Monitoring inout signal transactions

Discussion in 'VHDL' started by valtih1978, Jul 30, 2011.

  1. valtih1978

    valtih1978 Guest

    entity SPY is
    port (A : inout Std_Logic);
    end SPY;

    architecture ARCH of SPY is

    process begin
    report time'image(now) & ": a = " & std_logic'image(a);
    wait on a'transaction;
    end process;

    end architecture;

    architecture TB is
    signal A: std_logic;
    SPY_I: entity SPY(a)
    process begin
    wait for 1 fs;
    a <= 'Z'; wait for 1 ps;
    a <= '1'; wait for 5 ns;
    a <= '0'; wait for 20 ns;
    end process

    I do not drive the signal from the Spy, so output must be (U, Z, 1, 0).
    Right? Yet, simulator tells 'a = U' all four times!

    I observed this hacking the Zero-Ohm model. Ben Cohen temporarly assigns
    a and b to Z there. I do not understand why but that is a solution to
    get the normal output (U,Z,1,0). Another is to change port type inout ->
    in. Is is supposed VHDL behaviour? IMO, it is strange furthermore
    because I consider normal, architecture-declared signals, as inout
    because you can read and write them but no problem happens when the
    monitoring process is located within the same architecture that drives
    the signal rather than in a separate entity.
    valtih1978, Jul 30, 2011
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  2. valtih1978

    Guest Guest

    Signal A in the Testbench TB has multiple sources[1]: spy's inout port
    and the driver from the process in the testbench. The inout source from
    spy is initialized with 'U' by default. That 'U' overdrives the other
    assignments like usual.

    If you initialize the inout port using 'Z', spy will report the values
    assigned by the testbench process.

    port (A: inout std_logic := 'Z');

    [1]: LRM(93) line 183

    Guest, Jul 30, 2011
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