HDLBits 12-hour-clock答案

题目要求：使用8bit的2位16进制数表示时分秒，实现一个带am/pm指示的12小时制时钟

思路：

        先理清需要进位的情况，再用条件分情况对output信号进行赋值；

        需注意分和秒的进位方法，8'h09->8'h10，8'h19->8'h20...；小时的进位8'12->8'01；

解法：

module top_module(
    input clk,
    input reset,
    input ena,
    output pm,
    output [7:0] hh,
    output [7:0] mm,
    output [7:0] ss); 
    
    wire ss_carry = (ss == 8'h59) ? 1'b1:1'b0;
    wire mm_carry = (mm == 8'h59) ? 1'b1:1'b0;
    always @(posedge clk) begin
        if(reset) begin
            pm <= 1'b0;
            hh <= 8'h12;
            mm <= 8'h00;
            ss <= 8'h00;
        end else if (!ena) begin
            pm <= 1'b0;
            hh <= hh;
            mm <= mm;
            ss <= ss;
        end else begin
            if (!ss_carry) begin
                if (ss[3:0] == 4'h9) begin
                    pm <= pm;
                    hh <= hh;
                    mm <= mm;
                    ss <= {ss[7:4]+4'h1,4'h0};
                end else begin
                    pm <= pm;
                    hh <= hh;
                    mm <= mm;
                    ss <= ss + 8'h1;
                end
            end
            else if (ss_carry &!mm_carry) begin
                if (mm[3:0]==4'h9) begin
                    pm <= pm;
                    hh <= hh;
                    mm <= {mm[7:4]+4'h1,4'h0};
                    ss <= 8'h00;
                end else begin
                	pm <= pm;
                    hh <= hh;
                    mm <= mm + 8'h1;
                    ss <= 8'h00;
                end
            end
            else if (ss_carry & mm_carry) begin
                mm <= 8'h00;
                ss <= 8'h00;
                if (hh < 8'h09 | hh == 8'h10) begin
                    pm <= pm;
                    hh <= hh + 8'h1;
                end else if (hh == 8'h9) begin
                    pm <= pm;
                    hh <= 8'h10;
                end else if (hh == 8'h11) begin
                	pm <= ~pm;
                    hh <= 8'h12;
                end else begin
                    pm <= pm;
                    hh <= 8'h01;
                end
            end
           
        end
            
    end

endmodule