ldpc_optical/tb/tb_ldpc_decoder.sv

// Standalone Verilator testbench for LDPC decoder
// Tests the decoder core directly via Wishbone (no Caravel dependency)
//
// Test 1: Read VERSION register (expect 0x1D010001)
// Test 2: Decode all-zero codeword with strong +31 LLRs

`timescale 1ns / 1ps

module tb_ldpc_decoder;

    // =========================================================================
    // Clock and reset
    // =========================================================================

    logic        clk;
    logic        rst_n;
    logic        wb_cyc_i;
    logic        wb_stb_i;
    logic        wb_we_i;
    logic [7:0]  wb_adr_i;
    logic [31:0] wb_dat_i;
    logic [31:0] wb_dat_o;
    logic        wb_ack_o;
    logic        irq_o;

    // 50 MHz clock (20 ns period)
    initial clk = 0;
    always #10 clk = ~clk;

    // =========================================================================
    // DUT instantiation
    // =========================================================================

    ldpc_decoder_top dut (
        .clk       (clk),
        .rst_n     (rst_n),
        .wb_cyc_i  (wb_cyc_i),
        .wb_stb_i  (wb_stb_i),
        .wb_we_i   (wb_we_i),
        .wb_adr_i  (wb_adr_i),
        .wb_dat_i  (wb_dat_i),
        .wb_dat_o  (wb_dat_o),
        .wb_ack_o  (wb_ack_o),
        .irq_o     (irq_o)
    );

    // =========================================================================
    // VCD dump
    // =========================================================================

    initial begin
        $dumpfile("tb_ldpc_decoder.vcd");
        $dumpvars(0, tb_ldpc_decoder);
    end

    // =========================================================================
    // Watchdog timeout
    // =========================================================================

    int cycle_cnt;

    initial begin
        cycle_cnt = 0;
        forever begin
            @(posedge clk);
            cycle_cnt++;
            if (cycle_cnt > 100000) begin
                $display("TIMEOUT: exceeded 100000 cycles");
                $finish;
            end
        end
    end

    // =========================================================================
    // Wishbone tasks
    // =========================================================================

    task automatic wb_write(input logic [7:0] addr, input logic [31:0] data);
        @(posedge clk);
        wb_cyc_i = 1'b1;
        wb_stb_i = 1'b1;
        wb_we_i  = 1'b1;
        wb_adr_i = addr;
        wb_dat_i = data;

        // Wait for ack
        do begin
            @(posedge clk);
        end while (!wb_ack_o);

        // Deassert
        wb_cyc_i = 1'b0;
        wb_stb_i = 1'b0;
        wb_we_i  = 1'b0;
    endtask

    task automatic wb_read(input logic [7:0] addr, output logic [31:0] data);
        @(posedge clk);
        wb_cyc_i = 1'b1;
        wb_stb_i = 1'b1;
        wb_we_i  = 1'b0;
        wb_adr_i = addr;

        // Wait for ack
        do begin
            @(posedge clk);
        end while (!wb_ack_o);

        data = wb_dat_o;

        // Deassert
        wb_cyc_i = 1'b0;
        wb_stb_i = 1'b0;
    endtask

    // =========================================================================
    // Test variables
    // =========================================================================

    int pass_cnt;
    int fail_cnt;
    logic [31:0] rd_data;

    // =========================================================================
    // Main test sequence
    // =========================================================================

    initial begin
        pass_cnt = 0;
        fail_cnt = 0;

        // Initialize Wishbone signals
        wb_cyc_i = 1'b0;
        wb_stb_i = 1'b0;
        wb_we_i  = 1'b0;
        wb_adr_i = 8'h00;
        wb_dat_i = 32'h0;

        // Reset
        rst_n = 1'b0;
        repeat (10) @(posedge clk);
        rst_n = 1'b1;
        repeat (5) @(posedge clk);

        // =================================================================
        // TEST 1: Read VERSION register
        // =================================================================

        $display("[TEST 1] Read VERSION register");
        wb_read(8'h54, rd_data);

        if (rd_data === 32'h1D01_0001) begin
            $display("  PASS: VERSION = 0x%08X", rd_data);
            pass_cnt++;
        end else begin
            $display("  FAIL: VERSION = 0x%08X (expected 0x1D010001)", rd_data);
            fail_cnt++;
        end

        // =================================================================
        // TEST 2: Decode clean all-zero codeword
        // =================================================================

        $display("[TEST 2] Decode clean all-zero codeword");

        // Write 52 LLR words at addresses 0x10..0xDC
        // Each word = 5x +31 packed: {6'h1F, 6'h1F, 6'h1F, 6'h1F, 6'h1F}
        // = 0x1F | (0x1F<<6) | (0x1F<<12) | (0x1F<<18) | (0x1F<<24)
        // = 0x1F7DF7DF
        begin
            int i;
            for (i = 0; i < 52; i++) begin
                wb_write(8'h10 + i * 4, 32'h1F7D_F7DF);
            end
        end

        // Start decode: write CTRL
        // bit[0]=1 (start), bit[1]=1 (early_term), bits[12:8]=0x1E=30 (max_iter)
        // 0x00001E03
        wb_write(8'h00, 32'h0000_1E03);

        // Poll STATUS (addr 0x04) until busy (bit[0]) = 0
        // Allow a few cycles for busy to assert first
        repeat (5) @(posedge clk);

        begin
            int poll_cnt;
            poll_cnt = 0;
            do begin
                wb_read(8'h04, rd_data);
                poll_cnt++;
                if (poll_cnt > 10000) begin
                    $display("  FAIL: decoder stuck busy after %0d polls", poll_cnt);
                    fail_cnt++;
                    $display("=== %0d PASSED, %0d FAILED ===", pass_cnt, fail_cnt);
                    $finish;
                end
            end while (rd_data[0] == 1'b1);
        end

        // Check convergence: bit[1] of STATUS
        if (rd_data[1] == 1'b1) begin
            $display("  converged=1 (OK)");
        end else begin
            $display("  FAIL: converged=0 (expected 1)");
            fail_cnt++;
        end

        // Check syndrome weight: bits[23:16] of STATUS
        if (rd_data[23:16] == 8'd0) begin
            $display("  syndrome_weight=0 (OK)");
        end else begin
            $display("  FAIL: syndrome_weight=%0d (expected 0)", rd_data[23:16]);
            fail_cnt++;
        end

        // Check iterations used: bits[12:8] of STATUS
        $display("  iterations_used=%0d", rd_data[12:8]);

        // Read DECODED register (addr 0x50)
        wb_read(8'h50, rd_data);

        if (rd_data === 32'h0000_0000) begin
            $display("  PASS: decoded=0x%08X", rd_data);
            pass_cnt++;
        end else begin
            $display("  FAIL: decoded=0x%08X (expected 0x00000000)", rd_data);
            fail_cnt++;
        end

        // =================================================================
        // Summary
        // =================================================================

        $display("");
        if (fail_cnt == 0) begin
            $display("=== ALL %0d TESTS PASSED ===", pass_cnt);
        end else begin
            $display("=== %0d PASSED, %0d FAILED ===", pass_cnt, fail_cnt);
        end

        $finish;
    end

endmodule