How to assign value to bidirectional port in verilog?

Question

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;
 begin
 var = 1;
 value = 1;
 end
 endtask
 endmodule

and the error that I'm getting is:

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

Thanks.

Answer 1

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;
endmodule

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.

Answer 2

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.

Answer 3

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