AVR USART Example

main.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 8000000
#endif

#define FOSC 8000000
#define BAUDRATE 9600
#define UBRR (FOSC/(BAUDRATE<<4))-1

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

/**
 * Called when USART completes receiving data
 *
 * It checks if there's no error, and if the character r is received,
 * and ADC conversion is started
 */
ISR(USART_RX_vect)
{
	// Called when data received from USART

	// Read UDR register to reset flag
	unsigned char data = UDR0;

	// Check for error
	if((UCSR0A & ((1 << FE0) | (1 << DOR0) | (1 << UPE0))) == 0)
	{
		// No error occured
		if(data == 'r')
		{
			// Start ADC Conversion
			ADCSRA = (1 << ADSC);
		}
	}
}

/**
 * Called when the data register accepts new data
 *
 * When this occurs, we can write new data through the USART,
 * and in this case we write the ADCH value.
 */
ISR(USART_UDRE_vect)
{
	// Write ADC value
	UDR0 = ADCH;

	// Disable this interrupt
	UCSR0B &= ~(1 << UDRIE0);
}

/**
 * Called when the ADC completes a conversion.
 *
 * It enables the USART Data register empty interrupt,
 * so when ready, the uC can send this value back to the computer
 */
ISR(ADC_vect)
{
	static uint8_t i = 0;

	UCSR0B |= (1 << UDRIE0);

	i++;

	if(i < 6)
	{
		// Start a new conversion
		ADCSRA = (1 << ADSC);
	}
	else
	{
		i = 0;
	}
}

void usart_init(uint16_t ubrr_value)
{
	UBRR0 = ubrr_value;

	// 9600-8-E-1
	// That is, baudrate of 9600bps
	// 8 databits
	// Even parity
	// 1 stopbit
	UCSR0B = (1 << TXEN0) | (1 << RXEN0) | (1 << RXCIE0); // And enable interrupts
	UCSR0C = (1 << UPM01) | (1 << UCSZ01) | (1 << UCSZ00);
}

void adc_init()
{
	// Setup ADC
	// Enable left adjust a resolution of 8 bits is enough
	// and select first ADC channel
	ADMUX = (1 << ADLAR);

	// Enable the ADC unit, and use a prescaler
	// of 64 which gives us a fadc of 125kHz
	ADCSRA = (1 << ADEN) | (1 << ADPS2) | (1 << ADPS1);
}

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

	// Setup ADC
	adc_init();

	// Enable interrupts
	sei();

	while(1)
	{
		// Nothing to do here
	}

	return 0;
}

where the ADC is interrupt mode is enabled.