stm32f303单片机串口发数实验，从串口输入一个数发送到单片机（0-999999），然后给上位机发出来0-999999个循环的小写字母-stm32f303 microcontroller serial number issued experiment, enter a number from the serial port to send to the microcontroller (0-999999), and then sent to the host computer to 0-99