help in making engine digital RPM meter

Using Arduino Programming Questions
27

OK, I'll assume you are using an Arduino Uno. This can generate interrupt 0 on pin 2 and interrupt 1 on pin 3. I'll assume you are feeding the ignition pulse (cleaned up if necessary) to pin 2. Here is a sketch to calculate the rpm:

const int ignitionPin = 2;
const int ignitionInterrupt = 0;
const unsigned int pulsesPerRev = 1;

unsigned long lastPulseTime = 0;
unsigned long rpm = 0;

void ignitionIsr()
{
  unsigned long now = micros();
  unsigned long interval = now - lastPulseTime;
  if (interval > 2000)
  {
     rpm = 60000000UL/(interval * pulsesPerRev);
     lastPulseTime = now;
  }  
}

void setup()
{
  pinMode(ignitionPin, INPUT);
  attachInterrupt(ignitionInterrupt, &ignitionIsr, RISING);
}

void loop()
{
  // insert code here to show the RPM on the display and delay for e.g. 0.2 seconds
}

I've included some pulse cleanup in the software (i.e. ignore pulses that are less than 2ms apart).