#include <avr/interrupt.h>
#include <avr/sleep.h>

#define LED PB0
#define LED_POWER 100 // From 0 to 255

volatile uint8_t flag = 0;

// See Table 9-3. Watchdog Timer Prescale Select in ATtiny9 datasheet for Watchdog Timeout values
ISR(WDT_vect) {
	// Toggle Port B pin 0 output state
	if(flag) {
		// Set the LED pin to output
		DDRB = (1<<LED);
		// Set the watchdog to wake up in 1 second
		WDTCSR = (1<<WDIE) | (1<<WDP2) | (1<<WDP1);
	} else {
		// Set the LED pin to input (disable the LED)
		DDRB = 0;
		// Set the watchdog to wake up in 0.5 second
		WDTCSR = (1<<WDIE) | (1<<WDP2) | (1<<WDP0);
	}
	flag = !flag;
}

void main() {
	// Set up Port B as Input and set the LED pin to output
	DDRB = 0 | (1<<DDB0);

    // Setup comparator to work in simple PWM mode
	TCCR0A |= (1<<WGM00) | (1<<COM0A1);
	TCCR0B |= (1<<WGM02) | (1<<CS00);
    // Set comparator value to the requested LED_power value
	OCR0A = LED_POWER;

	// Set timer to 0.5s
	WDTCSR = (1<<WDIE) | (1<<WDP2) | (1<<WDP0);

	// Enable global interrupts
	sei();

	set_sleep_mode(SLEEP_MODE_IDLE);

	while(1) {
		// Go to sleep and wait for interrupt...
		sleep_mode();
	}
}