RM0444 Rev 5 1351/1390
RM0444 HDMI-CEC controller (CEC)
1364
Figure 419. Arbitration phase
Figure 420 shows an example for a SFT of three nominal bit periods.
Figure 420. SFT of three nominal bit periods
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.
MS31008V1
Arbitration phase
high impedance
START
BIT
INITIATOR[3:0] DESTINATION[3:0] EOM ACK
MS31009V1
last bit of previous frame
Start bit
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