Problems of initializing MSP430F5529 with external oscillator of 20 Mhz

Hi, I am working on a project that needs to use an external oscillator of 20Mhz. The chip outputs the MCLK to P7.7.

The clock seems running OK if I commented "analog_tx();" in the while loop,  but it becomes distorted or unstable when I put that line back in. If I add more lines in subroutine

analog_tx(), the chip wouldn't even run.

I wonder if I have initialized the clock wrong, or it was an oscillator problem. Thank you for your help!

The code is:

#include "msp430f5529.h"

void SetVCoreUp(unsigned int level);
void analog_tx (void);

unsigned int level1 = 1;
unsigned int level2 = 2;
unsigned int level3 = 3;

void main(void)
{
    WDTCTL = WDTPW + WDTHOLD;                 // Stop watchdog timer
   
    SetVCoreUp(level1);
    SetVCoreUp(level2);
    SetVCoreUp(level3);
    
    P2DIR |= BIT2;                            // SMCLK set out to pins
    P2SEL |= BIT2;                        
    P7DIR |= BIT7;                            // MCLK set out to pins
    P7SEL |= BIT7;
   
    P1SEL &= ~BIT0; // P1.0 set as I/O
    P1DIR |= BIT0;  // P1.0 as output
      
      P1SEL &= ~BIT1; // P1.1 set as I/O
    P1DIR |= BIT1;  // P1.1 as output
 
    P5SEL |= BIT2+BIT3;                       // Port select XT2
    UCSCTL6 |= XT2BYPASS;
    UCSCTL6 &= ~XT2OFF; //enables xt2

   
    do {
        UCSCTL7 &= ~XT2OFFG;        // Clear XT2 fault flags
        __delay_cycles(10000);
    }while (UCSCTL7&XT2OFFG);
   
    UCSCTL5 = 0x0000; // divide by 1
    UCSCTL4 = SELA_5 + SELS_5 + SELM_5;

 
    __bis_SR_register(GIE);
      
    while(1) {
        analog_tx();
    }
   
}
void SetVCoreUp(unsigned int level)
{
  // Open PMM registers for write
  PMMCTL0_H = PMMPW_H;          
  // Set SVS/SVM high side new level
  SVSMHCTL = SVSHE + SVSHRVL0 * level + SVMHE + SVSMHRRL0 * level;
  // Set SVM low side to new level
  SVSMLCTL = SVSLE + SVMLE + SVSMLRRL0 * level;
  // Wait till SVM is settled
  while ((PMMIFG & SVSMLDLYIFG) == 0);
  // Clear already set flags
  PMMIFG &= ~(SVMLVLRIFG + SVMLIFG);
  // Set VCore to new level
  PMMCTL0_L = PMMCOREV0 * level;
  // Wait till new level reached
  if ((PMMIFG & SVMLIFG))
    while ((PMMIFG & SVMLVLRIFG) == 0);
  // Set SVS/SVM low side to new level
  SVSMLCTL = SVSLE + SVSLRVL0 * level + SVMLE + SVSMLRRL0 * level;
  // Lock PMM registers for write access
  PMMCTL0_H = 0x00;
}
void analog_tx (void)
{
    P1OUT ^= 0x01;
    P1OUT ^= 0x02;
}