const byte digit[10] = {
  0b01000000, // 0
  0b01111001, // 1
  0b00100100, // 2
  0b00110000, // 3
  0b00011001, // 4
  0b00010010, // 5
  0b00000010, // 6
  0b01111000, // 7
  0b00000000, // 8
  0b00010000  // 9
};

int count = 0; 
int direction = 1; 

void setup() {
  DDRD = 0xFF; //  (2-7)
    PORTD = 0xFF; 
}

void loop() {
  displayDigit(count); 
  delay(1000); 

  count += direction; 

 
  if (count >= 9) {
    direction = -1; 
  } else if (count <= 0) {
    direction = 1; 
  }
}

void displayDigit(int num) {
  uint8_t segments = digit[num];
    PORTD = (PORTD & 0x00) | segments; 
}