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    Connection for unused dual purpose pins of CPLD

    Hi,

    I am trying to make hardware with a CPLD (XC2C256). It has a few dual purpose pins. If I am not using them, what should I do with it. Should I gnd them for better ESD? For eg. Pin 1-4 have dual function of global output enable (VQ100 package- pin out on page 18 of datasheet). If I am not using these pins as IO pins and gnd them, will it not disable the output of the entire IC? Also, if I want to use the pins as IO pins and give logic 0 as input, again will it not disable the output of the entire IC?

    Datasheet link : Here

    Thanks,
    Hobbyiclearner

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    Re: Connection for unused dual purpose pins of CPLD

    The "data sheet" is really more of an overview. Try this one: User Guide
    Look at the section for "Terminations" and I think all of your questions will be answered, plus allow you some options that might not have occurred to you.


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    Re: Connection for unused dual purpose pins of CPLD

    OK. Thanks for your answer. I could not reply earlier as I was away on some work.

    As I understand from the user guide, I can select one or more termination techniques from the software options. That’s fine if I don’t want to use any of the dual purpose pins. In that case, from hardware point of view, I can simply connect these pins to ground and then unselect the Xilinx software option – ‘use global output enable’, ‘use global set/reset’ etc. (picture attached for reference).

    But suppose I want to use only one of the 4 global output enable pins of XC2C128 (VQ100 package). Then I would still have to keep the software option ‘use global output enable’ checked. In that case the remaining 3 pins will also work as global output enable pins. If I don’t want to use any other of these 3 pins in my hardware, will the unused pins get internally terminated as per the option selected (keeper -by default)? In this case what should I do with the pins in hardware – connect them to ground anyways? If I do connect them to ground, the outputs would get disabled. Isnt it? Hence, from PCB point of view, what should I do in this situation?

    Also, if I want to use the remaining 3 pins as I/O pins, how can I do so? As I understand, they would be functioning as global output enable pins only.

    Datasheet of XC2C128 here. Pinout of VQ100 on page 15.

    Thanking You,
    Hobbyiclearner.

    Click image for larger version. 

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    Re: Connection for unused dual purpose pins of CPLD

    Hi,

    I usually don't externally connect unused pins to GND (or any other signal).
    You are more flexible to config these pins by program.

    ESD means "electrostatic discharge". If you fear that ESD may harm unused pins you should fear that ESD may harm used pins, too.
    But usually ESD is caused by external connections/wires.

    Klaus
    Please don´t contact me via PM, because there is no time to respond to them. No friend requests. Thank you.



  5. #5
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    Re: Connection for unused dual purpose pins of CPLD

    OK... just to confirm, is there any way to use one of four global output enable pins as it is and the rest as I/O pins?

    Thanks and Regards,
    Hobbyiclearner



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