Build A Digital Tachometer/RPM Counter

The Software
           There are two main portions of code that we are concerned with:
    -CCP Interrupt Service Routine
    -RPM Calculation & Display On LCD


Displaying the formula results on the LCD is done through pre-built functions. See the Interfacing a 16x2 LCD to PIC tutorial if you want a more indepth explaination of how it works. For this tutorial we will focus on how to convert the ccp data to something we can display on the LCD.

CCP Interrupt Service Routine

void InterruptHandlerHigh(void) // Declaration of InterruptHandler
{
            //Check If TMR1 Interrupt Flag Is Set
     if(PIR1bits.CCP1IF){
            //Get Timer1 Value For the 4th Detected Rising Edge;
     cap_val = ReadTimer1();
            //Clear Timer
     WriteTimer1( 0x0000 );
            //Clear CCP1 Overflow Flag Bit
       PIR1bits.CCP1IF = 0;
     }           //Check If CCP1 Interrupt Flag Is Set
     else if(PIR1bits.TMR1IF){
       //Timer Over Flowed: Add 0xFFFF To Current Capture Value
       //This Is Not Used, But Would Be Necessary
       //For Tracking Less Than 1200 RPMs
     cap_val += 65535;
            //Clear Timer1 Overflow Flag Bit
     PIR1bits.TMR1IF = 0;
     }
//Re-enable Global Interrupts
INTCONbits.GIE = 1;
}

           The bold code above is what happens when a rising edge is detected. The value from Timer1 was read and stored in a variable. This value is the period for 1 fan blade to interrupt the photo-detector circuit.
           One important note: I actually set this up so that every 4th rising edge triggers interrupt, then divide that value by 4 which gives a moving average for the period. This makes the data less sloppy and smoothed out.

RPM Calculation & Display On LCD

...
...
      cur_cap_val = 60000000 /((cap_val>>2)*7);
         //******************************************************
         //Don't Show Junk Data
         //RPMs Should Always Be between 1200 -> 3200
         //******************************************************
      if( cur_cap_val > 4000 || cur_cap_val < 1000){
      array_2[4] = '\0';
      array_2[3] = '\0';
      array_2[2] = '\0';
      array_2[1] = '\0';
      array_2[0] = '0';
      }
      else{
      ultoa(cur_cap_val,array_2);
      }

      if( isdigit( (unsigned char) array_2[4] )){
      LCD_WRITE( (unsigned char) array_2[4],8);
      LCD_WRITE( (unsigned char) array_2[3],7);
      LCD_WRITE( (unsigned char) array_2[2],6);
      LCD_WRITE( (unsigned char) array_2[1],5);
      LCD_WRITE( (unsigned char) array_2[0],4);
      }
      else if( isdigit( (unsigned char) array_2[3] )){
...
...

           Now that the current period value is stored in a global variable we can use, we plug it into the formula to find the current RPM for the digital tachometer and then convert that integer to a character array. After we have the character array, we output the number to the 16x2 LCD display via the LCD_WRITE() function.