Resistors when driving a CMOS input high or low?

[tecmec]

If I want to tie a CMOS input high or low (for example, a CS pin) permanently, is there any reason I'd need a resistor to do so? I see the value of using a resistor if it was an I/O pin of a microcontroller or something, and the device might try to drive the pin, but is there any need for a resistor if the pin is strictly an input (like a CS)? CMOS is high impedance, right? So why would I need a resistor?

Thanks for any input (no pun intended ),

Brad

 

[KerimF]

This resistor is usually added as a series damping resistance between the MOSFET input capacitance and the possible inductance of the driving wire (to damp possible oscillation during the fast signal transition). In static mode (or for relatively low frequencies), this resistor may be removed.

Kerim

 

[kak111]

In digital electronics the engineers use the terms pull-up or pull-down resistor,
this is a simple resistor usually between 10k and 100k, to define the input state
when no signal source is connected, this way overriding the noise signal, common sense
dictates that when you have potentially larger noise then a smaller resistor is needed,
but don’t be careless about it, don’t place a 100Ohm resistor because your signal source
must be able to “defeat” the pull-up(down) resistor. A rule of thumb is to use at least
10x larger pull-up(down) resistor then your signal source impedance.

The pull-up term is used when the resistor connects the pin to the + and pull-down
when connects to the ground, but then which should you choose?
Good question, in these days it doesn’t really mater, your application or the pcb design should dictate,
the older generation digital IC’s had asymmetrical current sink/source capability,
they could sink more current and because of that the pull-up resistor was more common.


https://www.edaboard.com/threads/217524/#post922937

Regards KAK

 

[tecmec]

In digital electronics the engineers use the terms pull-up or pull-down resistor,
this is a simple resistor usually between 10k and 100k, to define the input state
when no signal source is connected, this way overriding the noise signal, common sense
dictates that when you have potentially larger noise then a smaller resistor is needed,
but don’t be careless about it, don’t place a 100Ohm resistor because your signal source
must be able to “defeat” the pull-up(down) resistor. A rule of thumb is to use at least
10x larger pull-up(down) resistor then your signal source impedance.

The pull-up term is used when the resistor connects the pin to the + and pull-down
when connects to the ground, but then which should you choose?
Good question, in these days it doesn’t really mater, your application or the pcb design should dictate,
the older generation digital IC’s had asymmetrical current sink/source capability,
they could sink more current and because of that the pull-up resistor was more common.


https://www.edaboard.com/threads/217524/#post922937

Regards KAK

I understand what pull up/down resistors are, that wasn't my question at all. That's okay though, Kerim nailed it. Thanks Kerim

 

[FvM]

I understand what pull up/down resistors are, that wasn't my question at all. That's okay though, Kerim nailed it.

I believe, that Kerim guessed your question right, but reading your original post strictly, it seems to ask for something different. You are exactly asking for tying a CMOS input high or low. In my opinion, this means permanently, so there won't be a need for damping resistors. You'll however see statically driven CMOS inputs connected through resistors in many designs.

It's an option for in-circuit tests, allowing to drive the input to the opposite state. In so far, it's working similar to a pull-up or pull-down resistor.

 

[tecmec]

I believe, that Kerim guessed your question right, but reading your original post strictly, it seems to ask for something different. You are exactly asking for tying a CMOS input high or low. In my opinion, this means permanently, so there won't be a need for damping resistors. You'll however see statically driven CMOS inputs connected through resistors in many designs.

It's an option for in-circuit tests, allowing to drive the input to the opposite state. In so far, it's working similar to a pull-up or pull-down resistor.

You're right. You understood exactly what I was asking: permanent. Kerim answered though when he mentioned the slow or static signals.

Thanks for mentioning the in-circuit test though. I hadn't really considered that.