do CMOS chips need resistors on "1" inputs?

[maark6000]

Hi all, I'm starting to work with CMOS chips having learned everything on TTL ones. I seem to remember reading somewhere that I need to be careful of tying the Vdd directly to the inputs of CMOS chips... for example if on a counter you wanted to set a preset, or in indicating whether the chip counts up or down. Am I just making that up in my head?

Thanks,

Mark
Los Angeles

 

[kripacharya]

You're just making it up in your head.

It's the TTL's which require the resistors.

 

[maark6000]

ha, of course. so is there a standard value to use on TTLs, like 1k? And I'm assuming it's just for where you apply a digital "1"? You don't need to use a resistor on the +V in, right?

 

[kripacharya]

There is no 'standard' value, though my R of choice used to be 5k-10k for minimising current draw.

Obviously (i would think) its for applying only "1".

Lastly - what is +Vin ?

Is it the supply pin ? If so, then at best you would put in a smoothing/ decoupling R, L and / or C here. But definitely NOT a 1K.

 

[dick_freebird]

TTL has a natural pullup and, if the pin is truly open, will
assume a weak (but adequate) "1" state. Of course that
key assumption should not really be relied upon, in the
field.

CMOS inputs tehd to be very high impedance with ESD
clamps roughly balanced, and you have no good idea
where pin voltage will end up unless you do something
explicit about it. If I can conceive of a fault scenario
that would open up the pin, or I want to let the user
not-connect it safely, a pullup or pulldown I will apply
on-chip.