msp430f5310 XT2外部晶振无法工作，求解！！！

//外部晶振 8M 电容 20pF     

// MCLK=SMCLK=XT2(8M)  ACLK=32768KHz

    P5SEL |= 0x0C;                              // Port select XT2
   
    UCSCTL6 &= ~(XT2OFF);                       // Set XT2 On
    UCSCTL3 |= SELREF_2;
    UCSCTL4 |= SELA_2;
    UCSCTL6 |= XT2BYPASS;
  // Loop until XT1,XT2 & DCO stabilizes
  do
  {
    UCSCTL7 &= ~(XT2OFFG + XT1LFOFFG + DCOFFG);// Clear XT2,XT1,DCO fault flags
    SFRIFG1 &= ~OFIFG;                      // Clear fault flags
    for(unsigned  char t=0xFF;t>0;t--);
  }while (SFRIFG1&OFIFG);                   // Test oscillator fault flag
      
   UCSCTL6 &= ~ XT2DRIVE1;                   //01:8MHz~16MHz , dafault is 11:26MHz~32MHz
   UCSCTL4 |= SELS__XT2CLK + SELM__XT2CLK;   //SOURCE SEL
   UCSCTL5 |= DIVM__1 + DIVS__1 ;

结果：SMCLK才1MHz ，怀疑可能还是使用的是内部1MHz，求解！！！谢谢