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Register Pipeline (Simple)

Pipelines data words through a number of register stages. Common uses include pipeline alignment, and pipelining inputs so the registers can retime forward into logic to allow a faster clock.

This module is a simplification of the Register Pipeline, which allows parallel input/output and initial values other than zero, but does not support a PIPE_DEPTH of zero.

Each cycle clock_enable is high, the pipeline shifts by one from pipe_in towards pipe_out. clear sets all registers to zero. If PIPE_DEPTH is zero, pipe_in becomes directly wired to pipe_out and no logic is inferred.

`default_nettype none

module Register_Pipeline_Simple
#(
    parameter WORD_WIDTH =  0,
    parameter PIPE_DEPTH = -1
)
(
    // If PIPE_DEPTH is zero, these are unused
    // verilator lint_off UNUSED
    input   wire                        clock,
    input   wire                        clock_enable,
    input   wire                        clear,
    // verilator lint_on  UNUSED
    input   wire    [WORD_WIDTH-1:0]    pipe_in,
    output  reg     [WORD_WIDTH-1:0]    pipe_out
);

    localparam WORD_ZERO = {WORD_WIDTH{1'b0}};

    initial begin
        pipe_out = WORD_ZERO;
    end

    genvar i;
    generate
        if (PIPE_DEPTH == 0) begin
            always @(*) begin
                pipe_out = pipe_in;
            end
        end
        else if (PIPE_DEPTH > 0) begin

We strip out first iteration of Register instantiations to avoid having to refer to index -1 in the generate loop, and also to connect to pipe_in rather than the output of a previous register.

            wire [WORD_WIDTH-1:0] pipe [PIPE_DEPTH-1:0];

            Register
            #(
                .WORD_WIDTH     (WORD_WIDTH),
                .RESET_VALUE    (WORD_ZERO)
            )
            input_stage
            (
                .clock          (clock),
                .clock_enable   (clock_enable),
                .clear          (clear),
                .data_in        (pipe_in),
                .data_out       (pipe[0])
            );

Now repeat over the remainder of the pipeline stages, starting at stage 1, connecting each pipeline stage to the output of the previous pipeline stage.

            for (i=1; i < PIPE_DEPTH; i=i+1) begin: pipe_stages
                Register
                #(
                    .WORD_WIDTH     (WORD_WIDTH),
                    .RESET_VALUE    (WORD_ZERO)
                )
                pipe_stage
                (
                    .clock          (clock),
                    .clock_enable   (clock_enable),
                    .clear          (clear),
                    .data_in        (pipe[i-1]),
                    .data_out       (pipe[i])
                );
            end

And finally, connect the output of the last register to the module pipe output.

            always @(*) begin
                pipe_out = pipe[PIPE_DEPTH-1];
            end
        end
    endgenerate

endmodule

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