说明：  数字钟的设计用到，数制、基本逻辑运算、逻辑门电路、组合逻辑电路、触发器、时序逻辑电路、脉冲的产生等数字电路知识。
(Digital clock design used, number system, basic logic operations, logic gates, combinational logic circuits, flip-flops, sequential logic circuits, pulse generator and digital circuit knowledge.)