初始化

  /******rcc章节初始化********/                             |
  //1.使能gpiob组控制器                                     |
  RCC->MP_AHB4ENSETR |=(0X1<<1);                            |
  //2.使能gpiog组控制器                                     |
  RCC->MP_AHB4ENSETR |=(0x1<<6);                            |
  //3.使能uart4组寄存器                                     |
  RCC->MP_APB1ENSETR |=(0X1<<16);                           |
  /******gpio章节初始化********/                            |
  //1.GPIO引脚为PB2和PG11设置为复用功能模式                 |
  GPIOB->MODER &=~(0X3<<4);//先清零                         |
  GPIOB->MODER |=(0X1<<5); //再置1                          |
  GPIOG->MODER &=~(0X3<<22);//先清零                        |
  GPIOG->MODER |=(0X1<<23); //再置1                         |
                                                            |
  //2.设置PB2引脚的复用功能为UART4_RX                       |
  GPIOB->AFRL &=~(0XF<<8);//先清零                          |
  GPIOB->AFRL |=(0X1<<11);//再置1                           |
                                                            |
  //3.设置PG11引脚的复用功能为UART4_TX                      |
  GPIOG->AFRH &=~(0XF<<12);//先清零                         |
  GPIOG->AFRH |=(0X3<<13);//再置1                           |
                                                            |
  /******uart章节初始化********/                            |
  //很多都是ue=0才能设置，所以串口先设置为0                 |
  USART4->CR1 &=~(0X1);                                     |
  //1.设置8位数据位                                         |
  USART4->CR1 &=~(0X1<<28);                                 |
  USART4->CR1 &=~(0X1<<12);                                 |
  //2.设置16倍采样率                                        |
  USART4->CR1 &=~(0X1<<15);                                 |
  //3.设置无校验位                                          |
  USART4->CR1 &=~(0X1<<10);                                 |
  //4.设置发送位使能                                        |
  USART4->CR1 |=(0X1<<3);                                   |
  //5.设置接收位使能                                        |
  USART4->CR1 |=(0X1<<2);                                   |
  //6.设置停止位                                            |
  USART4->CR2 &=~(0X3<<12);                                 |
  //7.设置分频器                                            |
  USART4->PRESC &=~(0XF);                                   |
  //8.设置波特率                                            |
  USART4->BRR &=~(0xffff);                                  |
  USART4->BRR |=0x22b;                                      |
  //9.设置串口使能                                          |
  USART4->CR1 |=(0X1);                                      |
                                                            |
                                                            |

功能函数

                                                  
//  发送一个字符                                  
void hal_put_char(const char str)                 
{                                                 
    while(!(USART4->ISR&(0X1<<7)));               
    USART4->TDR=str;                              
    while(!(USART4->ISR&(0X1<<6)));               
                                                  
}                                                 
                                                  
//  发送字符串                                    
void hal_put_string(const char *s)                
{                                                 
                                                  
    while(*s)                                     
    {                                             
        hal_put_char(*s);                         
        s++;                                      
    }                                             
    hal_put_char('\r');                           
    hal_put_char('\n');                           
}                                                 
                                                  
//  接收字符                                      
char hal_get_str()                                
{                                                 
    char a;                                       
    while(!(USART4->ISR&(0x1<<5)));               
    a=USART4->RDR;                                
    return a;                                     
}                                                 
                                                  
//  接收字符串                                    
void hal_get_string()                             
{                                                 
    int i=0;                                      
    char s[100];                                  
    while(1)                                      
    {                                             
        s[i]=hal_get_str();                       
        if(s[i]=='\r')                            
        {                                         
          break;                                  
        }                                         
        hal_put_char(s[i]);                       
        i++;                                      
    }                                             
    s[i]='\0';                                    
    hal_put_char('\r');                           
    hal_put_char('\n');                           
    hal_put_string(s);                            
                                                  
}                                                 
                                                  
                                                  
                                                  
                                                  

实验现象：

字符

字符串