ISR(SIG_OUTPUT_COMPARE0B){} und TCNT0 Problem

[SprinterSB]

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

volatile uint16_t ImpulsZaehler1;
uint16_t  a = 50;  //mitte 1,6ms


// Timer interrupt 4000Hz (4Mhz Quarz)
void Timer0B_ON (void)
{
	// Mode #2 (CTC)
	TCCR0B = (1 << CS00) | (1 << WGM01);
	OCR0B  = 200;
	TIMSK |= (1 << OCIE0B);
}


ISR (SIG_OUTPUT_COMPARE0B)
{
    if (++ImpulsZaehler1 >= 100)
    {
	    ImpulsZaehler1 = 0;
    }
}

int main(void)
{
	Timer0B_ON();
	DDRD = (1 << PD6);
	
	sei();
	
	while(1)
	{
		uint16_t iz;
		
		// atomic
		cli();
		iz = ImpulsZaehler1;
		sei();
		
		if (iz < a) 
		{
			PORTD &= ~(1 << PD6);
		}
		else
		{
			PORTD |= (1 << PD6);
		}
	} // while (1)
} // main