ST STM32WL5 Series Reference Manual page 1149

RM0453
These modes are the following:
•
Mode 0: Any character starting with a bit at '1'.
In this case the USART measures the duration of the start bit (falling edge to rising
edge).
•
Mode 1: Any character starting with a 10xx bit pattern.
In this case, the USART measures the duration of the Start and of the 1st data bit. The
measurement is done falling edge to falling edge, to ensure a better accuracy in the
case of slow signal slopes.
•
Mode 2: A 0x7F character frame (it may be a 0x7F character in LSB first mode or a
0xFE in MSB first mode).
In this case, the baud rate is updated first at the end of the start bit (BRs), then at the
end of bit 6 (based on the measurement done from falling edge to falling edge: BR6).
Bit0 to bit6 are sampled at BRs while further bits of the character are sampled at BR6.
•
Mode 3: A 0x55 character frame.
In this case, the baud rate is updated first at the end of the start bit (BRs), then at the
end of bit0 (based on the measurement done from falling edge to falling edge: BR0),
and finally at the end of bit6 (BR6). Bit 0 is sampled at BRs, bit 1 to bit 6 are sampled at
BR0, and further bits of the character are sampled at BR6. In parallel, another check is
performed for each intermediate RX line transition. An error is generated if the
transitions on RX are not sufficiently synchronized with the receiver (the receiver being
based on the baud rate calculated on bit 0).
Prior to activating the auto baud rate detection, the USART_BRR register must be initialized
by writing a non-zero baud rate value.
The automatic baud rate detection is activated by setting the ABREN bit in the USART_CR2
register. The USART then waits for the first character on the RX line. The auto baud rate
operation completion is indicated by the setting of the ABRF flag in the USART_ISR
register. If the line is noisy, the correct baud rate detection cannot be guaranteed. In this
case the BRR value may be corrupted and the ABRE error flag is set. This also happens if
the communication speed is not compatible with the automatic baud rate detection range
(bit duration not between 16 and 65536 clock periods (oversampling by 16) and not between
8 and 65536 clock periods (oversampling by 8)).
The auto baud rate detection can be re-launched later by resetting the ABRF flag (by writing
a '0').
When FIFO management is disabled and an auto baud rate error occurs, the ABRE flag is
set through RXNE and FE bits.
When FIFO management is enabled and an auto baud rate error occurs, the ABRE flag is
set through RXFNE and FE bits.
If the FIFO mode is enabled, the auto baud rate detection should be made using the data on
the first RXFIFO location. So, prior to launching the auto baud rate detection, make sure
that the RXFIFO is empty by checking the RXFNE flag in USART_ISR register.
Note: The BRR value might be corrupted if the USART is disabled (UE = 0) during an auto baud
rate operation.
Universal synchronous/asynchronous receiver transmitter (USART/UART)
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