5529的launchpad串口通信问题

#include <msp430f5529.h>

/*
 * main.c
 */
/*int main(void) {
    WDTCTL = WDTPW | WDTHOLD; // Stop watchdog timer
 
 return 0;
}*/
void main(void)
{
 WDTCTL = WDTPW | WDTHOLD; // Stop watchdog timer
 P4SEL = BIT4+BIT5;                            // P5_6和P5_7第二功能打开，设置方向
     UCA1CTL1 |= UCSWRST;          // 首先使RST位置位，只有这样后面的设置才有效
     UCA1CTL1 |= UCSSEL_2;                  // SMCLK，为系统时钟4MHz
     UCA1BR0 = 0x02;                            // 4MHz 115200
     UCA1BR1 = 0;                               // 4MHz 115200
     UCA1MCTL |= UCBRS_3 + UCBRF_2 + UCOS16;        // 设置调整参数UCBRSx=1, UCBRFx=0
     UCA1CTL1 &= ~UCSWRST;                // RST复位
     UCA1IE |= UCRXIE + UCTXIE;
     _EINT();
    while (1)
     {
      //
     }

}

#pragma vector=USCI_A1_VECTOR
__interrupt void USCI_A1_ISR(void)
{
 switch(__even_in_range(UCA1IV,4))
 {
 case 0:break;
 case 2:
  while (!(UCA1IFG&UCTXIFG));
  UCA1TXBUF = UCA1RXBUF;
  break;
 case 4:break;
 default: break;

 }
}

为什么这段代码运行的时候  用串口通信助手发数据没有回复= =