Teensy USB Interrupts
Your ISR should not be disabling and re-enabling interrupts and it should not be delaying.

Try:

ISR(TIMER0_OVF_vect)
{
  PORTD ^= 1&lt;&lt;6;
}


Or even:

ISR(TIMER0_OVF_vect)
{
    if (overflow_count &lt; 0xFFFF)
        overflow_count++;
    else
    {
        overflow_count = 0;
        PORTD ^= 1&lt;&lt;6;
    }
}


Then configure the timer interrupt frequency by changing TCCR0B.

To begin the timer, you may also need to clear the counter:

TCNT0 = 0