void change_tempo(uint16_t set_tempo) {
  uint16_t t3_prescale;
  uint32_t num_instr;
  uint16_t top_num_instr;

  if (set_tempo > MAX_TEMPO) {
    set_tempo = MAX_TEMPO;
  }
  if (set_tempo < MIN_TEMPO) {
    set_tempo = MIN_TEMPO;
  }

  // figure out what the interrupt should be!
  // (use counter 3 for finest resolution!)
  num_instr = F_CPU * 60;

  num_instr /= set_tempo*48;
  
  
  top_num_instr = num_instr >> 16;
  if (!top_num_instr) {
    t3_prescale = 1;
    timer3_init = num_instr;
    TCCR3B = 1;
  } else if ((top_num_instr & ~0x7) == 0) {
    t3_prescale = 8;
    timer3_init = num_instr >> 3;
    TCCR3B = 2;
  }
/* 
    else if ((top_num_instr & ~0xF) == 0) {
    t3_prescale = 16; 
    timer3_init = num_instr >> 4;
    TCCR3B = 6;
  } else if ((top_num_instr & ~0x1F) == 0) {
    t3_prescale = 32;
    timer3_init = num_instr >> 5;
    TCCR3B = 7;
  }
*/
    else if ((top_num_instr & ~0x3F) == 0) {
    t3_prescale = 64;
    timer3_init = num_instr >> 6;
    TCCR3B = 3;
  } else if ((top_num_instr & ~0xFF) == 0) {
    t3_prescale = 256;
    timer3_init = num_instr >> 8;
    TCCR3B = 4;
  } else if ((top_num_instr & ~0x3FF) == 0) {
    t3_prescale = 1024;
    timer3_init = num_instr >> 10;
    TCCR3B = 5;
  } else {
    t3_prescale = 0;
    TCCR3B = 0;
  }
  
  timer3_init *= -1;
  
  //printf("T3 Prescale: %d. Init: 0x%x\n\r", t3_prescale, timer3_init);
  TCNT3 = timer3_init;
}