int Htime;              //integer for storing high time
int Ltime;                //integer for storing low time
float Ttime;            // integer for storing total time of a cycle
float frequency;        //storing frequency

void setup() {
  pinMode(8, INPUT_PULLUP);
  Serial.begin(9600);
}
void loop() {

  Serial.println("Frequency of signal");
  Htime = pulseIn(8, HIGH);   //read high time
  Ltime = pulseIn(8, LOW);     //read low time
  Ttime = Htime + Ltime;
  frequency = 1000000 / Ttime; //getting frequency with Ttime is in Micro seconds
  Serial.print(frequency);
  Serial.println(" Hz");
  delay(1000);
}