어? 의외로 RX busy는 없네?

UART Flag에 BUSY는

어라? 전송에 대한 것?

UART Busy When this bit is 1, the UART is busy transmitting data. This bit remains set until the complete byte, including all stop bits, has been sent from the shift register. This bit is set as soon as the transmit FIFO becomes non-empty (regardless of whether UART is enabled).