key_debounce/rtl/key_debounce.v

module key_debounce(
    input   wire  sys_clk   ,
    input   wire  sys_rst_n ,
    input   wire  key_in    ,

    output  reg   key_out
);
parameter MAX = 20'd999_999;
reg [19: 0] cnt_20ms;
always @(posedge sys_clk or negedge sys_rst_n) begin
    if(!sys_rst_n)begin
        cnt_20ms <= 20'd0;
    end 
    else if(key_in == 1'b1)begin
        cnt_20ms <= 20'd0;
    end 
    else if((cnt_20ms == MAX) && (key_in == 1'b0))begin
        cnt_20ms <= cnt_20ms;
    end 
    else begin
        cnt_20ms <= cnt_20ms + 1'd1;
    end 
end

always @(posedge sys_clk or negedge sys_rst_n)begin
    if(!sys_rst_n)begin
        key_out <= 1'b0;
    end 
    else if(cnt_20ms == MAX - 1'd1)begin
        key_out <= 1'b1;
    end 
    else begin
        key_out <= 1'b0;
    end 
end 
endmodule