用内部ADC测量MSP430F6721的主电源AVCC的电压有0.04~0.06V的偏差

Hi All,

我用MSP430F6721内部的ADC测量主电源AVCC的电压，发现有0.04~0.06v的偏差，

但是如果我把电压切换到辅助电源AUXVCC1 （3v）, 这时候再来测AVCC电压，是非常准的。

不知为何， 请大侠指教。

附检测电压的代码：

float get_vccVoltage(void)
{
int i = 0;
int count = 0;
long lAdcVal = 0;
float voltage=0;
float averageVol = 0;

REFCTL0 |= REFMSTR | REFON;
ADC10CTL0 = ADC10SHT_8 + ADC10ON;
ADC10CTL1 = ADC10SHP;
ADC10CTL2 |= ADC10RES |ADC10SR;
ADC10MCTL0 = ADC10SREF_1 + ADC10INCH_12;
ADC10CTL0 |= ADC10ENC;

AUXADCCTL = AUXADC;

for (i = 0; i < 3; i++)
{
while(ADC10CTL0 & ADC10BUSY);
ADC10CTL0 |= ADC10ENC;
ADC10CTL0 |= ADC10SC; //start convert

while (ADC10CTL1 & ADC10BUSY)//(!(ADC10IFG & ADC10IFG0))
{
count++;
_NOP();
}
lAdcVal = ADC10MEM0;
lAdcVal = lAdcVal*CAL_ADC_GAIN_FACTOR/32768 + CAL_ADC_OFFSET;
if (lAdcVal <=0)
lAdcVal = 0;
_NOP();
voltage = (float)1.5*3 * lAdcVal/1023;
//lcd_display((int)(voltage * 1000));
//lcd_display((int)(lAdcVal));
POWER_PRINTF(printf("temp: %u, vol:%f\n", lAdcVal, voltage));
averageVol += voltage;
}

REFCTL0 = REFTCOFF;
ADC10CTL0 = 0;
AUXADCCTL = 0;
ADC10MCTL0 = 0;

return (averageVol/3);
}

Thanks,

Asky