I don't think you can make the DS3234 alarm every 15 minutes. You would have to set an alarm to go off every minute and then, when the alarm does go off, read the minutes register to see if it is 0, 15 , 30 or 45.Pete
I really need save battery.
Some sleep modes keep the timers active, These timers can be programmed to trigger an ISR.This ISR can wake the system when appropriate
That info Is on the same page - http://www.gammon.com.au/forum/?id=11497 - search for "Waking from sleep with a timer"