AT91SAM7S256 的AD转换的程序，帮看哪里出错了

#define ADC_TRIGGERENABLE (0x00 & 0x01)
#define ADC_TRIGGESELECT (0x00 & 0x07)
#define ADC_RESOLUtiON (0x00 & 0x01)
#define ADC_SLEEPMODE (0x00 & 0x01)
#define ADC_PRESCALERATE (0x08 & 0x3F)
#define ADC_STARTUPTIME (0x07 & 0x1F)
#define ADC_SAMPHOLDTIME (0x03 & 0x0F)
/* ADC register access struct for all onboard ADCs */
static AT91PS_ADC pADC = AT91C_BASE_ADC;
/*
ower management controller register access struct */
static AT91PS_PMC pPMC = AT91C_BASE_PMC;
#define MAX_ADC_CHANS 8
void adc_init()
{
unsigned int adcModeRegVal;
/* Enable the ADC clock in the
MC */
pPMC->PMC_PCER = 0x10;
/* Configure the ADC's by setting up the ADC Mode register */
adcModeRegVal = ADC_TRIGGERENABLE
+ (ADC_TRIGGESELECT << 1)
+ (ADC_RESOLUTION << 4)
+ (ADC_SLEEPMODE << 5)
+ (ADC_PRESCALERATE << 8)
+ (ADC_STARTUPTIME << 16)
+ (ADC_SAMPHOLDTIME << 24);
pADC->ADC_MR = adcModeRegVal ;
}
adc_enablechan(unsigned int chan)
{
pADC->ADC_CHER = (unsigned int)(1 << chan);
}
adc_disablechan(unsigned int chan)
{
pADC->ADC_CHDR = (unsigned int)(1 << chan);
}
int adc_readchan(unsigned int chan,unsigned int *adcData)
{
int retVal = 0;
adc_init(); // init ADCs;
adc_enablechan(chan); // Eneable chenal;
pADC->ADC_CR = 0x02; // Start acquisition
// Wait for data-ready bit to be set
do
{
;
} while (!(pADC->ADC_SR & ((unsigned int) 0x1 << chan)));
switch(chan)
{ case 6: /* MAC_ADC_AMB_TEMP - ambient temperature of board */
*adcData = pADC->ADC_CDR6;
retVal=*adcData;
break;
default:
retVal = -1;
}
adc_disablechan(chan);
return(retVal);
}
ARM是：AT91SAM7S256 的ADC， 参考电压是2.7v，用的是通道6， 我在通道6上加 2v的 模拟电压
断点看 转换后的寄存器里的值 不对 ，一会是1023，一会是345，一会是429，算出来的值和 2v 差距比较大
是不是初始化 ADC 有问题? 还是其他什么地方有问题谢谢