During transmission, data is written into the transmit FIFO. When the UART is enabled, it causes a data frame to start transmitting with the parameters indicated in the Line Control register, UART LINE_CTRL . Data continues to be transmitted until there is no data left in the transmit FIFO. The BUSY signal goes High as soon as data is written to the transmit FIFO (that is, the FIFO is non-empty) and remains asserted High while data is being transmitted. BUSY is negated only when the transmit FIFO is empty, and the last character has been transmitted from the shift register, including the stop bits.