Loop through 00 .. 99 with a pair of IV-16 numitron's

numitron-counter.ino

/*
 * Pair of IV-16 Numitron's daisy chained on a pair of 74HC595 shift registers.
 * This sketch simply counts from 00 to 99 repeatedly. Numitron's are wired as
 * follows:
 *
 *   +-f-+       0b 00 00 00 00
 *   e   a       -> -d cg ef ba
 *   +-g-+
 *   d   b
 *   +-c-+
 */

const int latchPin = 8;  // 74HC595:ST_CP  
const int clockPin = 12; // 74HC595:SH_CP
const int dataPin = 11;  // 74HC595:DS

//
// Digit encoding with -dcgefba pin mapping
uint8_t DIGITS[] = {
  0b00010000, // 0             
  0b01111100, // 1
  0b00001010, // 2
  0b01001000, // 3
  0b01100100, // 4
  0b01000001, // 5
  0b00000001, // 6
  0b01111000, // 7
  0b00000000, // 8
  0b01100000 // 9
};

void setup() {                                                                                                                                                                                                                                                                                                                                                                                                         
  pinMode(latchPin, OUTPUT);
  pinMode(dataPin, OUTPUT);  
  pinMode(clockPin, OUTPUT);
  clear();
  delay(5000);
}

void loop() {
    for (int i = 0; i < 100; i++) {
        display(i);
        delay(1000);
    }
}

void clear() {
    digitalWrite(latchPin, LOW);
    shiftOut(dataPin, clockPin, LSBFIRST, 255 | 0b10000000);  
    shiftOut(dataPin, clockPin, LSBFIRST, 255 | 0b10000000);  
    digitalWrite(latchPin, HIGH);
}

void display(int i){
    digitalWrite(latchPin, LOW);
    shiftOut(dataPin, clockPin, LSBFIRST, DIGITS[(int)(i / 10)] | 0b10000000);  
    shiftOut(dataPin, clockPin, LSBFIRST, DIGITS[(int)(i % 10)] | 0b10000000);  
    digitalWrite(latchPin, HIGH);
}