Universal synchronous asynchronous receiver transmitter (USART) RM0351
1342/1830 DocID024597 Rev 5
Figure 429. RS232 CTS flow control
Note: For correct behavior, CTS 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 allows 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] bit fields in the
USART_CR1 control register. The de-assertion 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] bit fields 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 duration,
depending on the oversampling rate).
40.5.17 Wakeup from Stop mode using USART
The USART is able to wake up the MCU from Stop mode when the UESM bit is set and the
USART clock is set to HSI16 or LSE (refer to Section Reset and clock control (RCC)).
• USART source clock is HSI
If during stop mode the HSI clock is switched OFF, when a falling edge on the USART
receive line is detected, the USART interface requests the HSI clock to be switched
ON. The HSI clock is then used for the frame reception.
– If the wakeup event is verified, the MCU wakes up from low-power mode and data
reception goes on normally.
– If the wakeup event is not verified, the HSI clock is switched OFF again, the MCU
is not waken up and stays in low-power mode and the clock request is released.
• USART source clock is LSE
Same principle as described in case of USART source clock is HSI with the difference
that the LSE is ON in stop mode, but the LSE clock is not propagated to USART if the
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