Hi Mahdy,

Best is sofware via a state machine. Sample all the contacts every 100mS; if any 1 is closed, decode the contact immediately, then wait (100mS per loop) until *all* are off again (assuming simple keyboard). Why 100mS when the processor is running at 12MHz+?? Just show me a human moving faster than 10Hz!! (you're not building a PC keyboard, I suppose!). If your process has to run faster than that, don't use mechanical contacts!
If more than 1 contact can be on at one time, react to the *changes* by comparing current state to previous state, every 100mS.

Regards,

Richard