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Lets write simple example on using AVR comparator module. Program is written in C and compiled with AVR-GCC compiler(WinAVR). Program example illustrates the case when Positive Comparator input is connected to Internal voltage regulator 1.23V by setting ACGB bitin ACSR register. Also as negative comparator input is taken ADC3 input via Multiplexer. As indicator LED is connected to PORTD0 pin. If input voltage is lower than 0.23V then Diode is OFF and if voltage exceed 1.23V diode lights ON. Diode state is changed with ANA_COMP_vect interrupt service routine on ACO bit toggle in ACSR register. 
Working program code:
// *********************************************************** // Project: Analog Comparator example // Author: winavr.scienceprog.com // Module description: Analog comparator example with positive comparator // input connected to Vref 1.23V. When compared voltage exceed 1.23V LED lighst on. // When voltage drops bellow - LED turns OFF. Comparator inputis connected to ADC3 pin. // *********************************************************** #include <avr\io.h> #include <avr\interrupt.h> #define AINpin PA3 #define LED PD0 void Init(){ DDRA&=~(1<<AINpin);//as input PORTA&=~(1<<AINpin);//no Pull-up DDRD|=(1<<LED); //Led pin as output PORTD|=(1<<LED);//Initally LED is OFF SFIOR|=(1<<ACME);//enable multiplexer ADCSRA&=~(1<<ADEN);//make sure ADC is OFF ADMUX|=(0<<MUX2)|(1<<MUX1)|(1<<MUX0); //select ADC3 as negative AIN ACSR|= (0<<ACD)| //Comparator ON (1<<ACBG)| //Connect 1.23V reference to AIN0 (1<<ACIE)| //Comparator Interrupt enable (0<<ACIC)| //input capture disabled (0<<ACIS1)| //set interrupt on output toggle (0<<ACIS0); sei();//enable global interrupts } // Interrupt handler for ANA_COMP_vect // ISR(ANA_COMP_vect) { if bit_is_clear(ACSR, ACO) PORTD&=~(1<<LED);//LED is ON else PORTD|=(1<<LED);//LED is OFF } // *********************************************************** // Main program // int main(void) { Init(); while(1) { // Infinite loop; interrupts do the rest } }
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