16F877A Pullups/Pulldowns and unused pins


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  1. #1
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    Default 16F877A Pullups/Pulldowns and unused pins

    I've gotten some good advice so far about tieing unused pins to ground. I have another question concerning that and would appreciate the advice. If I enable the pullups on port b so I can use some of those, not all, pins as switches what do others normally do with those unused pins? Do you do the same and tie those to ground with a 10k resistor or do you leave those float considering the internal pullups are enabled?
    Thanks
    David

  2. #2
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    I never trust pull-up at all. Those are kinda good when you apply power on your board, but what happen when you power it down?

    WOOHOO, you got it, they're gone, so you input are there, taking a beer or twelve waiting for the next incoming sparks to blow you chip.

    I know I will make friends with that one
    Steve

    It's not a bug, it's a random feature.
    There's no problem, only learning opportunities.

  3. #3
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    Quote Originally Posted by mister_e View Post
    I never trust pull-up at all. Those are kinda good when you apply power on your board, but what happen when you power it down?

    WOOHOO, you got it, they're gone, so you input are there, taking a beer or twelve waiting for the next incoming sparks to blow you chip.

    I know I will make friends with that one
    Mister E, Thanks but I am not sure I understand what you mean. If I am going to use the RB0, RB0 and RB1 as simple N.O. Pushbuttons if I power down my board there will be nothing there. If no power the PB isn't going to do anything right?
    Thanks
    David

  4. #4
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    Think about it, in the worst case scenario... If this was true, ESD bags, wrist, mat, handling and such wouldn't exist either.

    Now how crazy you have to come to avoid ESD discharge? Well it's up to you. For me, it's never to much.

    If you have a pull-up/pull down... you provide a return path somewhere all the time.
    Steve

    It's not a bug, it's a random feature.
    There's no problem, only learning opportunities.

  5. #5
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    Quote Originally Posted by mister_e View Post
    Think about it, in the worst case scenario... If this was true, ESD bags, wrist, mat, handling and such wouldn't exist either.

    Now how crazy you have to come to avoid ESD discharge? Well it's up to you. For me, it's never to much.

    If you have a pull-up/pull down... you provide a return path somewhere all the time.
    Ok, let me rephrase my question. What is likely to happen if I enable the internal pullups and at the same time tie the pins to ground? Is this creating a short that could cause the PIC or other components to overheat or burn out?

    Thanks
    David

  6. #6
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    Ha, I see. There will have no problem then.
    Steve

    It's not a bug, it's a random feature.
    There's no problem, only learning opportunities.

  7. #7
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    I can't find it quickly, but the Weak Pull-Up in in the neighborhood of microAmps. If you apply any harder signal, you just over-ride the WPU.

    Just enough to keep the pin stable without any outside influence.

    Bo

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