#include <Servo.h>
#include <SoftwareSerial.h>

const int exti_pin = 2;
Servo servo;

volatile unsigned long now, past = 0, diff = 0;
volatile boolean flag = false;

void setup() {
  servo.attach(12);
  pinMode(exti_pin, INPUT);
  attachInterrupt(digitalPinToInterrupt(exti_pin), PPM, CHANGE);
  Serial.begin(115200);
  Serial.println();
}

void loop() {
  servo.write(90);
  delay(100);
  if(flag) {
    Serial.println(diff);
    flag = false;
  }
}

void PPM() {
  if (digitalRead(exti_pin)) {
    past = micros();
  }
  else {
    now = micros();
    diff = now - past;
    flag = true;
  }
}