#define clk 2
#define HalfPulseWidth 5

void setup() {
  // put your setup code here, to run once:
pinMode(clk, OUTPUT);
}

void loop() {
  // put your main code here, to run repeatedly:
digitalWrite(clk, HIGH);
delay(HalfPulseWidth);
digitalWrite(clk, LOW);
delay(HalfPulseWidth);
}