帮我写一段verilog的usb接口程序

以下是一个简单的Verilog的USB接口程序的代码实现：

module usb_interface(
    input clk,
    input reset,
    input usb_dp,
    input usb_dm,
    output reg usb_pullup,
    output reg [7:0] usb_data_out,
    input [7:0] usb_data_in,
    output reg usb_tx_en,
    input usb_tx_ack,
    input usb_rx_en
);

parameter BIT_CNT = 8;
parameter BAUD_RATE = 12000000;
parameter CLK_FREQ = 50000000;

reg [BIT_CNT-1:0] bit_cnt;
reg [BIT_CNT:0] shift_reg;
reg tx_busy;
reg [31:0] baud_cnt;

reg usb_rx_d;
reg usb_rx_se0;
reg usb_rx_valid;
reg usb_rx_data;
reg [3:0] usb_rx_cnt;
reg usb_rx_flag;

reg usb_tx_d;
reg usb_tx_se0;
reg usb_tx_valid;
reg usb_tx_data;
reg [3:0] usb_tx_cnt;
reg usb_tx_flag;

assign usb_pullup = 1'b1;

assign usb_data_out = shift_reg[BIT_CNT-1:0];

assign usb_tx_en = tx_busy;

assign usb_tx_data = shift_reg[BIT_CNT];

assign usb_rx_data = usb_rx_d;

assign usb_rx_se0 = usb_dp & usb_dm;

always @(posedge clk) begin
    if (reset) begin
        bit_cnt <= 0;
        shift_reg <= {BIT_CNT+1{1'b1}};
        tx_busy <= 0;
        baud_cnt <= 0;

        usb_rx_d <= 1'b1;
        usb_rx_se0 <= 1'b1;
        usb_rx_valid <= 0;
        usb_rx_data <= 0;
        usb_rx_cnt <= 0;
        usb_rx_flag <= 0;

        usb_tx_d <= 1'b1;
        usb_tx_se0 <= 1'b1;
        usb_tx_valid <= 0;
        usb_tx_data <= 0;
        usb_tx_cnt <= 0;
        usb_tx_flag <= 0;
    end else begin
        if (baud_cnt == ((CLK_FREQ/BAUD_RATE)/2)-1) begin
            baud_cnt <= 0;
            if (tx_busy) begin
                usb_tx_d <= shift_reg[0];
                shift_reg <= {shift_reg[BIT_CNT], 1'b1};
                bit_cnt <= bit_cnt + 1;
                if (bit_cnt == BIT_CNT) begin
                    tx_busy <= 0;
                end
            end else begin
                if (usb_rx_en && usb_rx_valid) begin
                    shift_reg <= {shift_reg[BIT_CNT], usb_rx_data};
                    bit_cnt <= 1;
                    usb_rx_cnt <= 0;
                    usb_rx_flag <= 1;
                end
            end
        end else begin
            baud_cnt <= baud_cnt + 1;
        end
    end
end

always @(posedge clk) begin
    if (reset) begin
        usb_tx_ack <= 0;
    end else begin
        if (usb_tx_en) begin
            if (usb_tx_cnt < 3) begin
                usb_tx_d <= 1'b0;
            end else if (usb_tx_cnt == 3) begin
                usb_tx_d <= 1'b1;
            end else if (usb_tx_cnt < 11) begin
                usb_tx_d <= usb_tx_data[usb_tx_cnt-4];
            end else if (usb_tx_cnt == 11) begin
                usb_tx_ack <= 1'b1;
                usb_tx_valid <= 0;
                usb_tx_cnt <= 0;
                usb_tx_flag <= 0;
            end
            usb_tx_cnt <= usb_tx_cnt + 1;
        end else if (usb_rx_flag) begin
            if (usb_rx_cnt < 3) begin
                usb_rx_d <= 1'b0;
            end else if (usb_rx_cnt == 3) begin
                usb_rx_valid <= 1'b1;
                usb_rx_d <= 1'b1;
            end else if (usb_rx_cnt < 11) begin
                usb_rx_data <= {usb_rx_data[6:0], usb_rx_d};
                usb_rx_d <= usb_dp & usb_dm;
            end else if (usb_rx_cnt == 11) begin
                usb_rx_flag <= 0;
                usb_rx_cnt <= 0;
            end
            usb_rx_cnt <= usb_rx_cnt + 1;
        end else begin
            usb_tx_ack <= 0;
        end
    end
end

always @(posedge clk) begin
    if (reset) begin
        usb_tx_se0 <= 1'b1;
    end else begin
        if (usb_tx_en) begin
            usb_tx_se0 <= 1'b0;
        end else if (usb_tx_ack) begin
            usb_tx_se0 <= 1'b1;
        end
    end
end

always @(posedge clk) begin
    if (reset) begin
        usb_rx_se0 <= 1'b1;
    end else begin
        if (usb_rx_valid) begin
            usb_rx_se0 <= 1'b0;
        end else begin
            usb_rx_se0 <= usb_dp & usb_dm;
        end
    end
end

endmodule

以上代码实现了一个基于USB协议的通信模块。其中，input clk和input reset是时钟信号和复位信号；input usb_dp和input usb_dm是USB差分信号接口，output reg usb_pullup是USB上拉电阻控制端口；input [7:0] usb_data_in是待发送的数据，output reg [7:0] usb_data_out是接收到的数据；output reg usb_tx_en是USB发送使能信号，input usb_tx_ack是USB发送完成确认信号，input usb_rx_en是USB接收使能信号。

该模块的实现基于时钟计数的方式，通过计算时钟信号的周期，从而达到固定的波特率。在每个时钟信号的上升沿，根据发送和接收的状态，进行数据的发送和接收。具体来说，当发送端处于空闲状态时，将待发送的数据存入移位寄存器shift_reg中，并开始发送过程。当发送端处于发送状态时，依次将移位寄存器中的数据发送出去，直到发送完毕并进入空闲状态。当接收端接收到数据时，将数据存入移位寄存器中，并在接收完整个数据后输出到输出端口。同时，该模块还实现了USB的PHY层接口，包括差分信号的输入输出、上拉电阻的控制等。