your main problem here is, making the timer in software makes your sleep mode useless, the moment the interrupt is called, the controller leaves it's sleep mode immediately to execute your Interrupt Service Routine (ISR)
you need to make it a bit more complicated to achive this with maximum efficiency
Code:
loop_code(){
if(buttonisreleased())
prepare_deep_sleep();
sleeping = TRUE;
//enter sleep, store the sleep status to some variable like sleeping = TRUE (important!!!)
while(sleeping){
execute_sleep();
}
}
}
ISR(external_interrupt)
{
if(buttonhasbeenpressed()){
start_timer();
prepare_low_power_sleep_with_timer();
else if(buttonhasbeenreleased()){
stop_timer();
prepare_deep_sleep();
sleeping = TRUE;
}
}
ISR{timer_reached_3s)
{
sleeping = FALSE;
}
when the button is not pressed, the cpui enters deep sleep until you press the button
whenever an ISR is returning, it re-enters the normal execution, the call after your sleep(), so when the button is pressed but sleep remains true(because your time dod not run out yet), you automatically returns to lo power mode, waiting for either the timer to execute an interrupt or the button to be released
when you release the button too early, the timer is stopped and deep sleep is re-entered
when the timer runs up, the sleeping variable is set to false and the sleep loop is left until you again release the button!
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