求助！msp430f5438 内部参考电压输出不正确

想输出内部参考电压2.5V

P6SEL |= 0x01; // Enable A/D channel A0

P5SEL |= 0x01;

ADC12CTL0 = ADC12ON+ADC12SHT02+ADC12REFON+ADC12REF2_5V;

ADC12CTL1 = ADC12SHP;

ADC12MCTL0 = ADC12SREF_1;

ADC12CTL2 |= ADC12REFOUT;

for ( i=0; i<0x30; i++);                  // Delay for reference start-up

ADC12CTL0 |= ADC12ENC;                    // Enable conversions

while (1)

 {

   ADC12CTL0 |= ADC12SC;                   // Start conversion

   while (!(ADC12IFG & BIT0));

   __no_operation();                       // SET BREAKPOINT HERE

 }

用示波器测端口P5.0，测得电压一直不对(测得3.1V左右)，到底哪里错了？各位帮帮忙啊