Arbitration; Figure 435. Bit Timings; Figure 436. Signal Free Time - ST STM32F446 Series Reference Manual

HDMI-CEC controller (HDMI-CEC)
32.4

Arbitration

All devices that have to transmit - or retransmit - a message onto the CEC line have to
ensure that it has been inactive for a number of bit periods. This signal free time is defined
as the time starting from the final bit of the previous frame and depends on the initiating
device and the current status as shown in the table below.
Since only one initiator is allowed at any one time, an arbitration mechanism is provided to
avoid conflict when more than one initiator begins transmitting at the same time.
CEC line arbitration commences with the leading edge of the start bit and continues until the
end of the initiator address bits within the header block. During this period, the initiator shall
monitor the CEC line, if whilst driving the line to high impedance it reads it back to 0, it then
assumes it has lost arbitration, stops transmitting and becomes a follower.
1268/1328

Figure 435. Bit timings

Figure 436. Signal free time

RM0390 Rev 4
RM0390