Four-digit counter on an Arduino Uno

// Four-digit counter: a bare 5641AS display, multiplexed by the sketch.
//
//   segments A B C D E F G DP -> pins 2 to 9, each through its own 220 ohm
//   digits DIG1 to DIG4       -> pins 10 to 13
//
// Twelve legs for four digits: each segment leg goes to that segment in all
// four digits, and each digit has one common leg. So only one digit can be
// shown at a time, and the sketch shows them in turn, four milliseconds each,
// fast enough that the eye sees all four at once, each a quarter as bright.
//
// The display is common cathode: a digit is selected by pulling its common
// LOW, and a segment lit by driving its leg HIGH. Every common goes off before
// the next digit's segments change, or the old digit shows a faint ghost of
// the new one. Try moving that line and look closely.
//
// The count is tenths of a second, 000.0 to 999.9, with the point lit on the
// third digit.

const uint8_t SEGMENT_PINS[8] = {2, 3, 4, 5, 6, 7, 8, 9};
const uint8_t DIGIT_PINS[4] = {10, 11, 12, 13};

// Bit 0 is segment A, bit 6 is G, bit 7 the point.
const uint8_t FONT[10] = {0x3F, 0x06, 0x5B, 0x4F, 0x66, 0x6D, 0x7D, 0x07, 0x7F, 0x6F};

void setup() {
  for (uint8_t i = 0; i < 8; i++) pinMode(SEGMENT_PINS[i], OUTPUT);
  for (uint8_t i = 0; i < 4; i++) {
    pinMode(DIGIT_PINS[i], OUTPUT);
    digitalWrite(DIGIT_PINS[i], HIGH);  // common cathode: HIGH is off
  }
}

static void show(uint8_t digit, uint8_t pattern) {
  for (uint8_t d = 0; d < 4; d++) digitalWrite(DIGIT_PINS[d], HIGH);  // all off first
  for (uint8_t s = 0; s < 8; s++) {
    digitalWrite(SEGMENT_PINS[s], (pattern >> s) & 1 ? HIGH : LOW);
  }
  digitalWrite(DIGIT_PINS[digit], LOW);  // then this one on
  delay(4);
}

void loop() {
  unsigned long tenths = (millis() / 100) % 10000;
  show(0, FONT[tenths / 1000]);
  show(1, FONT[tenths / 100 % 10]);
  show(2, (uint8_t)(FONT[tenths / 10 % 10] | 0x80));
  show(3, FONT[tenths % 10]);
}