#define ECHO_PIN 34
#define TRIG_PIN 25

volatile unsigned long pulseInTimeBegin = micros();
volatile unsigned long pulseInTimeEnd = micros();
volatile bool newPulseDurationAvailable = false;

void sendTrigPulse()  {
  digitalWrite(TRIG_PIN, LOW);
  delayMicroseconds(2);
  digitalWrite(TRIG_PIN, HIGH);
  delayMicroseconds(10);
  digitalWrite(TRIG_PIN, LOW);
  //delayMicroseconds(2);
}

void ARDUINO_ISR_ATTR echoInterrupt()
{
  if (digitalRead(ECHO_PIN) == HIGH) {
    // start measuring
    pulseInTimeBegin = micros();
  }
  else {
    // stop measuring
    pulseInTimeEnd = micros();
    newPulseDurationAvailable = true;
  }
}

void setup() {
  Serial.begin(9600);
  pinMode(ECHO_PIN, INPUT);
  pinMode(TRIG_PIN, OUTPUT);

  attachInterrupt(digitalPinToInterrupt(ECHO_PIN),
                  echoInterrupt,
                  CHANGE);
}

void loop() {
  if (newPulseDurationAvailable) {
    newPulseDurationAvailable = false;
    unsigned long pulseDuration = pulseInTimeEnd - pulseInTimeBegin;
    Serial.println((pulseDuration * 0.034) / 2);
  }

  sendTrigPulse();
  delay(1000);
}