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When to use Pull Up/Down resistors on CMOS

S

steveandaugie

Jan 1, 1970
0
Should every CMOS input be pulled high or low? What about inputs fed
by other CMOS outputs? Should unused outputs be allowed to float or
do they need to be tied to a known signal?
 
D

D from BC

Jan 1, 1970
0
Should every CMOS input be pulled high or low? What about inputs fed
by other CMOS outputs? Should unused outputs be allowed to float or
do they need to be tied to a known signal?


Ans. 1: Depends on what's driving the input...
Ans. 2: Generally none.
Ans. 3: Best not to float.

D from BC
 
D

David L. Jones

Jan 1, 1970
0
Should every CMOS input be pulled high or low?

Yes, every CMOS input should be tied either high or low, or driven by
another gate, your pick, just don't leave inputs floating.
What about inputs fed by other CMOS outputs?

No problem and no pull-up needed, unless the input to the gate driving
the input is left floating!
If the driving gate is an "open drain" type however, then you need a
pull-up.

Also, as a general rule you don't connect the inputs of unused gates
to another output that switches, as this just increases the total chip
current consumption.
Should unused outputs be allowed to float or do they need to be tied to a known signal?

You can leave outputs unconnected. Tying them with pull-ups can just
waste unnecessary current, there is no need to do this.

Dave.
 
B

Benj

Jan 1, 1970
0
David said:
Yes, every CMOS input should be tied either high or low, or driven by
another gate, your pick, just don't leave inputs floating.

Nobody explained why there is this rule so for your information: CMOS
inputs are VERY high impedance. Therefore if left floating they can
pick up charge and develop some voltage. The problem comes when they
develop a voltage in the middle of the logic range. In the middle
switching range the current through the transistors in the given gate
tend to draw much more current than when the input is full high or
full low. Therefore the distinct advantage of CMOS using little
current in static cases is lost. So if, say, you were developing some
low speed battery powered logic, some floating gates could very
drastically shorten battery life *even if those gates were not
connected to anything but power*!
 

neon

Oct 21, 2006
1,325
Joined
Oct 21, 2006
Messages
1,325
jONES IS RIGHT but he forget to mention that usualy they may begin to oscillate at very hi frequency [it depends on how trashy is gnd signals and so forth] generating inerference and heat. a good working system can become inoperative because of it.
 
S

steveandaugie

Jan 1, 1970
0
Yes, every CMOS input should be tied either high or low, or driven by
another gate, your pick, just don't leave inputs floating.


No problem and no pull-up needed, unless the input to the gate driving
the input is left floating!
If the driving gate is an "open drain" type however, then you need a
pull-up.

Also, as a general rule you don't connect the inputs of unused gates
to another output that switches, as this just increases the total chip
current consumption.


You can leave outputs unconnected. Tying them with pull-ups can just
waste unnecessary current, there is no need to do this.

Dave.

Thanks for everyone's answers. They are mostly as I expected but I
was losing confidence after placing nth resistor in circuit. Other
things I've read covered the topic but didn't necessarily leave me
with definitive answers. I do have one more question along these
lines though. If I have several inputs of the same type (like presets
or resets) that will not be switched, can they share a pull up/down
resistor or do they need to be isolated from each other as well?

Maybe I should be asking this in sci.electronics.basics. Thanks for
your time.
 
R

Richard Henry

Jan 1, 1970
0
Should every CMOS input be pulled high or low? What about inputs fed
by other CMOS outputs? Should unused outputs be allowed to float or
do they need to be tied to a known signal?

Read the datasheets carefully. The standard CMOS input requires
pullup or pulldown to keep the gate from a power-wasting and/or
oscillating middle state. However, some devices have "weak" pullups
built in to simplify this task (adding an external pullup will not
hurt the part but will cost power consumption, resistor cost and board
space); and some programmable CMOS parts (FPGAs, uCs, etc) allow
selection of internal pullups/pulldowns as an option.
 
D

D from BC

Jan 1, 1970
0
Thanks for everyone's answers. They are mostly as I expected but I
was losing confidence after placing nth resistor in circuit. Other
things I've read covered the topic but didn't necessarily leave me
with definitive answers. I do have one more question along these
lines though. If I have several inputs of the same type (like presets
or resets) that will not be switched, can they share a pull up/down
resistor or do they need to be isolated from each other as well?

Maybe I should be asking this in sci.electronics.basics. Thanks for
your time.

Unused CMOS inputs can be tied to a com or power rail depending on
the logic needed for proper operation.

For example, if you're driving your CMOS input with say an open
collector comparator and need to drive 2 CMOS inputs ...then yeah..it
looks like a pull up is shared.

The CMOS output configuration is push pull. So CMOS outputs driving
CMOS inputs don't need any resistors on the path...
D from BC
 
M

me

Jan 1, 1970
0
Thanks for everyone's answers. They are mostly as I expected but I
was losing confidence after placing nth resistor in circuit. Other
things I've read covered the topic but didn't necessarily leave me
with definitive answers. I do have one more question along these
lines though. If I have several inputs of the same type (like presets
or resets) that will not be switched, can they share a pull up/down
resistor or do they need to be isolated from each other as well?

Maybe I should be asking this in sci.electronics.basics. Thanks for
your time.
You can tie all of the inputs high with one resistor (withing reason)
 
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