HDMI-CEC controller (CEC) RM0444
1348/1390 RM0444 Rev 5
39.3 HDMI-CEC functional description
39.3.1 HDMI-CEC pin
The CEC bus consists of a single bidirectional line that is used to transfer data in and out of
the device. It is connected to a +3.3 V supply voltage via a 27 kΩ pull-up resistor. The output
stage of the device must have an open-drain or open-collector to allow a wired-AND
connection.
The HDMI-CEC controller manages the CEC bidirectional line as an alternate function of a
standard GPIO, assuming that it is configured as alternate function open drain. The 27 kΩ
pull-up must be added externally to the microcontroller.
To not interfere with the CEC bus when the application power is removed, it is mandatory to
isolate the CEC pin from the bus in such conditions. This can be done by using a MOS
transistor, as shown on Figure 414.
39.3.2 HDMI-CEC block diagram
Figure 414. HDMI-CEC block diagram
39.3.3 Message description
All transactions on the CEC line consist of an initiator and one or more followers. The
initiator is responsible for sending the message structure and the data. The follower is the
recipient of any data and is responsible for setting any acknowledgment bits.
A message is conveyed in a single frame that consists of a start bit followed by a header
block and optionally an opcode and a variable number of operand blocks.
Table 242. HDMI pin
Name Signal type Remarks
CEC Bidirectional
Two states:
– 1 = high impedance
– 0 = low impedance
A 27 kΩ resistor must be added externally.
MSv35920V2
CEC interrupt
Core
CEC
device
27 kΩ
3.3V
PAD
HSI
LSE
RCC
32 kHz
CEC
Kernel
AHB
Event
control
CEC
ITF
TX
RXCLK
Wake-int
G
S
D
CEC line
STM32
HDMI_CEC
controller
Cortex
APB
3.3V
Remote
CEC
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