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`timescale 1ns / 1ps
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//说明：当三个独立按键的某一个被按下后，相应的LED被点亮；
//		再次按下后，LED熄灭，按键控制LED亮灭

module sw_debounce(
    		clk,rst_n,
			sw1_n,sw2_n,sw3_n,sw4_n,
	   		led_d1,led_d2,led_d3,led_d4,
    		);

input   clk;	//主时钟信号，50MHz
input   rst_n;	//复位信号，低有效
input   sw1_n,sw2_n,sw3_n,sw4_n; 	//三个独立按键，低表示按下
output  led_d1,led_d2,led_d3,led_d4;	//发光二极管，分别由按键控制

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

always @(posedge clk  or negedge rst_n)
    if (!rst_n) key_rst <= 3'b111;
    else key_rst <= {sw4_n,sw3_n,sw2_n,sw1_n};

reg[3:0] 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[3:0] key_an = key_rst_r & ( ~key_rst);

//---------------------------------------------------------------------------
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[3:0] 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 <= {sw4_n,sw3_n,sw2_n,sw1_n};
      
//---------------------------------------------------------------------------
reg  [3:0] 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;
   
//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[3:0] led_ctrl = low_sw_r[3:0] & ( ~low_sw[3:0]);

reg d1;
reg d2;
reg d3;
reg d4;
  
always @ (posedge clk or negedge rst_n)
    if (!rst_n) begin
        d1 <= 1'b0;
        d2 <= 1'b0;
        d3 <= 1'b0;
        d4 <= 1'b0;
      end
    else begin		//某个按键值变化时，LED将做亮灭翻转
        if ( led_ctrl[0] ) d1 <= ~d1;	
        if ( led_ctrl[1] ) d2 <= ~d2;
        if ( led_ctrl[2] ) d3 <= ~d3;
        if ( led_ctrl[3] ) d4 <= ~d4;
      end

assign led_d4 = d4 ? 1'b1 : 1'b0;
assign led_d3 = d1 ? 1'b1 : 1'b0;		//LED翻转输出
assign led_d2 = d2 ? 1'b1 : 1'b0;
assign led_d1 = d3 ? 1'b1 : 1'b0;
  
endmodule