module usb_data_interface (
    input wire clk,              // Main clock (100MHz recommended)
    input wire reset_n,
    
    // Radar data inputs
    input wire [31:0] range_profile,
    input wire range_valid,
    input wire [15:0] doppler_real,
    input wire [15:0] doppler_imag,
    input wire doppler_valid,
    input wire cfar_detection,
    input wire cfar_valid,
    
    // FT601 Interface (Slave FIFO mode)
    // Data bus
    inout wire [31:0] ft601_data,    // 32-bit bidirectional data bus
    output reg [3:0] ft601_be,       // Byte enable (4 lanes for 32-bit mode)
    
    // Control signals
    output reg ft601_txe_n,          // Transmit enable (active low)
    output reg ft601_rxf_n,          // Receive enable (active low)
    input wire ft601_txe,             // Transmit FIFO empty
    input wire ft601_rxf,             // Receive FIFO full
    output reg ft601_wr_n,            // Write strobe (active low)
    output reg ft601_rd_n,            // Read strobe (active low)
    output reg ft601_oe_n,            // Output enable (active low)
    output reg ft601_siwu_n,          // Send immediate / Wakeup
    
    // FIFO flags
    input wire [1:0] ft601_srb,       // Selected read buffer
    input wire [1:0] ft601_swb,       // Selected write buffer
    
    // Clock
    output wire ft601_clk_out,        // Output clock to FT601 (forwarded via ODDR)
    input wire ft601_clk_in           // Clock from FT601 (60/100MHz)
);

// USB packet structure (same as before)
localparam HEADER = 8'hAA;
localparam FOOTER = 8'h55;

// FT601 configuration
localparam FT601_DATA_WIDTH = 32;
localparam FT601_BURST_SIZE = 512;    // Max burst size in bytes

// State definitions (Verilog-2001 compatible)
localparam [2:0] IDLE                = 3'd0,
                 SEND_HEADER         = 3'd1,
                 SEND_RANGE_DATA     = 3'd2,
                 SEND_DOPPLER_DATA   = 3'd3,
                 SEND_DETECTION_DATA = 3'd4,
                 SEND_FOOTER         = 3'd5,
                 WAIT_ACK            = 3'd6;

reg [2:0] current_state;
reg [7:0] byte_counter;
reg [31:0] data_buffer;
reg [31:0] ft601_data_out;
reg ft601_data_oe;  // Output enable for bidirectional data bus

// ========== CDC INPUT SYNCHRONIZERS (clk domain -> ft601_clk_in domain) ==========
// The valid signals arrive from clk_100m but the state machine runs on ft601_clk_in.
// Even though both are 100 MHz, they are asynchronous clocks and need synchronization.

// 2-stage synchronizers for valid signals
(* ASYNC_REG = "TRUE" *) reg [1:0] range_valid_sync;
(* ASYNC_REG = "TRUE" *) reg [1:0] doppler_valid_sync;
(* ASYNC_REG = "TRUE" *) reg [1:0] cfar_valid_sync;

// Synchronized data captures (registered in ft601_clk_in domain)
reg [31:0] range_profile_cap;
reg [15:0] doppler_real_cap;
reg [15:0] doppler_imag_cap;
reg cfar_detection_cap;

wire range_valid_ft;
wire doppler_valid_ft;
wire cfar_valid_ft;

always @(posedge ft601_clk_in or negedge reset_n) begin
    if (!reset_n) begin
        range_valid_sync   <= 2'b00;
        doppler_valid_sync <= 2'b00;
        cfar_valid_sync    <= 2'b00;
        range_profile_cap  <= 32'd0;
        doppler_real_cap   <= 16'd0;
        doppler_imag_cap   <= 16'd0;
        cfar_detection_cap <= 1'b0;
    end else begin
        // Synchronize valid strobes
        range_valid_sync   <= {range_valid_sync[0],   range_valid};
        doppler_valid_sync <= {doppler_valid_sync[0], doppler_valid};
        cfar_valid_sync    <= {cfar_valid_sync[0],    cfar_valid};

        // Capture data on rising edge of synchronized valid
        if (range_valid_sync[0] && !range_valid_sync[1])
            range_profile_cap <= range_profile;
        if (doppler_valid_sync[0] && !doppler_valid_sync[1]) begin
            doppler_real_cap <= doppler_real;
            doppler_imag_cap <= doppler_imag;
        end
        if (cfar_valid_sync[0] && !cfar_valid_sync[1])
            cfar_detection_cap <= cfar_detection;
    end
end

// Rising-edge detect on synchronized valid (pulse in ft601_clk_in domain)
assign range_valid_ft   = range_valid_sync[0]   && !range_valid_sync[1];
assign doppler_valid_ft = doppler_valid_sync[0]  && !doppler_valid_sync[1];
assign cfar_valid_ft    = cfar_valid_sync[0]     && !cfar_valid_sync[1];

// FT601 data bus direction control
assign ft601_data = ft601_data_oe ? ft601_data_out : 32'hzzzz_zzzz;

always @(posedge ft601_clk_in or negedge reset_n) begin
    if (!reset_n) begin
        current_state <= IDLE;
        byte_counter <= 0;
        ft601_data_out <= 0;
        ft601_data_oe <= 0;
        ft601_be <= 4'b1111;  // All bytes enabled for 32-bit mode
        ft601_txe_n <= 1;
        ft601_rxf_n <= 1;
        ft601_wr_n <= 1;
        ft601_rd_n <= 1;
        ft601_oe_n <= 1;
        ft601_siwu_n <= 1;
        // NOTE: ft601_clk_out is driven by the clk-domain always block below.
        // Do NOT assign it here (ft601_clk_in domain) — causes multi-driven net.
    end else begin
        case (current_state)
            IDLE: begin
                ft601_wr_n <= 1;
                ft601_data_oe <= 0;  // Release data bus
                if (range_valid_ft || doppler_valid_ft || cfar_valid_ft) begin
                    current_state <= SEND_HEADER;
                    byte_counter <= 0;
                end
            end
            
            SEND_HEADER: begin
                if (!ft601_txe) begin  // FT601 TX FIFO not empty
                    ft601_data_oe <= 1;
                    ft601_data_out <= {24'b0, HEADER};
                    ft601_be <= 4'b0001;  // Only lower byte valid
                    ft601_wr_n <= 0;     // Assert write strobe
                    current_state <= SEND_RANGE_DATA;
                end
            end
            
            SEND_RANGE_DATA: begin
                if (!ft601_txe) begin
                    ft601_data_oe <= 1;
                    ft601_be <= 4'b1111;  // All bytes valid for 32-bit word
                    
                    case (byte_counter)
                        0: ft601_data_out <= range_profile_cap;
                        1: ft601_data_out <= {range_profile_cap[23:0], 8'h00};
                        2: ft601_data_out <= {range_profile_cap[15:0], 16'h0000};
                        3: ft601_data_out <= {range_profile_cap[7:0], 24'h000000};
                    endcase
                    
                    ft601_wr_n <= 0;
                    
                    if (byte_counter == 3) begin
                        byte_counter <= 0;
                        current_state <= SEND_DOPPLER_DATA;
                    end else begin
                        byte_counter <= byte_counter + 1;
                    end
                end
            end
            
            SEND_DOPPLER_DATA: begin
                if (!ft601_txe && doppler_valid_ft) begin
                    ft601_data_oe <= 1;
                    ft601_be <= 4'b1111;
                    
                    case (byte_counter)
                        0: ft601_data_out <= {doppler_real_cap, doppler_imag_cap};
                        1: ft601_data_out <= {doppler_imag_cap, doppler_real_cap[15:8], 8'h00};
                        2: ft601_data_out <= {doppler_real_cap[7:0], doppler_imag_cap[15:8], 16'h0000};
                        3: ft601_data_out <= {doppler_imag_cap[7:0], 24'h000000};
                    endcase
                    
                    ft601_wr_n <= 0;
                    
                    if (byte_counter == 3) begin
                        byte_counter <= 0;
                        current_state <= SEND_DETECTION_DATA;
                    end else begin
                        byte_counter <= byte_counter + 1;
                    end
                end
            end
            
            SEND_DETECTION_DATA: begin
                if (!ft601_txe && cfar_valid_ft) begin
                    ft601_data_oe <= 1;
                    ft601_be <= 4'b0001;
                    ft601_data_out <= {24'b0, 7'b0, cfar_detection_cap};
                    ft601_wr_n <= 0;
                    current_state <= SEND_FOOTER;
                end
            end
            
            SEND_FOOTER: begin
                if (!ft601_txe) begin
                    ft601_data_oe <= 1;
                    ft601_be <= 4'b0001;
                    ft601_data_out <= {24'b0, FOOTER};
                    ft601_wr_n <= 0;
                    current_state <= WAIT_ACK;
                end
            end
            
            WAIT_ACK: begin
                ft601_wr_n <= 1;
                ft601_data_oe <= 0;  // Release data bus
                current_state <= IDLE;
            end
        endcase
    end
end

// ============================================================================
// FT601 clock output forwarding
// ============================================================================
// Forward ft601_clk_in back out via ODDR so that the forwarded clock at the
// pin has the same insertion delay as the data outputs (both go through the
// same BUFG). This makes the output delay analysis relative to the generated
// clock at the pin, where insertion delays cancel.

`ifndef SIMULATION
ODDR #(
    .DDR_CLK_EDGE("OPPOSITE_EDGE"),
    .INIT(1'b0),
    .SRTYPE("SYNC")
) oddr_ft601_clk (
    .Q(ft601_clk_out),
    .C(ft601_clk_in),
    .CE(1'b1),
    .D1(1'b1),
    .D2(1'b0),
    .R(1'b0),
    .S(1'b0)
);
`else
// Simulation: behavioral clock forwarding
reg ft601_clk_out_sim;
always @(posedge ft601_clk_in or negedge reset_n) begin
    if (!reset_n)
        ft601_clk_out_sim <= 1'b0;
    else
        ft601_clk_out_sim <= 1'b1;
end
// In simulation, just pass the clock through
assign ft601_clk_out = ft601_clk_in;
`endif

endmodule