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

//

// Filename:    bigsub.v

//

// Project:     OpenArty, an entirely open SoC based upon the Arty platform

//

// Purpose:     To subtract two 64-bit numbers, while maintaining

//              synchronization to whatever purpose these numbers had

//      originally.  For this reason, there is an i_sync input and an o_sync

//      output.  If i_sync is true with a particular set of data, o_sync will

//      then be true when that data is placed on the output.

//

//      If we needed to slow this down even more, I suppose we could register

//      the inputs before we used them ...

//

// Creator:     Dan Gisselquist, Ph.D.

//              Gisselquist Technology, LLC

//

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

//

// Copyright (C) 2016, Gisselquist Technology, LLC

//

// This program is free software (firmware): you can redistribute it and/or

// modify it under the terms of  the GNU General Public License as published

// by the Free Software Foundation, either version 3 of the License, or (at

// your option) any later version.

//

// This program is distributed in the hope that it will be useful, but WITHOUT

// ANY WARRANTY; without even the implied warranty of MERCHANTIBILITY or

// FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

// for more details.

//

// You should have received a copy of the GNU General Public License along

// with this program.  (It's in the $(ROOT)/doc directory, run make with no

// target there if the PDF file isn't present.)  If not, see

// <http://www.gnu.org/licenses/> for a copy.

//

// License:     GPL, v3, as defined and found on www.gnu.org,

//              http://www.gnu.org/licenses/gpl.html

//

//

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

//

//

module  bigsub(i_clk, i_sync, i_a, i_b, o_r, o_sync);

        parameter       NCLOCKS = 1;

        input                   i_clk, i_sync;

        input           [63:0]   i_a, i_b;

        output  wire    [63:0]   o_r;

        output  wire    o_sync;

        generate

        if (NCLOCKS == 0)

        begin

                assign  o_sync= i_sync;

                assign  o_r = i_a - i_b;

        end else if (NCLOCKS == 1)

        begin

                reg             r_sync;

                reg     [63:0]   r_out;

                always @(posedge i_clk)

                        r_sync <= i_sync;

                always @(posedge i_clk)

                        r_out <= i_a - i_b;

                assign  o_sync = r_sync;

                assign  o_r = r_out;

        end else // if (NCLOCKS == 2)

        begin

                reg             r_sync, r_pps;

                reg     [31:0]   r_hi, r_low;

                reg     [63:0]   f_r;

                reg             f_sync;

                wire    [63:0]   i_b_n;

                assign          i_b_n = ~i_b;

                initial r_sync = 1'b0;

                always @(posedge i_clk)

                        r_sync <= i_sync;

                always @(posedge i_clk)

                        { r_pps, r_low } <= i_a[31:0] + i_b_n[31:0] + 1'b1;

                always @(posedge i_clk)

                        r_hi <= i_a[63:32] + i_b_n[63:32];

                initial f_sync = 1'b0;

                always @(posedge i_clk)

                        f_sync <= r_sync;

                always @(posedge i_clk)

                        f_r[31:0] <= r_low;

                always @(posedge i_clk)

                        f_r[63:32] <= r_hi + { 31'h00, r_pps };

                assign  o_sync = f_sync;

                assign  o_r    = f_r;

        end endgenerate

endmodule