What to do with these input and outputs

Discussion in 'General Electronics Chat' started by Dawud Beale, Jan 5, 2013.

  1. Dawud Beale

    Thread Starter Member

    Feb 10, 2012
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    If you have some input pins and output pins that you dont need and you want to remain low but you can delete them for a good reason, is there any way to deal with them so they arent left loose?

    My software doesnt allow pins that arent connected to anything but I need to connect them without them affecting the circuit. As I am actually implementing this circuit on hardware I'd prefer not to create any extra pins on it to allow a permanently low connection to the pin, if there is a way to internally deal with it and attach it to something then that is preferable. Is there a logical way of achieving this? Like for example connecting it in a way that it is impossible to ever have an On signal?
     
  2. MrChips

    Moderator

    Oct 2, 2009
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    What software are you using?
     
  3. sheldons

    Active Member

    Oct 26, 2011
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    tie your unused logic inputs either to supply or to ground(your choice)with say a 1k to 10k
    resistor...depending on your logic type this will force the output of your unused gate high or low...
     
  4. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    I find it hard to believe that a PCB/schematic software (assume thats what you meant when you said software) doesn't allow NC pins/pads.. I'd read the manual/help files in your software.. Or check/correct the design rule checker to allow this.. It just doesn't make sense that a program can't allow that.. If so get a new program
     
  5. nerdegutta

    Moderator

    Dec 15, 2009
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    If I'm working on a PIC project and have not used all pins, I tie them to a PAD and call it "Test pad for future use". If I decide to develop an already made prototye, it's easier when the pad is already there.
     
  6. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    You both must be using the same garbage PCB program..
     
  7. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    That's one good way to handle them.

    While there are bad ways to handle unused pins (ie, leave them as inputs and let them float) there is no best way to handle unused pins.

    You can make it as an input and connect to + or -, but if one day you want to cut and jumper it back into the circuit you will have grief.

    You can set it as an output and leave it safely floating free for rework, but if your probe slips you can sometimes get a bad short no matter if you set it as 1 or 0.

    Thus there is no universal best way to handle unused pins! You pick the compromise that suits your needs best.
     
  8. JMac3108

    Active Member

    Aug 16, 2010
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    I do the same thing. Every unused pin gets tied to a small SMD pad that is defined as a single pin part called a TEST POINT. The pad is just big enough to solder a 30AWG wire to it if needed later, but not big enough to get in the way when routing.

    BTW, your software is probably flagging a single-pin-net error, but most packages allow you to "accept" or "over-ride" an error.
     
  9. nerdegutta

    Moderator

    Dec 15, 2009
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    Hey!!!

    I'm not complaining...

    My software can handle un-used pins.:)
     
  10. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    But you do spend the time to add a maybe never needed extra pad :p
     
  11. nerdegutta

    Moderator

    Dec 15, 2009
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    Yes, but I choose to do that.:p
     
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