（原创）高精度计时器电路原理图。采用AT89S52加DP8573，实现两个功能：带掉电保持功能的日历时钟、由外部开关信号触发的高精度计时。-(Original) high-precision timer circuit schematic. Using AT89S52 plus DP8573, realize two functions: to maintain power-down function with calendar clock, by an external switch to t