Problem with while loops while detecting frequency of a signal

Question

I just started to use the unusual part of Arduino language (avr) like interrupts, accessing registers, etc in Arduino IDE. I wrote a program to calculate the frequency of a digital signal using external interrupts.

The following code is to calculate timeperiod of a digital signal, the output of the color sensor (TCS3200) which depends on the color filter choosen via S2 and S3 pins, as in the code. That is, color sensor produces 50% duty cycle square pulses of frequency dependent on the power of the light incident on it. So when sensor faces red color, output signal's frequency in red color mode (by setting S2, S3 pins) is more than the frequency in blue and green modes. Thus we can detect the likeliness of the color which it faces w.r.t R,G,B.

When this code is uploaded in the Arduino, it failed to print the values as expected in the serial monitor i.e. its stuck in while loops. But when Serial.print(e) statement in the while loops are uncommented, it is able to print the required time periods. I am unable to figure out the problem here, which is making Arduino to be inside the while loop when no statements are in it.

// INT0 interupt is used i.e. digital pin 2
int ti=0,tim=0,e=0,S2=4,S3=5;

void Interruptinit() //initializing interrupt INT0
{
  cli();
  EICRA =  0x02; //Control register
  EIMSK =  0x01; //Mask register
  sei();
}

void setup()
{
  pinMode(2,INPUT);
  digitalWrite(2,HIGH); //enabling pull up register
  pinMode(S2,OUTPUT);
  pinMode(S3,OUTPUT);
  Interruptinit();
  Serial.begin(9600);
}

ISR(INT0_vect)
{
  tim=micros()-ti;
  ti=micros();
  e++;
}

void loop()
{ 
  digitalWrite(S2,0); digitalWrite(S3,0); //switching over red filtered photodiodes
  e=0;
  while(e<2)
  //Serial.print(e) //waiting till interrupt is addressed 2 times     
  ;
  Serial.print("red:");
  Serial.print(tim);

  digitalWrite(S2,0); digitalWrite(S3,1); //switching over blue filtered photodiodes
  e=0;
  while(e<2)
  //Serial.print(e)
  ;
  Serial.print(" blue:");
  Serial.print(tim);

  digitalWrite(S2,1); digitalWrite(S3,1); //switching over green filtered photodiodes
  e=0;
  while(e<2)
  //Serial.print(e)
  ;
  Serial.print(" green:");
  Serial.println(tim);

}

Answer 1

Whenever you modify non-local variables in an ISR, it's important that they are declared as volatile. This tells the compiler that the contents may change unexpectedly, ensuring it doesn't use out-of-date cached copies somewhere else. If you don't do this, the code can behave in unpredictable ways, such as outputting the wrong value under some circumstances.

Since the variables are bigger than 1 byte, it's also necessary to disable interrupts around non-ISR code which uses them (e.g. in loop()). This is because interrupts could be triggered at any time, potentially half way through an operation which reads/writes them.

You can use noInterrupts() and interrupts() to disable/enable interrupt handling.