Orcad question - connecting output pins to an input pin

Discussion in 'General Electronics Chat' started by EngIntoHW, May 15, 2010.

  1. EngIntoHW

    Thread Starter Member

    Apr 24, 2010
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    I need to connect two output pins to an input pin.

    However, If I do it directly, Orcad issues an error that 2 output pins are connected to each other (as they are both connected to the same input pin).

    How should it be done correctly?

    Thank you.
     
  2. retched

    AAC Fanatic!

    Dec 5, 2009
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    You may consider using a diode on the 1st output pin. This should keep the signal from routing from the 2nd output pin into the 1st.

    But all depending on the reason you would need to do this (Which doesnt seem to make sense to me yet)

    Why would you want to connect 2 output pins to 1 input pin on the same IC?

    Is this a microcontroller?

    If so, you dont want to do this. The input pins can typically sink 25ma and each output can source 25ma. So you will be trying to sink 50ma into one input. Not a good idea.
     
  3. EngIntoHW

    Thread Starter Member

    Apr 24, 2010
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    Hi Retched.
    Thanks for your helpful comment.

    I'll explain why I want to connect two output pins to an input pin.
    The input pin is the RSTn pin of an 8051 Uc.

    Output pin #1 is a pull-up switch (when the switch is held down, it grounds the RSTn pin of the Uc).

    Output pin #2 is a RSTn signal of an evaluation board, which I use in order to burn firmware into the Uc and to debug the Uc.
    The EV board has 5 signals going to the Uc - RSTn, DD (Debug Data), DC, GND, VDD.

    Does it make sense to you now?

    Thanks.
     
  4. retched

    AAC Fanatic!

    Dec 5, 2009
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    I may not be the best person for this thread, as I have 0 experience with the 8051.

    But, It the #1 pin is a pull-up, and not a pull-down, It will not pull the rst pin to ground. Now if it is both, cool, ive never seen that.

    If you need RST held low, you should just tie it to ground with a pull-down resistor. If you need variability, I would connect it to ground through a transistor. The transistor can be switched by the pull up switch. When it is active, pin 2 will be pulled to ground.

    There may be an internal need for the 8051 to have things this way, but it seems unlikely.

    Look how it is tied to ground here:
    http://www.kmitl.ac.th/~kswichit/easy1puiu/easyp12.htm
     
  5. EngIntoHW

    Thread Starter Member

    Apr 24, 2010
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    Hi again Retched.

    This is the pull-up switch I'm using:

    [​IMG]

    As you can see, when the switch is held down, the Uc's RSTn pin is grounded, so I believe it functions well.

    I'm not sure why a pull-down resistor is needed here.

    However, as I said, there is also another output pin that is connected to the Uc's RSTn pin - that is the RSTn signal of the EV board.

    But ORCAD issues an error when the two output pins are connected to each other.
     
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  6. retched

    AAC Fanatic!

    Dec 5, 2009
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    Let me see the whole schematic.
     
  7. EngIntoHW

    Thread Starter Member

    Apr 24, 2010
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    Hi Retched.

    I can't upload the whole schematics as it belongs to the company which I work for.

    However, These are the only parts of the schematics that relate to the RSTn pin:

    [​IMG]

    [​IMG]

    The RSTn signal of the EV board connects to the mechanical hole.

    I hope its clear enough and am sorry for the confidentiality.
     
  8. retched

    AAC Fanatic!

    Dec 5, 2009
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    Are you tying VDD to the reset pin and ground?
    I think you may be confusing which pins are set as inputs and outputs as a default before anything is programmed.
     
  9. EngIntoHW

    Thread Starter Member

    Apr 24, 2010
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    Hi.

    I hope its now clear which pin is an output and which pin is an input.

    [​IMG]

    [​IMG]

    The VDD is connected to the RSTn input pin of the Uc through a 43KΩ pull-up resistor.

    The ground is connected to the RSTn input pin of the Uc only when the switch is held down.
     
  10. retched

    AAC Fanatic!

    Dec 5, 2009
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    .....Yes, i understand that. What I am not seeing is the 2nd pin from the uC.

    The only reference I see to a second pin is here:
    [​IMG]

    That tells me you are running your VDD to the reset pin, while trying to pull it to ground
     
  11. EngIntoHW

    Thread Starter Member

    Apr 24, 2010
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    Oh I see.

    Well, the button is normally open, so normally, no current flows through the pull-up resistor.
    Meaning, currently, when no one presses the button, the voltage of the Uc's RSTn pin is VDD.

    However, when the button is pressed, current is flowing through the resistor (I = VDD / 43KΩ) and therefore the voltage of the Uc's RSTn pin is 0V (GND).

    All the RST_N nets are the same net.
     
  12. retched

    AAC Fanatic!

    Dec 5, 2009
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    That could be what orcad is having the problem with. I would add the pull-down resistor to the pin.

    See if that does it. The uC may require a load (hence the pull-down)

    It is a simulator, not a 'Real-life-inator"
     
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  13. EngIntoHW

    Thread Starter Member

    Apr 24, 2010
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    Hey.

    How would adding a pull-down resistor prevent the two output pins from being connected to each other?
     
  14. retched

    AAC Fanatic!

    Dec 5, 2009
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    It wouldnt. But the RST_N pin will be floating without it. It actually needs a LOAD to determine state.
     
  15. EngIntoHW

    Thread Starter Member

    Apr 24, 2010
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    Hi.
    I thank you for your help.

    I don't see how the RSTn pin of the Uc is floating, as its either connected to VDD (throught a pull-up resistor) or connected to ground when the switch is pressed.

    I already tested the functionality of this circuit and it works well.
    The problem is that ORCAD issues an error as 2 output pins are connected to each other in the schematics.
     
  16. retched

    AAC Fanatic!

    Dec 5, 2009
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    I suppose its just an error in orcad. I was suggesting the resistor in orcad only, not real life.
     
  17. EngIntoHW

    Thread Starter Member

    Apr 24, 2010
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    Well, I think that we agreed that adding a resistor won't prevent the two output pins from being connected to each other.
    Moreover, the RSTn pin isn't floating so I dont see why a pull-down resistor is needed.

    Also, I'm intending to send the ORCAD schematics to a PCB designer and I want the schematics to be with no errors (and everything that is placed there will appear on the PCB eventually).
     
  18. retched

    AAC Fanatic!

    Dec 5, 2009
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    Good idea.
     
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