這部分加了接收模組，將接收和傳送模組一起進行模擬驗證

一、設計檔案

module uart_rx
#(
    parameter CLK_FRE = 50,      //clock frequency(Mhz)
    parameter BAUD_RATE = 115200 //serial baud rate ？？？？？？為什麼是序列波特率
)
(
    input                        clk,              //clock input
    input                        rst_n,            //asynchronous reset input, low active 
 

    output reg[7:0]              rx_8bit,          //received serial data
    output reg                   rx_done,    //received serial data is valid
    input                        rx_en,    //data receiver module ready
    input                        rx_1bit            //serial data input
);
//calculates the clock cycle for baud rate 
 

localparam                       CYCLE = CLK_FRE * 1000000 / BAUD_RATE;
//state machine code
localparam                       S_IDLE      = 1;
localparam                       S_START     = 2; //start bit
localparam                       S_REC_BYTE  = 3; //data bits
localparam                       S_STOP      = 4
 
; //stop bit
localparam                       S_DATA      = 5;

reg[2:0]                         state;
reg[2:0]                         next_state;
reg                              rx_d0;            //delay 1 clock for rx_1bit
reg                              rx_d1;            //delay 1 clock for rx_d0
wire                             rx_negedge;       //negedge of rx_1bit
reg[7:0]                         rx_bits;          //temporary storage of received data
reg[15:0]                        cycle_cnt;        //baud counter
reg[2:0]                         bit_cnt;          //bit counter

// 邊沿檢測
assign rx_negedge = rx_d1 && ~rx_d0;

// 非同步串列埠通訊：這裡需要將輸入訊號打2拍
// 目的：將輸入訊號與系統時鐘同步，防止出現亞穩態
always@(posedge clk or negedge rst_n)
begin
    if(rst_n == 1'b0)
    begin
        rx_d0 <= 1'b0;
        rx_d1 <= 1'b0;    
    end
    else
    begin
        rx_d0 <= rx_1bit;
        rx_d1 <= rx_d0;
    end
end

//第一段：同步時序，描述狀態轉移
always@(posedge clk or negedge rst_n)
begin
    if(rst_n == 1'b0)
        state <= S_IDLE;
    else
        state <= next_state;
end


//（*）第二段：組合邏輯判斷狀態轉移條件、描述狀態轉移規律
always@(*) 
begin
    case(state)
        S_IDLE:
            if(rx_negedge)
                next_state <= S_START;
            else
                next_state <= S_IDLE;
        S_START:
            if(cycle_cnt == CYCLE - 1)//one data cycle 
                next_state <= S_REC_BYTE;
            else
                next_state <= S_START;
        S_REC_BYTE:
            if(cycle_cnt == CYCLE - 1  && bit_cnt == 3'd7)  //receive 8bit data
                next_state <= S_STOP;
            else
                next_state <= S_REC_BYTE;
        S_STOP:
            if(cycle_cnt == CYCLE/2 - 1)//half bit cycle,to avoid missing the next byte receiver
                next_state <= S_DATA;
            else
                next_state <= S_STOP;
        S_DATA:
            if(rx_en)    //data receive complete
                next_state <= S_IDLE;
            else
                next_state <= S_DATA;
        default:
            next_state <= S_IDLE;
    endcase
end

always@(posedge clk or negedge rst_n)
begin
    if(rst_n == 1'b0)
        rx_done <= 1'b0;
    else if(state == S_STOP && next_state != state)
        rx_done <= 1'b1;
    else if(state == S_DATA && rx_en)
        rx_done <= 1'b0;
end


//第三段：同步時序，描述每個狀態的輸出
always@(posedge clk or negedge rst_n)
begin
    if(rst_n == 1'b0)
        rx_8bit <= 8'd0;
    else if(state == S_STOP && next_state != state)
        rx_8bit <= rx_bits;//latch received data
end

// 接收的資料計數位
always@(posedge clk or negedge rst_n)
begin
    if(rst_n == 1'b0)
        begin
            bit_cnt <= 3'd0;
        end
    else if(state == S_REC_BYTE)
        if(cycle_cnt == CYCLE - 1)
            bit_cnt <= bit_cnt + 3'd1;
        else
            bit_cnt <= bit_cnt;
    else
        bit_cnt <= 3'd0;
end

// 波特率計數器
always@(posedge clk or negedge rst_n)
begin
    if(rst_n == 1'b0)
        cycle_cnt <= 16'd0;
    else if((state == S_REC_BYTE && cycle_cnt == CYCLE - 1) || next_state != state)//next_state != state:狀態發生跳轉
        cycle_cnt <= 16'd0;
    else
        cycle_cnt <= cycle_cnt + 16'd1;    
end
//receive serial data bit data
always@(posedge clk or negedge rst_n)
begin
    if(rst_n == 1'b0)
        rx_bits <= 8'd0;
    else if(state == S_REC_BYTE && cycle_cnt == CYCLE/2 - 1)
        rx_bits[bit_cnt] <= rx_1bit;//串並轉換
    else
        rx_bits <= rx_bits; 
end
endmodule 

二、測試檔案

`timescale 1ns/1ns

// 第二步：測試接收模組能否將序列資料轉成並行資料

module tb_step_02;

reg clk;
reg rst_n;

// tx輸入
reg [7:0] tx_8bit ;
reg tx_en ;

// tx輸出
wire tx_1bit;
wire tx_done;

//// rx輸入
reg rx_1bit;
reg rx_en;

// rx輸出
wire [7:0]rx_8bit;
wire rx_done;

initial begin
clk = 1'b0;
rst_n = 1'b0;
#12;
rst_n = 1'b1;
begin 

tx_8bit = 8'd0;
tx_en = 1'b0;

// 產生輸入資料
# 1000;//延遲50個時鐘週期
tx_8bit = 8'b0100_1111;
tx_en = 1'b1;//tx_en拉高，傳送端開始接收資料，當tx_en拉高1000ns後將其拉低，等資料傳完後，等著tx_done拉高，就完成一個H的傳送
#1000;
tx_en = 1'b0;
@(posedge tx_done)

tx_8bit = 8'b0110_0101;
tx_en = 1'b1;
# 100000;
tx_en = 1'b0;

end

end

always #10 clk = ~ clk;


// 呼叫傳送模組
uart_tx uart_tx_inst
(
.clk(clk),              //clock input
.rst_n(rst_n),            //asynchronous reset input, low active 
.tx_8bit(tx_8bit),          //data to send
.tx_en(tx_en),    //data to be sent is valid

.tx_done(tx_done),    //send ready
.tx_1bit(tx_1bit)            //serial data output
);

uart_rx uart_rx_inst
(
    .clk(clk),              //clock input
    .rst_n(rst_n),            //asynchronous reset input, low active
    
    .rx_8bit(rx_8bit),          //received serial data
    .rx_done(rx_done),    //received serial data is valid
    
    .rx_en(tx_done),    //data receiver module ready
    .rx_1bit(tx_1bit)            //serial data input
);
endmodule 

三、波形圖