//---------方法一（我的写法）-----------------------------------------------

//--------------4999_9999+1=5000_0000----------------------------

reg [:] cnt_1s; //需要26位寄存器来放置4999_9999

    always @ (posedge clk or negedge rst_n)

    begin

    if(!rst_n)

        cnt_1s <= 'd0;

    else if(cnt_1s < 'd4999_9999) //需要减一

        cnt_1s <= cnt_1s + 'b1;

    else

        cnt_1s <= cnt_1s;

end

wire cnt_done1s = (cnt_1s == 'd4999_9999);

//---------需要加上使能信号的方法----------------------------------

reg [:] cnt_1s; //需要26位寄存器来放置4999_9999

    always @ (posedge clk or negedge rst_n)

    begin

    if(!rst_n)

        cnt_1s <= 'd0;

else if (delay_en)     //外界使能信号输入，启动计时

             begin

            if(cnt_1s < 'd4999_9999)

              cnt_1s <= cnt_1s + 'b1;

        else

        cnt_1s <= cnt_1s;

             end

end

wire cnt_done1s = (cnt_1s == 'd4999_9999);

//---------方法二------------------------------------------------

wire cnt_done1s;

reg [:] cnt_1s;

    always @ (posedge clk or negedge rst_n)

    begin

    if(!rst_n)

        cnt_1s <= 'd0;

    else if(cnt_1s < 'd4999_9999)

        cnt_1s <= cnt_1s + 'b1;

    else

        cnt_1s <= cnt_1s;

end

assign cnt_done1s = (cnt_1s == 'd4999_9999);

//------------方法三--------------------------------------

Parameter t_one = 4999_9999;     //需要减一

reg[:] delay_cnt;

always @(posedge clk or negedge rst_n)

    if(!rst_n)

      delay_cnt <= 'd0;

      if(delay_cnt == t_one)

        delay_cnt <= 'd0;

      else

        delay_cnt <= delay_cnt +'b1;

wire delay_done = (delay_cnt == t_one)