unsigned long t_f;
unsigned long t_r;
void setup() {
  // put your setup code here, to run once:
  Serial1.begin(115200);
  Serial1.println("Hello, Raspberry Pi Pico!");
  pinMode(16, INPUT_PULLUP);
  attachInterrupt(0, isr_f, FALLING);
  attachInterrupt(0, isr_r, RISING);

}
void isr_f(void){
  t_f = millis();
  t_r = 0;
}
void isr_r(void){
  t_f = 0;
  t_r = millis();
}
void loop() {
  // put your main code here, to run repeatedly:
  delay(10); // this speeds up the simulation
  Serial1.print(t_f);
  Serial1.print(" - ");
  Serial1.print(t_r);
  Serial1.print(" = ");
  Serial1.println(t_r - t_f);
}