#define FAN_WIRES   4
#define PWM_PIN     3
#define SENSOR_PIN  2

#if FAN_WIRES == 4
volatile unsigned int revs;
#endif

void setup()
{
  Serial.begin(115200);
  Serial.println("FAN CONTROL");
  // prescaller for 31.372KHz
//  TCCR2B &= B11111000;
//  TCCR2B |= (0 << CS22) | (0 << CS21) | (1 << CS20);
  digitalWrite(PWM_PIN, LOW);
  pinMode(PWM_PIN, OUTPUT);
#if FAN_WIRES == 4
  attachInterrupt(digitalPinToInterrupt(SENSOR_PIN), interruptHandler, FALLING);
#endif
}

void loop()
{
  int pwm, pwmGpio;

  for (pwm = 0 ; pwm <= 100 ; pwm += 10) {
    Serial.print(pwm);
    Serial.print("%: (");
    pwmGpio = pwm * 2.5;
    Serial.print(pwmGpio);
    Serial.print(") ");
    analogWrite(PWM_PIN, pwmGpio);
    delay(2000);
#if FAN_WIRES == 4
    Serial.print(" speed = "); Serial.println(getSpeed());
#else
    Serial.println();
#endif
  }
  for (pwm = 90 ; pwm >= 0 ; pwm -= 10) {
    Serial.print(pwm);
    Serial.print("%: (");
    pwmGpio = pwm * 2.5;
    Serial.print(pwmGpio);
    Serial.print(") ");
    analogWrite(PWM_PIN, pwmGpio);
    delay(2000);
#if FAN_WIRES == 4
    Serial.print(" speed = "); Serial.println(getSpeed());
#else
    Serial.println();
#endif
  }
}

#if FAN_WIRES == 4

unsigned int getSpeed()
{
  static unsigned long lastMillis;
  static unsigned int lastRead;
  unsigned long elapsed = millis() - lastMillis;
  if (elapsed > 1000) {
    detachInterrupt(digitalPinToInterrupt(SENSOR_PIN));
    double correctionFactor = 1000.0 / elapsed;
    lastRead = correctionFactor * revs / 2 * 60;
    revs = 0;
    lastMillis = millis();
    attachInterrupt(digitalPinToInterrupt(SENSOR_PIN), interruptHandler, FALLING);
    return lastRead;
  }
}

void interruptHandler(void)
{
  revs++;
}

#endif