lab5.c

#define F_CPU 1000000UL

#include <avr/io.h>
#include <util/delay.h>
#include <stdint.h>
#include <stdio.h>

// 2400 baud @ 1 MHz: UBRR = F_CPU/(16*BAUD) - 1 = 25
#define UBRR_VAL 25

void serial_init(void)
{
	UCSRA = 0x00;
	UCSRB = (1 << TXEN);
	UCSRC = (1 << URSEL) | (1 << UCSZ1) | (1 << UCSZ0);  // async, 8N1
	UBRRL = UBRR_VAL;
	UBRRH = 0x00;
}

int serial_send(char c, FILE *stream)
{
	while (!(UCSRA & (1 << UDRE)));
	UDR = c;
	return 0;
}

FILE uart_out = FDEV_SETUP_STREAM(serial_send, NULL, _FDEV_SETUP_WRITE);

void adc_init(void)
{
	DDRA  &= ~(1 << DDA6);  // PA6 as input
	PORTA &= ~(1 << PA6);   // no pull-up

	ADMUX  = (1 << REFS0) | (1 << MUX1) | (1 << MUX0);       // AVcc ref, ADC6
	ADCSRA = (1 << ADEN)  | (1 << ADPS1) | (1 << ADPS0);     // enable, /8 prescaler
}

uint16_t adc_read(void)
{
	ADCSRA |= (1 << ADSC);
	while (ADCSRA & (1 << ADSC));
	
	uint8_t low = ADCL;
	uint8_t high = ADCH;
	return ((uint16_t)high << 8) | low;
}

int main(void)
{
	serial_init();
	stdout = &uart_out;

	adc_init();
	adc_read();  // discard first result

	uint16_t digital;
	uint32_t mv;

	while (1)
	{
		digital = adc_read();
		mv = ((uint32_t)digital * 5000UL) / 1024UL;

		printf("ADC Digital Value = %4u  |  Input Voltage = %4lu mV\r\n",
		digital, (unsigned long)mv);

		_delay_ms(500);
	}

	return 0;
}