Code

volatile int count = 0;
void isrResponse()// Interrupt Service Routine (ISR)
{
	count = 1000;// Reset the counter;
}

void setup()
{
	pinMode(PB0,OUTPUT);//
	pinMode(PB3,OUTPUT);//WHT LED Interrupt felt
	//attachInterrupt (0, isrResponse, HIGH);  // Works - attach interrupt handler
	attachInterrupt (INT0, isrResponse, HIGH);  // Fails - attach interrupt handler
	count = 0;
}

void loop()
{
	if (count >10)
	{
		digitalWrite(PB3,HIGH);//Dead Man Turn on LED
		delay(1000);

		digitalWrite(PB3,LOW);//Dead Man Turn off LED
		count = 0;
	}
	digitalWrite(PB0,HIGH);  // Proof of life
	delay(500);
	digitalWrite(PB0,LOW);  // Proof of life
	delay(500);
	count ++; 
}