Figure 267. Rts Flow Control; Figure 268. Cts Flow Control - ST STM32F412 Reference Manual

Universal synchronous receiver transmitter (USART) /universal asynchronous receiver transmit-
RTS flow control
If the RTS flow control is enabled (RTSE=1), then nRTS is asserted (tied low) as long as the
USART receiver is ready to receive a new data. When the receive register is full, nRTS is
deasserted, indicating that the transmission is expected to stop at the end of the current
frame. Figure 267
RX
nRTS
CTS flow control
If the CTS flow control is enabled (CTSE=1), then the transmitter checks the nCTS input
before transmitting the next frame. If nCTS is asserted (tied low), then the next data is
transmitted (assuming that a data is to be transmitted, in other words, if TXE=0), else the
transmission does not occur. When nCTS is deasserted during a transmission, the current
transmission is completed before the transmitter stops.
When CTSE=1, the CTSIF status bit is automatically set by hardware as soon as the nCTS
input toggles. It indicates when the receiver becomes ready or not ready for communication.
An interrupt is generated if the CTSIE bit in the USART_CR3 register is set. The figure
below shows an example of communication with CTS flow control enabled.
nCTS
Transmit data register
TDR
TX
798/1163
shows an example of communication with RTS flow control enabled.

Figure 267. RTS flow control

Start
Data 1
bit

Figure 268. CTS flow control

Data 2 empty
Stop
Start
Data 1
bit
bit
Writing data 3 in TDR
RM0402 Rev 6
Stop Start
Idle
bit bit
RXNE
Data 1 read
Data 2 can now be transmitted
CTS
Data 3
Stop
Data 2
bit
Transmission of Data 3 is
delayed until nCTS = 0
Stop
Data 2
bit
RXNE
MSv31168V1
CTS
empty
Start
Idle
Data 3
bit
MSv31167V1