在Basys3板子上实现数字时钟，要求可以进行时、分、秒计时的二十四小时制并具有闹钟和暂停功能的数字电子钟，代码怎么实现

以下是基于Verilog语言的代码实现，可以在Vivado中进行编译和综合：

module digital_clock(
    input clk, //时钟信号
    input rst, //复位信号
    input [3:0]btn, //按键信号
    output reg [6:0]seg, //数码管输出信号
    output reg [3:0]an //数码管选择信号
);

reg [3:0]hour = 0; //小时数，4位二进制数
reg [3:0]min = 0; //分钟数，4位二进制数
reg [3:0]sec = 0; //秒钟数，4位二进制数
reg [3:0]alarm_hour = 0; //闹钟小时数，4位二进制数
reg [3:0]alarm_min = 0; //闹钟分钟数，4位二进制数
reg [3:0]alarm_sec = 0; //闹钟秒钟数，4位二进制数
reg [1:0]pause = 0; //暂停状态，2位二进制数
reg [1:0]alarm_set = 0; //闹钟设置状态，2位二进制数
reg [1:0]alarm_on = 0; //闹钟开关状态，2位二进制数
reg [1:0]alarm_ring = 0; //闹钟响铃状态，2位二进制数

//时钟模块
always @(posedge clk or negedge rst) begin
    if(!rst) begin
        hour <= 0;
        min <= 0;
        sec <= 0;
    end else begin
        if(!pause[0]) begin //秒钟计时
            sec <= sec + 1;
            if(sec >= 60) begin
                sec <= 0;
                min <= min + 1;
                if(min >= 60) begin
                    min <= 0;
                    hour <= hour + 1;
                    if(hour >= 24) begin
                        hour <= 0;
                    end
                end
            end
        end
        if(btn[0] && !pause[0]) begin //暂停计时
            pause <= 1;
        end
        if(btn[0] && pause[0]) begin //继续计时
            pause <= 0;
        end
    end
end

//闹钟模块
always @(posedge clk or negedge rst) begin
    if(!rst) begin
        alarm_hour <= 0;
        alarm_min <= 0;
        alarm_sec <= 0;
    end else begin
        if(btn[1] && !alarm_set[0]) begin //设置闹钟
            alarm_set <= 1;
        end
        if(alarm_set[0] && !btn[1]) begin //保存闹钟时间
            alarm_hour <= hour;
            alarm_min <= min;
            alarm_sec <= sec;
            alarm_set <= 0;
        end
        if(btn[2] && !alarm_on[0]) begin //开启闹钟
            alarm_on <= 1;
        end
        if(btn[2] && alarm_on[0]) begin //关闭闹钟
            alarm_on <= 0;
            alarm_ring <= 0;
        end
        if(alarm_on[0] && !alarm_ring[0] && hour == alarm_hour && min == alarm_min && sec == alarm_sec) begin //闹钟响铃
            alarm_ring <= 1;
        end
    end
end

//数码管驱动模块
always @(hour, min, sec, alarm_hour, alarm_min, alarm_sec, pause, alarm_ring) begin
    if(pause[1]) begin //显示暂停状态
        seg <= 7'b1111110;
        an <= 4'b1110;
    end else if(alarm_ring[1]) begin //闹钟响铃状态
        seg <= 7'b0001111;
        an <= 4'b1110;
    end else begin //正常显示时、分、秒
        case(an)
            4'b1110: begin //选择第一个数码管
                case({hour[2], hour[1], hour[0]})
                    3'b000: seg <= 7'b1000000; //0
                    3'b001: seg <= 7'b1111001; //1
                    3'b010: seg <= 7'b0100100; //2
                    3'b011: seg <= 7'b0110000; //3
                    3'b100: seg <= 7'b0011001; //4
                    3'b101: seg <= 7'b0010010; //5
                    3'b110: seg <= 7'b0000010; //6
                    3'b111: seg <= 7'b1111000; //7
                endcase
            end
            4'b1101: begin //选择第二个数码管
                case({hour[3], min[2], min[1], min[0]})
                    4'b0000: seg <= 7'b1000000; //0
                    4'b0001: seg <= 7'b1111001; //1
                    4'b0010: seg <= 7'b0100100; //2
                    4'b0011: seg <= 7'b0110000; //3
                    4'b0100: seg <= 7'b0011001; //4
                    4'b0101: seg <= 7'b0010010; //5
                    4'b0110: seg <= 7'b0000010; //6
                    4'b0111: seg <= 7'b1111000; //7
                    4'b1000: seg <= 7'b0000000; //8
                    4'b1001: seg <= 7'b0010000; //9
                endcase
            end
            4'b1011: begin //选择第三个数码管
                case({min[3], sec[2], sec[1], sec[0]})
                    4'b0000: seg <= 7'b1000000; //0
                    4'b0001: seg <= 7'b1111001; //1
                    4'b0010: seg <= 7'b0100100; //2
                    4'b0011: seg <= 7'b0110000; //3
                    4'b0100: seg <= 7'b0011001; //4
                    4'b0101: seg <= 7'b0010010; //5
                    4'b0110: seg <= 7'b0000010; //6
                    4'b0111: seg <= 7'b1111000; //7
                    4'b1000: seg <= 7'b0000000; //8
                    4'b1001: seg <= 7'b0010000; //9
                endcase
            end
            4'b0111: begin //选择第四个数码管
                if(alarm_set[1]) begin //闹钟设置状态
                    case({alarm_hour[2], alarm_hour[1], alarm_hour[0]})
                        3'b000: seg <= 7'b1000000; //0
                        3'b001: seg <= 7'b1111001; //1
                        3'b010: seg <= 7'b0100100; //2
                        3'b011: seg <= 7'b0110000; //3
                        3'b100: seg <= 7'b0011001; //4
                        3'b101: seg <= 7'b0010010; //5
                        3'b110: seg <= 7'b0000010; //6
                        3'b111: seg <= 7'b1111000; //7
                    endcase
                end else begin //正常显示状态
                    case({alarm_on[1], alarm_hour[3], alarm_min[2], alarm_min[1], alarm_min[0]})
                        5'b00000: seg <= 7'b0000000; //闹钟未开启，无闹钟设置
                        5'b00001: seg <= 7'b0000000; //闹钟未开启，有闹钟设置
                        5'b00010: seg <= 7'b1000000; //闹钟开启，无闹钟设置
                        5'b00011: seg <= 7'b1111001; //闹钟开启，有闹钟设置
                        5'b00100: seg <= 7'b0100100; //闹钟关闭，无闹钟设置
                        5'b00101: seg <= 7'b0010010; //闹钟关闭，有闹钟设置
                        5'b00110: seg <= 7'b0000010; //闹钟响铃，无闹钟设置
                        5'b00111: seg <= 7'b0001111; //闹钟响铃，有闹钟设置
                    endcase
                end
            end
        endcase
    end
end

endmodule

该代码实现了数字时钟的计时、暂停、闹钟设置、开关和响铃等功能，同时可以实现24小时制的时、分、秒计时。在Basys3开发板上，将该代码烧录到FPGA芯片中，即可完成数字时钟的实现。