MSP430串口0通信，回显收到的字符。波特率19200.-USART0, UART 19200 Echo ISR, XT2 HF XTAL ACLK. Echo a received character, RX ISR used. Normal mode is LPM0,USART0 RX interrupt triggers TX Echo. Though not required, MCLK = XT2.ACLK = n/a, MCLK = SMCLK = UCLK0 = XT2 = 8MHz