上电后，串口软件显示一次字符串‘请发送你要开始计数的数字’。然后PC端发出一个数字，根据这个数字，每隔2秒显示递增2的数字。（数字超过9就回0）(When the power is on, the serial software displays a string. "Please send the number you want to start counting.". Then the PC side issues a number, which, according t