When to use Pull Up/Down resistors on CMOS

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

D from BC

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.

signal?

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

Dave.

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*!

known signal?

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.

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.

known signal?

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

steveandaugie wrote in news: snipped-for-privacy@x40g2000prg.googlegroups.com:

You can tie all of the inputs high with one resistor (withing reason)

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