const uint8_t LATCH = 2;
const uint8_t SER = 4;
const uint8_t CLK = 7;
const uint8_t PWM_PIN = 5;

void init_transfer_2_shift_reg(uint8_t LATCH, uint8_t SER, uint8_t CLK){
  pinMode(LATCH, OUTPUT);
  pinMode(SER, OUTPUT);
  pinMode(CLK, OUTPUT);
  pinMode(PWM_PIN, OUTPUT);
}

void transfer_2_shift_reg(uint8_t LATCH, uint8_t SER, uint8_t CLK, uint8_t dataSlow, uint8_t dataHigh){
  digitalWrite(LATCH, LOW);
  shiftOut(SER, CLK, MSBFIRST, dataSlow);
  shiftOut(SER, CLK, MSBFIRST, dataHigh);
  digitalWrite(LATCH, HIGH);
}

void tranfer_num_2_shift_reg(uint8_t num, uint8_t LATCH, uint8_t SER, uint8_t CLK){
  static const uint8_t DIGITS[] = {
    0b01101111,
    0b00001001,
    0b01010111,
    0b01011101,
    0b00111001,
    0b01111100,
    0b01111110,
    0b01001001,
    0b01111111,
    0b01111101,
  };

  uint8_t digit_0 = num % 10;
  uint8_t digit_1 = num / 10;

  transfer_2_shift_reg(LATCH, SER, CLK, DIGITS[digit_0], DIGITS[digit_1]);
}

void setup() {
  init_transfer_2_shift_reg(LATCH, SER, CLK);
}

void loop() {
  analogWrite(PWM_PIN, 125);

  for(uint8_t i = 0; i < 100; i++){
    tranfer_num_2_shift_reg(i, LATCH, SER, CLK);

    delay(500);
  }
}