Verilog nested for loop not behaving as expected

Question

I am having trouble with a simulation of an 8-bit full adder i wrote in verilog. Basically I have two integers that I feed into the full adder, I add them together and I check if the result is as expected. Here is the code:

full_adder_Nb_tb.v:

`timescale 1ns / 1ps
`include "config.v"
module full_adder_Nb_tb #(parameter N = 8)();
reg [N-1:0] a;
reg [N-1:0] b;
wire [N-1:0] s;
wire co;
integer i, j;
integer error_cnt;
full_adder_Nb #(N) test_module (a, b, s, co); 
initial
begin
 $display("TESTING full_adder_Nb.v");
 error_cnt = 0;
 for (i = 0; i < 2^(N-1); i=i+1) begin : outer_loop
 a = i;
 for (j = 0; j < 2^(N-1); j=j+1) begin : inner_loop
 b = j;
 #5;
 if (s != (i+j)%(2^(N-1)) || co != (i+j)/(2^(N-1)))
 begin
 error_cnt = error_cnt + 1;
 `ifdef VERBOSE
 $display("There was an ERROR. Inputs: a=%d,b=%d, outputs: s=%d, co=%d. Expecected value: s=%d, co=%d", a, b, s, co, (i+j)%(2^(N-1)), (i+j)/(2^(N-1)));
 `endif 
 end 
 end : inner_loop 
 end : outer_loop
 // SIMULATION FINAL OUTPUT
 if (error_cnt == 0)
 begin
 $display("full_adder_Nb.v WORKS correctly.");
 end
 else
 begin
 $display("full_adder_Nb.v does NOT WORK. It has %d errors!", error_cnt);
 end
 #20;
 $finish;
end
endmodule

This is the result of the simulation: simulation_result

As you can see integer i and reg a are both stuck at 0, while j and b are behaving as inteded. I do realise that this does not fully test this circuit (I intend to add negative numbers simulation).

I am grateful for any hints.

• The Photon: Is this Verilog or SystemVerilog?
• Oldfart: You realize that ^ is the exor operator. not power!
• Jure Vreča: No, I didn't realize that...
• Jure Vreča: This is verilog, not system verilog.

Answer 1

You have problems with the Operator Precedence.

The comparison ( < ) has higher precedence then you EXOR operator ( ^ ).

Thus first j < 2 is calculated as true (1) or false (0).
Then that is EXOR-ed with 7, which gives either 6 or 7.
But it never gives 0 (false) so the loop does not finish.

Try: j < (2^(N-1)) or what you probably want: j < (1<<(N-1))