#include <LiquidCrystal.h>
LiquidCrystal lcd(3, 5, 9, 10, 11, 12);

volatile float time = 0;
volatile float time_last = 0;
volatile int rpm_array[5] = {0,0,0,0,0};

void setup()
{
   //Digital Pin 2 Set As An Interrupt
  attachInterrupt(0, fan_interrupt, FALLING);

   // set up the LCD's number of columns and rows:  
   lcd.begin(16, 2);
   // Print a message to the LCD.
   lcd.print("Current RPM:");
}

void loop()
{
   int rpm = 0;
    
   while(1){     

      //Slow Down The LCD Display Updates
   delay(400);
    
   //Clear The Bottom Row
   lcd.setCursor(0, 1);
   lcd.print("                ");    
    
   //Update The Rpm Count
   lcd.setCursor(0, 1);
   lcd.print(rpm);    

   ////lcd.setCursor(4, 1);
   ////lcd.print(time);    

   //Update The RPM
   if(time > 0)
   {
     //5 Sample Moving Average To Smooth Out The Data
       rpm_array[0] = rpm_array[1];
       rpm_array[1] = rpm_array[2];
       rpm_array[2] = rpm_array[3];
       rpm_array[3] = rpm_array[4];
       rpm_array[4] = 60*(1000000/(time*ХХХХХ));     
     //Last 5 Average RPM Counts Eqauls....
       rpm = (rpm_array[0] + rpm_array[1] + rpm_array[2] + rpm_array[3] + rpm_array[4]) / 5;
   }
   
  }
}

void fan_interrupt()
{
    time = (micros() - time_last);  
    time_last = micros();
}