代码：uart4.c

#include "uart4.h"

void uart4_config()
{
    //使能GPIO\GPIOG\UART4的外设时钟
    RCC->MP_AHB4ENSETR |= (0x1<<1);
    RCC->MP_AHB4ENSETR |= (0x1<<6);
    RCC->MP_APB1ENSETR |= (0x1<<16);

    //设置PB2和PG11为管脚复用功能
    //PB2
    GPIOB->MODER &= (~(0x3<<4));
    GPIOB->MODER |= (0x2<<4);
    //PG11
    GPIOG->MODER &= (~(0x3<<22));
    GPIOG->MODER |= (0x2<<22);

    //设置PG11为UART4_TX功能
    GPIOG->AFRH &= (~(0xf<<12));
    GPIOG->AFRH |= (0x6<<12);

    //设置PB2为UART4_RX功能
    GPIOB->AFRL &= (~(0xf<<8));
    GPIOB->AFRL |= (0x8<<8);
    
    //设置串口不使能
    USART4->CR1 &= (~0x1);
    
    //设置8位数据位
    USART4->CR1 &= (~(0x1<<12));
    USART4->CR1 &= (~(0x1<<28));

    //设置没有校验位
    USART4->CR1 &= (-(0x1<<10));
    //设置时钟频率不分频
    USART4->CR1 &= (~0xf);
    //设置16倍过采样
    USART4->CR1 &= (~(0x1<<15));
    //设置1位停止位
    USART4->CR1 &= (-(0x3<<12));

    //设置波特率为115200
    USART4->BRR = 0x22B;
    //使能发送器
    USART4->CR1 |= (0x1<<3);
    //使能接收器
    USART4->CR1 |= (0x1<<2);
    //使能串口
    USART4->CR1 |= (0x1<<0);
}
void putchar(char dat)
{
    //当发送数据寄存器中没有数据时可以发送
    while(!(USART4->ISR & (0x1<<7)));  //当TDR中有数据时阻塞等待没有数据
    USART4->TDR = dat;  //向发送数据寄存器中写入数据
    //等待传输完成函数结束
    while(!(USART4->ISR & (0x1<<6)));
}

char getchar()
{
    //当接受数据寄存器中有有效数据时读取
    while(!(USART4->ISR & (0x1<<5)));
    //将读取到的数据返回
    return USART4->RDR;
}

//发送字符串
void puts(char *s)
{
    while(1)
    {
        if(*s == '\0')
            break;
        putchar(*s);
        s++;
    }
    putchar('\n');
}

void gets(char *s)
{
    while(1)
    {
        if(*s== '\r' )
            break;
        *s = getchar();
        putchar(*s);
        s++;
    }
    *s = '\0';
    putchar('\n');
}