Using the RTC SQW pin to interrupt the Arduino

I trying to write a sketch which runs once a second. I can't use millis because i'm already using it within the once e very second code. This will be my first attempt at interrupts. I assume (hah) I can connect the SQW pin to pin 21 ( I'm already using pins 2 and 3). I need to know how I enable the SQW signal.The only way I've found involves setting all the time values also ( 8th send). I don't want to do that, I want all the time values left alone. Would triggering on rising edge be the best way? I've been searching the web but have only gotten more confused.
Jim

Do you mean, you want to sleep for most of that second? Or is it not that complicated?

I have some writeups on timers here:

You could set timer 1 to interrupt you once a second. However that won't work so well if the CPU is asleep.

Nick asked

No, The sketch will be rather busy processing data, with delays using millis to ensure the processing completes before going on. I plan on all the processing to complete before the next 1 second interrupt starts it all over.

Jim

Seems overcomplicated to me to use interrupts. Just do what you have to do, and then loop until millis() returns 1000 more than last time, and you know the second is up.

I mean, all the interrupt will do is tell you the second is up. Well you can find that out for yourself.

if (millis () - lastMillis >= 1000)
  {
  // time to start again ...
  }

I don't understand, are you saying that you can have multiple instances of milli? I can use milli to time my small delays while still using milli for my one second time delay? I need the whole thing to start as exactly on one second as possible to sync with an external process.

Jim

are you saying that you can have multiple instances of milli?

No. There is one function that returns how long the Arduino has been running.

I can use milli to time my small delays while still using milli for my one second time delay?

Yes.

alfiesty:
I can use milli to time my small delays while still using milli for my one second time delay?

Well, can you use your watch to time the main meal, and also the carrots? Or do you go and buy two watches?

Seriously, you note the start time, and from that various different elapsed times can be noted also.