Pin name/function (Newbie question)

Discussion in 'General Electronics Chat' started by NY10, Aug 4, 2010.

  1. NY10

    Thread Starter Member

    Jul 30, 2010
    Hi, all

    I have a EP3CxE144 and some pin#'s indicate I/O.

    As far as I understand, I/O means input and output. so does this mean the particular pin#'s do input and output functions at the same time. (or sometimes it does input and other time it does output functions)

    or am I misunderstanding ?

    I am going to attach the document, so that you can understand it better.

  2. tom66

    Senior Member

    May 9, 2009
    Yes they are known as tristate pins. They can be an input, an output high, or an output low, they have three states. Input mode is sometimes called high-Z or high-impedance mode, because little current flows in when in high-Z mode; it's almost as if it were disconnected from the circuit.
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  3. BMorse

    AAC Fanatic!

    Sep 26, 2009

    I beg to differ about you explanation for a tri state I/O pin:

    This is what Wiki says it is:
    Tri-State has nothing to do with the pin being an input....

    B. Morse
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  4. kingdano


    Apr 14, 2010
    BMorse is correct

    I/O pins which can be "tri-stated" can be used as an input to the device, output from the device (either exclusively, or if you are advanced in your coding, coincidentally in the same program) or in a 3rd state (hence tri) which is high-Z (high impedance) such that if you are in a sensitive circuit you can block the additional load of the devices I/O pin if need be.

    This is helpful for protecting the device on start-up as well.
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  5. tom66

    Senior Member

    May 9, 2009
    An input/output pin can be tristated. An output pin can also be tristated (you can buy buffers which do this.) But guess which has an input in high-Z mode? I'll give an example with PIC microcontrollers. The register files for example are named TRISA, TRISB. This terminology comes from TRIState port A, B, and so on. I was giving an example relevant to the OP's FPGA, as it does not have tristate output buffers.
    Last edited: Aug 4, 2010
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  6. Ghar

    Active Member

    Mar 8, 2010