Verilog Always statement for 4 Demux

Question

This is my Verilog code for the procedural modeling of the 4 Demux:

//4 Bit demux in Gate level
module HW3_PM(input [3:0] I, input Sel, output [3:0] A, output [3:0] B);
    always@(*)
        begin
            case(Sel)
                1'b0: begin
                        A = ~Sel&I;
                    end
                1'b1: begin
                        B = Sel&I;
                    end
                default: begin
                            end
            endcase
        end
endmodule

And I input to receive this error:

eos$ ncverilog +access+r HW3_PM.v HW2_Demux4_tb.v +gui
ncverilog: 09.20-s019: (c) Copyright 1995-2010 Cadence Design Systems, Inc.
file: HW3_PM.v
                                                A = ~Sel&I;
                                                |
ncvlog: *E,WANOTL (HW3_PM.v,8|6): A net is not a legal lvalue in this context [9.3.1(IEEE)].
                                                B = Sel&I;
                                                |
ncvlog: *E,WANOTL (HW3_PM.v,11|6): A net is not a legal lvalue in this context [9.3.1(IEEE)].
        module worklib.HW3_PM:v
                errors: 2, warnings: 0

file: HW2_Demux4_tb.v
ncverilog: *E,VLGERR: An error occurred during parsing.  
Review the log file for errors with the code *E and fix 
those identified problems to proceed.  Exiting with code (status 1).

I've tried changing and adding A and B as both reg and wire but it causes higher errors. I tried changing to posedge of the clock and get a different error. Your help is appreciated.

NEW: Changed code:

//4 Bit demux in Gate level
module HW3_PM(input [3:0] I, input Sel, output [3:0] A, output [3:0] B);

reg A, B;

always@(*)
    begin
        case(Sel)
            1'b0: begin
                    A = ~Sel&I;
                end
            1'b1: begin
                    B = Sel&I;
                end
            default: begin
                        end
        endcase
    end

endmodule

Received error:

file: HW3_PM.v

module HW3_PM(input [3:0] I, input Sel, output [3:0] A, output [3:0] B);

                                                     |
ncvlog: *E,BADIOO (HW3_PM.v,2|53): input/output/inout 'A' declared as vector, then redeclared as scalar [3.3(IEEE)].

        reg A, B;
            |
ncvlog: *W,ILLPDX (HW3_PM.v,4|5): Multiple declarations for a port not allowed in module with ANSI list of port declarations (port 'A') [12.3.4(IEEE-2001)].

module HW3_PM(input [3:0] I, input Sel, output [3:0] A, output [3:0] B);
                                                                     |
ncvlog: *E,BADIOO (HW3_PM.v,2|69): input/output/inout 'B' declared as vector, then redeclared as scalar [3.3(IEEE)].

        reg A, B;
               |
ncvlog: *W,ILLPDX (HW3_PM.v,4|8): Multiple declarations for a port not allowed in module with ANSI list of port declarations (port 'B') [12.3.4(IEEE-2001)].

        module worklib.HW3_PM:v

                errors: 2, warnings: 2

file: HW2_Demux4_tb.v

ncverilog: *E,VLGERR: An error occurred during parsing.  
Review the log file for errors with the code *E and fix 
those identified problems to proceed.  Exiting with code (status 1)

What does A equal when sel and what does B equal when ~sel? — N8TRO

N8TRO N8TRO · Accepted Answer · 2014-09-17T06:07:07

module HW3_PM(
   input [3:0] I, 
   input Sel,
   output reg [3:0] A, 
   output reg [3:0] B
   );

always@(*) 
begin
   if(~Sel) begin 
      A = I;
      B = 0; 
   end else begin 
      A = 0; 
      B = I; 
   end
end

endmodule

or

module HW3_PM(
   input [3:0] I, 
   input Sel,
   output wire [3:0] A, 
   output wire [3:0] B
   );

assign A = ~sel&I;
assign B =  sel&I; 

endmodule

or

module HW3_PM(
   input [3:0] I, 
   input Sel,
   output reg [3:0] A, 
   output reg [3:0] B
   );

always@(*)
begin
   case(Sel)
   0: begin
         A = I;
         B = 0;
      end
   1: begin
         A = 0;
         B = I;
      end
   endcase
end

endmodule

or

module HW3_PM(input [3:0] I,input Sel,output wire [3:0] A, output wire [3:0] B);

and selA[3:0] (A, ~sel, I);
and selB[3:0] (B,  sel, I);

endmodule

Verilog Always statement for 4 Demux

1 Answers