Subversion Repositories zipcpu

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

//

// Filename:    busdelay.v

//

// Project:     Zip CPU -- a small, lightweight, RISC CPU soft core

//

// Purpose:     Delay any access to the wishbone bus by a single clock.

//

//      When the first Zip System would not meet the timing requirements of

//      the board it was placed upon, this bus delay was added to help out.

//      It may no longer be necessary, having cleaned some other problems up

//      first, but it will remain here as a means of alleviating timing

//      problems.

//

//      The specific problem takes place on the stall line: a wishbone master

//      *must* know on the first clock whether or not the bus will stall.

//

//

//

//

// Creator:     Dan Gisselquist, Ph.D.

//              Gisselquist Tecnology, LLC

//

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

//

// Copyright (C) 2015, 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.

//

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

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

//

//

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

//

module  busdelay(i_clk,

                // The input bus

                i_wb_cyc, i_wb_stb, i_wb_we, i_wb_addr, i_wb_data,

                        o_wb_ack, o_wb_stall, o_wb_data,

                // The delayed bus

                o_dly_cyc, o_dly_stb, o_dly_we, o_dly_addr, o_dly_data,

                        i_dly_ack, i_dly_stall, i_dly_data);

        parameter       AW=32, DW=32;

        input   i_clk;

        // Input/master bus

        input                           i_wb_cyc, i_wb_stb, i_wb_we;

        input           [(AW-1):0]       i_wb_addr;

        input           [(DW-1):0]       i_wb_data;

        output  reg                     o_wb_ack;

        output  wire                    o_wb_stall;

        output  reg     [(DW-1):0]       o_wb_data;

        // Delayed bus

        output  reg                     o_dly_cyc, o_dly_stb, o_dly_we;

        output  reg     [(AW-1):0]       o_dly_addr;

        output  reg     [(DW-1):0]       o_dly_data;

        input                           i_dly_ack;

        input                           i_dly_stall;

        input           [(DW-1):0]       i_dly_data;

        initial o_dly_cyc = 1'b0;

        initial o_dly_stb = 1'b0;

        always @(posedge i_clk)

                o_dly_cyc <= i_wb_cyc;

        always @(posedge i_clk)

                if (~o_wb_stall)

                        o_dly_stb <= i_wb_stb;

        always @(posedge i_clk)

                if (~o_wb_stall)

                        o_dly_we  <= i_wb_we;

        always @(posedge i_clk)

                if (~o_wb_stall)

                        o_dly_addr<= i_wb_addr;

        always @(posedge i_clk)

                if (~o_wb_stall)

                        o_dly_data <= i_wb_data;

        always @(posedge i_clk)

                o_wb_ack  <= (i_dly_ack)&&(o_dly_cyc)&&(i_wb_cyc);

        always @(posedge i_clk)

                o_wb_data <= i_dly_data;

        // Our only non-delayed line, yet still really delayed.  Perhaps

        // there's a way to register this?

        // o_wb_stall <= (i_wb_cyc)&&(i_wb_stb) ... or some such?

        assign  o_wb_stall = ((i_wb_cyc)&&(o_dly_cyc)&&(i_dly_stall)&&(o_dly_stb));

endmodule