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HDMI-CEC controller (CEC) RM0444

1352/1390 RM0444 Rev 5

In case of SFTOPT = 1 a bus-event condition starting the SFT timer is detected in the

following cases:

• In case of a regular end of transmission/reception, when TXEND/RXEND bits are set at

the minimum nominal data bit duration of the last bit in the message (ACK bit).

• In case of a transmission error detection, SFT timer starts when the TXERR

transmission error is detected (TXERR = 1).

• In case of a missing acknowledge from the CEC follower, the SFT timer starts when the

TXACKE bit is set, that is at the nominal sampling time of the ACK bit.

• In case of a transmission underrun error, the SFT timer starts when the TXUDR bit is

set at the end of the ACK bit.

• In case of a receive error detection implying reception abort, the SFT timer starts at the

same time the error is detected. If an error bit is generated, then SFT starts being

counted at the end of the error bit.

• In case of a wrong start bit or of any uncodified low impedance bus state from idle, the

SFT timer is restarted as soon as the bus comes back to hi-impedance idleness.

39.5 Error handling

39.5.1 Bit error

If a data bit - excluding the start bit - is considered invalid, the follower is expected to notify

such error by generating a low bit period on the CEC line of 1.4 to 1.6 times the nominal

data bit period (3.6 ms nominally).

Figure 421. Error bit timing

39.5.2 Message error

A message is considered lost and therefore may be retransmitted under the following

conditions:

• a message is not acknowledged in a directly addressed message

• a message is negatively acknowledged in a broadcast message

• a low impedance is detected on the CEC line while it is not expected (line error)

Three kinds of error flag can be detected when the CEC interface is receiving a data bit:

39.5.3 Bit rising error (BRE)

BRE (bit rising error) is set when a bit rising edge is detected outside the windows where it is

expected (see Figure 422). BRE flag also generates a CEC interrupt if the BREIE = 1.

MS31010V1

3.6 ms +/-0.24 ms

ERROR BIT

high impedance

low impedance

ST STM32G0 1 Series User Manual

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