Message Description; Table 135. Hdmi Pin; Figure 268. Cec Line Connection - ST STM32F100 Series Reference Manual

RM0041 High-definition multimedia interface-consumer electronics control controller (HDMI™-
Name
CEC
1. When configured as output open-drain alternate function, the Schmitt trigger is still activated.
Note: 1 If you do not need the power-off state, you may simply connect the STM32F100xx
microcontroller to the CEC line.
2 If the power-off state is needed in the application, the external pull-up circuit has to be
disconnected from the CEC line when the device is off. For example, this can be
implemented by connecting an isolating diode between the CEC line and the external pull-
up circuit, such that the diode is reverse-biased in the off state with an external device
pulling up the CEC line. A bidirectional isolation buffer is also needed to comply with the
HDMI 1.3a specification. For more details refer to AN3127: "CEC networking using
STM32F100xx value line microcontrollers".
24.3.2

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 acknowledgement 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.
All these blocks are made of a 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. In the event
that a message contains additional blocks after an EOM is indicated, those additional blocks
should be ignored. The EOM bit may be set in the header block to "ping" other devices, to
ascertain if they are active.
Signal type
Bidirectional

Figure 268. CEC line connection

HDMI_CEC
CEC_RX
controler
APB
bus
CEC_TX
STM32F100xx

Table 135. HDMI pin

Two states:
1 = high impedance
0 = low impedance
Ω
A 27 k pull-up resistor must be added externally.
CEC
GPI/O configured
as output open-drain
alternate function (1)
RM0041 Rev 6
Remarks
3.3 V
27 kΩ
CEC line
CEC device
ai17314
649/709
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