#define PIN_ULTRASONIC_ECHO (2)
#define PIN_ULTRASONIC_TRIGGER (7)
#define PIN_INTERUPT_BUTTON (2)

void isr_us(void);
void trigger(void);

unsigned long timestamp;
unsigned long us_start;
unsigned long us_end;
bool measurementReady = false;

void setup() {
  // put your setup code here, to run once:
  Serial.begin(115200);
  pinMode(PIN_ULTRASONIC_ECHO, INPUT);
  pinMode(PIN_ULTRASONIC_TRIGGER, OUTPUT);
  attachInterrupt(digitalPinToInterrupt(PIN_ULTRASONIC_ECHO),isr_us, CHANGE);
  
  timestamp = millis();
}

void loop() {
  // put your main code here, to run repeatedly:
  if(millis() >= timestamp){
    trigger();
    timestamp += 500;
  } 
  
  if (measurementReady){
    measurementReady = false;
    unsigned long timeDelta =  us_end - us_start;
    Serial.println(timeDelta / 58);
  }
}

void isr_us(void){
  if(digitalRead(PIN_ULTRASONIC_ECHO) == HIGH){
    us_start = micros();
  }
  else{
    us_end = micros();
    measurementReady = true;
  }

}

void trigger(void){
  digitalWrite(PIN_ULTRASONIC_TRIGGER, HIGH);
  delayMicroseconds(10);
  digitalWrite(PIN_ULTRASONIC_TRIGGER, LOW);
}