利用ADC0809采集电位器的电压值，将采集的电压值通过8255显示在七段数码管上，显示数据保留小数点后2位，利用8253定时数据更新，每隔0.5秒数据更新一次。扩展要求：具有设置报警阈值功能，当超过阈值时，点亮一个LED灯。(Using ADC0809 to collect the voltage value of potentiometer, the collected voltage value is displayed on seven digital tubes through 8255. The display data is kept at two decimal points and updated with 8253 timing data every 0.5 seconds. Expansion requirements: With the function of setting alarm threshold, when exceeding the threshold, light an LED lamp.)