Basic Arduino loop() function

gistfile1.txt

uint32_t counter = 0; /* ostensibly, OM */
char counter_buff[21];

/*
 * cmd_byte 
 * One of:
 *  I - On
 *  O - Off
 */
int cmd_byte = 0; 
int led = 13;
int light_status = 0;


/* change the light state. */
void blinky() {
  if(light_status == 0) {
     digitalWrite(led, HIGH);
     light_status = 1;
  } else {
    digitalWrite(led, LOW);
    light_status = 0;
  }
  counter+=1;
}

void loop() {
  if(Serial.available() > 0) {
     cmd_byte = Serial.read();
     if(cmd_byte == 'R') {
        sprintf(counter_buff, "%lu", counter);
        Serial.println(counter_buff);
     } else if(cmd_byte == 'Z') {
        counter = 0;
     }
  }
}

void setup() {
  pinMode(led, OUTPUT);
  attachInterrupt(0, blinky, FALLING);

  Serial.begin(9600);
  while(Serial.available() <= 0) {
    Serial.println("READY");
    delay(500);
  }
}