ST STM32F423 Reference Manual page 886

Universal synchronous receiver transmitter (USART) /universal asynchronous receiver transmit-
RX: Receive Data Input is the serial data input. Oversampling techniques are used for data
recovery by discriminating between valid incoming data and noise.
TX: Transmit Data Output. When the transmitter is disabled, the output pin returns to its I/O
port configuration. When the transmitter is enabled and nothing is to be transmitted, the TX
pin is at high level. In single-wire and smartcard modes, this I/O is used to transmit and
receive the data (at USART level, data are then received on SW_RX).
Through these pins, serial data is transmitted and received in normal USART mode as
frames comprising:
•
An Idle Line prior to transmission or reception
•
A start bit
•
A data word (8 or 9 bits) least significant bit first
•
0.5,1, 1.5, 2 Stop bits indicating that the frame is complete
•
This interface uses a fractional baud rate generator - with a 12-bit mantissa and 4-bit
fraction
•
A status register (USART_SR)
•
Data Register (USART_DR)
•
A baud rate register (USART_BRR) - 12-bit mantissa and 4-bit fraction.
•
A Guardtime Register (USART_GTPR) in case of Smartcard mode.
Refer to
The following pin is required to interface in synchronous mode:
•
SCLK: Transmitter 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 (e.g. LCD drivers). The clock phase and polarity
are software programmable. In smartcard mode, SCLK can provide the clock to the
smartcard.
The following pins are required in Hardware flow control mode:
•
nCTS: Clear To Send blocks the data transmission at the end of the current transfer
when high
• nRTS: Request to send indicates that the USART is ready to receive a data (when
low).
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Section 28.6: USART registers for the definition of each bit.
RM0430 Rev 8