实验02：按键控制LED（key_led）

module key_led(
    input            clk,
    input            rst,
    input      [3:0] key,
    output reg [3:0] led
);
reg [23:0] counter;
reg [1:0]  led_switch;
parameter TIME = 24'd10_000_000;    //0.2s
// counter 计数0.2秒
always @(posedge clk or negedge rst) begin
    if(!rst)
        counter <= 24'd0;
    else if(counter > TIME)
        counter <= 24'd0;
    else
        counter <= counter + 1'b1;
end
// led_switch 状态计数器切换
always @(posedge clk or negedge rst) begin
    if(!rst)
        led_switch <= 2'd0;
    else if(counter == TIME) begin
        led_switch <= led_switch + 1'b1;
//        if(led_switch == 2'b11)     // led_switch溢出复位
//            led_switch <= 2'b00;
//        else
//            led_switch <= led_switch;
    end
    else
        led_switch <= led_switch;
end
// 按键控制 LED 状态
always @(posedge clk or negedge rst) begin
    if(!rst)
        led[3:0] <= 4'b0000;
    else begin
        if(key[0] == 0) begin   // KEY1:自右向左流水灯
            case(led_switch)
                2'b00:  led[3:0] <= 4'b1000;
                2'b01:  led[3:0] <= 4'b0100;
                2'b10:  led[3:0] <= 4'b0010;
                2'b11:  led[3:0] <= 4'b0001;
                default:led[3:0] <= 4'b0000;
            endcase
        end
        else if(key[1] == 0) begin   // KEY2:自左向右流水灯
            case(led_switch)
                2'b00:  led[3:0] <= 4'b0001;
                2'b01:  led[3:0] <= 4'b0010;
                2'b10:  led[3:0] <= 4'b0100;
                2'b11:  led[3:0] <= 4'b1000;
                default:led[3:0] <= 4'b0000;
            endcase
        end
        else if(key[2] == 0) begin   // KEY3:四灯同时闪烁
            case(led_switch)
                2'b00:  led[3:0] <= 4'b1111;
                2'b01:  led[3:0] <= 4'b0000;
                2'b10:  led[3:0] <= 4'b1111;
                2'b11:  led[3:0] <= 4'b0000;
                default:led[3:0] <= 4'b0000;
            endcase
        end
        else if(key[3] == 0) begin   // KEY4:四灯全亮
            led[3:0] <= 4'b1111;
        end
        else begin
            led[3:0] <= 4'b0000;     // 默认四灯全灭
        end
    end
end
endmodule