Source

Signed Binary Accumulator

Adds the signed increment to the signed accumulated_value every cycle increment_valid is high. load_valid overrides increment_valid and instead loads the accumulator with load_value. clear overrides both increment_valid and load_valid and puts the accumulator back at INITIAL_VALUE.

If the accmulator increments past the max or min signed integer value it can hold, the accumulator will roll-over and set the signed_overflow bit. The overflow bit is cleared at the next non-overflowing increment, or if the accumulator is cleared or loaded.

When chaining accumulators, which may happen if you are incrementing in unusual bases where each digit has its own accumulator, AND the carry_out of the previous accumulator with the signal fed to the increment_valid input of the next accumulator. The carry_in is kept for generality.

`default_nettype none

module Accumulator_Binary
#(
    parameter                   WORD_WIDTH      = 0,
    parameter [WORD_WIDTH-1:0]  INITIAL_VALUE   = 0
)
(
    input   wire                        clock,
    input   wire                        clear,
    input   wire    [WORD_WIDTH-1:0]    increment,
    input   wire                        increment_valid,
    input   wire    [WORD_WIDTH-1:0]    load_value,
    input   wire                        load_valid,
    input   wire                        carry_in,
    output  wire                        carry_out,
    output  wire    [WORD_WIDTH-1:0]    accumulated_value,
    output  wire                        signed_overflow
);

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

Apply the increment to the current accumulator value.

    wire [WORD_WIDTH-1:0] incremented_value;

    Adder_Subtractor_Binary
    #(
        .WORD_WIDTH (WORD_WIDTH)
    )
    add_increment
    (
        .add_sub    (1'b0), // 0/1 -> A+B/A-B
        .carry_in   (carry_in),
        .A_in       (accumulated_value),
        .B_in       (increment),
        .sum_out    (incremented_value),
        .carry_out  (carry_out)
    );

Then, let's reconstruct the carry-in into the last (most-significant) bit position of the result. If it differs from the carry_out, then a signed overflow/underflow happened, where we either added past the largest positive value or subtracted past the smallest negative value.

    wire final_carry_in;

    CarryIn_Binary
    #(
        .WORD_WIDTH (1)
    )
    calc_final_carry_in
    (
        .A          (accumulated_value  [WORD_WIDTH-1]),
        .B          (increment          [WORD_WIDTH-1]),
        .sum        (incremented_value  [WORD_WIDTH-1]),
        .carryin    (final_carry_in)
    );

    reg signed_overflow_internal = 1'b0;

    always @(*) begin
        signed_overflow_internal = (carry_out != final_carry_in);
    end

Update the accumulator if load or increment is valid. Load overrides increment. Clear the overflow if loading.

    reg [WORD_WIDTH-1:0]    next_value              = WORD_ZERO;
    reg                     enable_accumulator      = 1'b0;
    reg                     enable_overflow         = 1'b0;
    reg                     clear_overflow          = 1'b0;

    always @(*) begin
        next_value          = (load_valid == 1'b1) ? load_value : incremented_value;
        enable_accumulator  = (increment_valid == 1'b1) || (load_valid == 1'b1);
        enable_overflow     = (increment_valid == 1'b1);
        clear_overflow      = (load_valid      == 1'b1) || (clear      == 1'b1);
    end

Finally, the accumulator and signed_overflow registers.

    Register
    #(
        .WORD_WIDTH     (WORD_WIDTH),
        .RESET_VALUE    (INITIAL_VALUE)
    )
    accumulator
    (
        .clock          (clock),
        .clock_enable   (enable_accumulator),
        .clear          (clear),
        .data_in        (next_value),
        .data_out       (accumulated_value)
    );

    Register
    #(
        .WORD_WIDTH     (1),
        .RESET_VALUE    (1'b0)
    )
    overflow
    (
        .clock          (clock),
        .clock_enable   (enable_overflow),
        .clear          (clear_overflow),
        .data_in        (signed_overflow_internal),
        .data_out       (signed_overflow)
    );

endmodule

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