As the others have said - staying in an ISR for a long time is counter-productive. Maybe to solve your problem you need to approach it differently. I gather you want to do *something* as long as the input is active and this input is most likely a switch of some sort as you have a debounce time. You are probably better off using a timer to poll this input at a regular interval - say 10mS. Also, try to limit calling subroutines from the ISR - it causes more register usage to keep track of and bloats up the stack. As Erik mentions - keep the ISRs short and sweet. Using an interrupt input for a switch input is normally not a good idea - except if you want to wake up the cpu from sleep mode!