I'm trying to use a bidirectional port in Verilog so I can send and receive data through it. My problem is that when I try to assign a value to the port inside a task, I keep getting an error.

What is the correct way to assign a value to these types of variables?

My code is as follows:

    module test(value,var);

    inout value;
    output var;
    reg var,value;

    task sendValue;
        var = 1;
        value = 1;


and the error that I'm getting is:

Error: C:/[...]: (vlog-2110) Illegal reference to a net "value".



3 Answers 3


If you must use any port as inout, Here are few things to remember:

  1. You can't read and write inout port simultaneously, hence kept highZ for reading.
  2. inout port can NEVER be of type reg.
  3. There should be a condition at which it should be written. (data in mem should be written when Write = 1 and should be able to read when Write = 0).

For e.g. I'll write your code in following way.

module test (value, var);
  inout value;
  output reg var;

  assign value = (condition) ? <some value / expression> : 'bz;

  always @(<event>)
    var = value;


BTW When var is of type wire, you can read it in following fashion:

assign var = (different condition than writing) ? value : [something else];

Hence as you can see there is no restriction how to read it but inout port MUST be written the way shown above.

I hope that explains it to you.


Check if the library you're using includes any tristate drivers - they're what I always used in this situation.

If you're forced to try to design your own tristate driver, remember that it's output must be Z whenever it should allow input instead.


Here my 2cents,

wire [32-1:0] DATA;//bidireccional port in VERILOG/VHDL
logic put_input_in_bidi;

assign m_sram_if.SRAM_DATA_READ = DATA;//read from inout port
assign DATA = (put_input_in_bidi==1'b1)? m_sram_if.SRAM_DATA_WRITE : 32'hZZZZ_ZZZZ;//write in bidi iout port
assign put_input_in_bidi =     (blabla == 0)  &&  (bobbob == 1);//control write in bidi using master information

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