Logic IC's

When one is using cmos logic IC's does one need to use pull up/down resistors? e.g., if I connect several and, or, nand, and inverter gates together to do I need to use pull up/down's on them so there output doesn't float if the inputs float for some reason(which they shouldn't but just in case). Can it hurt?

Thanks, Jon

If the input is fed from a CMOS output, then a resistor does nothing for you but waste power. Of course, inputs that would float otherwise need to be pulled up or down - there are sometimes discussions about using a resistor vs. tying it to the rail - the best argument I've heard in favor of a resistor is that if you need to mod the circuit, you don't have to hack traces or clip the pin. :-)

Hope This Helps! Rich

Only the inputs would need some pull, the outputs do not need it and can provide the pull for the input of another stage of CMOS gates. You would only need a pull on an input that is expected to be in a floating state. For example, an input on a board from some device else where or, an un-used input.

Well, not necessarily. If one of the gates get messed up then the resistors could pull the logic to a valid value instead of leaving it floating.

Thats what I was thinking but that assumes every gate in the path works? What if a gate goes down? (know its unlikely but a few resistors after the bulk of the gates might be ok too?)

Every gate in your path *must* work, otherwise your circuit (usually) won't work.

Then your circuit won't work any more. So having a pull-up or pull- down the next gate down the chain isn't going to help you at all, in all but the most unusual circumstances.

The extra resistors are just a waste of time. They add parts count, cost, board area, and could even possibly have a detrimental affect on your overall circuit reliability in some circumstances.

Dave.

That's not a good idea (IM!HO) because you could be masking the malfunction, which really should be repaired.

Cheers! Rich

heh, not if the malfunction is worse when it cannot be repaired immediately... you need some type of fail safe.

Obviously there is the possibility that when a gate fails it might go high but if I put pull ups before and after the logic then my hope is that it will provide some type of secondary fail safe incase the logic gates go bad. It would not mask the malfunction because it provides a "default" value to the logic and that value is that it doesn't do anything. (or maybe you could provide a state that triggers an "alarm" or something but my alarm is that the device will not work so it can be repaired instead, say, of self descructing)

point being: If the logic goes bad and supplies a certain output then the device could self-destruct. By supplying some pull up/down's I can potentially have it go into a safe state in some cases which is better than none.