I have the following Verilog modules, which I'm simulating with Icarus Verilog. It seems that the ternary operator doesn't work, or has a bug:

main module (i2c_app_tb.v)

`timescale 1ns/100ps

module i2c_app_tb;

    wire t1;
    wire [7:0] t2;

    reg rst = 0;
    reg clk = 0;


    pullupdown_t1 pt1(t1, t2, rst, clk);

    always #5 clk <= ~clk;

    initial begin
        $dumpvars(1, i2c_app_tb);

            rst <= 1;
        #20 rst <= 0;

    wire _i0_reg;
    assign _i0_reg = pt1.i0_reg;



module pullupdown_t1 (
    inout wire i0,
    inout wire [7:0] i1,
    input rst,

    input clk
    reg i0_reg;

    always @ (posedge clk or posedge rst) begin
        if (rst) begin
            i0_reg <= 0;
        end else begin
            i0_reg <= (i0_reg == 0 ? (1'bz) : 0);

    assign i0 = i0_reg;


When I simulate the above, I get:

enter image description here

However, if I replace

i0_reg <= (i0_reg == 0 ? (1'bz) : 0);


if (i0_reg == 0) begin
    i0_reg <= 1'bz;
end else begin
    i0_reg <= 0;

(which is equivalent), I get:

enter image description here

Am I missing something? Is this a bug?

  • \$\begingroup\$ Check the data types — not every type supports ‘z’ which is defined as not driven. Only a tri-state signal path can physically support the z state. \$\endgroup\$
    – MarkU
    Dec 30, 2021 at 22:26

1 Answer 1


If you want to use the ternary (conditional) operator, you can use case equality (===) instead of logical equality (==):

i0_reg <= (i0_reg === 0 ? (1'bz) : 0);

Refer to IEEE Std 1800-2017, section 11.4.5 Equality operators.

The behavior you observed is not a bug and it is not specific to iverilog; it is observed on other simulators as well.

In your original ternary code, when i0_reg has the value 1'bz, then the expression i0_reg == 0 evaluates to 1'bx, and i0_reg is assigned the value 1'bx.


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