Electronics > Projects, Designs, and Technical Stuff
Is it safe to connect inputs to VCC directly
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ataradov:
Ok, I did some searching and I can't find a good definitive answer.

Is it safe to connect to inputs of the 74-series CMOS logic directly to VCC instead of using pull-ups? What are realistic pros and cons?

I'm not interested in what may happen if device fails catastrophically and the input becomes an output and stuff like that.  I'm only interested in normal operation.

I'm also not interested in TTL logic. The devices will never be substituted on accident. Seriously, some people still make this argument.

I guess the question may be rephrased as: are there realistic use cases where it is not safe to connect inputs directly to VCC?
EEEnthusiast:
Direct shorting of inputs to Vcc or Ground can lead to latchup in some CMOS families. It is always safe to use some pull-up to limit this current.
ataradov:
What are "some". I see things like this said a lot, but no concrete evidence or an actual description of the process of that latch up.

Plus I have never seen anyone use pull-down resistors, except for some flight-critical avionics, but they do this stuff there because of very-very old standards.
digsys:
I've probably done it several 100+ times, over ~35yrs, and I've seen it done in 100s devices. In my cases, I've never seen latch-ups. As long as the Vcc lines are clean (nicely decoupled), I doubt you'll ever see a problem
james_s:
I've seen it lots of times too and done it in some of my own projects, never saw a problem anywhere.
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