Universal synchonous receiver transmitter (USART) RM0444
1044/1390 RM0444 Rev 5
Figure 338. RS232 CTS flow control
Note: For correct behavior, nCTS must be asserted at least 3 USART clock source periods before
the end of the current character. In addition it should be noted that the CTSCF flag may not
be set for pulses shorter than 2 x PCLK periods.
RS485 driver enable
The driver enable feature is enabled by setting bit DEM in the USART_CR3 control register.
This enables the user to activate the external transceiver control, through the DE (Driver
Enable) signal. The assertion time is the time between the activation of the DE signal and
the beginning of the start bit. It is programmed using the DEAT [4:0] bitfields in the
USART_CR1 control register. The deassertion time is the time between the end of the last
stop bit, in a transmitted message, and the de-activation of the DE signal. It is programmed
using the DEDT [4:0] bitfields in the USART_CR1 control register. The polarity of the DE
signal can be configured using the DEP bit in the USART_CR3 control register.
In USART, the DEAT and DEDT are expressed in sample time units (1/8 or 1/16 bit time,
depending on the oversampling rate).
MSv31167V1
Start
bit
Stop
bit
TX
TDR
CTS
Data 1
Data 2
Stop
bit
Idle
Start
bit
Data 2 Data 3
Data 3
empty empty
CTS
nCTS
Transmit data register
Writing data 3 in TDR
Transmission of Data 3 is
delayed until nCTS = 0
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