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I have a 74HC14 Schmitt trigger IC. I only use one of its I/O pairs, and everything else is not used.

I understand that in TTL logic the unused inputs should be set to either high or low to prevent floating pins.

But I wonder, can I set the outputs to high instead? Will the effect be the same, i.e prevent floating pins? Is this a bad idea?

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2 Answers 2

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The 74HC14 isn't TTL, it's CMOS, and what matters is that the input pins be kept from floating.

The reason is because the impedance of the input pins is so high when they're floating that random charges accumulating there can/will cause unpredictable internal behavior in the chip, including oscillation and high power dissipation.

The outputs don't need to be terminated since they're low impedance and they never really float, (being connected to either the high or low rail internally) their states being determined by whether the inputs are terminated to the positive or negative rail.

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  • \$\begingroup\$ Thank you, that is a great answer also because you explain the "why" and corrected the TTL error in my question. \$\endgroup\$ Commented Jun 5, 2015 at 10:01
  • \$\begingroup\$ @futureshocked: You're welcome, and thanks for the accept. :) \$\endgroup\$
    – EM Fields
    Commented Jun 5, 2015 at 11:22
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The output pins are being driven by the schmitt trigger; you should never tie a (non-tristated) output to a rail, and it won't do you any good here.

Because these are schmitt triggers, leaving the inputs floating is less harmful than would be the case for regular logic, but it's still a good idea (and free!) to tie them all to ground or VCC.

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  • \$\begingroup\$ On a breadboard or for testing it would be OK to leave the inputs floating but for a properly engineered design you must always tie floating inputs to ground or Vdd. Yes also for schmitt-triggers because you never know what state they will be in. So force them in a state and be done with it. \$\endgroup\$ Commented Jun 5, 2015 at 9:24
  • \$\begingroup\$ @Rimpelbekkie Isn't that what I just said? \$\endgroup\$ Commented Jun 5, 2015 at 10:03

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