CMOS 74HC164 Shift Register

[nbrucew]

Novice electronics hobbyist. Building toy for grandkid using 74HC164 shift register wired as ring counter to circulate some LED lights. Only using outputs Q to Q5, leaving outputs Q6-Q7 floating. Question ... realize with CMOS chips you should not have any input gates floating ... but how about outputs ??? Works well as is and now curious from electronics sense if leaving outputs Q6-Q7 floating is correct ...

Thanks for any input

[Chris Smith]

From memory it was the inputs that were critical.

Stray inductance set them off and they could flip flop, heat up, re transmit emf, damage, spike, or what ever.

[Robert Reed]

Hello Nbruce
The input resistance to C-Mos devices can approach the 12th order of magnitude. When left open it is indeterminate as to what voltage they will assume, and the device may even oscillate due to noise and other perturberations on its input. So yes by all means, tie them to ground or the 5 volt rail, The outputs now being very stable in one logic state or the other (due to it's inputs being tied) can just sit there unterminated forever with no ill effects.However, I believe in your case, the inputs to Q6,7 are already internally tied .

[haklesup]

To add to the already correct answer above, floating inputs not only leaves them undertemined (and thus also their associated outputs) but it also tends to needlessly raise the supply current of the device. This is particularly important in battery operated devices.

For example, a 74HCT00 with all inputs high or low and all outputs floating will consume almost no supply current (<10nA ) But float an input and the supply current will increse by at least 5mA for each input floated. Float all 8 inputs and the supply current can exceed 150mA. (I can explain why too but left it out for now, just ask)

The magnitude of the efffect depends on the device and pin type but it is generally true for CMOS input only pins.

Unused I/O and output pins can be left floating as can pins specifically designated as having pullup or pulldown resistors. Outputs should never be tied to VDD, Ground or to another output pin (not counting tri state output, they are really I/O pins)

[Chris Smith]

Unused inputs can act like a flip flop, JKs, and all the rest of the gate devices that go into full mode and trigger at any moment, causing anything from spiking, over heating, run away effects, or RF creation that causes other parts to trigger, etc.

Some require shielding if the RF effect gets too big.

When I was building flip flops to run diodes and laser diodes at their maximum speeds in the NS range, they suggested a minimal PF capacitor or a 914 diode on the unused and feed back circiutry [and often the used] inputs to oscillate at its maximum frequency and still be safe. [Oscillator]

With out them they suggested in the foot notes all of the above can happen including over heating, spiking the chip, and false triggering.