better_countdown.ino

#define SEC_PER_MIN (60)
#define SEC_PER_HOUR (60ul * 60)
#define SEC_PER_DAY (24ul * 60 * 60)

unsigned long countdownTime = 0;

void setup(){  
  Serial.begin(9600);
  delay(20);
  Serial.println(F("Enter Days"));
  int days = getInputFromUser();
  Serial.println(F("Enter Hours"));
  int hours = getInputFromUser();
  Serial.println(F("Enter Minutes"));
  int minutes = getInputFromUser();
  Serial.println(F("Enter Seconds"));
  int seconds = getInputFromUser();
  
  countdownTime = seconds + (minutes * SEC_PER_MIN) + (hours * SEC_PER_HOUR) + (days * SEC_PER_DAY); 
}

void loop(){
  
  static unsigned long lastTick = millis();  
  unsigned long currentMillis = millis();
  
//  Way more accurate timing this way than with delay
//  Also allows for code to do other things during countdown
  if (currentMillis - lastTick >= 1000){
    lastTick += 1000;
    countdownTime--;
    displayTime(countdownTime);
  }
  
  if (countdownTime == 0){
    Serial.println(F("Countdown Finished"));
    while(1);  // infinite loop...  lock up program until reset
  }  
}

// blocks program for input from user via serial
// returns the number entered as an int
int getInputFromUser(){
  char buf[5];
  int index = 0;
  while(Serial.available() < 1);  // DO nothing until serial arrives
  
  buf[index] = Serial.read();
  
  while (buf[index] != '\n'){
    if(Serial.available()){
      buf[++index] = Serial.read();
    }
    //  Don't run over the end of buf
    if (index == 4){
      buf[index] = '\n';
      break;
    }
  }
  // Received a null, Serial transmission is over  
  return atoi(buf);
}


//Prints time to serial in D:H:M:S format  

void displayTime(unsigned long aTime){
  int seconds = aTime % SEC_PER_MIN;
  int minutes = (aTime % (SEC_PER_HOUR)) / SEC_PER_MIN;
  int hours = (aTime % (SEC_PER_DAY) / (SEC_PER_HOUR));
  int days = aTime / (SEC_PER_DAY);
  
  Serial.println(F("tick"));
  Serial.print(days);
  Serial.print(F(" : "));
  Serial.print(hours);
  Serial.print(F(" : "));
  Serial.print(minutes);
  Serial.print(F(" : "));
  Serial.println(seconds);
}