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`timescale 1ns / 1ps

// ============================================================================

//      (C) 2007,2012  Robert Finch

//  robfinch<remove>@opencores.org

//      All rights reserved.

//

//      PSGFilter.v

//      Version 1.1

//

// This source file is free software: you can redistribute it and/or modify 

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

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

// (at your option) any later version.                                      

//                                                                          

// This source file is distributed in the hope that it will be useful,      

// but WITHOUT ANY WARRANTY; without even the implied warranty of           

// MERCHANTABILITY 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.  If not, see <http://www.gnu.org/licenses/>.    

//

//

//        16-tap digital filter

//

//    Currently this filter is only partially tested. The author believes that

//    the approach used is valid however.

//      The author opted to include the filter because it is part of the design,

//      and even this untested component can provide an idea of the resource

//      requirements, and device capabilities.

//              This is a "how one might approach the problem" example, at least

//      until the author is sure the filter is working correctly.

//        

//      Time division multiplexing is used to implement this filter in order to

//      reduce the resource requirement. This should be okay because it is being

//      used to filter audio signals. The effective operating frequency of the

//      filter depends on the 'cnt' supplied (eg 1MHz)

//

//      Spartan3

//      Webpack 9.1i xc3s1000-4ft256

//      158 LUTs / 88 slices / 73.865MHz

//      1 MULT

//============================================================================

//

module PSGFilter(rst, clk, cnt, wr, adr, din, i, o);

parameter pTaps = 16;

input rst;

input clk;

input [7:0] cnt;

input wr;

input [3:0] adr;

input [12:0] din;

input [14:0] i;

output [14:0] o;

reg [14:0] o;

reg [30:0] acc;                 // accumulator

reg [14:0] tap [0:pTaps-1];     // tap registers

integer n;

// coefficient memory

reg [11:0] coeff [0:pTaps-1];   // magnitude of coefficient

reg [pTaps-1:0] sgn;            // sign of coefficient

initial begin

        for (n = 0; n < pTaps; n = n + 1)

        begin

                coeff[n] <= 0;

                sgn[n] <= 0;

        end

end

// update coefficient memory

always @(posedge clk)

    if (wr) begin

        coeff[adr] <= din[11:0];

        sgn[adr] <= din[12];

    end

// shift taps

// Note: infer a dsr by NOT resetting the registers

always @(posedge clk)

    if (cnt==8'd0) begin

        tap[0] <= i;

        for (n = 1; n < pTaps; n = n + 1)

                tap[n] <= tap[n-1];

    end

wire [26:0] mult = coeff[cnt[3:0]] * tap[cnt[3:0]];

always @(posedge clk)

    if (rst)

        acc <= 0;

    else if (cnt==8'd0)

        acc <= sgn[cnt[3:0]] ? 0 - mult : 0 + mult;

    else if (cnt < pTaps)

        acc <= sgn[cnt[3:0]] ? acc - mult : acc + mult;

always @(posedge clk)

    if (rst)

        o <= 0;

    else if (cnt==8'd0)

        o <= acc[30:16];

endmodule