使用外设：串口线一跟，通过COM1连接PC

** 实验名：【B】USART_TEST
** keil MDK-ARM Standard Version：V4.22A
** 软件库(keil) Version: V2.0.1
** 固件库(ST) Version: V
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** 实验现象：
　 1、首先进行常规配置(RCC、NVIC、GPIO)，然后配置串口1(115200baud 8位数据位 1位停止位 无奇偶校验位)

　 2、通过对USART1写操作，完成了向PC发送字符串的功能

　 3、此实验对串口的操作都是通过调用ST固件库，
** 实验说明：
　 1、打开超级终端，进行相同的设置

　 2、通过JLINK烧写程序，按下“RESET”键，可以看到PC接收到的字符串
　　　“welcome to STM32”(using peripherals: the serial port line is a heel, and the PC is connected to the COM1
* * * experimental Name: [B] USART_TEST
* * * keil MDK-ARM Standard Version:V4.22A
* * software library (keil) Version: V2.0.1
* * firmware library (ST) Version: V
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* * experimental phenomena:
1, first of all the regular configuration (RCC, NVIC, GPIO), and then the serial port 1 (115200baud 8 bit data bit, 1 bit stop bit and no parity bit)
2. By writing to USART1, the function of sending strings to PC is completed
3. The operation of this experiment on the serial port is by calling the ST firmware library.
* * the experiment shows that:
1. Open the super terminal and make the same setup
2, through the JLINK burning program, press the "RESET" key, you can see the string received by PC
Welcome to STM32)