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I'm currently using an ultrasonic proximity sensor to measure how much of the water in a tank is filled up to. I'm starting by simply trying to receive a distance measurement between the sensor and the object in which the wave is reflected by. I've tried building my own code (in C language) however I'm not quite sure where is my issue. I have the code written to display on the LCD. The numbers in which I am getting are 669, A45, h45. Below is the commented function that I have written to perform this task. 

void distance_sensor(void)
{
    DDRC= 0x0F;    //enables only one nybble to be an output

    double timecnt;
    double distance;
    int echo;

    char distb [16] = {"Container up to:"};
    lcd_write(distb, 16, line_1);  //displays banner on LCD

    echo = (PINC & 0x10);  //declares the input echo from the 2nd nybble

    distance= 0;           //initialization
    timecnt = 0;

    PORTC = 0x00;
    _delay_ms(1.0);

    PORTC = 0x0F;
    _delay_ms(5.2);        //creating a 10 us output square wave and sending it out through port c
    PORTC = 0x00;

    while(echo==0) 
    {
        echo = (PINC & 0x10);   //rechecking the input
        timecnt = timecnt + 1;  //time taken for the wave to reflect back to the sensor             
        _delay_us(100.0);
    }

    timecnt= timecnt*100;
    distance = (timecnt*0.000343)/2;  
    hextodec(distance);         //function made to split the value into msd, nsd, lsd and displays on LCD

    return; 
}
Mary
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    Have you tested this against a known distance? If you're seeing "h45" on the display then fix your display code first, 'h' isn't a valid number unless you're using base-18 or above. – yano Feb 04 '20 at 00:44
  • yes, I have but the confusing part is when had the object at that same distance and reloaded the code I get a different number every time. I also tried using the equation distance = (timcnt /2)/29;(supposed to be distance in cm) but still got a weird number. – Mary Feb 04 '20 at 00:49
  • Don't forget that the time you get for an echo is round-trip, so divide in half for distance. – stark Feb 04 '20 at 00:52
  • displaying 'h' where a number should be still makes no sense. Forget the sensor and figure that out first. Maybe you're passing a `double` to a function that expects an `int`? Can you hardcode 100.0, 324.534, 24.5, and they all show up on the display correctly? – yano Feb 04 '20 at 00:59
  • The maths in the code seems right to me (assuming the sound is travelling through air at 20 degrees Celsius and that you want distance in meters). – Brendan Feb 04 '20 at 01:02
  • i think that can be my problem to displaying the value. My function hextodec does expect an int not a double (will fix that). but if the rest of the code is right, why would reading not change while I move the object forward and backwards? – Mary Feb 04 '20 at 01:34
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    Read the datasheet that came with the sensor. – TomServo Feb 04 '20 at 02:16
  • Not related to the problem, but need to be fixed: `char distb [16] = {"Container up to:"};` is 1 character short because of the terminating '\0'. Simply use `char distb [] = "Container up to:";` and the compiler allocates enough memory; additionally you don't need the curly braces. -- You don't need the `return` at the end, the compiler inserts one automatically. – the busybee Feb 04 '20 at 06:51

1 Answers1

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As soon as a high level on the TRIG input appears, the ultrasonic module sets the low level at the ECHO output and emits a burst of sound pulses. If there is an obstacle, the sound burst is being reflected back as an echo. As soon as the module detects the echo, it sets the high level at the ECHO output.

Therefore, the distance equals time between emitting sound and receiving echo multiplied by the speed of sound in air and divided by two (because the sound travels the distance forth and back)

FYI in dry air at +20 °C, the speed of sound is 343 meters per second.

The formula is your code is correct

But there are several issues in the code.

1)

    PORTC = 0x0F;
    _delay_ms(5.2);        //creating a 10 us output square wave and sending it out through port c
    PORTC = 0x00;

Do not confuse ms (milliseconds) and μs (microsecods).

within 5.2 ms a sound travels 178 cm, therefore you can get wrong results when you start counting echo.

By the why, why you set the high level on 4 outputs? You need to set only at that pin, where TRIG is connected. (I don't know which is it, let's say it is PC2)

PORTC = 0x0F;

    PORTC |= (1 << 2); // set high level at PC2
    _delay_us(10);        //creating a 10 us output square wave and sending it out through port c
    PORTC &= ~(1 << 2); // set low level at PC2

2) You're initializing echo as 

echo = (PINC & 0x10);  //declares the input echo from the 2nd nybble

That is wrong. First, because of the ECHO signal has no sense before TRIG is triggered, and second, because ECHO remains high since the previous measurement. Therefore the echo variable will be initialized to non-zero and while(echo==0) loop will never be entered.

3) If the module was not able to detect the reflected echo, it will never set the high level on the ECHO output. Therefore you need to limit somehow how long the loop is waiting, otherwise, it could never end.

4) I don't know what is hextodec and how it works when a floating-point number is passed to it. Are you sure it will correctly display floating-point numbers less than 1.0? I'm pretty sure it will not try to scale the number up and convert it to an integer:

hextodec(lround(distance * 1000));
AterLux
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