#include <avr/io.h>
#include <avr/interrupt.h>
#include <avr/sleep.h>

void setup_adc() {
    //enable interrupts
    sei();
    // setup external reference voltage.
    ADMUX &= ~(3UL << REFS0);
    ADMUX |= (1UL << REFS0);
    // set left adjusted ADC result so we can easily only use the upper 8 bits of the result
    ADCSRB |= (1UL << ADLAR);
    // enable ADC interrupt
    ADCSRA |= (1UL << ADIE);
}

void setup_timers() {
    //use system clock for timers, no prescaler.
    TCCR0B &= ~(7UL << CS00);
    TCCR0B |= (1UL << CS00);

    //set mode 5 (PWM, phase correct, top defined by OCRA)
    TCCR0A &= ~(3UL << WGM00);
    TCCR0B &- ~(1UL << WGM02);
    TCCR0A |= (1UL << WGM00);
    TCCR0B |= (1UL << WGM02);
}

void enable_ADC() {
    PRR &= ~(1UL << PRADC);
    ADCSRA |= (1UL << ADEN);
}

void disable_ADC() {
    ADCSRA &= ~(1UL << ADEN);
    PRR |= (1UL << PRADC);
}

void select_ADC_channel(int channel) {
    ADMUX &= ~(7UL << MUX0);
    ADMUX |= (channel << MUX0);
}

uint8_t get_ADC_result() {
    uint8_t result = ADCL;
    //conversion would be blocked if we don't also read ADCH although we won't use the upper 2 bits.
    uint8_t unblock = ADCH; 
    return result;
}

void start_ADC() {
    ADCSRA |= (1UL << ADSC);
}

volatile uint8_t fan1;
volatile uint8_t fan2;

void start_polling_fans() {
    PRR &= ~(1UL << PRADC);
    enable_ADC();
    select_ADC_channel(ADC1D);
    start_ADC();
}

ISR(ADC_vect) {
    if(ADMUX & (7UL << MUX0) == ADC1D) {
        fan1 = get_ADC_result();
        select_ADC_channel(ADC2D);
        start_ADC();
        return;
    }

    if(ADMUX & (7UL << MUX0) == ADC2D) {
        fan2 = get_ADC_result();
        disable_ADC();
        return;
    }
}

int main() {
    set_sleep_mode(0);
    setup_adc();
    setup_timers();

    while(1);
}