RA_SERIAL_CLKDIV
(RA_UART_FREQ / RA_SERIAL_CLKDIV / RA_BAUDRATE)
(RA_UART_FREQ / RA_SERIAL_CLKDIV / RA_BAUDRATE) & 0xffff);
((RA_UART_FREQ / RA_SERIAL_CLKDIV / RA_BAUDRATE) & 0xffff) >> 8);