split_add.v

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// baeckler - 09-01-2006
//
// Example of adder using 2 carry chains, with the carry between them on standard
// routing.  This technique is useful when very long chains are causing placement
// difficulty.  Should use exactly WIDTH+2 cells.

module split_add (a,b,o);

parameter LS_WIDTH = 10;
parameter MS_WIDTH = 10;
parameter WIDTH = LS_WIDTH + MS_WIDTH;

input [WIDTH-1:0] a,b;
output [WIDTH-1:0] o;
wire [WIDTH-1:0] o;

// Build the less significant adder with an extra bit on the top to get
// the carry chain onto the normal routing.  The keep pragma prevents
// synthesis from undoing the split.

wire [LS_WIDTH-1+1:0] ls_adder;
wire cross_carry = ls_adder[LS_WIDTH] /* synthesis keep */;
assign ls_adder = {1'b0,a[LS_WIDTH-1:0]} + {1'b0,b[LS_WIDTH-1:0]};

// Build the more significant adder with an extra low bit to incorporate
// the carry from the split lower chain.

wire [MS_WIDTH-1+1:0] ms_adder;
assign ms_adder = {a[WIDTH-1:WIDTH-MS_WIDTH],cross_carry} + 
		{b[WIDTH-1:WIDTH-MS_WIDTH],cross_carry};

// collect the sum back together, drop the two internal bits
assign o = {ms_adder[MS_WIDTH:1],ls_adder[LS_WIDTH-1:0]};

endmodule

/////////////////////////////////

module split_add_tb ();
    parameter LS_WIDTH = 15;
    parameter MS_WIDTH = 20;
    parameter WIDTH = LS_WIDTH + MS_WIDTH;

    reg [WIDTH-1:0] a,b;
    wire [WIDTH-1:0] oa,ob;

    assign ob = a + b; // functional model
    split_add s (.a(a),.b(b),.o(oa));
        defparam s .LS_WIDTH = LS_WIDTH;
        defparam s .MS_WIDTH = MS_WIDTH;

    always begin
        #100 
		a = {$random,$random};
		b = {$random,$random}; 
		#10 if (oa !== ob) begin
           $display ("Mismatch at time %d",$time);
           $stop();
        end
    end

    initial begin
        #1000000 $display ("PASS");
        $stop();
    end
endmodule