TachDuino alpha 1

#include <avr/interrupt.h>
#define PIN 2
#define NUM_BLADES 2
float RPM = 0.0f;
int interrupted_times = 0;
unsigned long last_time = 0, elapsed_time;
int state = 0;

void setup()
{
    DDRD &= ~(1 << _BV(PIN)); //Set PIN as Input
    PORTD &= ~(1 << _BV(PIN)); //Pull-Down

    cli();                    // Disable global interrupts
    EIMSK |= (1 << INT0);     // Enable external interrupt INT0
    //Rising edge
        EICRA |= (1 << ISC00);
        EICRA |= (1 << ISC01);

    sei();                    // Enable global interrupts

    Serial.begin(9600);
    pinMode(13, OUTPUT);
    digitalWrite(13, state);

    last_time = micros();
}

void loop()
{
    Serial.println(RPM);
    delay(100);
}

ISR(INT0_vect)
{
    digitalWrite(13, (state ^= 1));

    interrupted_times++;
    if(interrupted_times >= NUM_BLADES)
    {
        elapsed_time = (micros() - last_time);
        RPM = 1.0f/(((((float)elapsed_time)/1000.0f)/1000.0f)/60.0f);
        interrupted_times = 0;
        last_time = micros();
    }
}