18.按键消抖模块设计（使用状态机，独热码编码）
（1）设计意义：按键消抖主要针对的时机械弹性开关，当机械触点断开、闭合时，由于机械触点的弹性作用，一个按键开关在闭合时不会马上稳定地接通，在断开时也不会一下子就断开。因而在闭合以及断开的瞬间会伴随有一连串的抖动，为了保证系统正确的识别到按键的开关，就必须对按键的抖动进行处理，这就是按键消抖。
(2)Verilog实现代码：
module key_filter(clk,reset_n,key_in,key_p_flag,key_r_flag,key_state);input clk;input reset_n;input key_in;output reg key_p_flag;output reg key_r_flag;output reg key_state;reg key_in1;reg key_in2;reg key_in3;reg [3:0]STATE;
//抖动时间往往小于20ms，20ms = 20_000_000ns = 20ns * 1_000_000; 需要一个20位的寄存器reg [19:0]cnt;reg en_cnt;wire podge;wire nedge;wire arrive_time_20ms;//状态设计parameter IDLE = 4'b0001;parameter P_SHAKE = 4'b0010;parameter DOWN = 4'b0100;parameter R_SHAKE = 4'b1000;//异步输入key_in信号的同步化————“打两拍”always@(posedge clk)beginkey_in1 <= key_in;key_in2 <= key_in1;end//上升沿、下降沿设计always@(posedge clk)key_in3 <= key_in2;assign podge = key_in2 && (!key_in3);assign nedge = (!key_in2) && key_in3;//20ms计数器模块设计 always@(posedge clk or negedge reset_n)if(!reset_n)cnt <= 20'd0;else if(en_cnt &&(cnt == 20'd999_999))cnt <= 20'd0;else if(en_cnt)cnt <= cnt + 20'd1;else cnt <= 20'd0;//计满20ms信号设计 assign arrive_time_20ms = (cnt == 20'd999_999);//状态机主程序设计always@(posedge clk or negedge reset_n)if(!reset_n)beginkey_r_flag <= 1'd0;key_p_flag <= 1'd0;key_state <= 1'd1;STATE <= IDLE;endelse begincase(STATE)IDLE:beginkey_r_flag <= 1'd0;key_state <= 1'd1;if(nedge)beginSTATE <= P_SHAKE;en_cnt <= 1'd1;endelse STATE <= STATE;endP_SHAKE:beginif(arrive_time_20ms)beginSTATE <= DOWN;en_cnt <= 1'd0;key_p_flag <= 1'd1;key_state <= 1'd0;endelse if(podge)beginSTATE <= IDLE;en_cnt <= 1'd0;endelse STATE <= STATE; endDOWN:beginkey_p_flag <= 1'd0;key_state <= 1'd0;if(podge)beginSTATE <= R_SHAKE;en_cnt <= 1'd1;endelse STATE <= STATE; endR_SHAKE:beginif(arrive_time_20ms)beginSTATE <= IDLE;en_cnt <= 1'd0;key_r_flag <= 1'd1;key_state <= 1'd1;endelse if(nedge)beginSTATE <= DOWN;en_cnt <= 1'd0;endelse STATE <= STATE; enddefault:beginkey_r_flag <= 1'd0;key_p_flag <= 1'd0;key_state <= 1'd1;STATE <= IDLE;endendcaseendendmodule
(3)仿真文件代码：
`timescale 1ns / 1psmodule key_filter_tb;reg clk;reg reset_n;reg key_in;wire key_p_flag;wire key_r_flag;wire key_state;key_filter key_filter_inst(.clk(clk),.reset_n(reset_n),.key_in(key_in),.key_p_flag(key_p_flag),.key_r_flag(key_r_flag),.key_state(key_state));initial clk = 1'd1;always #10 clk = ~clk;initial beginreset_n <= 1'd0;key_in <= 1'd1;#15;reset_n <= 1'd1;#2000;key_in <= 1'd0;#1500;key_in <= 1'd1;#20000;key_in <= 1'd0;#40_000_000;key_in <= 1'd1;#1000;key_in <= 1'd0;#200;key_in <= 1'd1;#1800;key_in <= 1'd0;#25000;key_in <= 1'd1;#30_000_000;$stop;endendmodule
(4)仿真波形分析：
