XOR signal in verilog

Question

I have a signal like this for example:
wire [18:0] A;

I need to XOR it with itself with multiple XOR gates that have up to 4 inputs. So this signal would be broken down like this:

XOR the first 4 bits [3:0] 
XOR the next 4 bits [7:4] 
XOR the next 4 bits [11:8] 
XOR the next 4 bits [15:12] 
XOR the remaining 3 bits together [18:16]

How do I XOR a signal with itself like this? Also, what is the best way to do this?

Answer 1

There are a couple of ways of doing this. One way could be to build a 4-input XOR module, and then instantiate multiple copies.

module myXOR4 ( input a, input b, input c, input d, output f);
    assign f = a ^ b ^ c ^ d; // ^ is the XOR operator
endmodule
// ...

myXOR4 xor1 (A[0],A[1],A[2],A[3],out[0]);
myXOR4 xor2 (A[4],A[5],A[6],A[7],out[1]);
// etc

Another way would be to use a for-loop. This won't work with all the cases, since you don't have an evenly-divisible number of wires.

reg [4:0] out;
integer i;
always@(*) begin
    for (i=0; i<4; i=i+1) 
        out[i]=A[4*i] ^ A[4*i+1] ^ A[4*i+2] ^ A[4*i+3];
    out[4] = A[18] ^ A[17] ^ A[16];
end

There are some other tricks (like 0-padding the upper-bits of A to make it evenly divisible, which makes the loops simpler) but they'll all end up with the same result, so this should be enough to get you started.