USART non interrupt driven example

usart.c

/**
 * Voltmeter based on atmega168, sends Analog2Digital conversion
 * results over RS232 to the connected computer
 *
 * Created by Lucas van Dijk
 * http://www.return1.net
 */

#ifndef F_CPU
#define F_CPU 3686400
#endif

#define BAUDRATE 9600
#define UBRR (F_CPU/(BAUDRATE*16UL))-1

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

void usart_init(uint16_t ubrr_value)
{
	UBRRH = (unsigned char) (ubrr_value >> 8);
	UBRRL = (unsigned char) ubrr_value;

	// 9600-8-E-1
	// That is, baudrate of 9600bps
	// 8 databits
	// Even parity
	// 1 stopbit
	UCSRB = (1 << TXEN) | (1 << RXEN);
	UCSRC = (1 << URSEL) | (1 << UPM1) | (1 << UCSZ1) | (1 << UCSZ0);
}

void adc_init()
{
	// Setup ADC

	// Enable the ADC unit, and use a prescaler
	// of 32 which gives us a fadc of 115kHz
	ADCSRA = (1 << ADEN) | (1 << ADPS2) | (1 << ADPS0);
}

char usart_receive()
{
	// Wait until receive is complete
	while(!(UCSRA & (1 << RXC)));

	return UDR;
}

void usart_send(char data)
{
	// Wait until receive is complete
	while(!(UCSRA & (1 << UDRE)));

	UDR = data;
}

void usart_send_string(char * str)
{	
	char current;
	do
	{
		current = *str;
		usart_send(current);

		str++;
	}
	while(current != '\0');
}

int main()
{
	// Setup USART
	usart_init(UBRR);

	// Setup ADC
	adc_init();

	// Enable interrupts
	sei();

	char data;

	usart_send_string("Started");

	while(1)
	{
		data = usart_receive();
		usart_send(data); // echo commands

		if(data == 'r')
		{
			// Start ADC Conversion
			ADCSRA |= (1 << ADSC);

			// Wait untill conversion is completed
			while(!(ADCSRA & (1 << ADIF)));

			// Convert ADC result to ASCII
			// And send it through USART
			char result[5];
			uint16_t adc_value = ADC;
			itoa(adc_value, result, 10);

			// Reset ADC Flag
			ADCSRA |= (1 << ADIF);

			usart_send_string(result);
		}
	}

	return 0;
}