gistfile1.c

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

int main(void) {
	cli(); // disable interrupts for prescaler set
	CLKPR = (1 << CLKPCE); // set prescaler 1x
	CLKPR = 0;
	sei();
	DDRD |= (1 << 0) | (1 << 1); //portd0 portd1 output
	PORTD &= ~(1 << 0); //ensure portd0 low
	DDRB |= (1 << 1); //portb1 (ocr1a) output
	PRR &= ~(1 << PRADC); //ensure adc power is on
	ADCSRA |= (1 << ADEN) | (1 << ADIE) | (1 << ADATE); //adc interrupt
	ADCSRB = 0; //free running mode
	ADMUX = 0; //zero admux to ensure selection of adc0
	ADMUX |= (1 << ADLAR) | (1 << REFS0); //left-adjust adc result
	ADCSRA |= (1 << ADSC); //start conversion
	TCCR1A |= (1 << COM1A0); //ocr1a compare on toggle
	OCR1A = 1024;
	TCCR1B |= (1 << CS10) | (1 << WGM12); //timer1 ctc mode /1 scale
	for (;;) {
	}
}

ISR(ADC_vect) {
	OCR1A = ADCH;
}