Negative values??

Forum 2005-2010 (read only) General Frequently-Asked Questions
#1 
void loop()
{
  val = analogRead(analogPin);    // read the input pin
  for(y=0;y<300;y++) {
    val=val+analogRead(analogPin);
    }
  ;

  Serial.println(val);             // debug value
  
  
 delay (500);

}

Why do I get a negative value in this loop? If I write 200 instead of 300 I get positive values. There’s no reason for me to get a negative value as analogRead(analogPin) is always positive. My objective is to meke an average of the readings but these negative values are ruining my program…

#2

What type is val? If it’s a signed 8-bit type you’ll see -ve amounts.

Oh, it could be a signed type of any bitlength, and still overflow. What does val’s declaration look like?

#3

It’s just
int val = 0;

I’m a bit noob… What am I doing wrong?

#4

If I change it to
long val=0
it works ok. :slight_smile:
I guess this was the problem.

#5

analogRead returns an integer value in the range of 0 to 1023:
http://www.arduino.cc/en/Reference/AnalogRead

But if val is an int type, and if its value exceeds 32,767, “they ‘roll over’ back to their minimum capacitiy”:
http://www.arduino.cc/en/Reference/Int

So you can use long type for val. Its limits is 2,147,483,647. Then yous sum will not exceed limits when using 300.

[edit]I didn’t see your last post. Almost at same time…[/edit]

#6

I need to verify it, but I believe that there is an issue with Serial.println printing negative values for unsigned long inputs.

#7

Yes, the problem was that I need to use a long instead of int. Thank you all.

#8