byte ms = 10;

void setup() {
  // Set pins for OUTPUT
  DDRB = DDRB | B00111111; // Port B pins OUTPUT  x  x  8  9 10 11 12 13
  DDRC = DDRC | B11111111; // Port C pins OUTPUT 21 20 19 18 17 16 15 14 (A7 - A0)
  DDRD = DDRD | B11111100; // Port D pins OUTPUT  7  6  5  4  3  2  1  0

  // Set DIO pins HIGH
  PORTB = PORTB | B00111111; // Port B pins HIGH  x  x 13 12 11 10  9  8
  PORTC = PORTC | B11111111; // Port C pins HIGH 21 20 19 18 17 16 15 14 (A7 - A0)
  PORTD = PORTD | B11111100; // Port D pins HIGH  7  6  5  4  3  2  1  0

  walk();
}

void loop() {}

void walk() { // clear pins from Pin 2 to Pin 21
  for (int i = 2; i < 8; i++) { // Port D pins 2 to 7
    PORTD = PORTD | 1 << i; // shift bit left one register
    delay (ms);
    PORTD = 0;
    delay (ms);
  }

  for (int i = 0; i < 6; i++) { // Port B pins 8 to 13
    PORTB = PORTB | 1 << i;
    delay (ms);
    PORTB = 0;
    delay (ms);
  }

  for (int i = 0; i < 8; i++) { // Port C pins 14 to 21
    PORTC = PORTC | 1 << i;
    delay (ms);
    PORTC = 0;
    delay (ms);
  }
}