pull-up and pull-down doubts and questions....

Thread Starter

NOISEBOB

Joined Jul 7, 2019
48
hello

i have many questions and doubts about this project i am working on.

i'm fairly sure i'm pulling up correctly, i am doing this:


pullupper.png







but as for pull-down, i am not sure if either of this is correct:



pulldowner23333.png






pulldowner1.png






so, how am i doing?
thanks,
BOB
 

Thread Starter

NOISEBOB

Joined Jul 7, 2019
48
thanks..

so, what you are telling me is that this is all wrong?



i just want to give the chip there (a 4052) a high signal by default...
 

sagor

Joined Mar 10, 2019
479
Many ICs require unused inputs to be pulled either to Vcc or to ground. For unused pins of gates as you show,, they can be pulled down with one resistor. Same for unused gates for pullups instead, no need for individual resistors. Any input signal lines you want to hold high (or low) can use one resistor if it is a fixed, constant value.
Now for outputs, you should have individual pullup/pulldown for each line as required, such as in open collector or open drain type of outputs. Each circuit has its own requirements, depending on chips that are being used, and what the circuit is for. In some cases, like I2C signal lines (and many others), pullups are required. For some MOSFETs, pulldowns are used on the input gates to help discharge any gate charge (depends on what is driving it)
 

metermannd

Joined Oct 25, 2020
211
The practice depends on what is intended.

If you need an input permanently low (like tying off unused logic gate inputs), you can connect them directly to ground.

Likewise, if you're pulling inputs permanently high, you can tie them all together and use a single resistor to VCC.

On the other hand, if you have an input that's normally low or high, but can be pulled in the other direction at times, then you want a resistor between the input and whichever voltage would be the normal state for that pin.
 
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