Unsigned long and delayed actions using millis()

Projects Programming
1

Hi, I have a strange "non-problem" with my code. For simplification I've written a smaller program. The program should control two LED's. Each LED will turn on for 1 second, and then turn off for 3 seconds. My initial problem was that I wanted one LED to turn on after the second LED has been off for 1 second.

So basically it would look like: both off, LED1 on, both off, LED2 on, both off, LED1 on, both off... and so on.

The following code seems to work, but I feel like it's not supposed to.

int led1 = 2;
int led2 = 3; 
long off_interval = 3000;
long on_interval = 1000;
int d = 2000; //delay

unsigned long prev_ms1 = 0;
unsigned long prev_ms2 = -d; //delay added

void setup()
{
  Serial.begin(9600); 
  pinMode(led1,OUTPUT);
  pinMode(led2,OUTPUT);
  digitalWrite(led1, LOW);
  digitalWrite(led2, LOW);
}

void loop()
{
  unsigned long curr_ms = millis(); 
  if((curr_ms - prev_ms1) > off_interval)
  {
    digitalWrite(led1, HIGH);
  }
  if((curr_ms - prev_ms1) > off_interval+on_interval)
  {
    digitalWrite(led1, LOW);
    prev_ms1 = millis();
  }
  if((curr_ms - prev_ms2) > off_interval)
  {
    digitalWrite(led2, HIGH);
  }
  if((curr_ms - prev_ms2) > off_interval+on_interval)
  {
    digitalWrite(led2, LOW);
    prev_ms2 = millis();
  }
}

The lines I am worried about are the if statements involving prev_ms2:

 if((curr_ms - prev_ms2) > off_interval)

I thought this line should not work, since I defined int d=2000, and unsigned long prev_ms2 = -d. I was trying to "shift" the output of the second LED so that it would blink when the first LED was off. I feel like this line should give me an error, since at the beginning, wouldn't we have a negative value - and thus some sort of rollover - in an unsigned long? Somehow I'm not having this issue and the code works fine. Could someone please tell me why? Am I accidentally casting something?

2

wouldn't we have a negative value - and thus some sort of rollover - in an unsigned long?

No you are not going to get a negative value in an unsigned long, by definition.

3

It's probably easier to keep the timing separate (rather than adding interval values together). Look at the demo several things at a time.

Use one activity to start another. For example the code that turns one LED off also starts the timer for another action.

I find it very useful to write down all the steps - each on a separate line - so I can see clearly what I want to happen before I start writing code. See also planning and implementing a program.

...R

4

Thank you for the suggestion, Robin2. I hadn't quite thought that I could use one activity to start the second LED.