an 8-bit payload (most significant bit transmitted first) followed
by an end-of-message (EOM) bit and an acknowledge (ACK) bit.
The EOM bit is set in the last block of a message and kept cleared in all others. If a message
contains additional blocks after the EOM, these blocks should be ignored. The EOM bit may
be set in the header block to “ping” other devices and make sure that they are active.
The acknowledge bit is always brought to high impedance by the initiator. It can therefore be
driven low by either the follower that has read its own address in the header, or the follower
that needs to reject a broadcast message.
The header consists of the source logical address field, and the destination logical address
field. Note that the address 0xF is specially used for broadcast messages.
1.3 Bit
timing
The format of the start bit is unique and identifies the start of a message. It should be
validated by its low duration and its total duration.
All remaining data bits in the message, after the start bit, have a consistent timing. The high-
to-low transition at the end of the data bit is the start of the next data bit except for the final
bit where the CEC line remains high.
shows the timings of the start bit and the different data bits.
Figure 4.
Bit timings
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