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 (DriverEnable) signal. The assertion time is the time between the activation of the DE signal andthe beginning of the START bit. It is programmed using the DEAT [4:0] bit fields in theUSART_CR1 control register. The de-assertion time is the time between the end of the laststop bit, in a transmitted message, and the de-activation of the DE signal. It is programmedusing the DEDT [4:0] bit fields in the USART_CR1 control register. The polarity of the DEsignal 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).