说明：  A single sample is made on A0 with reference to AVcc. Software sets ADC12SC to start sample and conversion - ADC12SC automatically cleared at EOC. ADC12 internal oscillator times sample (16x) and conversion. In Mainloop MSP430 waits in LPM0 to save power until ADC12 conversion is complete. ADC12_ISR forces exit from LPM0 in Mainloop on reti. If A0 > 0.5*AVcc P5.1 is set, else it s reset. ACLK = n/a, MCLK = SMCLK = default DCO, ADC12CLK = ADC12OSC