Assume that you have written "1" to a port pin used as input.

READ: Wait here till the port pin goes low

Debonce : Delay for 20mSec or So.

Check : Is the port pin still low ?

If yes, then you got a valid keypress. Do something based on it.

If no, it was a invalid keypress. Go back to READ.

Note : The 20ms is typical. You may have to vary it upwards till you get the optimum value.

And this is not a good way to read a switch. Holding a processor to read a switch only is bad. Sample shown just to explain the debounce. Otherwise ideally done with interrupts. Read the 8051 Bible for info on that.

Raghu