#define D 18
#define CLK 17

void setup() {
  pinMode(CLK, OUTPUT);
  pinMode(D, OUTPUT);
}

void loop() {
  static int i = 0; 
  i++;
  digitalWrite(D, i % 100 < 50 ? HIGH : LOW);
  digitalWrite(CLK, i % 2 == 0 ? HIGH : LOW);
  delay(10);
}