Author Topic: Is it safe to connect inputs to VCC directly  (Read 9939 times)

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Online ataradovTopic starter

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Is it safe to connect inputs to VCC directly
« on: February 04, 2020, 03:56:01 am »
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?
Alex
 
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Offline EEEnthusiast

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Re: Is it safe to connect inputs to VCC directly
« Reply #1 on: February 04, 2020, 04:08:58 am »
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.
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Online ataradovTopic starter

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Re: Is it safe to connect inputs to VCC directly
« Reply #2 on: February 04, 2020, 04:14:00 am »
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.
Alex
 

Offline digsys

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Re: Is it safe to connect inputs to VCC directly
« Reply #3 on: February 04, 2020, 04:18:17 am »
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
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Offline james_s

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Re: Is it safe to connect inputs to VCC directly
« Reply #4 on: February 04, 2020, 05:53:51 am »
I've seen it lots of times too and done it in some of my own projects, never saw a problem anywhere.
 

Offline obiwanjacobi

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Re: Is it safe to connect inputs to VCC directly
« Reply #5 on: February 04, 2020, 06:27:59 am »
The only reason I always use pullups is because I can remove them if I need the gate to botch a problem.

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Offline TimFox

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Re: Is it safe to connect inputs to VCC directly
« Reply #6 on: February 04, 2020, 08:56:45 pm »
Connecting inactive inputs to ground or Vcc through appropriate resistors allows testing the IC in-circuit by driving the input from a “stiff” test voltage on a test jig.  This is part of design-for-test process.
 

Online ataradovTopic starter

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Re: Is it safe to connect inputs to VCC directly
« Reply #7 on: February 04, 2020, 09:00:40 pm »
Connecting inactive inputs to ground or Vcc through appropriate resistors allows testing the IC in-circuit by driving the input from a “stiff” test voltage on a test jig.  This is part of design-for-test process.
This argument is just strange. Why would you ever want to override an input, which value is fixed at the design stage? What are you testing here exactly?

Why don't you decouple the inputs that are driven by other outputs? So you could override those as well for testing?

What is so special about pins that must have fixed value?
Alex
 
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Online ejeffrey

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Re: Is it safe to connect inputs to VCC directly
« Reply #8 on: February 04, 2020, 09:15:58 pm »
On a straight 74 series CMOS logic?  No, just connect to Vcc.  On a more complicated IC like an MCU There are lots of reasons you might not want to: ease of rework / modification, that the pins might have some use during function test, or to prevent excess current if the pin is accidentally switched to an output.  But these are all unlikely to apply on a simple logic chip, and only really apply for R&D prototypes anyway.  On anything you make a second time, those resistors are just a waste of space and another thing to go wrong.
 

Online wraper

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Re: Is it safe to connect inputs to VCC directly
« Reply #9 on: February 04, 2020, 09:35:29 pm »
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.
How resistor is supposed to limit current through MOSFET gates when latch-up does not even cause current flowing through them?
 

Online wraper

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Re: Is it safe to connect inputs to VCC directly
« Reply #10 on: February 04, 2020, 09:40:15 pm »
 

Offline TomS_

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Re: Is it safe to connect inputs to VCC directly
« Reply #11 on: February 04, 2020, 10:07:12 pm »
I asked about this some time ago and got some reasonably good answers:

https://www.eevblog.com/forum/projects/pull-up-resistors-technically-necessary-vs-preference/msg1699586/

Short answer: I dont bother with pull up/down resistors any more except when I might need to override a signal.
 
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Offline tggzzz

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Re: Is it safe to connect inputs to VCC directly
« Reply #12 on: February 04, 2020, 10:08:51 pm »
Connecting inactive inputs to ground or Vcc through appropriate resistors allows testing the IC in-circuit by driving the input from a “stiff” test voltage on a test jig.  This is part of design-for-test process.
This argument is just strange. Why would you ever want to override an input, which value is fixed at the design stage? What are you testing here exactly?

Not testing the design; that was done earlier.

Testing that this instance of the design has been manufactured correctly, e.g. components inserted correctly, no short circuits etc. That requires a completely different set of tests.
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Offline tkamiya

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Re: Is it safe to connect inputs to VCC directly
« Reply #13 on: February 04, 2020, 10:32:47 pm »
The idea is, you don't want input of a CMOS gate floating around.  You can tie it to ground, VCC, via or without registers.  Just make sure the voltage you tie to is equal to or less than power to the chip.  I usually tie to VCC or ground, whichever is closer.
 

Offline David Hess

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Re: Is it safe to connect inputs to VCC directly
« Reply #14 on: February 04, 2020, 11:12:36 pm »
If multiple Vcc supplies are used then that would be a good reason to use series input resistors to limit the current but in the common case, I do not know of any reason the input cannot be directly connected to either power rail.

Early TTL is a special case because of its common base multiple emitter input structure which required series resistors if an input was pulled up to Vcc.

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.

How resistor is supposed to limit current through MOSFET gates when latch-up does not even cause current flowing through them?

It does not have anything to do with the gate; it is insulating after all.  Latchup occurs when carriers are injected into the substrate and they activate the parasitic SCR structure present in a junction isolated CMOS process.  This can happen if the input or output ESD protection diodes are forward biased with enough current.

I have personally witnessed it a few times when static jumped between a finger and a CMOS logic board and one CMOS IC became an SCR crowbar across the power supply.
 

Online MarkF

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Re: Is it safe to connect inputs to VCC directly
« Reply #15 on: February 04, 2020, 11:28:30 pm »
Very very old...

 

Online ataradovTopic starter

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Re: Is it safe to connect inputs to VCC directly
« Reply #16 on: February 04, 2020, 11:29:24 pm »
But what does SCR latch-up has to do with connecting inputs to the VCC directly?
Alex
 

Online wraper

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Re: Is it safe to connect inputs to VCC directly
« Reply #17 on: February 04, 2020, 11:40:36 pm »
If multiple Vcc supplies are used then that would be a good reason to use series input resistors to limit the current but in the common case, I do not know of any reason the input cannot be directly connected to either power rail.

Early TTL is a special case because of its common base multiple emitter input structure which required series resistors if an input was pulled up to Vcc.

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.

How resistor is supposed to limit current through MOSFET gates when latch-up does not even cause current flowing through them?

It does not have anything to do with the gate; it is insulating after all.  Latchup occurs when carriers are injected into the substrate and they activate the parasitic SCR structure present in a junction isolated CMOS process.  This can happen if the input or output ESD protection diodes are forward biased with enough current.

I have personally witnessed it a few times when static jumped between a finger and a CMOS logic board and one CMOS IC became an SCR crowbar across the power supply.
Exactly, you need to bring terminals above Vdd or below GND. Direct input connection to the power rail actually prevents it. So I don't get what current is supposed to be limited by using resistor instead of direct connection. Certainly not current which flows through IC when latch-up already happened.
« Last Edit: February 04, 2020, 11:43:02 pm by wraper »
 

Offline tkamiya

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Re: Is it safe to connect inputs to VCC directly
« Reply #18 on: February 04, 2020, 11:45:59 pm »
And.....  Why do input of TTL logic gate have to be pulled up or down?  It is completely safe to just leave it unconnected.  At least I've never done it and never seen any one else do it that way.  I thought the practice was for ?mos type devices only.
 

Offline james_s

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Re: Is it safe to connect inputs to VCC directly
« Reply #19 on: February 05, 2020, 12:05:04 am »
It's just good practice to do so, it can prevent strange problems. A lot of modern "TTL" stuff is actually CMOS.
 

Online wraper

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Re: Is it safe to connect inputs to VCC directly
« Reply #20 on: February 05, 2020, 12:13:40 am »
It's just good practice to do so, it can prevent strange problems.
More like stupid practice how to increase manufacturing costs and problems due to higher component count. Also you reduce noise immunity or robustness against board contamination or tin whiskers. Say CMOS output (connected to input) has much lower impedance than a few k resistor you would use as pull-up.
EDIT: It was about adding resistors. Apparently james_s meant connecting to power rail.
Quote
A lot of modern "TTL" stuff is actually CMOS.
Haven't seen anything claiming to be TTL. There might be written something like TTL voltage level.
« Last Edit: February 05, 2020, 02:43:29 am by wraper »
 

Offline MasterT

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Re: Is it safe to connect inputs to VCC directly
« Reply #21 on: February 05, 2020, 12:30:02 am »
But what does SCR latch-up has to do with connecting inputs to the VCC directly?
In normal condition doesn't matter if there is a resistor installed or not. But a good designer have to predict all abnormal situation as well, like:
1. over-voltage to the IC
2. ESD discharge
3. current injection to/from neighboring inputs/ outputs that may get signal from other modules/ boards etc.
4. power pin connection loss
5. strong RF pulse - EMP warfare.

Inputs are not designed to pass much current, so there is "probability"  latching-up, and it's easier to install a resistor than estimate 12-20 inputs/outputs interference to each other .
 

Offline iMo

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Re: Is it safe to connect inputs to VCC directly
« Reply #22 on: February 05, 2020, 12:31:11 am »
It is a good practice, originated in times where a 2 layer pcb board was 40x80cm large with 500 DIL14/16 chips on it. On such a board with a dozen of XXcm long parallel straight Vcc and Gnd rails, following the never ending rows of DIL sockets, the term "connected directly to Vcc or Gnd" may not always mean "connected directly to Vcc or Gnd".. :)
Readers discretion is advised..
 

Online ataradovTopic starter

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Re: Is it safe to connect inputs to VCC directly
« Reply #23 on: February 05, 2020, 12:34:37 am »
 As I said, I'm not interested in catastrophic failures. If there is over-voltage or power pin connection loss, then device is broken. It does not really matter what happens after that.

I don't buy this "good designer" stuff.

I see this matter for aerospace or military applications, but there you have to think about much more than just a simple pull-up.
Alex
 

Online wraper

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Re: Is it safe to connect inputs to VCC directly
« Reply #24 on: February 05, 2020, 12:38:50 am »
Inputs are not designed to pass much current, so there is "probability"  latching-up, and it's easier to install a resistor than estimate 12-20 inputs/outputs interference to each other .
Again, please explain how MOSFET gates tied to Vdd/Vss are supposed to pass any current. Latch-up does not result in current passing through the gates.
 


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