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UARTs
198
Serial Interfaces
The method used to identify the speed is:
1) Detect the transition 1 → 0 on the received data. This happens as soon
as a stop-to-start bit transition occurs. The transition is valid after a
majority vote on 3 sampling periods.
2) Sample the start bit duration with 115 200 *16-Hz clock frequency as long
as there is no rising edge. A transition 0 → 1 is considered as valid after
a majority vote on 3 sampling periods.
3) Compare the sampled value with a table. If the sampled value is outside
a valid range, an error is reported (no speed identified) and the hardware
goes back to the first state (1).
4) Else store the first data bit in the received register (for serial to parallel
conversion) and go to frame format identification.
The next received bits are sampled according to the programmed baud rate.
However, after 7 bits reception, the speed identification must be restarted
because several a or A characters may have been received before a valid t or
T character.
The autobauding mode is selected when MDR1[2:0] = 010. In the UART
autobauding mode, DLL, DLH, and LCR[5:0] settings are not used. Instead,
UASR is updated with the configuration detected by the autobauding logic.
SPRU760B
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