Basic AVR blink for attiny13a

#define F_CPU 1200000UL
#include <util/delay.h>
#define LED PB0
#define LED_ON  PORTB |= (1<<LED)
#define LED_OFF PORTB &= ~(1<<LED)

int main()
{
    DDRB |= (1<<LED);
    while(1)
    {
        LED_ON;
        _delay_ms(500);
        LED_OFF;
        _delay_ms(500);
    }
}