完整代码：

module johnson(
			clk,
			rst_n,
			led,
			sw1_n,
			sw2_n,
			sw3_n
		);

input clk;		                  //时钟信号，50MHz
input rst_n;	                  //复位信号，低电平有效
output[3:0] led;	              //LED控制，1--灭，0--亮
input sw1_n,sw2_n,sw3_n; 	      //三个独立按键，低表示按下

reg led_dir;	                  //1'b1--left,1'b0--right
reg led_on;	                      //1'b1--on,1'b0--off

//------------------------------------
reg[23:0] cnt24;

always @(posedge clk or negedge rst_n)
	if(!rst_n) cnt24 <= 24'd0;
	else cnt24 <= cnt24+1'b1;

//------------------------------------
reg[3:0] led_r;

always @(posedge clk or negedge rst_n)
	if(!rst_n) led_r <= 4'b0001;
	else if(cnt24 == 24'hffffff && led_on) begin
			if(led_dir) led_r <= {led_r[0],led_r[3:1]};	//left
			else led_r <= {led_r[2:0],led_r[3]};	//right
		end

assign led = led_r;
	

//------------------------------------
//---------------------------------------------------------------------------
reg[2:0] key_rst;  

always @(posedge clk  or negedge rst_n)
    if (!rst_n) key_rst <= 3'b111;
    else key_rst <= {sw3_n,sw2_n,sw1_n};

reg[2:0] key_rst_r;       //每个时钟周期的上升沿将low_sw信号锁存到low_sw_r中

always @ ( posedge clk  or negedge rst_n )
    if (!rst_n) key_rst_r <= 3'b111;
    else key_rst_r <= key_rst;
   
//当寄存器key_rst由1变为0时，led_an的值变为高，维持一个时钟周期 
wire[2:0] key_an = key_rst_r & (~key_rst);
/*
key_rst     1 1 1 0 0 1
~key_rst    0 0 0 1 1 0
key_rst_n     1 1 1 0 0 1
key_an        0 0 1 0 0
*/
//---------------------------------------------------------------------------
reg[19:0]  cnt;	//计数寄存器

always @ (posedge clk  or negedge rst_n)
    if (!rst_n) cnt <= 20'd0;	//异步复位
	else if(key_an) cnt <=20'd0;
    else cnt <= cnt + 1'b1;
  
reg[2:0] low_sw;

always @(posedge clk  or negedge rst_n)
    if (!rst_n) low_sw <= 3'b111;
    else if (cnt == 20'hfffff) 	//满20ms，将按键值锁存到寄存器low_sw中	 cnt == 20'hfffff
      low_sw <= {sw3_n,sw2_n,sw1_n};
      
//---------------------------------------------------------------------------
reg  [2:0] low_sw_r;       //每个时钟周期的上升沿将low_sw信号锁存到low_sw_r中

always @ ( posedge clk  or negedge rst_n )
    if (!rst_n) low_sw_r <= 3'b111;
    else low_sw_r <= low_sw;
/*          clk clk clk clk clk clk 
low_sw		111 111 111 110 110 110  
~low_sw     000 000 000 001 001 001
low_sw_r        111 111 111 110 110 110

led_ctrl	000 000 000 001 000 000 
   */
//当寄存器low_sw由1变为0时，led_ctrl的值变为高，维持一个时钟周期 
wire[2:0] led_ctrl = low_sw_r[2:0] & ( ~low_sw[2:0]);


  
always @ (posedge clk or negedge rst_n)
    if (!rst_n) begin
		led_on = 1'b0;
		led_dir = 1'b0;
      end
    else begin		//某个按键值变化时，LED将做亮灭翻转
        if ( led_ctrl[0] ) led_on <= ~led_on;	
        if ( led_ctrl[1] ) led_dir <= 1'b0;
        if ( led_ctrl[2] ) led_dir <= 1'b1;
      end



endmodule

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