35.UART（通用异步收发传输器）-RS232（2）
（1）RS232接收模块visio框图：
（2）接收模块Verilog代码编写:
/*
常见波特率： 4800、9600、14400、115200
在系统时钟为50MHz时，对应计数为：
(1/4800) * 10^9 /20 -1 = 10416
(1/9600) * 10^9 /20 -1 = 5207
(1/14400) * 10^9 /20 -1 = 3471
(1/115200) * 10^9 /20 -1 = 433
*/module rs232_rx
(input [16:0] baud_set ,input clk ,input reset_n ,input rx ,input rx_start ,output reg[7:0] rx_data ,output reg rx_done );reg [15:0] BAUD_MCNT ;reg rx_reg0 ;reg rx_reg1 ;reg rx_reg2 ;reg en_baud_cnt ;reg [15:0] baud_cnt ;reg [3:0] bit_cnt ;reg [7:0] r_rx_data ;wire nedge ;wire w_rx_done ;//波特最大计数设计always@(posedge clk)begincase(baud_set)17'd4800 :BAUD_MCNT <= 16'd10416;17'd9600 :BAUD_MCNT <= 16'd5207;17'd14400 :BAUD_MCNT <= 16'd3471;17'd115200 :BAUD_MCNT <= 16'd433;default :BAUD_MCNT <= 16'd5207; //当输入baud_set为其他值时，一律当成9600处理。endcaseend//输入信号打拍处理always@(posedge clk)beginrx_reg0 <= rx;rx_reg1 <= rx_reg0;end//下降沿设计always@(posedge clk)rx_reg2 <= rx_reg1;assign nedge = (rx_reg2) && (!rx_reg1);//波特率计数使能信号设计 always@(posedge clk or negedge reset_n)if(!reset_n)en_baud_cnt <= 1'd0;else if(!rx_start)en_baud_cnt <= 1'd0;else if(nedge)en_baud_cnt <= 1'd1;else if((bit_cnt == 4'd0) && (baud_cnt == BAUD_MCNT/2)&&(rx_reg2)) //防止起始位只是一个抖动en_baud_cnt <= 1'd0;else if((bit_cnt == 4'd9) && (baud_cnt == BAUD_MCNT/2))en_baud_cnt <= 1'd1;else en_baud_cnt <= en_baud_cnt;//波特计数器模块设计always@(posedge clk or negedge reset_n)if(!reset_n)baud_cnt <= 16'd0;else if(!en_baud_cnt)baud_cnt <= 16'd0;else if(baud_cnt == BAUD_MCNT)baud_cnt <= 16'd0;else baud_cnt <= baud_cnt + 16'd1;//位计数器模块设计always@(posedge clk or negedge reset_n)if(!reset_n)bit_cnt <= 4'd0;else if((baud_cnt == BAUD_MCNT) &&(bit_cnt == 4'd9))bit_cnt <= 4'd0;else if(baud_cnt == BAUD_MCNT)bit_cnt <= bit_cnt + 4'd1;else bit_cnt <= bit_cnt;//rx_data设计always@(posedge clk or negedge reset_n)if(!reset_n)r_rx_data <= 8'd0;else if(baud_cnt == BAUD_MCNT/2)begincase(bit_cnt)4'd1: r_rx_data[0] <= rx_reg2;4'd2: r_rx_data[1] <= rx_reg2;4'd3: r_rx_data[2] <= rx_reg2;4'd4: r_rx_data[3] <= rx_reg2;4'd5: r_rx_data[4] <= rx_reg2;4'd6: r_rx_data[5] <= rx_reg2;4'd7: r_rx_data[6] <= rx_reg2;4'd8: r_rx_data[7] <= rx_reg2;default:r_rx_data <= r_rx_data;endcaseendelse r_rx_data <= r_rx_data;always@(posedge clk or negedge reset_n)if(!reset_n)rx_data <= 8'd0;else if(w_rx_done)rx_data <= r_rx_data;else rx_data <= rx_data;//w_rx_done和rx_done信号设计assign w_rx_done = (bit_cnt == 4'd9) && (baud_cnt == BAUD_MCNT/2) ;always@(posedge clk or negedge reset_n)if(!reset_n)rx_done <= 1'd0;else rx_done <= w_rx_done;endmodule
（3）仿真文件代码：
`timescale 1ns / 1psmodule rs232_tx_tb;reg [16:0] baud_set ;
reg clk ;
reg reset_n ;
reg rx ;
reg rx_start ;wire [7:0] rx_data ;
wire rx_done ;initial clk = 1'd1;always #10 clk = ~clk;initial begin baud_set <= 17'd9600;rx_start <= 1'd0;#200;rx_start <= 1'd1;reset_n <= 1'd0;rx <= 1'd1;#15;reset_n <= 1'd1;@(posedge clk)set_bit(8'd1 );set_bit(8'd3 );set_bit(8'd7 );set_bit(8'd15);$stop;endrs232_rx rs232_rx_inst
(.baud_set ( baud_set ) ,.clk ( clk ) ,.reset_n ( reset_n ) ,.rx ( rx ) ,.rx_start ( rx_start ) ,.rx_data ( rx_data ) ,.rx_done ( rx_done ) );task set_bit(input [7:0]data
);integer i;for(i=0;i<10;i=i+1)begincase(i)0: rx <= 1'd0;1: rx <= data[0];2: rx <= data[1];3: rx <= data[2];4: rx <= data[3];5: rx <= data[4];6: rx <= data[5];7: rx <= data[6];8: rx <= data[7];9: rx <= 1'd1;endcase#(5208 * 20); end
endtaskendmodule
（4）仿真波形：
