STM32 serial port debugging printf and rewriting function fputc

Rewrite the serial port experiment program of punctual atomic STM32F103ZET6 today.
The source program uses serial port 1 for experiments, and it is modified to use serial port 3 for experiments.
Because of carelessness, a strange error occurred. It took an hour with the help of a friend to understand the cause of the error. .
(Actually, I have long known that redefining the function fputc from USART1 to USART3 can solve the problem, but I found the cause of the error step by step through the breakpoint in the main function. I have mastered the basic method of program debugging. Here, Thanks friends!)

process:

(1) Replace "USART1" in main.c, usart.h and usart.c with "USART3";
(2) Enable GPIOB and USART3 of serial port 3 in usart.c.

question:

In the redefinition function in usart.c, USART1 has not been changed:

//重定义fputc函数
int fputc(int ch, FILE *f)
{
    
          
	while((USART1->SR&0X40)==0)
;//循环发送，直到发送完毕 
    USART1->DR = (u8) ch;      
	return ch;
}

results in the program:

else
		{
    
    
			times++;
			if(times%5000==0)
			{
    
    
				printf("\r\n串口实验\r\n");
				printf("正点原子\r\n\r\n");
			}
			if(times%200==0)
			{
    
    
			printf("请输入数据，以回车键结束\n"); 
			}	 
			if(times%2==0)LED0=!LED0;//
			delay_ms(100);   
		}

When times == 200, execute the printf statement and enter the redefined function fputc, causing the program to enter an infinite loop.

solve:

Of course, change the USART1 in the redefine function fputc to USART3.