Arif,

You should re-read the posting that Kai made to this thread. Do not let inputs float!! As a derivative comment to that - Do not let floating (or open) inputs be a design criteria that you try to depend upon for your electronics implementation.

Even in days when I still used "LS" logic it was an unacceptable practice to leave inputs open. These days when CMOS type parts are the prevalent choice it is EVEN MORE IMPORTANT to not let inputs be open of float. Kai gave many of the reasons why. The most worrisome ones being those that could blow out the input gates of the part due to the 10^^12 ohm input impedance on this type of part.

Do note that on some parts the chip designer has taken care to provide some pull-up or pull-down resistors so that unused inputs can be left open in some instances. Take for example the MAX232 chip (http://www.maxim-ic.com/datasheet...p/id/1798). It has 400K ohm pullups on its inputs on the CMOS input side so it is generally safe to leave these open if there is NOTHING connected to the package pin. Do note that if there is any copper connection to the open input of such chip with the on-board 400K resistor or if the part usage scenario is such that there is a lot of noise or switching activity on adjacent copper connections then it would be a wise idea to provide for a lower impedance drive to the pins using the output of some other gate or a pullup resistor with a much lower value. Size selection of course is design specific and depends on a number of factors including power source, whether there is another driving source that must compete with the pullup and whether you are trying to pull in the opposite direction than the on-chip pull-up/pull-down.

Michael Karas