`timescale 1ns / 1ps

module tb_cdc_modules;

    // ── Clock periods (reflecting real system) ─────────────────
    localparam SRC_CLK_PERIOD = 2.5;    // 400 MHz (ADC domain)
    localparam DST_CLK_PERIOD = 10.0;   // 100 MHz (processing domain)
    // For handshake tests, use different ratio
    localparam HS_SRC_PERIOD  = 10.0;   // 100 MHz
    localparam HS_DST_PERIOD  = 7.0;    // ~143 MHz (non-integer ratio)

    // ── Test bookkeeping ───────────────────────────────────────
    integer pass_count;
    integer fail_count;
    integer test_num;
    integer i;

    // ── Check task ─────────────────────────────────────────────
    task check;
        input cond;
        input [511:0] label;
        begin
            test_num = test_num + 1;
            if (cond) begin
                $display("[PASS] Test %0d: %0s", test_num, label);
                pass_count = pass_count + 1;
            end else begin
                $display("[FAIL] Test %0d: %0s", test_num, label);
                fail_count = fail_count + 1;
            end
        end
    endtask

    // ════════════════════════════════════════════════════════════
    // MODULE 1: cdc_adc_to_processing (Gray-code multi-bit CDC)
    // ════════════════════════════════════════════════════════════
    reg         m1_src_clk;
    reg         m1_dst_clk;
    reg         m1_src_reset_n;
    reg         m1_dst_reset_n;
    reg  [7:0]  m1_src_data;
    reg         m1_src_valid;
    wire [7:0]  m1_dst_data;
    wire        m1_dst_valid;

    always #(SRC_CLK_PERIOD/2) m1_src_clk = ~m1_src_clk;
    always #(DST_CLK_PERIOD/2) m1_dst_clk = ~m1_dst_clk;

    cdc_adc_to_processing #(
        .WIDTH(8),
        .STAGES(3)
    ) uut_m1 (
        .src_clk     (m1_src_clk),
        .dst_clk     (m1_dst_clk),
        .src_reset_n (m1_src_reset_n),
        .dst_reset_n (m1_dst_reset_n),
        .src_data    (m1_src_data),
        .src_valid   (m1_src_valid),
        .dst_data    (m1_dst_data),
        .dst_valid   (m1_dst_valid)
    );

    // ════════════════════════════════════════════════════════════
    // MODULE 2: cdc_single_bit
    // ════════════════════════════════════════════════════════════
    reg         m2_src_clk;
    reg         m2_dst_clk;
    reg         m2_reset_n;
    reg         m2_src_signal;
    wire        m2_dst_signal;

    always #(SRC_CLK_PERIOD/2) m2_src_clk = ~m2_src_clk;
    always #(DST_CLK_PERIOD/2) m2_dst_clk = ~m2_dst_clk;

    cdc_single_bit #(
        .STAGES(3)
    ) uut_m2 (
        .src_clk  (m2_src_clk),
        .dst_clk  (m2_dst_clk),
        .reset_n  (m2_reset_n),
        .src_signal(m2_src_signal),
        .dst_signal(m2_dst_signal)
    );

    // ════════════════════════════════════════════════════════════
    // MODULE 3: cdc_handshake
    // ════════════════════════════════════════════════════════════
    reg          m3_src_clk;
    reg          m3_dst_clk;
    reg          m3_reset_n;
    reg  [31:0]  m3_src_data;
    reg          m3_src_valid;
    wire         m3_src_ready;
    wire [31:0]  m3_dst_data;
    wire         m3_dst_valid;
    reg          m3_dst_ready;

    always #(HS_SRC_PERIOD/2) m3_src_clk = ~m3_src_clk;
    always #(HS_DST_PERIOD/2) m3_dst_clk = ~m3_dst_clk;

    cdc_handshake #(
        .WIDTH(32)
    ) uut_m3 (
        .src_clk  (m3_src_clk),
        .dst_clk  (m3_dst_clk),
        .reset_n  (m3_reset_n),
        .src_data (m3_src_data),
        .src_valid(m3_src_valid),
        .src_ready(m3_src_ready),
        .dst_data (m3_dst_data),
        .dst_valid(m3_dst_valid),
        .dst_ready(m3_dst_ready)
    );

    // ── Main test sequence ─────────────────────────────────────
    initial begin
        $dumpfile("tb_cdc_modules.vcd");
        $dumpvars(0, tb_cdc_modules);

        // Init all clocks and signals
        m1_src_clk   = 0; m1_dst_clk   = 0;
        m1_src_reset_n = 0; m1_dst_reset_n = 0;
        m1_src_data  = 0; m1_src_valid  = 0;
        m2_src_clk   = 0; m2_dst_clk   = 0; m2_reset_n   = 0;
        m2_src_signal = 0;
        m3_src_clk   = 0; m3_dst_clk   = 0; m3_reset_n   = 0;
        m3_src_data  = 0; m3_src_valid  = 0; m3_dst_ready = 0;
        pass_count   = 0; fail_count   = 0; test_num     = 0;

        // ════════════════════════════════════════════════════════
        // SECTION A: cdc_adc_to_processing tests
        // ════════════════════════════════════════════════════════
        $display("\n=== Section A: cdc_adc_to_processing (Gray-code CDC) ===");

        // ── A1: Reset behaviour ────────────────────────────────
        $display("\n--- A1: Reset Behaviour (split-domain reset) ---");
        m1_src_reset_n = 0; m1_dst_reset_n = 0;
        #100;  // let both clocks run
        check(m1_dst_valid === 1'b0, "M1: dst_valid = 0 during reset");
        check(m1_dst_data === 8'd0, "M1: dst_data = 0 during reset");

        // Release reset
        @(posedge m1_dst_clk);
        m1_src_reset_n = 1; m1_dst_reset_n = 1;
        @(posedge m1_src_clk);

        // ── A2: Single value transfer ──────────────────────────
        $display("\n--- A2: Single Value Transfer ---");
        m1_src_data  = 8'hA5;
        m1_src_valid = 1;
        @(posedge m1_src_clk); #1;
        m1_src_valid = 0;

        // Wait for CDC propagation (3 stages × dst_clk + margin)
        begin : a2_wait
            integer wait_cycles;
            reg saw_valid;
            saw_valid = 0;
            for (wait_cycles = 0; wait_cycles < 20; wait_cycles = wait_cycles + 1) begin
                @(posedge m1_dst_clk); #1;
                if (m1_dst_valid) begin
                    saw_valid = 1;
                    disable a2_wait;
                end
            end
        end
        check(m1_dst_valid === 1'b1, "M1: dst_valid asserts after CDC");
        check(m1_dst_data === 8'hA5, "M1: data 0xA5 transferred correctly");

        // ── A3: Multiple sequential values ─────────────────────
        $display("\n--- A3: Multiple Sequential Values ---");
        m1_src_reset_n = 0; m1_dst_reset_n = 0;
        #100;
        m1_src_reset_n = 1; m1_dst_reset_n = 1;
        @(posedge m1_src_clk);

        begin : a3_block
            reg [7:0] received_values [0:31];
            integer rx_count;
            integer tx_count;
            integer total_wait;
            reg all_received;

            rx_count = 0;

            // Send 8 values, one per src_clk cycle
            // At 400:100 ratio, src sends 4x faster than dst can sample
            // Gray-code CDC may miss intermediate values — that's expected.
            // What matters is that received values are VALID (not corrupted).
            for (tx_count = 0; tx_count < 8; tx_count = tx_count + 1) begin
                m1_src_data  = tx_count * 37 + 10;  // 10, 47, 84, 121, 158, 195, 232, 13
                m1_src_valid = 1;
                @(posedge m1_src_clk); #1;
            end
            m1_src_valid = 0;

            // Collect outputs
            for (total_wait = 0; total_wait < 40; total_wait = total_wait + 1) begin
                @(posedge m1_dst_clk); #1;
                if (m1_dst_valid) begin
                    received_values[rx_count] = m1_dst_data;
                    rx_count = rx_count + 1;
                end
            end

            $display("  Sent 8 values at 400MHz, received %0d valid values at 100MHz",
                     rx_count);
            check(rx_count > 0, "M1: At least one value received from burst");

            // Verify last received value matches last sent value
            // (The CDC should eventually stabilize to the last written value)
            if (rx_count > 0) begin
                // Check that every received value is one of the sent values
                begin : verify_received
                    reg [7:0] sent_vals [0:7];
                    reg found;
                    integer j, k;
                    reg all_valid;

                    for (j = 0; j < 8; j = j + 1) begin
                        sent_vals[j] = j * 37 + 10;
                    end

                    all_valid = 1;
                    for (j = 0; j < rx_count; j = j + 1) begin
                        found = 0;
                        for (k = 0; k < 8; k = k + 1) begin
                            if (received_values[j] === sent_vals[k]) found = 1;
                        end
                        if (!found) begin
                            all_valid = 0;
                            $display("  [WARN] Received value 0x%02x not in sent set",
                                     received_values[j]);
                        end
                    end
                    check(all_valid, "M1: All received values are valid (no corruption)");
                end
            end else begin
                check(1'b0, "M1: All received values are valid (no corruption)");
            end
        end

        // ── A4: Slow sender (one value every 4 dst_clk cycles) ─
        $display("\n--- A4: Slow Sender ---");
        m1_src_reset_n = 0; m1_dst_reset_n = 0;
        #100;
        m1_src_reset_n = 1; m1_dst_reset_n = 1;
        @(posedge m1_src_clk);

        begin : a4_block
            reg [7:0] expected_vals [0:7];
            reg [7:0] got_vals [0:7];
            integer tx_idx, rx_idx, wait_cnt;
            reg all_match;

            rx_idx = 0;

            for (tx_idx = 0; tx_idx < 4; tx_idx = tx_idx + 1) begin
                expected_vals[tx_idx] = (tx_idx + 1) * 50;  // 50, 100, 150, 200
                m1_src_data  = expected_vals[tx_idx];
                m1_src_valid = 1;
                @(posedge m1_src_clk); #1;
                m1_src_valid = 0;

                // Wait long enough for CDC to propagate
                for (wait_cnt = 0; wait_cnt < 15; wait_cnt = wait_cnt + 1) begin
                    @(posedge m1_dst_clk); #1;
                    if (m1_dst_valid && rx_idx < 8) begin
                        got_vals[rx_idx] = m1_dst_data;
                        rx_idx = rx_idx + 1;
                    end
                end
            end

            $display("  Slow send: sent 4, received %0d", rx_idx);
            check(rx_idx == 4, "M1: All 4 slow-sent values received");

            all_match = 1;
            for (i = 0; i < rx_idx && i < 4; i = i + 1) begin
                if (got_vals[i] !== expected_vals[i]) begin
                    all_match = 0;
                    $display("  [WARN] Slow rx[%0d]: got 0x%02x, exp 0x%02x",
                             i, got_vals[i], expected_vals[i]);
                end
            end
            check(all_match, "M1: Slow-sent values match exactly");
        end

        // ── A5: Split-Domain Reset — Src resets while dst stays active ──
        $display("\n--- A5: Split-Domain Reset (src resets, dst active) ---");
        m1_src_reset_n = 0; m1_dst_reset_n = 0;
        m1_src_data = 0; m1_src_valid = 0;
        #100;

        // Release dst_reset_n first, src stays in reset
        m1_dst_reset_n = 1;
        begin : a5_dst_idle
            integer wait_cycles;
            reg saw_valid;
            saw_valid = 0;
            for (wait_cycles = 0; wait_cycles < 10; wait_cycles = wait_cycles + 1) begin
                @(posedge m1_dst_clk); #1;
                if (m1_dst_valid) saw_valid = 1;
            end
            check(!saw_valid, "M1: dst_valid stays 0 while src is in reset");
        end

        // Now release src_reset_n
        m1_src_reset_n = 1;
        @(posedge m1_src_clk);

        // Send data and verify transfer works after staggered reset
        m1_src_data  = 8'h3C;
        m1_src_valid = 1;
        @(posedge m1_src_clk); #1;
        m1_src_valid = 0;

        begin : a5_wait
            integer wait_cycles;
            for (wait_cycles = 0; wait_cycles < 20; wait_cycles = wait_cycles + 1) begin
                @(posedge m1_dst_clk); #1;
                if (m1_dst_valid) disable a5_wait;
            end
        end
        check(m1_dst_valid === 1'b1, "M1: dst_valid asserts after staggered src reset release");
        check(m1_dst_data === 8'h3C, "M1: data 0x3C correct after staggered src reset");

        // ── A6: Split-Domain Reset — Dst resets while src stays active ──
        // KEY test: catches the original P0 bug where a single reset from
        // the src domain was used to reset dst-domain registers.
        $display("\n--- A6: Split-Domain Reset (dst resets, src active) ---");
        m1_src_reset_n = 1; m1_dst_reset_n = 1;
        m1_src_data = 0; m1_src_valid = 0;
        @(posedge m1_src_clk);

        // Send data and verify it arrives (baseline)
        m1_src_data  = 8'hF0;
        m1_src_valid = 1;
        @(posedge m1_src_clk); #1;
        m1_src_valid = 0;

        begin : a6_baseline
            integer wait_cycles;
            for (wait_cycles = 0; wait_cycles < 20; wait_cycles = wait_cycles + 1) begin
                @(posedge m1_dst_clk); #1;
                if (m1_dst_valid) disable a6_baseline;
            end
        end
        check(m1_dst_data === 8'hF0, "M1: Baseline data 0xF0 received before dst-only reset");

        // Assert ONLY dst_reset_n (src keeps running)
        m1_dst_reset_n = 0;
        begin : a6_check_reset
            integer wait_cycles;
            reg dst_cleared;
            dst_cleared = 0;
            for (wait_cycles = 0; wait_cycles < 10; wait_cycles = wait_cycles + 1) begin
                @(posedge m1_dst_clk); #1;
                if (m1_dst_data === 8'd0 && m1_dst_valid === 1'b0)
                    dst_cleared = 1;
            end
            check(dst_cleared, "M1: dst_data=0 and dst_valid=0 after dst-only reset");
        end

        // Deassert dst_reset_n
        m1_dst_reset_n = 1;
        repeat (3) @(posedge m1_dst_clk);

        // Send new data from src, verify it arrives correctly
        m1_src_data  = 8'h55;
        m1_src_valid = 1;
        @(posedge m1_src_clk); #1;
        m1_src_valid = 0;

        begin : a6_recovery
            integer wait_cycles;
            for (wait_cycles = 0; wait_cycles < 20; wait_cycles = wait_cycles + 1) begin
                @(posedge m1_dst_clk); #1;
                if (m1_dst_valid) disable a6_recovery;
            end
        end
        check(m1_dst_valid === 1'b1, "M1: dst_valid asserts after dst-only reset recovery");
        check(m1_dst_data === 8'h55, "M1: data 0x55 correct after dst-only reset recovery");

        // ── A7: Staggered Reset Deassertion ────────────────────
        $display("\n--- A7: Staggered Reset Deassertion ---");
        m1_src_reset_n = 0; m1_dst_reset_n = 0;
        m1_src_data = 0; m1_src_valid = 0;
        #100;

        // Release src_reset_n first, start sending data immediately
        m1_src_reset_n = 1;
        @(posedge m1_src_clk);
        m1_src_data  = 8'hBB;
        m1_src_valid = 1;
        @(posedge m1_src_clk); #1;
        m1_src_valid = 0;

        // Wait 50ns with dst_reset_n still asserted
        #50;

        // Release dst_reset_n
        m1_dst_reset_n = 1;

        // Let sync chain clear through a few dst_clk cycles first
        repeat (5) @(posedge m1_dst_clk);

        // Src sends another value so dst can capture it fresh
        @(posedge m1_src_clk);
        m1_src_data  = 8'hCC;
        m1_src_valid = 1;
        @(posedge m1_src_clk); #1;
        m1_src_valid = 0;

        begin : a7_wait
            integer wait_cycles;
            for (wait_cycles = 0; wait_cycles < 40; wait_cycles = wait_cycles + 1) begin
                @(posedge m1_dst_clk); #1;
                if (m1_dst_valid) disable a7_wait;
            end
        end
        check(m1_dst_valid === 1'b1, "M1: dst_valid asserts after staggered deassertion");
        // Accept either 0xBB (if pipeline retained) or 0xCC (if fresh capture)
        check(m1_dst_data === 8'hBB || m1_dst_data === 8'hCC,
              "M1: Data not corrupted after staggered deassertion");

        // ── A8: Port Connectivity Check ────────────────────────
        $display("\n--- A8: Port Connectivity Check ---");
        m1_src_reset_n = 0; m1_dst_reset_n = 0;
        m1_src_data = 0; m1_src_valid = 0;
        #100;
        m1_src_reset_n = 1; m1_dst_reset_n = 1;
        repeat (5) @(posedge m1_dst_clk); #1;

        // After reset deassertion, outputs should not be X or Z
        check(m1_dst_data !== 8'bxxxxxxxx, "M1: dst_data is not X after reset");
        check(m1_dst_data !== 8'bzzzzzzzz, "M1: dst_data is not Z after reset");
        check(m1_dst_valid !== 1'bx, "M1: dst_valid is not X after reset");
        check(m1_dst_valid !== 1'bz, "M1: dst_valid is not Z after reset");

        // After a transfer, check again
        m1_src_data  = 8'h99;
        m1_src_valid = 1;
        @(posedge m1_src_clk); #1;
        m1_src_valid = 0;

        begin : a8_wait
            integer wait_cycles;
            for (wait_cycles = 0; wait_cycles < 20; wait_cycles = wait_cycles + 1) begin
                @(posedge m1_dst_clk); #1;
                if (m1_dst_valid) disable a8_wait;
            end
        end
        check(m1_dst_data !== 8'bxxxxxxxx, "M1: dst_data is not X after transfer");
        check(m1_dst_data !== 8'bzzzzzzzz, "M1: dst_data is not Z after transfer");
        check(m1_dst_valid !== 1'bx, "M1: dst_valid is not X after transfer");
        check(m1_dst_valid !== 1'bz, "M1: dst_valid is not Z after transfer");

        // ════════════════════════════════════════════════════════
        // SECTION B: cdc_single_bit tests
        // ════════════════════════════════════════════════════════
        $display("\n=== Section B: cdc_single_bit ===");

        // ── B1: Reset behaviour ────────────────────────────────
        $display("\n--- B1: Reset Behaviour ---");
        m2_reset_n    = 0;
        m2_src_signal = 0;
        #100;
        check(m2_dst_signal === 1'b0, "M2: dst_signal = 0 during reset");

        // ── B2: Signal propagation (low to high) ───────────────
        $display("\n--- B2: Low-to-High Propagation ---");
        m2_reset_n = 1;
        @(posedge m2_dst_clk);

        m2_src_signal = 1;
        begin : b2_wait
            integer wait_cycles;
            reg saw_high;
            saw_high = 0;
            for (wait_cycles = 0; wait_cycles < 10; wait_cycles = wait_cycles + 1) begin
                @(posedge m2_dst_clk); #1;
                if (m2_dst_signal === 1'b1) begin
                    saw_high = 1;
                    $display("  Signal propagated after %0d dst_clk cycles", wait_cycles + 1);
                    disable b2_wait;
                end
            end
        end
        check(m2_dst_signal === 1'b1, "M2: 0->1 propagates through sync chain");

        // ── B3: Signal propagation (high to low) ───────────────
        $display("\n--- B3: High-to-Low Propagation ---");
        m2_src_signal = 0;
        begin : b3_wait
            integer wait_cycles;
            for (wait_cycles = 0; wait_cycles < 10; wait_cycles = wait_cycles + 1) begin
                @(posedge m2_dst_clk); #1;
                if (m2_dst_signal === 1'b0) begin
                    $display("  Signal de-propagated after %0d dst_clk cycles", wait_cycles + 1);
                    disable b3_wait;
                end
            end
        end
        check(m2_dst_signal === 1'b0, "M2: 1->0 propagates through sync chain");

        // ── B4: Minimum pulse width ────────────────────────────
        $display("\n--- B4: Minimum Pulse Width ---");
        m2_reset_n = 0;
        #100;
        m2_reset_n = 1;
        @(posedge m2_dst_clk);

        // A single src_clk pulse may or may not be captured
        // At 400:100 ratio, 1 src_clk pulse = 2.5ns, dst_clk period = 10ns
        // A single src_clk pulse might be missed — that's expected behavior
        m2_src_signal = 1;
        @(posedge m2_src_clk); #1;
        m2_src_signal = 0;

        begin : b4_wait
            integer wait_cycles;
            reg saw_pulse;
            saw_pulse = 0;
            for (wait_cycles = 0; wait_cycles < 10; wait_cycles = wait_cycles + 1) begin
                @(posedge m2_dst_clk); #1;
                if (m2_dst_signal) saw_pulse = 1;
            end
            // Single src_clk pulse at 400MHz might be too short for 100MHz dst
            // This is a known limitation of single-bit synchronizers
            $display("  Single src_clk pulse captured: %b (may miss — expected for narrow pulse)",
                     saw_pulse);
            check(1'b1, "M2: Narrow pulse test completed (miss is acceptable)");
        end

        // ── B5: Long pulse always captured ─────────────────────
        $display("\n--- B5: Long Pulse Capture ---");
        m2_reset_n = 0;
        #100;
        m2_reset_n = 1;
        @(posedge m2_dst_clk);

        // Pulse held for 8 src_clk cycles = 20ns (> 2× dst_clk period)
        m2_src_signal = 1;
        repeat (8) @(posedge m2_src_clk);
        m2_src_signal = 0;

        begin : b5_wait
            integer wait_cycles;
            reg saw_pulse;
            saw_pulse = 0;
            for (wait_cycles = 0; wait_cycles < 10; wait_cycles = wait_cycles + 1) begin
                @(posedge m2_dst_clk); #1;
                if (m2_dst_signal) saw_pulse = 1;
            end
            check(saw_pulse, "M2: Long pulse (8 src_clk) always captured");
        end

        // ── B6: Reset clears sync chain ────────────────────────
        $display("\n--- B6: Reset Clears Sync Chain ---");
        m2_src_signal = 1;
        repeat (10) @(posedge m2_dst_clk);
        // dst_signal should be 1 now
        m2_reset_n = 0;
        repeat (2) @(posedge m2_dst_clk); #1;
        check(m2_dst_signal === 1'b0, "M2: Reset clears sync chain to 0");
        m2_reset_n = 1;
        m2_src_signal = 0;

        // ── B7: Port Connectivity ──────────────────────────────
        $display("\n--- B7: Port Connectivity ---");
        m2_reset_n = 0;
        m2_src_signal = 0;
        #100;
        m2_reset_n = 1;
        repeat (5) @(posedge m2_dst_clk); #1;

        check(m2_dst_signal !== 1'bx, "M2: dst_signal is not X after reset");
        check(m2_dst_signal !== 1'bz, "M2: dst_signal is not Z after reset");

        // Drive signal high and verify after propagation
        m2_src_signal = 1;
        repeat (8) @(posedge m2_dst_clk); #1;
        check(m2_dst_signal !== 1'bx, "M2: dst_signal is not X after propagation");
        check(m2_dst_signal !== 1'bz, "M2: dst_signal is not Z after propagation");
        m2_src_signal = 0;
        repeat (8) @(posedge m2_dst_clk);

        // ════════════════════════════════════════════════════════
        // SECTION C: cdc_handshake tests
        // ════════════════════════════════════════════════════════
        $display("\n=== Section C: cdc_handshake ===");

        // ── C1: Reset behaviour ────────────────────────────────
        $display("\n--- C1: Reset Behaviour ---");
        m3_reset_n   = 0;
        m3_src_valid = 0;
        m3_dst_ready = 0;
        #200;
        check(m3_src_ready === 1'b1, "M3: src_ready = 1 during reset (not busy)");
        check(m3_dst_valid === 1'b0, "M3: dst_valid = 0 during reset");

        // Release reset
        m3_reset_n = 1;
        @(posedge m3_src_clk);

        // ── C2: Single data transfer ───────────────────────────
        $display("\n--- C2: Single Data Transfer ---");
        m3_dst_ready = 1;  // destination always ready

        m3_src_data  = 32'hDEADBEEF;
        m3_src_valid = 1;
        @(posedge m3_src_clk); #1;
        m3_src_valid = 0;

        // Wait for data to appear at destination
        begin : c2_wait
            integer wait_cycles;
            for (wait_cycles = 0; wait_cycles < 30; wait_cycles = wait_cycles + 1) begin
                @(posedge m3_dst_clk); #1;
                if (m3_dst_valid) begin
                    disable c2_wait;
                end
            end
        end
        check(m3_dst_valid === 1'b1, "M3: dst_valid asserts for single transfer");
        check(m3_dst_data === 32'hDEADBEEF, "M3: data 0xDEADBEEF transferred correctly");

        // Wait for handshake to complete (src_ready goes back to 1)
        begin : c2_wait_ready
            integer wait_cycles;
            for (wait_cycles = 0; wait_cycles < 30; wait_cycles = wait_cycles + 1) begin
                @(posedge m3_src_clk); #1;
                if (m3_src_ready) disable c2_wait_ready;
            end
        end
        check(m3_src_ready === 1'b1, "M3: src_ready reasserts after transfer");

        // ── C3: Consecutive transfers ──────────────────────────
        $display("\n--- C3: Consecutive Transfers ---");
        m3_reset_n = 0;
        #200;
        m3_reset_n = 1;
        @(posedge m3_src_clk);
        m3_dst_ready = 1;

        begin : c3_block
            reg [31:0] sent_data [0:7];
            reg [31:0] recv_data [0:7];
            integer tx_idx, rx_idx;
            integer wait_cnt;
            reg all_match;
            reg src_wait_done, dst_wait_done;

            tx_idx = 0;
            rx_idx = 0;

            // Send 4 values with proper handshaking
            for (tx_idx = 0; tx_idx < 4; tx_idx = tx_idx + 1) begin
                sent_data[tx_idx] = (tx_idx + 1) * 32'h11111111;

                // Wait for src_ready
                src_wait_done = 0;
                for (wait_cnt = 0; wait_cnt < 50 && !src_wait_done; wait_cnt = wait_cnt + 1) begin
                    @(posedge m3_src_clk); #1;
                    if (m3_src_ready) begin
                        src_wait_done = 1;
                    end
                end

                m3_src_data  = sent_data[tx_idx];
                m3_src_valid = 1;
                @(posedge m3_src_clk); #1;
                m3_src_valid = 0;

                // Wait for dst_valid
                dst_wait_done = 0;
                for (wait_cnt = 0; wait_cnt < 50 && !dst_wait_done; wait_cnt = wait_cnt + 1) begin
                    @(posedge m3_dst_clk); #1;
                    if (m3_dst_valid) begin
                        recv_data[rx_idx] = m3_dst_data;
                        rx_idx = rx_idx + 1;
                        dst_wait_done = 1;
                    end
                end
            end

            $display("  Consecutive: sent %0d, received %0d", tx_idx, rx_idx);
            check(rx_idx == 4, "M3: All 4 consecutive transfers received");

            all_match = 1;
            for (i = 0; i < rx_idx && i < 4; i = i + 1) begin
                if (recv_data[i] !== sent_data[i]) begin
                    all_match = 0;
                    $display("  [WARN] Consec rx[%0d]: got 0x%08x, exp 0x%08x",
                             i, recv_data[i], sent_data[i]);
                end
            end
            check(all_match, "M3: All consecutive values match exactly");
        end

        // ── C4: Backpressure (dst_ready = 0) ───────────────────
        $display("\n--- C4: Backpressure ---");
        m3_reset_n = 0;
        #200;
        m3_reset_n = 1;
        @(posedge m3_src_clk);
        m3_dst_ready = 0;  // NOT ready

        // Send data
        m3_src_data  = 32'hCAFEBABE;
        m3_src_valid = 1;
        @(posedge m3_src_clk); #1;
        m3_src_valid = 0;

        // Wait — dst_valid should assert but data shouldn't be consumed
        begin : c4_wait_valid
            integer wait_cnt;
            for (wait_cnt = 0; wait_cnt < 30; wait_cnt = wait_cnt + 1) begin
                @(posedge m3_dst_clk); #1;
                if (m3_dst_valid) disable c4_wait_valid;
            end
        end
        check(m3_dst_valid === 1'b1, "M3: dst_valid asserts even without dst_ready");
        check(m3_dst_data === 32'hCAFEBABE, "M3: Correct data held during backpressure");

        // src should NOT be ready (busy with in-flight transfer)
        check(m3_src_ready === 1'b0, "M3: src_ready = 0 during backpressure");

        // Now assert dst_ready — transfer should complete
        m3_dst_ready = 1;
        begin : c4_wait_done
            integer wait_cnt;
            for (wait_cnt = 0; wait_cnt < 30; wait_cnt = wait_cnt + 1) begin
                @(posedge m3_src_clk); #1;
                if (m3_src_ready) disable c4_wait_done;
            end
        end
        check(m3_src_ready === 1'b1, "M3: src_ready reasserts after backpressure release");

        // ── C5: Data integrity with edge-case values ───────────
        $display("\n--- C5: Edge-Case Values ---");
        m3_reset_n = 0;
        #200;
        m3_reset_n = 1;
        @(posedge m3_src_clk);
        m3_dst_ready = 1;

        begin : c5_block
            reg [31:0] edge_vals [0:3];
            reg [31:0] edge_recv [0:3];
            integer tx_idx, rx_idx, wait_cnt;
            reg all_match;
            reg src_wait_done, dst_wait_done;

            edge_vals[0] = 32'h00000000;
            edge_vals[1] = 32'hFFFFFFFF;
            edge_vals[2] = 32'h80000000;
            edge_vals[3] = 32'h00000001;

            rx_idx = 0;

            for (tx_idx = 0; tx_idx < 4; tx_idx = tx_idx + 1) begin
                // Wait for src_ready
                src_wait_done = 0;
                for (wait_cnt = 0; wait_cnt < 50 && !src_wait_done; wait_cnt = wait_cnt + 1) begin
                    @(posedge m3_src_clk); #1;
                    if (m3_src_ready) src_wait_done = 1;
                end

                m3_src_data  = edge_vals[tx_idx];
                m3_src_valid = 1;
                @(posedge m3_src_clk); #1;
                m3_src_valid = 0;

                dst_wait_done = 0;
                for (wait_cnt = 0; wait_cnt < 50 && !dst_wait_done; wait_cnt = wait_cnt + 1) begin
                    @(posedge m3_dst_clk); #1;
                    if (m3_dst_valid) begin
                        edge_recv[rx_idx] = m3_dst_data;
                        rx_idx = rx_idx + 1;
                        dst_wait_done = 1;
                    end
                end
            end

            all_match = 1;
            for (i = 0; i < 4; i = i + 1) begin
                if (i < rx_idx && edge_recv[i] !== edge_vals[i]) begin
                    all_match = 0;
                    $display("  [WARN] Edge val[%0d]: got 0x%08x, exp 0x%08x",
                             i, edge_recv[i], edge_vals[i]);
                end
            end
            check(rx_idx == 4, "M3: All 4 edge-case values received");
            check(all_match, "M3: Edge-case values (0x0, 0xFFFF, 0x8000, 0x1) correct");
        end

        // ── C6: Port Connectivity + Reset Recovery ─────────────
        $display("\n--- C6: Port Connectivity + Reset Recovery ---");
        m3_reset_n = 0;
        m3_src_valid = 0;
        m3_dst_ready = 0;
        #200;
        m3_reset_n = 1;
        repeat (5) @(posedge m3_src_clk); #1;

        // Port connectivity: outputs should not be X or Z after reset
        check(m3_src_ready !== 1'bx, "M3: src_ready is not X after reset");
        check(m3_src_ready !== 1'bz, "M3: src_ready is not Z after reset");
        check(m3_dst_valid !== 1'bx, "M3: dst_valid is not X after reset");
        check(m3_dst_valid !== 1'bz, "M3: dst_valid is not Z after reset");
        check(m3_dst_data !== 32'hxxxxxxxx, "M3: dst_data is not X after reset");
        check(m3_dst_data !== 32'hzzzzzzzz, "M3: dst_data is not Z after reset");

        // Reset during active transfer: start a transfer, then assert reset mid-flight
        m3_dst_ready = 1;

        // Wait for src_ready
        begin : c6_wait_ready
            integer wait_cnt;
            for (wait_cnt = 0; wait_cnt < 30; wait_cnt = wait_cnt + 1) begin
                @(posedge m3_src_clk); #1;
                if (m3_src_ready) disable c6_wait_ready;
            end
        end

        m3_src_data  = 32'h12345678;
        m3_src_valid = 1;
        @(posedge m3_src_clk); #1;
        m3_src_valid = 0;

        // Wait a few cycles for transfer to be in-flight, then reset
        repeat (3) @(posedge m3_dst_clk);
        m3_reset_n = 0;
        #200;

        // Verify outputs are clean during reset
        check(m3_dst_valid === 1'b0, "M3: dst_valid = 0 after mid-transfer reset");

        // Release reset and verify recovery
        m3_reset_n = 1;
        @(posedge m3_src_clk);
        m3_dst_ready = 1;

        // Wait for src_ready to reassert (module recovered)
        begin : c6_recovery_wait
            integer wait_cnt;
            reg recovered;
            recovered = 0;
            for (wait_cnt = 0; wait_cnt < 50; wait_cnt = wait_cnt + 1) begin
                @(posedge m3_src_clk); #1;
                if (m3_src_ready) begin
                    recovered = 1;
                    disable c6_recovery_wait;
                end
            end
            check(recovered, "M3: src_ready reasserts after mid-transfer reset recovery");
        end

        // Verify a new transfer works after recovery
        m3_src_data  = 32'hABCD0000;
        m3_src_valid = 1;
        @(posedge m3_src_clk); #1;
        m3_src_valid = 0;

        begin : c6_post_reset_xfer
            integer wait_cnt;
            for (wait_cnt = 0; wait_cnt < 30; wait_cnt = wait_cnt + 1) begin
                @(posedge m3_dst_clk); #1;
                if (m3_dst_valid) disable c6_post_reset_xfer;
            end
        end
        check(m3_dst_valid === 1'b1, "M3: dst_valid asserts for post-recovery transfer");
        check(m3_dst_data === 32'hABCD0000, "M3: data 0xABCD0000 correct after reset recovery");

        // ════════════════════════════════════════════════════════
        // Summary
        // ════════════════════════════════════════════════════════
        $display("");
        $display("========================================");
        $display("  CDC MODULES TESTBENCH RESULTS");
        $display("  PASSED: %0d / %0d", pass_count, test_num);
        $display("  FAILED: %0d / %0d", fail_count, test_num);
        if (fail_count == 0)
            $display("  ** ALL TESTS PASSED **");
        else
            $display("  ** SOME TESTS FAILED **");
        $display("========================================");
        $display("");

        #100;
        $finish;
    end

endmodule