Hot!Can pins connected to an external crystal be used for general purpose I/O?

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craigacox
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2019/01/28 18:55:51 (permalink)
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Can pins connected to an external crystal be used for general purpose I/O?

I have a Curiosity High-Pin Count development board. For the 28-pin headers, a 32 kHz external crystal is connected to RC0 and RC1. This is not a crystal that I have added; it is hard-wired as part of the board. I do not need the external crystal for a current application, but don't want to try to remove it from the development board. If I use pins RC0 and RC1 as general purpose digital outputs, will it harm either the board or the PIC microcontroller I am using? I understand that doing so doesn't get me the external oscillator function; I don't need that. If I set RC0 digitally high and RC1 digitally low, for example, will that damage anything? Asked another way, will a constant voltage potential across a crystal harm either the crystal or the microcontroller supplying the voltage?

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    qhb
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    Re: Can pins connected to an external crystal be used for general purpose I/O? 2019/01/30 22:33:08 (permalink)
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    Are you sure it is actually fitted?
    The crystal and load capacitors are all labelled "DNP", which means "Do Not Place".
     
    #2
    Nikolay_Po
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    Re: Can pins connected to an external crystal be used for general purpose I/O? 2019/01/30 22:49:09 (permalink)
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    +1. Just a place on PCB to let the developer add a crystal himself. (But I hold not this board in my hands).
    #3
    Mysil
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    Re: Can pins connected to an external crystal be used for general purpose I/O? 2019/01/30 23:41:46 (permalink)
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    Hi,
    I have a Curiosity HPC board, but that board have no crystal mounted.
    There are pads on underside of the board, connecting to RC0 and RC1, but no components, neither crystal nor capacitors.
     
        Mysil
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    hexreader
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    Re: Can pins connected to an external crystal be used for general purpose I/O? 2019/01/31 02:59:36 (permalink)
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    Never tried it, and I am no expert, but I would have thought that even if crystal and caps are fitted, you could use the pins for many normal IO purposes.
     
    High speed signals might be affected, along with multiples of the crystal frequency, especially if the input or load are high impedance.
     
    Switches and LED indicators would be just fine, I imagine.
     
    I cannot imagine that any harm would be caused by trying it. 

    Experienced Hobbyist
    #5
    coffee critic
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    Re: Can pins connected to an external crystal be used for general purpose I/O? 2019/01/31 08:54:04 (permalink)
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    The pads for the crystal and the loading caps are on the bottom of the HPC. They are not populated by default.  If they are, someone added them after the fact.  Even so the crystal would be fairly easy to remove with two soldering irons if you are concerned about the crystal.  I wouldn't worry about it.  The crystal is very high Q and essentially open at anything other than resonant.  At resonant the ESR will be between 30k and 90k ohms depending on the specs of part placed.   

    n_*$
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    NKurzman
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    Re: Can pins connected to an external crystal be used for general purpose I/O? 2019/01/31 09:41:00 (permalink)
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    "essentially open"  But not exactly. A proper pulse will store energy in the crystal that will be released to the pins.
    so the pins would not be exactly isolated.
    #7
    coffee critic
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    Re: Can pins connected to an external crystal be used for general purpose I/O? 2019/01/31 09:45:45 (permalink)
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    Yep, a sharp edge will release a burst of 32kHz.  It's either that or try his luck with soldering iron. I still suspect the crystal is not there.

    n_*$
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    craigacox
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    Re: Can pins connected to an external crystal be used for general purpose I/O? 2019/01/31 17:36:25 (permalink)
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    Thank  you, everyone. I did not know what DNP meant. There are unpopulated pads on the bottom of the board, so there is likely no crystal on mine either.
    #9
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