unused pins on 4093 IC- what to do with them?

Unused logic inputs to CMOS chips should generally not be left open. (The only case where it is ok is in those where the manufacturer has designed in some on-chip pullup or pulldown scheme). In the case of this part the inputs to your spare gates should be tied to a legitimate logic high or logic low level. For simplicity on a hobby type project it would be suitable to simply ground pins 8 & 9 and also

12 & 13. Do not ground pins 10 and 11 since these are outputs and part damage could occur if you did so.

For the inputs you could actually do any of:

- ground as suggested

- pull down to ground via a resistor

- tie direct to VDD (same rail as pin 14)

- pullup to VDD through a resistor

- tie the pins to some other logic device output that is at a known idle level as a high or low.

- mkaras

On a production board it is often wise to pull the inputs up or down through resistors. That way if you need a spare gate you'll be able to get to it without lifting pins -- I always try to design my boards to make greenwire mods easy.

If you're _really_ paranoid you'll bring the spare outputs out to vias or pads, to have them available, too.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Posting from Google?  See http://cfaj.freeshell.org/google/

"Applied Control Theory for Embedded Systems" came out in April.
See details at http://www.wescottdesign.com/actfes/actfes.html