The user's manual: hardware states that the transmit data empty flag of the serial communications interface (SCI) is 1 when data is transferred from the TDR register to the TSR register. In the example of serial transmission flowchart, it seems that after data transmission is started, you wait for the generation of a TXI interrupt without even once writing data to the TDR.
If TXI interrupts are enabled while the TDR is empty, is an interrupt generated immediately?
If the transmit interrupt enable bit (TIE) is set to 1 in this state, a TXI interrupt is generated.
Write the first data in the TXI interrupt processing routine.