如何设置外部晶振为系统的默认时钟?

 尊敬的TI工程师，你们好，我想设置芯片F5172的外部晶振为系统的默认时钟，我看了芯片手册以及一些参考，配置了芯片，但是还存在问题，请工程师们给我纠正一下，以下是我的代码配置：

void init_clock()
{
  WDTCTL = WDTPW + WDTHOLD;                 // Stop WDT
   PJSEL |= BIT4+BIT5;                       // Port select XT1

  UCSCTL6 &= ~(XT1OFF);                     // XT1 On
  UCSCTL6 |= XCAP_0;                        // Internal load cap
  UCSCTL6 |= XT1DRIVE_3;
  UCSCTL3 = 0;                              // FLL Reference Clock = XT1

  // Loop until XT2 & DCO stabilizes - In this case loop until XT1 and DCo settle
  do
  {
    UCSCTL7 &= ~(XT1LFOFFG + XT1HFOFFG + DCOFFG);
                                            // Clear XT1,DCO fault flags
    SFRIFG1 &= ~OFIFG;                      // Clear fault flags
  }while (SFRIFG1&OFIFG);                   // Test oscillator fault flag
  
  UCSCTL6 &= ~(XT1DRIVE_3);                 // Xtal is now stable, reduce drive strength

  UCSCTL4 |= SELA_0;                        // ACLK = LFTX1 (by default)
    // UCSCTL4 |=SELA__XT1CLK+SELS__XT1CLK+SELM__XT1CLK;
  __bis_SR_register(GIE);             // Enter LPM3
  __no_operation(); 
}