apb总线的usart verilog代码

以下是一个简单的APB总线上USART模块的Verilog代码示例：

module apb_usart (
  input        apb_clk,
  input        apb_resetn,
  input        apb_psel,
  input        apb_penable,
  input [31:0] apb_paddr,
  input [31:0] apb_pwdata,
  output [31:0] apb_prdata,
  output       apb_pready,
  output       apb_pslverr,
  output       tx,
  input        rx
);

// 内部寄存器
reg [31:0] baud_rate;
reg [31:0] data_reg;
reg         tx_busy;

// 状态机状态
parameter IDLE    = 2'b00;
parameter START   = 2'b01;
parameter DATA    = 2'b10;
parameter STOP    = 2'b11;

// APB总线状态机状态
parameter IDLE_ST = 3'b000;
parameter ADDR_ST = 3'b001;
parameter DATA_ST = 3'b010;
parameter RESP_ST = 3'b011;

// APB总线状态机变量
reg [2:0] apb_state;
reg [31:0] apb_addr;
reg [31:0] apb_data;

// 状态机变量
reg [1:0] state;
reg [7:0] data_byte;
reg [3:0] bit_count;

// APB总线响应
parameter OKAY    = 2'b00;
parameter ERROR   = 2'b10;
reg [1:0] apb_resp;

// APB总线响应时间
parameter RESP_TIME = 2;

// 初始值
initial begin
  baud_rate <= 9600;
  state <= IDLE;
  apb_state <= IDLE_ST;
  apb_resp <= OKAY;
  tx_busy <= 1'b0;
  data_reg <= 0;
  apb_prdata <= 0;
  apb_pready <= 1'b1;
  apb_pslverr <= 1'b0;
end

// APB总线状态机
always @(posedge apb_clk) begin
  case (apb_state)
    IDLE_ST: begin
      if (apb_psel && apb_penable && !apb_pslverr) begin
        apb_state <= ADDR_ST;
        apb_addr <= apb_paddr;
      end
    end
    ADDR_ST: begin
      if (apb_penable && !apb_pslverr) begin
        apb_state <= DATA_ST;
        if (apb_paddr == 32'h0) begin
          apb_data <= baud_rate;
        end
        else if (apb_paddr == 32'h4) begin
          apb_data <= data_reg;
        end
        else if (apb_paddr == 32'h8) begin
          apb_data <= tx_busy;
        end
        else begin
          apb_data <= 32'h0;
          apb_resp <= ERROR;
        end
      end
    end
    DATA_ST: begin
      if (!apb_penable || apb_pslverr) begin
        apb_state <= RESP_ST;
      end
      else begin
        if (apb_paddr == 32'h0) begin
          baud_rate <= apb_pwdata;
        end
        else if (apb_paddr == 32'h4) begin
          data_reg <= apb_pwdata;
        end
        else if (apb_paddr == 32'h8) begin
          tx_busy <= apb_pwdata;
        end
        apb_data <= apb_pwdata;
      end
    end
    RESP_ST: begin
      apb_resp <= OKAY;
      apb_state <= IDLE_ST;
    end
  endcase
end

// 状态机
always @(posedge apb_clk) begin
  case (state)
    IDLE: begin
      if (!tx_busy && data_reg != 0) begin
        tx <= 1'b0;
        state <= START;
        bit_count <= 0;
        data_byte <= data_reg[7:0];
      end
      else begin
        tx <= 1'b1;
      end
    end
    START: begin
      tx <= 1'b0;
      if (bit_count == 0) begin
        state <= DATA;
      end
      bit_count <= bit_count + 1;
    end
    DATA: begin
      tx <= data_byte[0];
      data_byte <= {data_byte[6:0], 1'b0};
      bit_count <= bit_count + 1;
      if (bit_count == 9) begin
        state <= STOP;
      end
    end
    STOP: begin
      tx <= 1'b1;
      state <= IDLE;
      data_reg <= 0;
    end
  endcase
end

// 接收器
always @(posedge apb_clk) begin
  if (rx) begin
    data_reg <= {data_reg[6:0], 1'b1};
  end
end

// APB总线接口
assign apb_prdata = (apb_state == DATA_ST) ? apb_data : 32'h0;
assign apb_pready = (apb_state == IDLE_ST) ? 1'b1 : 1'b0;
assign apb_pslverr = 1'b0;

endmodule

这是一个简单的APB总线上USART模块，实现了数据的发送和接收。在代码中，APB总线和USART模块使用两个不同的状态机来处理它们的操作。USART模块使用一个状态机来处理数据的发送和接收，APB总线使用一个状态机来处理读写操作。总体而言，这个模块可以被用来与其他APB总线设备进行通信，并通过USART发送和接收数据。