说明：  no intro
(Using AT89C51 to design a 2-bit LED digital display as a "stopwatch": the display time is 00-99 seconds, adding 1 per second automatically, another "start" key and a "reset" key are designed. Through the scanning of the keyboard, the running time / stop of the clock is controlled. Timer T0 is used as the timer in the project. Every 10ms an interrupt occurs, and every 100 interrupts plus 1s. During this period, if the "start" button is pressed, the program will set tr0 to 1, so as to turn on the interrupt, and the clock will start to run; if the "reset" button is pressed, the program will set tr0 to 0, and at the same time clear the variables of the storage time, so as to stop the interrupt and realize the reset.)