写一个spi的verilog代码

以下是一个基本的SPI的Verilog代码，实现了一个简单的主机模式的SPI通信。

module spi_master(
input clk,
input rstn,
input ssel,
output mosi,
input miso,
output reg sclk
);

//状态定义
parameter IDLE = 2'b00;
parameter TRANSFER = 2'b01;

//状态寄存器和计数器
reg [1:0] state;
reg [3:0] count;

//发送数据和接收数据寄存器
reg [7:0] tx_data;
reg [7:0] rx_data;

//初始化
initial begin
state <= IDLE;
count <= 4'b0000;
tx_data <= 8'b00000000;
rx_data <= 8'b00000000;
sclk <= 1'b0;
end

//时钟上升沿
always @(posedge clk) begin
if (!rstn) begin
state <= IDLE;
count <= 4'b0000;
tx_data <= 8'b00000000;
rx_data <= 8'b00000000;
sclk <= 1'b0;
end else begin
case (state)
IDLE: begin
if (!ssel) begin
state <= TRANSFER;
count <= 4'b0001;
sclk <= 1'b0;
end
end
TRANSFER: begin
case (count)
4'b0001: begin
mosi <= tx_data[7];
count <= count + 1;
end
4'b0010: begin
mosi <= tx_data[6];
count <= count + 1;
end
4'b0100: begin
mosi <= tx_data[5];
count <= count + 1;
end
4'b1000: begin
mosi <= tx_data[4];
count <= count + 1;
end
4'b0001_0000: begin
rx_data[3] <= miso;
mosi <= tx_data[3];
count <= count + 1;
end
4'b0010_0000: begin
rx_data[2] <= miso;
mosi <= tx_data[2];
count <= count + 1;
end
4'b0100_0000: begin
rx_data[1] <= miso;
mosi <= tx_data[1];
count <= count + 1;
end
4'b1000_0000: begin
rx_data[0] <= miso;
mosi <= tx_data[0];
count <= count + 1;
end
4'b1111_0000: begin
sclk <= 1'b1;
count <= count + 1;
end
4'b1111_0001: begin
sclk <= 1'b0;
count <= 4'b0000;
state <= IDLE;
end
endcase
end
endcase
end
end

//数据输出
assign mosi = (state == TRANSFER) ? tx_data[count-1] : 1'b0;

endmodule