This should give you the idea of software debounce:

Assume you are polling a switch connected port P1.0 .The switch when closed will connect the port to ground..

<pre>
setb p1.0 ; port pin as input
NoChange: jb p1.0, $ ; wait to sense the first low
call Delay ; this is the debounce delay -say 30 millisecond delay routine.
jb p1.0, NoChange ; Not a valid closure. Go back and wait

YesClosed: (your code when switch closed )

Normally the delay can be between 20 to 50millisecond.

Raghu