verilog写spi三线制

以下是使用Verilog实现SPI三线制的代码示例：

module spi_master(
    input clk,
    input rst_n,
    output reg cs_n,
    output reg sclk,
    output reg mosi,
    input miso
);

parameter IDLE = 2'b00;
parameter READ = 2'b01;
parameter WRITE = 2'b10;

reg [1:0] state;
reg [7:0] tx_data;
reg [7:0] rx_data;
reg [2:0] bit_cnt;

assign miso = (state == READ) ? rx_data[bit_cnt] : 1'bZ;

always @(posedge clk or negedge rst_n) begin
    if (~rst_n) begin
        state <= IDLE;
        cs_n <= 1'b1;
        sclk <= 1'b0;
        mosi <= 1'b0;
        bit_cnt <= 0;
        tx_data <= 8'h00;
        rx_data <= 8'h00;
    end else begin
        case (state)
            IDLE: begin
                if (cs_n == 1'b0) begin
                    state <= WRITE;
                    sclk <= 1'b0;
                    mosi <= 1'b0;
                    bit_cnt <= 0;
                end
            end
            READ: begin
                sclk <= ~sclk;
                bit_cnt <= bit_cnt + 1;
                if (bit_cnt == 7) begin
                    state <= IDLE;
                    cs_n <= 1'b1;
                end
            end
            WRITE: begin
                sclk <= ~sclk;
                if (bit_cnt == 7) begin
                    state <= READ;
                    cs_n <= 1'b1;
                    bit_cnt <= 0;
                end else begin
                    mosi <= tx_data[bit_cnt];
                    bit_cnt <= bit_cnt + 1;
                end
            end
        endcase
    end
end

// 用于启动SPI传输
task spi_transfer;
    input [7:0] data;
    begin
        cs_n <= 1'b0;
        tx_data <= data;
        state <= WRITE;
    end
endtask

endmodule

该代码实现了一个SPI主设备，包括时钟信号`clk`、复位信号`rst_n`、片选信号`cs_n`、时钟信号`sclk`、主设备输出信号`mosi`和主设备输入信号`miso`。其中，`spi_transfer`任务用于启动SPI传输，`data`参数为要发送的数据。