atmega328p

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

volatile uint16_t counter;

int main(void)
{
    DDRB |= (1<<PB5);

    TCCR0B |= (1 << CS00) | (1 << CS01);
    TCNT0 = 0;
    TIMSK0 |= (1<<TOIE0);
    sei();                          // enable interrupts


    while (1)
    {
        if (TCNT0 >= 100)
        {
            PORTB ^= _BV(PB5);
            TCNT0 = 0;            // reset counter
        }
    }
}