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/* ============================================================================

        (C) 2007  Robert Finch

        All rights reserved.

        PSGFilter.v

        Version 1.1

    This source code is available for evaluation and validation purposes

    only. This copyright statement and disclaimer must remain present in

    the file.

        NO WARRANTY.

    THIS Work, IS PROVIDEDED "AS IS" WITH NO WARRANTIES OF ANY KIND, WHETHER

    EXPRESS OR IMPLIED. The user must assume the entire risk of using the

    Work.

    IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE FOR ANY

    INCIDENTAL, CONSEQUENTIAL, OR PUNITIVE DAMAGES WHATSOEVER RELATING TO

    THE USE OF THIS WORK, OR YOUR RELATIONSHIP WITH THE AUTHOR.

    IN ADDITION, IN NO EVENT DOES THE AUTHOR AUTHORIZE YOU TO USE THE WORK

    IN APPLICATIONS OR SYSTEMS WHERE THE WORK'S FAILURE TO PERFORM CAN

    REASONABLY BE EXPECTED TO RESULT IN A SIGNIFICANT PHYSICAL INJURY, OR IN

    LOSS OF LIFE. ANY SUCH USE BY YOU IS ENTIRELY AT YOUR OWN RISK, AND YOU

    AGREE TO HOLD THE AUTHOR AND CONTRIBUTORS HARMLESS FROM ANY CLAIMS OR

    LOSSES RELATING TO SUCH UNAUTHORIZED USE.

        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

// 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