unused ic pins

Discussion in 'The Projects Forum' started by Gadersd, Mar 26, 2013.

  1. Gadersd

    Thread Starter Member

    Dec 8, 2012
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    I have a ic with a couple of pins that I am not using. When I connect them to ground, do I need a resister. They are not output pins.
     
  2. MrChips

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    Oct 2, 2009
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    You have to tell us the chip number and which pins you are not using.
    We can't see your workbench, at least not yet.
     
  3. Gadersd

    Thread Starter Member

    Dec 8, 2012
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    I am using the SN74LS173A 4 bit d-type register. The unused pins are clear,M, and N.
     
  4. MrChips

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    The devil is in the details. It is important to read the datasheets and understand the function of each pin.

    CLR is an active-high control input. To avoid clearing the register, connect CLR to GND.

    M and N are active-low control inputs that control the tri-state output drivers.
    To enable the outputs, connect both M and N to GND.

    No resistors are necessary.
     
  5. Gadersd

    Thread Starter Member

    Dec 8, 2012
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    How would I tell which pins can be connected to ground without a resister? The data sheet never said anything about whether the pins needed a resister or not.
     
  6. nigelwright7557

    Senior Member

    May 10, 2008
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    They don't need a resistor.
     
  7. Gadersd

    Thread Starter Member

    Dec 8, 2012
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    Also, do the 1d-4d inputs need a resistor or will the internal resistors take care of it.
     
  8. Ron H

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    Are you not using all four D inputs?

    You don't need resistors on any inputs.
     
  9. Gadersd

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    Dec 8, 2012
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    Are you sure? The data sheet said the maximum high level input current was 40 nano amps.
     
  10. Ron H

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    The datasheet I looked at says IIH=20 microamps max when VI=2.7V. This is not a max allowable value. It is the maximum current the input will draw when the input is 2.7V. This spec is given so you will know how many inputs you can connect to the output of another device without overloading it.
    I repeat, YOU DO NOT NEED A RESISTOR between any input and ground, or vcc, or the output of any other logic device that is powered off the same supply.
     
  11. Gadersd

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    Dec 8, 2012
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    Is this true for many other ttl ics, including ram and binary counters?
     
  12. Ron H

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    I have never seen a TTL or CMOS IC that requires resistors to apply logic levels to undriven or unused inputs.
    I am 72 years old. I worked as a circuit design engineer for 40 years. TTL was invented about the time I started my career. CMOS came later. I have worked with them, as well as RTL, DTL, ECL, and probably other logic technologies.
     
    Last edited: Mar 27, 2013
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  13. ErnieM

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    Everything Ron said is sooth. It's true. Don't go looking for any other answers (cause you may find one and it's wrong).

    The only minor exception (and more of a clarification as Ron does say "logic levels") is the monostable timer chips do need a resistor and capacitor to set the time, but that is a different issue really and the only exception to the rule I can think of.

    Of course power pins get decoupling caps, that's a different rule entirely.

    Input pins go directly to ground or the power rail. No resistor needed.

    Here Endeth the Lesson.
     
  14. shortbus

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    I'll have to chime in here since my project uses some logic and am still learning. While most forums say no resistor is necessary, the three books I just bought on logic chips say the opposite.:)

    They say to connect the unused inputs to each other and then to a ~10k resistor to ground. So if the O/P is reading one of the newer logic how to books, I can see his confusion.
     
  15. takao21203

    Distinguished Member

    Apr 28, 2012
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    Normally they are just grounded. They are not supposed to be used with a resistor in the first place. That would slow them down a lot.
     
  16. Ron H

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    That will work with CMOS, but it is not necessary. With TTL, 10k resistor pulldowns WILL NOT WORK. The input currents will make the input voltages high enough to be perceived by the IC as logic highs.
    Resistor pullups will work with TTL, but are not necessary. I have seen a couple of reasons why some people think they are necessary, or at least possibly useful. One ostensible reason is to protect the input from voltage transients that could cause damaging current spikes. If your power supply is so noisy, and the layout is so poor, that the instantaneous voltage difference between an input and the vcc pin is that high, your power supply rail and/or layout needs some serious redesign.
    Another reason is to leave an unused pin available for future use, without the need of cutting a trace. This is a personal choice. 10k works fine for a pullup.
    If, for this same reason, you want to leave a grounded pin available, the value for 74LSXX logic must be less than 1k. For 74XX, this value is 250Ω. If you later want to drive one of those pins, you will have to remove the resistor, because a TTL output cannot reliably supply enough current to drive the resistor.
     
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  17. shortbus

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    Aren't all of the latest (produced in the last couple of years) TTL logic, Cmos or Dmos internally? Even though they are still called TTL?
     
  18. MrChips

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    Even though you are correct, sometimes things don't change for historical reasons.
    Most gates today are CMOS transistor-transistor logic.

    The term TTL (transistor-transistor logic) historically is used to describe 74XX00 series logic using bipolar junction transistors.
     
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  19. Ron H

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    Below is an excerpt from the 7400 Series wiki:
    Bipolar
    74 - the "standard TTL" logic family had no letters between the "74" and the specific part number.
    74L - Low power (compared to the original TTL logic family), very slow
    H - High speed (still produced but generally superseded by the S-series, used in 1970s era computers)
    S - Schottky (obsolete)
    LS - Low Power Schottky
    AS - Advanced Schottky
    ALS - Advanced Low Power Schottky
    F - Fast (faster than normal Schottky, similar to AS)

    CMOS
    C - CMOS 4–15 V operation similar to buffered 4000 (4000B) series
    HC - High speed CMOS, similar performance to LS, 12 ns
    HCT - High speed, compatible logic levels to bipolar parts
    AC - Advanced CMOS, performance generally between S and F
    AHC - Advanced High-Speed CMOS, three times as fast as HC
    ALVC - Low voltage - 1.65 to 3.3 V, Time Propagation Delay (TPD) 2 ns[7]
    AUC - Low voltage - 0.8 to 2.7 V, TPD < 1.9 ns@1.8 V[7]
    FC - Fast CMOS, performance similar to F
    LCX - CMOS with 3 V supply and 5 V tolerant inputs
    LVC - Low voltage – 1.65 to 3.3 V and 5 V tolerant inputs, tpd < 5.5 ns@3.3 V, tpd < 9 ns@2.5 V[7]
    LVQ - Low voltage - 3.3 V
    LVX - Low voltage - 3.3 V with 5 V tolerant inputs
    VHC - Very High Speed CMOS - 'S' performance in CMOS technology and power

    BiCMOS
    BCT - BiCMOS, TTL-compatible input thresholds, used for buffers
    ABT - Advanced BiCMOS, TTL-compatible input thresholds, faster than ACT and BCT
     
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  20. shortbus

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    Sep 30, 2009
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    If given a choice in a circuit design, which would you use, 74LSxx or 74HCxx? I'm at the development stage of a circuit and never done this before so this is all new to me. Was originally going to use 40xx logic originally.
     
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