Interfacing Ultrasonic Sensors with Atmega

About Ultrasonic Sensors:

An ultrasonic sensor module has an ultrasonic transmitter, receiver and a control circuit.
It has 4 pins:
     Trigger
     Echo
     Vcc
     Ground

When we give trigger signal, the module will send 8 cycles of 40kHz ultrasonic pulses. These pulses go into the air, and at the same time, the ECHO pin becomes HIGH.  After reflection from any any object/obstacle on its path they are reflected back and are received by the receiver. Now the ECHO pin goes to LOW state.

So, our code has to perform two task:
     1. Give Trigger Signal.
     2. Based on the time for which ECHO pin remains HIGH, calculate the distance.

We can keep a note of time by two methods:
    1. Polling. (Run a while loop).
    2. Interrupt. (Stop the timer when ECHO pin becomes LOW).

Calculating Distance:

   

       Velocity of ultrasonic signal =  velocity of sound waves = v = 340 m/s.

       Distance in cm = v * t (in microseconds) / 2. 

       d =  t/58 cm.

Based on the above calculations, the code for Atmega32 is:

#include<avr/io.h>

#include<util/delay.h>

#include<compat/deprecated.h>

#include"uart.c"

#include"uart.h"

#include<avr\interrupt.h>

#include<stdlib.h>

uint8_t counter;

void ultraj(void);

int calculate(void);

void main()

{

DDRC = 0xFF;

PORTC = 0;

uart_init(UART_BAUD_SELECT(9600,F_CPU));

while(1)

{

ultraj();

}

}

void ultraj()

{

PORTC = 0xFF;

   DDRA = 0x01;

char buf[40] = {0};

int distance;

cbi(PORTA,0);

_delay_us(10);

sbi(PORTA,0);

_delay_us(10);

cbi(PORTA,0);

TCNT0 = 0x00;

TIMSK = (1<<TOIE0);

cbi(PORTA,1);

sei();

while(!(PINA&(1<<1)));

TCCR0 = 0x03;

while(PINA&(1<<1));

TCCR0 = 0x00;

distance =  calculate();

itoa(distance,buf,10);

uart_puts("disttnace :  ");

uart_puts(buf);

uart_puts("\n\r");

_delay_ms(70);

}

int calculate()

{

int timea,timeb,timec,timed;

int dist;

char chara[20] = {0};

timea = counter*255;

timeb = TCNT0+timea;

timec = timeb*2.44;

timed = (timec/58);

itoa(timed,chara,10);

uart_puts(chara);

dist = timed;

return dist;

}

ISR(TIMER0_OVF_vect)

{

counter++;

}