//  MSP430G2x33/G2x53 Demo - ADC10, Sample A1, AVcc Ref, Set P1.0 if > 0.5*AVcc
//
//  Description: A single sample is made on A1 with reference to AVcc.
//  Software sets ADC10SC to start sample and conversion - ADC10SC
//  automatically cleared at EOC. ADC10 internal oscillator times sample (16x)
//  and conversion. In Mainloop MSP430 waits in LPM0 to save power until ADC10
//  conversion complete, ADC10_ISR will force exit from LPM0 in Mainloop on
//  reti. If A1 > 0.5*AVcc, P1.0 set, else reset.

#include <msp430.h>

int main(void)
{
  WDTCTL = WDTPW + WDTHOLD;                 // Stop WDT
  ADC10CTL0 = ADC10SHT_2 + ADC10ON + ADC10IE; // ADC10ON, interrupt enabled,16*ADC41CLKs
  ADC10CTL1 = INCH_1;                       // input A1
  ADC10AE0 |= 0x02;                         // PA.1 ADC option select
  P1DIR |= 0x01;                            // Set P1.0 to output direction
  P1OUT |= 0x01;

  for (;;)
  {
    ADC10CTL0 |= ENC + ADC10SC;             // Sampling and conversionstart
    __bis_SR_register(CPUOFF + GIE);        // LPM0, ADC10_ISR will force exit
    if (ADC10MEM < 0x1FF)
      P1OUT &= ~0x01;                       // Clear P1.0 LED off
    else
      P1OUT |= 0x01;                        // Set P1.0 LED on
  }
}

// ADC10 interrupt service routine
#pragma vector=ADC10_VECTOR
__interrupt void ADC10_ISR(void)
{
  __bic_SR_register_on_exit(CPUOFF);        // Clear CPUOFF bit from 0(SR)
}

请教大神们一个问题，A1通道与0.5倍的VCC比较，这个ADC10MEM<0x1FF不懂什么意思，求大神指点