Multiple IC pins with the same pull-up resistor?

I’ve searched across the forum but I didn’t find an answer. Maybe this is a too stupid question… :-[

I’m interfacing Arduino Pro Mini with an IC (i.e. an STN1110). Datasheet recommends to pull-up several unused pins to 3.3V by using the same resistor value: 100K Ohm.

Since I’m trying to save as much space as possible on my PCB board a question has raised to my mind: do I really need to solder several resistors? Could I use just one resistor commonly shared between all that pins?

For example, in the attached schematics, pin 4, 15, 21 and 22 could not share the same resistor pole, while the other is linked to +3.3V? :-![

(I thought: when using 4 distinct resistors the power line will “see” those resistors in parallel, not in series.)

(I thought: when using 4 distinct resistors the power line will "see" those resistors in parallel, not in series.)

Sorry, that is completely wrong. The power line "sees" only one end of each resistor. It is the other end that matters, and they are connected to four different circuits.

But you really have to dig into the IC specifications to determine why individual resistors were placed. It doesn't seem worthwhile.

Hi, This is not an unusual practice. I have used it on projects with dozens of ICs. I have seen it on other commercial products. The purpose of the pullups is to keep the input from floating where they may pick up noise.

It depends which pins - are they inputs or in/out ? Do they need a set current or are they just logic signals?

For example, in the attached schematics, pin 4, 15, 21 and 22 could not share the same resistor pole, while the other is linked to +3.3V?

IMHO, it is OK to use the same 100k resistor to pull-up the pins that, according to the datasheet, must be pulled up to VDD via 100k resistor - But you should have a closer look at the datasheet, because some of the pins you listed here (21 and 22 ...), must be pulled up to VDD (no resistor specified) .

terryking228: This is not an unusual practice.

Yes, it seems so also to me . Today, in place of going to the park (my wife is still angry), I spent time on google and I've found this kind of question on StackExchange too.

So, even if not orthodox, the "common resistor" solution seems natural and reasonable.

MarkT: It depends which pins - are they inputs or in/out ? Do they need a set current or are they just logic signals?

In my case, they all manage logic signal.

In the data sheet there is a table listing their type:

4 = Input = "Schmitt trigger input with CMOS levels analog input"

15 = #22 = Output = "open drain output"

21 = Input/Output

alnath: But you should have a closer look at the datasheet, because some of the pins you listed here (21 and 22 ...), must be pulled up to VDD (no resistor specified) .

Yeah, I know. VDD has a decoupling capacitor. Unfortunately, even datasheets are made by humans, so pin description must be integrated by reading the following chapter 5:

"Unused open drain outputs can only be terminated with a [u]resistor[/u] connected to VDD or 5V. Unused inputs can be terminated via a [u]resistor[/u] or direct connection to VSS or VDD."

(FWIW, the STN1110 work quite well also in the configuration that I've posted)

I would made my question a little bit more generic, to help people having similar doubts when looking at a IC schematic. Schematics are often populated with a lot of the same pull-up resistor flying everywhere.

I suspect that would be a bad idea as some of these nominated pins are clearly outputs. Unless they are guaranteed to be open-collector, accidental glitches (including program errors) could cause them to conflict as well as define unwanted states.

Input only pins can be paralleled to a single resistor with no issue ( I usually use a 10K though ). Anything that says output is not an unused input. Anything that says in/out is still and output. If you don't know the drive state use the 100K and keep it separate. Dwight