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Field TXE reader - Transmit data register empty TXE is set by hardware when the content of the USART_TDR register has been transferred into the shift register. It is cleared by writing to the USART_TDR register. The TXE flag can also be set by writing 1 to the TXFRQ in the USART_RQR register, in order to discard the data (only in Smartcard T = 0 mode, in case of transmission failure). An interrupt is generated if the TXEIE bit  = 1 in the USART_CR1 register.