Universal synchronous asynchronous receiver transmitter (USART) UM0306
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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.
Figure 162. CTS flow control
Start
Bit
Stop
Bit
Data 2 Idle
Start
Bit
Data 3
TX
nCTS
CTS
Transmission of Data 3
Data 1
Stop
Bit
is delayed until nCTS = 0
CTS
Data 2 Data 3empty empty
Transmit data register
TDR
Writing data 3 in TDR