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

Discussion in 'General Electronics Chat' started by gimpo, Jan 30, 2016.

  1. gimpo

    Thread Starter Member

    Jan 27, 2016
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    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)
     
  2. hp1729

    Well-Known Member

    Nov 23, 2015
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    Safest answer is no, you can't tie them together with one pullup. If they are all inputs, probably you could.
     
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  3. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
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    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.
     
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  4. gimpo

    Thread Starter Member

    Jan 27, 2016
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    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.
     
  5. andre_teprom

    Member

    Jan 17, 2016
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    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.
     
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  6. gimpo

    Thread Starter Member

    Jan 27, 2016
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    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.)
     
  7. andre_teprom

    Member

    Jan 17, 2016
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    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.
     
  8. gimpo

    Thread Starter Member

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

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

    Expert

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

    Thread Starter Member

    Jan 27, 2016
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    Nice to know
     
  11. andre_teprom

    Member

    Jan 17, 2016
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    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.
     
  12. djsfantasi

    AAC Fanatic!

    Apr 11, 2010
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    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.
     
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  13. andre_teprom

    Member

    Jan 17, 2016
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    Exactly, if someone wants to proceed other than common sense, the'll be doing it for his own peril.
     
  14. gimpo

    Thread Starter Member

    Jan 27, 2016
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    On stackexchange they say that this can be done for input permanently wired to HIGH signal:

    Sharing a pull up resistor

    mmmh... now I'm confused.... :confused:
     
  15. hp1729

    Well-Known Member

    Nov 23, 2015
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    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.
     
  16. gimpo

    Thread Starter Member

    Jan 27, 2016
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    Yeah, but what happens if the IC manufacturer states explicitly "Pull up to VDD via 100 kΩ resistor if unused." for those pins?
     
  17. Hypatia's Protege

    Distinguished Member

    Mar 1, 2015
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    --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: Jan 30, 2016
  18. Hypatia's Protege

    Distinguished Member

    Mar 1, 2015
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    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
     
  19. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    In the datasheet there is a typical configuration given.
    It clearly shows wich pins are input and wich are output:

    STN1110_typical_configuration.png

    Bertus
     
  20. hp1729

    Well-Known Member

    Nov 23, 2015
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    Manufacturer's specs override general ideas.
     
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