#define PIR_PIN 2
#define LED_PIN 8

volatile unsigned long tRise = 0, tLED = 0;
volatile bool eventFlag = false;

void onPirRise() {
  tRise = micros();
  digitalWrite(LED_PIN, HIGH);
  tLED = micros();
  eventFlag = true;
}

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

void loop() {
  if (eventFlag) {
    noInterrupts();
    unsigned long dt = tLED - tRise;
    eventFlag = false;
    interrupts();
    Serial.print("RESP_US=");
    Serial.println(dt);
  }
  if (digitalRead(PIR_PIN) == LOW && digitalRead(LED_PIN) == HIGH) {
    digitalWrite(LED_PIN, LOW);
  }
}