DocID024597 Rev 3 1181/1693
RM0351 Universal synchronous asynchronous receiver transmitter (USART)
1282
Serial data are transmitted and received through these pins in normal USART mode. The
frames are comprised of:
• An Idle Line prior to transmission or reception
• A start bit
• A data word (7, 8 or 9 bits) least significant bit first
• 0.5, 1, 1.5, 2 stop bits indicating that the frame is complete
• The USART interface uses a baud rate generator
• A status register (USARTx_ISR)
• Receive and transmit data registers (USARTx_RDR, USARTx_TDR)
• A baud rate register (USARTx_BRR)
• A guard-time register (USARTx_GTPR) in case of Smartcard mode.
Refer to Section 36.8: USART registers on page 1222 for the definitions of each bit.
The following pin is required to interface in synchronous mode and Smartcard mode:
• CK: Clock output. This pin outputs the transmitter data clock for synchronous
transmission corresponding to SPI master mode (no clock pulses on start bit and stop
bit, and a software option to send a clock pulse on the last data bit). In parallel, data
can be received synchronously on RX. This can be used to control peripherals that
have shift registers. The clock phase and polarity are software programmable. In
Smartcard mode, CK output can provide the clock to the smartcard.
The following pins are required in RS232 Hardware flow control mode:
• CTS: Clear To Send blocks the data transmission at the end of the current transfer
when high
• RTS: Request to send indicates that the USART is ready to receive data (when low).
The following pin is required in RS485 Hardware control mode:
• DE: Driver Enable activates the transmission mode of the external transceiver.
Note: DE and RTS share the same pin.
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