Universal synchronous/asynchronous receiver transmitter (USART/UART) RM0440
1604/2126 RM0440 Rev 4
Figure 533. TC/TXE behavior when transmitting
Note: When FIFO management is enabled, the TXFNF flag is used for data transmission.
Break characters
Setting the SBKRQ bit transmits a break character. The break frame length depends on the
M bit (see Figure 531).
If a ‘1’ is written to the SBKRQ bit, a break character is sent on the TX line after completing
the current character transmission. The SBKF bit is set by the write operation and it is reset
by hardware when the break character is completed (during the stop bits after the break
character). The USART inserts a logic 1 signal (stop) for the duration of 2 bits at the end of
the break frame to guarantee the recognition of the start bit of the next frame.
When the SBKRQ bit is set, the break character is sent at the end of the current
transmission.
When FIFO mode is enabled, sending the break character has priority on sending data even
if the TXFIFO is full.
Idle characters
Setting the TE bit drives the USART to send an idle frame before the first data frame.
37.5.6 USART receiver
The USART can receive data words of either 7 or 8 or 9 bits depending on the M bits in the
USART_CR1 register.
Start bit detection
The start bit detection sequence is the same when oversampling by 16 or by 8.
In the USART, the start bit is detected when a specific sequence of samples is recognized.
This sequence is: 1 1 1 0 X 0 X 0X 0X 0 X 0X 0.
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