Mini-Nav/hw/rtl/core/result_serializer.sv

`timescale 1ns / 1ps
`include "cam_params.svh"

//==============================================================================
// Result Serializer
//==============================================================================
// Serializes the packed Top-K array (rows and scores) into a rank-ordered
// handshake interface, one result per fire.
//
// Start is edge-triggered (sampled on the cycle start_i is high while not
// already active).  Once started, the module presents outputs for the
// current rank (starting at 0) and advances on each valid-ready fire.
// After the last rank (K-1) fires, done_o pulses for one cycle and the
// module returns to idle with rank reset to 0.
//
// Back-pressure: holds current output while result_ready_i is low.
//
// Supports K=1 cleanly (last rank = rank 0, done on first fire).
//==============================================================================

module result_serializer #(
    parameter int K         = `TOPK_K,
    parameter int ROW_BITS  = `ROW_BITS,
    parameter int SCORE_BITS = `SCORE_BITS,
    parameter int RANK_BITS = (K <= 1) ? 1 : $clog2(K)
) (
    input  logic                    clk,
    input  logic                    rst_n,

    input  logic                    start_i,
    output logic                    busy_o,
    output logic                    done_o,

    input  logic [K*ROW_BITS-1:0]   topk_rows_i,
    input  logic [K*SCORE_BITS-1:0] topk_scores_i,

    output logic                    result_valid_o,
    input  logic                    result_ready_i,
    output logic [RANK_BITS-1:0]    result_rank_o,
    output logic [ROW_BITS-1:0]     result_row_o,
    output logic [SCORE_BITS-1:0]   result_score_o,
    output logic                    result_last_o
);

    // ── State encoding ─────────────────────────────────────────────────────
    typedef enum logic {
        IDLE,
        ACTIVE
    } state_t;

    // ── Localparam for last-rank comparison (width-matched to avoid warnings) ─
    localparam [RANK_BITS-1:0] LAST_RANK = RANK_BITS'(K - 1);

    state_t state_q, state_next;

    logic [RANK_BITS-1:0] rank_q, rank_next;
    logic                 done_q, done_next;

    // Snapshot of packed inputs captured at start (holds during back-pressure)
    logic [K*ROW_BITS-1:0]   rows_snapshot_q, rows_snapshot_next;
    logic [K*SCORE_BITS-1:0] scores_snapshot_q, scores_snapshot_next;

    // ── Sequential: state, rank, done, and snapshot registers ─────────────
    always_ff @(posedge clk or negedge rst_n) begin
        if (!rst_n) begin
            state_q        <= IDLE;
            rank_q         <= '0;
            done_q         <= 1'b0;
            rows_snapshot_q   <= '0;
            scores_snapshot_q <= '0;
        end else begin
            state_q        <= state_next;
            rank_q         <= rank_next;
            done_q         <= done_next;
            rows_snapshot_q   <= rows_snapshot_next;
            scores_snapshot_q <= scores_snapshot_next;
        end
    end

    // ── Combinational: next-state, snapshot, and output logic ──────────────
    always_comb begin
        // Defaults: stay in current state, retain rank, no done pulse,
        //           hold snapshot (outputs stable under back-pressure)
        state_next      = state_q;
        rank_next       = rank_q;
        done_next       = 1'b0;
        rows_snapshot_next   = rows_snapshot_q;
        scores_snapshot_next = scores_snapshot_q;

        if (state_q == IDLE) begin
            // Edge-triggered start — capture packed inputs into snapshot
            if (start_i) begin
                state_next        = ACTIVE;
                rank_next         = '0;  // start from rank 0
                rows_snapshot_next   = topk_rows_i;
                scores_snapshot_next = topk_scores_i;
            end
        end else begin
            // ACTIVE: advance on fire
            if (result_valid_o && result_ready_i) begin
                // Is this the last rank?
                if (rank_q == LAST_RANK) begin
                    state_next = IDLE;
                    rank_next  = '0;
                    done_next  = 1'b1;
                end else begin
                    rank_next = rank_q + 1'b1;
                end
            end
        end
    end

    // ── Output assignments ─────────────────────────────────────────────────
    assign busy_o        = (state_q == ACTIVE);
    assign done_o        = done_q;           // registered — pulses after deactivation
    assign result_valid_o = (state_q == ACTIVE);

    // Slice the snapshot registers at the current rank
    assign result_row_o   = rows_snapshot_q[rank_q * ROW_BITS +: ROW_BITS];
    assign result_score_o = scores_snapshot_q[rank_q * SCORE_BITS +: SCORE_BITS];
    assign result_rank_o  = rank_q;

    // result_last_o: high only when active and on the last rank
    assign result_last_o  = (state_q == ACTIVE) && (rank_q == LAST_RANK);

endmodule