timer interrupt pogram for 8051

[vanam1]

here am designing a board for door interlocking system. if we press a button, we can open the door. if the door is kept open for 15 sec then a buzzer should be blown. so i need a timer interrupt program which runs for 20secs.
so please any one help me out with some particular example.

THANKS

 

[gajendra_808]

you put the counter in program........

 

[vanam1]

you put the counter in program........

please can u guide me some sort of example....!