Serial stops sending????

Hi, so basically I'm a complete noob at this, but I know that TX means that something is being sent to the serial monitor, however, when sometimes the TX light just turns off and no data is sent to the serial monitor (so there are no new lines.)
What is this problem and how can I fix it?
here is my code:

/*  Pulse Sensor Amped 1.4    by Joel Murphy and Yury Gitman   http://www.pulsesensor.com

----------------------  Notes ----------------------  ---------------------- 
This code:
1) Blinks an LED to User's Live Heartbeat   PIN 13
2) Fades an LED to User's Live HeartBeat
3) Determines BPM
4) Prints All of the Above to Serial

Read Me:
https://github.com/WorldFamousElectronics/PulseSensor_Amped_Arduino/blob/master/README.md   
 ----------------------       ----------------------  ----------------------
*/

//  Variables
int pulsePin = 0;                 // Pulse Sensor purple wire connected to analog pin 0
int blinkPin = 13;                // pin to blink led at each beat
int fadePin = 5;                  // pin to do fancy classy fading blink at each beat
int fadeRate = 0;                 // used to fade LED on with PWM on fadePin
int EmoLite = 12;

// Volatile Variables, used in the interrupt service routine!
volatile int BPM;                   // int that holds raw Analog in 0. updated every 2mS
volatile int Signal;                // holds the incoming raw data
volatile int IBI = 600;             // int that holds the time interval between beats! Must be seeded! 
volatile int PBI = 600;
volatile boolean Pulse = false;     // "True" when User's live heartbeat is detected. "False" when not a "live beat". 
volatile boolean QS = false;        // becomes true when Arduoino finds a beat.

// Regards Serial OutPut  -- Set This Up to your needs
static boolean serialVisual = true;   // Set to 'false' by Default.  Re-set to 'true' to see Arduino Serial Monitor ASCII Visual Pulse 


void setup(){
  pinMode(blinkPin,OUTPUT);         // pin that will blink to your heartbeat!
  pinMode(fadePin,OUTPUT);          // pin that will fade to your heartbeat!
  Serial.begin(115200);             // we agree to talk fast!
  interruptSetup();                 // sets up to read Pulse Sensor signal every 2mS 
  pinMode(EmoLite,OUTPUT);
   // IF YOU ARE POWERING The Pulse Sensor AT VOLTAGE LESS THAN THE BOARD VOLTAGE, 
   // UN-COMMENT THE NEXT LINE AND APPLY THAT VOLTAGE TO THE A-REF PIN
//   analogReference(EXTERNAL);   
}


//  Where the Magic Happens
void loop(){
  
    serialOutput() ;       
    
  if (QS == true){     // A Heartbeat Was Found
                       // BPM and IBI have been Determined
                       // Quantified Self "QS" true when arduino finds a heartbeat
        digitalWrite(blinkPin,HIGH);     // Blink LED, we got a beat. 
        fadeRate = 255;         // Makes the LED Fade Effect Happen
                                // Set 'fadeRate' Variable to 255 to fade LED with pulse
        serialOutputWhenBeatHappens();   // A Beat Happened, Output that to serial.     
        QS = false;                      // reset the Quantified Self flag for next time    
  }
     
  ledFadeToBeat();                      // Makes the LED Fade Effect Happen 
  delay(20);  //  take a break

  if (IBI - PBI <= -90){              //when the difference between the most recent IBI and the one before IBI is less than -90
        delay(1000);
        if (IBI - PBI<= -90) {
          delay(1000);
          if (IBI - PBI<= -90) {
            digitalWrite(EmoLite,HIGH);
             
          }
          else {     
            digitalWrite(EmoLite,LOW); 
     
          }
        }
        else {   
          digitalWrite(EmoLite,LOW);
     
        }
      }
      else {
        digitalWrite(EmoLite,LOW);
   
      }
  
}



void ledFadeToBeat(){
    fadeRate -= 15;                         //  set LED fade value
    fadeRate = constrain(fadeRate,0,255);   //  keep LED fade value from going into negative numbers!
    analogWrite(fadePin,fadeRate);          //  fade LED
  }

And this is the premade serial handling tab:

//////////
/////////  All Serial Handling Code, 
/////////  It's Changeable with the 'serialVisual' variable
/////////  Set it to 'true' or 'false' when it's declared at start of code.  
/////////

void serialOutput(){   // Decide How To Output Serial. 
 if (serialVisual == true){  
     arduinoSerialMonitorVisual('-', Signal);   // goes to function that makes Serial Monitor Visualizer
 } else{
      sendDataToSerial('S', Signal);     // goes to sendDataToSerial function
 }        
}


//  Decides How To OutPut BPM and IBI Data
void serialOutputWhenBeatHappens(){    
 if (serialVisual == true){            //  Code to Make the Serial Monitor Visualizer Work
    Serial.print("*** Heart-Beat Happened *** ");  //ASCII Art Madness
    Serial.print("BPM: ");
    Serial.print(BPM);
    Serial.print("  ");
 } else{
        sendDataToSerial('B',BPM);   // send heart rate with a 'B' prefix
        sendDataToSerial('Q',IBI);   // send time between beats with a 'Q' prefix
 }   
}



//  Sends Data to Pulse Sensor Processing App, Native Mac App, or Third-party Serial Readers. 
void sendDataToSerial(char symbol, int data ){
    Serial.print(symbol);

    Serial.println(data);                
  }


//  Code to Make the Serial Monitor Visualizer Work
void arduinoSerialMonitorVisual(char symbol, int data ){    
  const int sensorMin = 0;      // sensor minimum, discovered through experiment
const int sensorMax = 1024;    // sensor maximum, discovered through experiment

  int sensorReading = data;
  // map the sensor range to a range of 12 options:
  int range = map(sensorReading, sensorMin, sensorMax, 0, 11);

  // do something different depending on the 
  // range value:
  switch (range) {
  case 0:     
    Serial.println("");     /////ASCII Art Madness
    break;
  case 1:   
    Serial.println("---");
    break;
  case 2:    
    Serial.println("------");
    break;
  case 3:    
    Serial.println("---------");
    break;
  case 4:   
    Serial.println("------------");
    break;
  case 5:   
    Serial.println("--------------|-");
    break;
  case 6:   
    Serial.println("--------------|---");
    break;
  case 7:   
    Serial.println("--------------|-------");
    break;
  case 8:  
    Serial.println("--------------|----------");
    break;
  case 9:    
    Serial.println("--------------|----------------");
    break;
  case 10:   
    Serial.println("--------------|-------------------");
    break;
  case 11:   
    Serial.println("--------------|-----------------------");
    break;
  
  } 
}

Thanks very much

If you don't post the entire sketch, no one can re-create your problem and probably
won't bother.

For instance your snippets mention interrupts - but we don't get to see your ISR (so
many pitfalls there that might be relevant - can't tell)

MarkT:
If you don't post the entire sketch, no one can re-create your problem and probably
won't bother.

For instance your snippets mention interrupts - but we don't get to see your ISR (so
many pitfalls there that might be relevant - can't tell)

Yes. Sorry I was planning to do that but I forgot.

Here is the ISR:

volatile int rate[10];                    // array to hold last ten IBI values
volatile unsigned long sampleCounter = 0;          // used to determine pulse timing
volatile unsigned long lastBeatTime = 0;           // used to find IBI
volatile int P =512;                      // used to find peak in pulse wave, seeded
volatile int T = 512;                     // used to find trough in pulse wave, seeded
volatile int thresh = 525;                // used to find instant moment of heart beat, seeded
volatile int amp = 100;                   // used to hold amplitude of pulse waveform, seeded
volatile boolean firstBeat = true;        // used to seed rate array so we startup with reasonable BPM
volatile boolean secondBeat = false;      // used to seed rate array so we startup with reasonable BPM


void interruptSetup(){     
  // Initializes Timer2 to throw an interrupt every 2mS.
  TCCR2A = 0x02;     // DISABLE PWM ON DIGITAL PINS 3 AND 11, AND GO INTO CTC MODE
  TCCR2B = 0x06;     // DON'T FORCE COMPARE, 256 PRESCALER 
  OCR2A = 0X7C;      // SET THE TOP OF THE COUNT TO 124 FOR 500Hz SAMPLE RATE
  TIMSK2 = 0x02;     // ENABLE INTERRUPT ON MATCH BETWEEN TIMER2 AND OCR2A
  sei();             // MAKE SURE GLOBAL INTERRUPTS ARE ENABLED      
} 


// THIS IS THE TIMER 2 INTERRUPT SERVICE ROUTINE. 
// Timer 2 makes sure that we take a reading every 2 miliseconds
ISR(TIMER2_COMPA_vect){                         // triggered when Timer2 counts to 124
  cli();                                      // disable interrupts while we do this
  Signal = analogRead(pulsePin);              // read the Pulse Sensor 
  sampleCounter += 2;                         // keep track of the time in mS with this variable
  int N = sampleCounter - lastBeatTime;       // monitor the time since the last beat to avoid noise

    //  find the peak and trough of the pulse wave
  if(Signal < thresh && N > (IBI/5)*3){       // avoid dichrotic noise by waiting 3/5 of last IBI
    if (Signal < T){                        // T is the trough
      T = Signal;                         // keep track of lowest point in pulse wave 
    }
  }

  if(Signal > thresh && Signal > P){          // thresh condition helps avoid noise
    P = Signal;                             // P is the peak
  }                                        // keep track of highest point in pulse wave

  //  NOW IT'S TIME TO LOOK FOR THE HEART BEAT
  // signal surges up in value every time there is a pulse
  if (N > 250){                                   // avoid high frequency noise
    if ( (Signal > thresh) && (Pulse == false) && (N > (IBI/5)*3) ){        
      Pulse = true;                               // set the Pulse flag when we think there is a pulse
//      digitalWrite(blinkPin,HIGH);                // turn on pin 13 LED
      IBI = sampleCounter - lastBeatTime;         // measure time between beats in mS
      lastBeatTime = sampleCounter;               // keep track of time for next pulse

      if(secondBeat){                        // if this is the second beat, if secondBeat == TRUE
        secondBeat = false;                  // clear secondBeat flag
        for(int i=0; i<=9; i++){             // seed the running total to get a realisitic BPM at startup
          rate[i] = IBI;                      
        }
      }

      if(firstBeat){                         // if it's the first time we found a beat, if firstBeat == TRUE
        firstBeat = false;                   // clear firstBeat flag
        secondBeat = true;                   // set the second beat flag
        sei();                               // enable interrupts again
        return;                              // IBI value is unreliable so discard it
      }   


      // keep a running total of the last 10 IBI values
      word runningTotal = 0;                  // clear the runningTotal variable    

      for(int i=0; i<=8; i++){                // shift data in the rate array
        rate[i] = rate[i+1];                  // and drop the oldest IBI value 
        runningTotal += rate[i];              // add up the 9 oldest IBI values
      }

      PBI = rate[8];
      rate[9] = IBI;                          // add the latest IBI to the rate array
      runningTotal += rate[9];                // add the latest IBI to runningTotal
      runningTotal /= 10;                     // average the last 10 IBI values 
      BPM = 60000/runningTotal;               // how many beats can fit into a minute? that's BPM!
      QS = true;                              // set Quantified Self flag 
      // QS FLAG IS NOT CLEARED INSIDE THIS ISR
     
      // QS FLAG IS NOT CLEARED INSIDE THIS ISR
    }                       
  }
                         
  

  if (Signal < thresh && Pulse == true){   // when the values are going down, the beat is over
//    digitalWrite(blinkPin,LOW);            // turn off pin 13 LED
    Pulse = false;                         // reset the Pulse flag so we can do it again
    amp = P - T;                           // get amplitude of the pulse wave
    thresh = amp/2 + T;                    // set thresh at 50% of the amplitude
    P = thresh;                            // reset these for next time
    T = thresh;
  }

  if (N > 2500){                           // if 2.5 seconds go by without a beat
    thresh = 512;                          // set thresh default
    P = 512;                               // set P default
    T = 512;                               // set T default
    lastBeatTime = sampleCounter;          // bring the lastBeatTime up to date        
    firstBeat = true;                      // set these to avoid noise
    secondBeat = false;                    // when we get the heartbeat back
  }

  sei();                                   // enable interrupts when youre done!
}// end isr

You don't need to disable and enable interrupts in an ISR, this happens automatically for you.

Well the ISR will take a while, but it doesn't seem to have anything in it that could seize up the
processor. However there might be some risk its stealing enough processor cycles to affect other
things.

One trick is to set a pin on entering the ISR and clear it on leaving - then an LED and resistor on that pin
will show how often the processor is handling that ISR as you are efectively PWM'ing the LED.

MarkT:
One trick is to set a pin on entering the ISR and clear it on leaving - then an LED and resistor on that pin
will show how often the processor is handling that ISR as you are efectively PWM'ing the LED.

How would I do that?
Thanks

christychan14:
How would I do that?
Thanks

with digitalWrite() at the start and end of the ISR, writing to a pin that you've got a LED connected to?