Count from 00 to 99

// Count 00 to 99 on chained 4" seven-segment modules with a 74HC595.
//
// Wiring, at the module's IN edge (reading the back, IN is on the left):
//   12 V supply +   -> 12V            two-pin header
//   12 V supply -   -> GND            two-pin header
//   Uno 5V          -> 3V3/5V         six-pin header
//   Uno GND         -> GND            six-pin header
//   Uno D8          -> DATAIN (SER)
//   Uno D11         -> LATCH (RCLK)
//   Uno D12         -> CLOCK (SRCLK)
//   Uno D9          -> PWM            or leave PWM empty and solder the
//                                     PWM pad shut (then PWM_PIN is -1)
//
// The board ships with the PWM pad OPEN. With nothing on PWM the digit
// has no supply and stays dark, whatever this sketch sends.
//
// The 12 V supply's ground and the Uno's ground must be the same wire.
// For a second module, join all eight pins of the first module's OUT edge
// to the second module's IN edge (its DATAOUT goes to DATAIN).
//
// Arduino IDE: Tools > Board > Arduino AVR Boards > Arduino Uno, then
// Tools > Port and pick the port the board appears on. No libraries.

#define NUM_DIGITS 2   // how many modules are joined together

#define SER  8         // DATAIN (SER)
#define LAT 11         // LATCH  (RCLK)
#define CLK 12         // CLOCK  (SRCLK)
#define PWM_PIN 9      // PWM; use -1 if you shorted the PWM pad instead

#define STEP_MS 400    // how long each number stays on screen

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

// buf[0] is the leftmost digit. It goes out first, so it travels furthest
// down the chain and ends up in the module at the far end.
void updateDisplay(const uint8_t *buf) {
  digitalWrite(LAT, LOW);
  for (int i = 0; i < NUM_DIGITS; i++)
    shiftOut(SER, CLK, MSBFIRST, buf[i]);
  digitalWrite(LAT, HIGH);
}

void setup() {
  pinMode(SER, OUTPUT);
  pinMode(LAT, OUTPUT);
  pinMode(CLK, OUTPUT);

  if (PWM_PIN >= 0) {
    pinMode(PWM_PIN, OUTPUT);
    digitalWrite(PWM_PIN, HIGH);   // high = the digit is supplied
  }

  uint8_t blank[NUM_DIGITS];
  for (int i = 0; i < NUM_DIGITS; i++) blank[i] = 0x00;
  updateDisplay(blank);   // a register powers up holding anything
}

void loop() {
  uint8_t b[NUM_DIGITS];

  unsigned long limit = 1;
  for (int i = 0; i < NUM_DIGITS; i++) limit *= 10;
  limit -= 1;             // 9, 99, 999 …

  for (unsigned long n = 0; n <= limit; n++) {
    unsigned long t = n;
    for (int i = NUM_DIGITS - 1; i >= 0; i--) {
      b[i] = d[t % 10];   // the rightmost digit of what is left
      t /= 10;            // drop it
    }
    updateDisplay(b);
    delay(STEP_MS);
  }
}