Sft Option Bit; Figure 419. Arbitration Phase; Figure 420. Sft Of Three Nominal Bit Periods - ST STM32G0 1 Series Reference Manual

RM0444
high impedance START
Figure 420
A configurable time window is counted before starting the transmission.
In the SFT = 0 configuration, HDMI-CEC performs automatic SFT calculation ensuring
compliance with the HDMI-CEC standard:
•
2.5 data bit periods if the CEC is the last bus initiator with unsuccessful transmission
•
4 data bit periods if the CEC is the new bus initiator
•
6 data bit periods if the CEC is the last bus initiator with successful transmission
This is done to guarantee the maximum priority to a failed transmission and the lowest one
to the last initiator that completed successfully its transmission.
Otherwise there is the possibility to configure the SFT bits to count a fixed timing value.
Possible values are 0.5, 1.5, 2.5, 3.5, 4.5, 5.5, 6.5 data bit periods.
39.4.1

SFT option bit

In case of SFTOPT = 0 configuration, SFT starts being counted when the start-of-
transmission command is set by software (TXSOM = 1).
In case of SFTOPT = 1, SFT starts automatically being counted by the HDMI-CEC device
when a bus-idle or line error condition is detected. If the SFT timer is completed at the time
TXSOM command is set then transmission starts immediately without latency. If the SFT
timer is still running instead, the system waits until the timer elapses before transmission
can start.
Arbitration phase
INITIATOR[3:0]
BIT
shows an example for a SFT of three nominal bit periods.

Figure 420. SFT of three nominal bit periods

last bit of previous frame

Figure 419. Arbitration phase

DESTINATION[3:0]
RM0444 Rev 5
HDMI-CEC controller (CEC)
EOM ACK
MS31008V1
Start bit
MS31009V1
1351/1390
1364