#ifndef F_CPU
#define F_CPU 16000000UL
#endif

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

int main(void){
  DDRB|=(1<<PB1);
  TCCR1A=(1<<COM1A1)|(1<<WGM11);
  TCCR1B=(1<<WGM13)|(1<<WGM12)|(1<<CS11);
  ICR1=39999;
  for(;;){
    for(uint16_t us=1000; us<=2000; us+=5){ OCR1A=us*2; _delay_ms(15); }
    for(uint16_t us=2000; us>1000; us-=5){ OCR1A=us*2; _delay_ms(15); }
  }
}