以单片机为核心器件，设计一个数字电压表,,数字电压表的输入电压范围为0－50V,测量精度为20mV以上,用4个数码管显示电压值，小数点在第3位。请放心使用！-With the single chip processor as the core device, design a digital voltmeter, and the digital voltmeter input voltage range of 0-50 V, measurement accuracy for 20 mV above, with four digital pipe display voltage value, the decimal point in third place. Please feel free to use!