#define PIR_PIN 2
#define LED_PIN 8

volatile unsigned long tRise = 0, tCmd = 0, dt = 0;
volatile bool ready = false;

void isrChange() {
  if (digitalRead(PIR_PIN) == HIGH) {
    tRise = micros();
    digitalWrite(LED_PIN, HIGH);
    tCmd = micros();
    dt = tCmd - tRise;
    ready = true;
  } else {
    digitalWrite(LED_PIN, LOW);
  }
}

void setup() {
  pinMode(PIR_PIN, INPUT);
  pinMode(LED_PIN, OUTPUT);
  digitalWrite(LED_PIN, LOW);
  Serial.begin(115200);
  attachInterrupt(digitalPinToInterrupt(PIR_PIN), isrChange, CHANGE);
}

void loop() {
  if (ready) {
    noInterrupts();
    unsigned long x = dt;
    ready = false;
    interrupts();
    Serial.print("RESPONSE_MICROSECONDS=");
    Serial.println(x);
  }
}