HDLBits—Count clock

创建12小时制计数器。

当ena使能时（注意：复位最高优先级），时钟正常工作，否则始终所有状态不变

正常工作时，高电平同步复位，复位为12：00：00am，am时pm为0。

一个时钟秒加一，11：59：59时，下一个状态pm进行翻转，同时要设置计数器的进位

module top_module(
    input clk,
    input reset,
    input ena,
    output pm,
    output [7:0] hh,
    output [7:0] mm,
    output [7:0] ss); 
    wire s0_reset,s1_reset,m0_reset,m1_reset,s0_s1,s1_m0,m0_m1,h0_in,h1_in;
    wire[2:0] change;
    assign s0_reset = (ena&ss[3:0]==9)?(1):(reset);
    assign s1_reset = (ena&ss[7:4]==5&ss[3:0]==9)?(1):(reset);
    assign m0_reset = (ena&mm[3:0]==9&s1_m0)?(1):(reset);
    assign m1_reset = (ena&mm[7:4]==5&m0_m1)?(1):(reset);
    assign s0_s1 = (ss[3:0]==9)?(1):(0);
    assign s1_m0 = (ss[7:4]==5&ss[3:0]==9)?(1):(0);
    assign m0_m1 = (mm[3:0]==9&s1_m0)?(1):(0);
    assign h0_in = (mm[7:4]==5&m0_m1)?(1):(0);
    assign h1_in = (hh[3:0]==9&h0_in)?(1):(0);
    assign change = (hh[7:4]==1&hh[3:0]==2&h0_in)?(1):((hh[7:4]==1&hh[3:0]==1&h0_in)?(2):((hh[3:0]==9&h0_in)?(3):(0)));
    nine_zero s0(clk,s0_reset,0,1,ena,ss[3:0]);
    nine_zero s1(clk,s1_reset,0,s0_s1,ena,ss[7:4]);
    nine_zero m0(clk,m0_reset,0,s1_m0,ena,mm[3:0]);
    nine_zero m1(clk,m1_reset,0,m0_m1,ena,mm[7:4]);
    
    always@(posedge clk)begin
        if(reset)begin
            hh[3:0]<=2;
            hh[7:4]<=1;
            pm<=0;
        end
        else begin
            case (change)
            	2'b00:begin
                    hh[3:0] = hh[3:0] + h0_in;
            		hh[7:4] = hh[7:4] + h1_in;
                end
                2'b01:begin
                    hh[3:0]<=1;
                	hh[7:4]<=0;
                end
                2'b10:begin
                    pm<=~pm; 
                    hh[3:0]<=2;
                    hh[7:4]<=1;
                end
                2'b11:begin
                    hh[3:0]<=0;
                    hh[7:4]<=1;
                end
            endcase
        end
    end
endmodule

module nine_zero(input clk,input reset,input reset_to,input in,input ena,output reg[3:0] out);
    always@(posedge clk)begin
        if(reset)begin
        	out<=reset_to;
        end
        else begin
            if(!ena)begin
                out<=out; 
            end
            else begin
                out<=out+in;     
            end   
        end 
    end
endmodule