external reference SD16 for MSP430F2013OPW

I added  1.225V external reference to MSP430F2013  pin 5.  I using ADC channel A3, P1.6 pin 8. If I test SD16 with external reference it is works fine, when I modified code for external reference - the first reading is good then 0xffff . What could be wrong ?

void main(void)
{
  volatile unsigned int i;                  // Use volatile to prevent removal
 
  WDTCTL = WDTPW + WDTHOLD;                 // Stop watchdog timer

  P1OUT = 0;
  P1DIR |= 0xB7;                         // Set P1.0 to output direction

if(1)  // external ref
{
  P1SEL |= 0x08;                             // ext 1.2V ref,
  SD16CTL = SD16SSEL_1 ;                     // SMCLK+
}
if(0) // internal ref
{
  SD16CTL = SD16REFON + SD16SSEL_1;         // 1.2V ref, SMCLK
}

 
  SD16INCTL0 = SD16INCH_3;                  // A3+/-
  SD16CCTL0 =  SD16UNI + SD16IE + SD16OSR_1024;//+ SD16VMIDON;            // 256OSR, unipolar, interrupt enable+
  SD16AE = SD16AE3;                         // P1.7 A3+, A3- = VSS
  SD16CCTL0 |= SD16SC;                      // Set bit to start conversion+

  for (i = 0; i < 0x3600; i++);             // Delay for 1.2V ref startup

  _BIS_SR(LPM0_bits + GIE);
}

#pragma vector = SD16_VECTOR

__interrupt void SD16ISR(void)
{
  static   unsigned long int reading_mvolts;
  static   unsigned int reading_counts;

  reading_counts = SD16MEM0;
  reading_mvolts = ((1225 * reading_counts)) /65536;

  if (reading_counts < RANGE1_COUNTS)
  {
       P1OUT &= ~0x01;
  }
  else
  {
   P1OUT = 0x01;
  }
}