Do I need a pull up resistor?

Discussion in 'Embedded Systems and Microcontrollers' started by spinnaker, Dec 9, 2012.

  1. spinnaker

    Thread Starter AAC Fanatic!

    Oct 29, 2009
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    I need to tie an enable pin of a logic chip high. It will always stay high and never change. In general, is it safe and proper to pull a pin high by connecting it direct to VDD do I need a pull up resistor?
     
    Last edited: Dec 9, 2012
  2. JohnInTX

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    Jun 26, 2012
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    I've seen and done it both ways but a resistor is preferred to help protect the input against transients on the power bus. A single R is sufficient for all pulled up inputs. A current project I'm writing firmware for uses 1K.
     
  3. panic mode

    Senior Member

    Oct 10, 2011
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    you can do either.

    in my designs i leave power traces wider not just because of current draw but also to easier identify them. then when routing is done, any pin that was connected to GND or Vcc will likely have more copper around it and make any patchwork less appealing.

    i find that using resistor usually allows to make an easier fix/patch if you should ever change your mind. for example you may find that firmware change could expose oversights so a small delay on enable etc. could be useful etc (then adding "invisible" smt cap to gnd could save the day).

    if there is room, i leave place for options. zero ohm resistor or blob of solder is easy to apply or remove.
     
  4. nerdegutta

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    Dec 15, 2009
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    How many volts are VDD? Is it regulated?

    And, when the pin is pulled high, in other applications, what voltage is that?
     
  5. spinnaker

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    Oct 29, 2009
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    4.5V battery.
     
  6. MrChips

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    Oct 2, 2009
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    I once saw an article with arguments for and against a pullup resistor.
    I cannot remember the conclusion. I will try to find the article.

    In TTL it was common to use 3k3 to 4k7Ω for pullups.
    For CMOS I would go with 10k to 20kΩ.
     
    Last edited: Dec 9, 2012
  7. spinnaker

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    BTW Yes they are CMOS chips.
     
  8. spinnaker

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    Well it seems my question was not as stupid sounding as I first thought. :)
     
  9. John P

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    Oct 14, 2008
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    What logic series is this?

    As I recall it, you don't need a resistor on any logic type except plain 74 and its variants. 74LS, 74HC etc are all OK with direct connection to Vcc.
     
  10. spinnaker

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    Just two post up CMOS.

    CD series.
     
  11. panic mode

    Senior Member

    Oct 10, 2011
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    Part of TTL specification is to accept logic levels from 0V to Vcc. There is nothing wrong or unusual about connecting input to Vcc.
    http://en.wikipedia.org/wiki/Logic_level
     
  12. BillO

    Well-Known Member

    Nov 24, 2008
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    Tying inputs to Vcc/Vdd or Gnd is not an issue. Pull-up/pull-down resistors are not needed if there are no outputs or other signals connected to the input. This is both a common and best practice. The only logic families I can think of that might be exempt are ECL and maybe RTL, neither of which are used any more.
     
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