MSPG2553的一个ADC10的官方例程有一句没看懂，求大神指点
//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)
}
A1通道与0.5倍的VCC比较，通过LED的OFF或ON来显示，代码中的这个ADC10MEM<0x1FF不懂什么意思，求大神指点 10-bit 的ADC，数字表示的范围0x000~0x3fff，代表0～AVcc，0.5*AVcc电平数值为0x1ff，剩下的就是注文中的“ If A1 > 0.5*AVcc, P1.0 set, else reset.”
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