RM0367 Rev 7 775/1043
RM0367 Universal synchronous/asynchronous receiver transmitter (USART/UART)
872
If neither conditions a. or b. are met, the start detection aborts and the receiver returns to the
idle state (no flag is set).
Character reception
During an USART reception, data shifts in least significant bit first (default configuration)
through the RX pin. In this mode, the USART_RDR register consists of a buffer (RDR)
between the internal bus and the receive shift register.
Character reception procedure
1. Program the M bits in USART_CR1 to define the word length.
2. Select the desired baud rate using the baud rate register USART_BRR
3. Program the number of stop bits in USART_CR2.
4. Enable the USART by writing the UE bit in USART_CR1 register to 1.
5. Select DMA enable (DMAR) in USART_CR3 if multibuffer communication is to take
place. Configure the DMA register as explained in multibuffer communication.
6. Set the RE bit USART_CR1. This enables the receiver which begins searching for a
start bit.
For code example, refer to A.17.4: USART receiver configuration code example.
When a character is received:
• The RXNE bit is set to indicate that the content of the shift register is transferred to the
RDR. In other words, data has been received and can be read (as well as its
associated error flags).
• An interrupt is generated if the RXNEIE bit is set.
• The error flags can be set if a frame error, noise or an overrun error has been detected
during reception. PE flag can also be set with RXNE.
• In multibuffer, RXNE is set after every byte received and is cleared by the DMA read of
the Receive data Register.
• In single buffer mode, clearing the RXNE bit is performed by a software read to the
USART_RDR register. The RXNE flag can also be cleared by writing 1 to the RXFRQ
in the USART_RQR register. The RXNE bit must be cleared before the end of the
reception of the next character to avoid an overrun error.
For code example, refer to A.17.5: USART receive byte code example.
Break character
When a break character is received, the USART handles it as a framing error.
Idle character
When an idle frame is detected, there is the same procedure as for a received data
character plus an interrupt if the IDLEIE bit is set.
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