Do I really need all that bunch of pull-up resistors?

Thread Starter

gimpo

Joined Jan 27, 2016
123
Hello,

Very often I see ICs datasheets where unused input pin are recommended to be permanently pulled-up to high logic-value by using a resistor. Usually in the schematics every unused pin is drawn with his own transistor. Take the following IC as example:

stn_min.jpg

I'm always questioning my self: "do I really need to solder four resistor or I could use just one for all that pins???" o_O

BTW. In the schematic above the four pins are so classified:

#4 = input
#15 = output (open drain)
#21 = input and output (open drain)
#22 = output (open drain)
 

hp1729

Joined Nov 23, 2015
2,304
Hello,

Very often I see ICs datasheets where unused input pin are recommended to be permanently pulled-up to high logic-value by using a resistor. Usually in the schematics every unused pin is drawn with his own transistor. Take the following IC as example:

View attachment 99687

I'm always questioning my self: "do I really need to solder four resistor or I could use just one for all that pins???" o_O

BTW. In the schematic above the four pins are so classified:

#4 = input
#15 = output (open drain)
#21 = input and output (open drain)
#22 = output (open drain)
Safest answer is no, you can't tie them together with one pullup. If they are all inputs, probably you could.
 

blocco a spirale

Joined Jun 18, 2008
1,546
Clearly, some of those pins are outputs and may change state during normal operation. It would be unwise to connect these to inputs where these changes of state may invoke other actions within the chip.

I would speculate that on some devices where there are multiple identical unused inputs that it would be safe to connect those to a common pull-up and the same could apply to identical unused open drain/collector outputs.

But, it is always best to follow the manufacturers recommendations, they know far more about the internal workings of the device than we do.
 

Thread Starter

gimpo

Joined Jan 27, 2016
123
Clearly, some of those pins are outputs and may change state during normal operation.
mmmh... clever observation! ;) Why I didn't think about that???

Datasheets rarely goes in assembly details, especially for ICs that can be used in multiple configurations. Maybe authors think that the reader is smart enough to get out tricks by himself (not my case :oops:)

So, as hp1729 said above, seems that only "pure" input logic pins (i.e neither "O" nor "I/O" pins) can share the same resistor.
 

andre_teprom

Joined Jan 17, 2016
31
I admit that did not read the datasheet of this device in particuar, as well as in general I do not think about that once designing the circuits, but I would dare to say that as a general rule, assuming that these are high impedance inputs, the use of individua pull up/down resistors close to each input, tends to turn the design less sensitive to EMI noises.
 

Thread Starter

gimpo

Joined Jan 27, 2016
123
the use of individua pull up/down resistors close to each input, tends to turn the design less sensitive to EMI noises.
Interesting point of view too. I'm not an expert but... could really a simple resistor (alone) lower the EMI noises?

I also have a question: when the input pin is used for TTL logic levels (at 3.3 or 5V levels) are such EMI noises so strong enough to corrupt the logic value interpreted by the IC? (I hope that you can understand what I mean.)
 

andre_teprom

Joined Jan 17, 2016
31
Interesting point of view too. I'm not an expert but... could really a simple resistor (alone) lower the EMI noises?
I did not say that, what I meant is actually the use of a single resistor, indeed due to you have to route at the PCB layout several tracks comming to the same component, tends to create an "antenna" effect, whereas using pul -up individual's components confines the concentration of chargein a small area. Obviously, I'm mentioning issues reated to critical speed constraintes, which certainly is not the case of your design, in which perhaps gathering of several resistors into a single one would not represent any issue, just a guess.
 

Thread Starter

gimpo

Joined Jan 27, 2016
123
Ah! Ok. Now it's clear.

(Anyway, just for speaking, the STN1110 it's an OBD decoder chip running at 16 Mhz.)
 

Papabravo

Joined Feb 24, 2006
12,770
If the board is to be tested with an automated bed-of-nails tester, then the individual resistors are essential to the testing protocol.
 

andre_teprom

Joined Jan 17, 2016
31
it's an OBD decoder chip running at 16 Mhz.)
In this case, considering that there are pins with pull-up in opposite positions of the chip, and the routing in general go nearby several circuits, would not be at all a bad idea not to concentrate on a single resistor.
 

djsfantasi

Joined Apr 11, 2010
5,958
Some people have speculated that it may work. Others have pointed out problems that they speculate may occur. I'd worry that every time it was used, whether it would work would then be speculative.
 

Thread Starter

gimpo

Joined Jan 27, 2016
123
Resistors are not "required". In most instances you can connect the unused input to ground or VCC. Another option is to use an unused gate output.
Yeah, but what happens if the IC manufacturer states explicitly "Pull up to VDD via 100 kΩ resistor if unused." for those pins?
 
Resistors are not "required". In most instances you can connect the unused input to ground or VCC
--Emphasis added--

Tying unused inputs to Vcc/Vdd is not advisable for liability to latch-up (among other considerations) -- Don't do it!!!:eek::eek::eek::rolleyes:

Best regards
HP
 
Last edited:
Yeah, but what happens if the IC manufacturer states explicitly "Pull up to VDD via 100 kΩ resistor if unused." for those pins?
Would you believe that it means: "Specified operation is guaranteed only when said unused pin is pulled up to VDD via a 100 kΩ resistor"?:rolleyes:

Golly golly!o_O
HP
 
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