[Iverilog-devel] Force statement in Icarus

[Kenneth B. N. <sub...@br...>]

Hi all,

I initially tried to post this to the comp.lang.verilog group, but the
only answer I got was a redirection to this mailing list. Feel free to
continue the discussion in the news group, if the subject is not
specifically related to Icarus, but more to my understanding of
Verilog.

I'm trying to make a FORCE statement work in Icarus, but it doesn't
work as I expect it to.

I want to force a value to a node deep into a hierarchy.  The force
statement works on the value itself, but it has no effect on
subsequent calculations. E.g. if I force the input of a simple gate
somewhere in the hierarchy, I would expect its output to change value
to satisfy whatever input->output relation that might exist.

I made the following code for a simple example:
---
`timescale 1ns / 1ns

module FD_TOP();
   reg topa,topb;
   wire topc,topd;

   initial
     begin
        force DUT.a = 1'b1;
        #100;
        release DUT.a;
     end

   initial
     begin
        $dumpfile("debug.vcd");
        $dumpvars;
        topa=0;
        topb=0;
     end

   hierBlock DUT(topa,topb,topc,topd);

endmodule

module hierBlock(a,b,c,d);
   input a,b;
   output c,d;

   INV forced (c,a);
   INV unforced (d,b);

     initial
     begin
        #40 $display("%b",{a,b,c,d});
        #40 $display("%b",{a,b,c,d});
        #40 $display("%b",{a,b,c,d});
        $finish;
     end

endmodule // hierBlock

module INV (Z, A);

   output Z;
   input A;

   assign Z = ~A;

endmodule // IVLL
---

The code models a top block instantiating a hierarchical block (named
hierBlock), which consist of two inverters. From the top-level I use a
FORCE statement to manipulate the value of the 'a' node inside
hierBlock. This node 'a' is the input to one of the inverters and I
would expect the output of this inverter to be the inversion of the
forced value.

However, the output is not what I expect. I get the following
simulation in Icarus 0.9.5:
---
1011
1011
0011
---

Is it a bug in the simulator, or are my expectations to the behaviour
wrong? Any idea to achieve the behaviour I want?

Best regards,
Kenneth