What to do with extra pins on 74xx

[RJDog]

I am designing a circuit which requires multiplexing some digital signals... I originally designed the circuit by trying together the outputs of some tri-state non-inverting buffers (74HCT125), but then I discovered there is actually a 74xx that does what I want already... 74HCT157.  Neat!

Anyway, 74HCT157 has four discrete circuits... I only need two, so what do I do with the pins for the other two circuits?  Tie all the pins to ground?  Leave them unconnected?  What is generally accepted as the de-facto way to handle this situation?

[basinstreetdesign]

With CMOS gates such as the Quad two-input mux 74HCT157 its best to tie the inputs to all unused gates or logic blocks to ground or Vcc and leave unused outputs open.  This stops the unused inputs from floating into the active range and possibly causing the gate/logic to oscillate and so draw excessive power.

If you have unused inputs on the gates/logic which you ARE using then you have to tie them either high or low depending on the logic involved in order for your circuit to work correctly.  Unused outputs on same should, again, just be left open.

This rule holds for both CMOS logic families and real TTL families such as 74LSXX, 74XX, 74Sxx, etc but in the case of TTL the power draw with unused gates can be minimized by tying the inputs high directly to Vcc or through a 1K resistor.  As many as 20 inputs can be tied to the same resistor.

[RJDog]

Thanks; I figured tying the input pins to ground would be a good idea, but wasn't sure, and really wasn't sure about the output pins.  I guess nothing's going to hurt it by not having a load on the output.

[T3sl4co1l]

Tying them to VCC will consume slightly less current than GND, but either option is better than none.

In a pinch, TTL inputs can be left open.  They will float to logic high.  They will be susceptible to ambient noise, though.

Tim