Modular 8 bit Ripple Carry Adder (Help!)- Verilog

Code Verilog - [expand]
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
//1-bit full adder
module RCASlice (CO, S, A, B, CI);
 
input A, B, CI;
output S, CO;
wire X1;
wire P1, P2, P3;
 
xor #(2,2)
    U1a (X1, A, CI),
    U1b (S , X1, B);
    
and #(2,2)
    U2a (P1, A, B),
    U2b (P2, A, CI),
    U2c (P3, B, CI);
    
or #(2,2)
    U3a (CO, P1, P2, P3);
    
endmodule
 
//Ripple Carry Adder of 8 bits
module RCAnBITS (CO,S,X,Y,CI);
parameter nBITS = 8;
input [nBITS-1:0] X,Y;
input CI;
output [nBITS-1:0] S;
output CO;
 
wire [nBITS:0] C; //output requires 9bits: C[8] is MSB
genvar i;
 
generate
    assign C[0] = CI;
    assign CO = C[nBITS];
    for (i=0; i<nBITS; i=i+1)
    begin:RCA
        RCASlice m(C[i+1],S[i],X[i],Y[i],C[i]);
        end
    endgenerate
    endmodule

Code Verilog - [expand]
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
//
// Testbench for 8-bit Ripple Carry Adder
//
 
 
module GoldenDevice(s, x, y);
parameter NBITS = 8;
input [NBITS-1:0] x,y;
output [NBITS:0] s;
 
assign s = x + y;
endmodule
 
 
 
 
module Adder8BitTest;
integer FirstInput, SecondInput;
parameter NBITS = 8;
reg [NBITS-1:0] x, y;
 
// response from Golden device
wire [NBITS:0] sum; 
 
// response from DUT
wire [NBITS:0] s;
 
reg Error;
 
task ReportError;
begin
$display("*** Error: x = %2h, y = %2h, observed p = %4h, expected p =%4h\n",x,y,s,sum);
end
endtask
 
 
//8 bit Ripple Carry Adder
RCAnBITS t1(CO,S,X,Y,CI);
GoldenDevice #(.NBITS(8)) GD(sum, x, y);
 
 
initial
begin
Error = 0;
 
 
 
for (FirstInput = 0; FirstInput < (1 << NBITS); FirstInput = FirstInput+1)       // iterate over all values
    begin
    x = FirstInput;
    for (SecondInput = 0; SecondInput< (1 << NBITS); SecondInput = SecondInput+1)        // iterate over all values
        begin
        y = SecondInput;
        #1000
        if (sum !== s)
            begin
            ReportError;
            Error = 1;
            end
        end
    end
 
if (!Error)
    $display("*** Congratulations!   No errors detected\n");
else
    $display("*** Sorry.    Try again.\n");
    
$finish();
end
endmodule