One-digit countdown on a 74HC595 (pins 2, 4 and 3), on an Arduino Uno

// A seven-segment digit counting down from 9, through a 74HC595.
//
//   74HC595 DS -> pin 2    SHCP -> pin 4    STCP -> pin 3
//   74HC595 VCC and MR -> 5V      GND and OE -> GND
//   Q0 to Q6 -> segments a to g, Q7 -> the decimal point
//   the display's common cathode -> 220 ohm -> GND
//
// A seven-segment display is eight LEDs in one package: segments a to g and
// the dot. Each digit is a pattern of lit segments, one bit per segment, so
// the whole digit fits in one byte and goes out through the shift register
// with a single shiftOut().
//
// The patterns below are written with segment a in the lowest bit.

const int DATA_PIN = 2;
const int LATCH_PIN = 3;
const int CLOCK_PIN = 4;

//                   gfedcba
const byte DIGITS[10] = {
    0b00111111,  // 0
    0b00000110,  // 1
    0b01011011,  // 2
    0b01001111,  // 3
    0b01100110,  // 4
    0b01101101,  // 5
    0b01111101,  // 6
    0b00000111,  // 7
    0b01111111,  // 8
    0b01101111,  // 9
};

void show(byte pattern) {
  digitalWrite(LATCH_PIN, LOW);
  shiftOut(DATA_PIN, CLOCK_PIN, MSBFIRST, pattern);
  digitalWrite(LATCH_PIN, HIGH);
}

void setup() {
  pinMode(DATA_PIN, OUTPUT);
  pinMode(LATCH_PIN, OUTPUT);
  pinMode(CLOCK_PIN, OUTPUT);
  Serial.begin(9600);
  Serial.println("Mokxi Uno: one digit on a 74HC595 (pins 2, 4 and 3)");
}

void loop() {
  for (int n = 9; n >= 0; n--) {
    show(DIGITS[n]);
    Serial.println(n);
    delay(1000);
  }
  delay(1000);
}