Some Datasheet fu - (hw) hello world using timer in attiny.

ledtiny85.c

#include <avr/interrupt.h>
#include <avr/io.h>
#include <util/atomic.h>

#define F_CPU 8000000

#include <util/delay.h>

volatile uint8_t ctr = 0;
volatile uint16_t ms = 0;
volatile uint16_t s = 0;

ISR (TIMER0_COMPA_vect) {
     ctr ++;
     /* 8Mhz/8 and once 100 uS = 0.1ms */
     if (ctr == 10) {
	  ctr = 0;
	  ms ++;
	  if (ms == 1000) {
	       ms = 0;
	       s ++;
	       if (s == 1000)
		    s = 0;
	  }
     }

     TCNT0 = 0;
     TIFR |= 0x02;
}

inline uint16_t getctr () {
     uint16_t c;
     ATOMIC_BLOCK(ATOMIC_FORCEON)
     {
	  c = ms;
     }
     return c;
}
 
int main (void)
{
     DDRB = 0x1F;
     PORTB = 0x1F;

     cli();

     TCCR0A = 0x00; /* No outputs on compare match (COM0nx), no modes of
		     * outputs (WGMx) */
     TCCR0B = 0x02; /* FOCXn = 0, WGM2 = 0, Prescaler 2 = io_clk/8, 5
		     * = io_clk / 1024 */

     OCR0A = 100;  /* Compare value */
     TCNT0 = 0x00; /* Counter */

     TIMSK = 0x10; /* OCIE0A = 1, TOIE0 =0  */
     TIFR = 0x10; /* OCF0A = 1, TOV0 = 0 */
 
     sei();

     while (1) {
	  uint16_t c = getctr();
	  if (c > 500)
	       PORTB = 0x1F;
	  else
	       PORTB = 0;
     }
     return 1;
}