说明：  简易数字电压表的设计,利用单片机AT89S51与ADC0809设计一个数字电压表，能够测量0－5V之间的直流电压值，四位数码显示，但要求使用的元器件数目最少。
(Simple digital voltage meter design, the use of single-chip AT89S51 with ADC0809 to design a digital voltage meter, capable of measuring between 0-5V DC voltage value, four digital display, but requires the use of minimum number of components.)