效果：(字符串的收发还是有问题，只能实现预置字符串的发送，无法实现输入字符串回响)

代码:

#include "uart4.h"

void hal_uart4_init()
{
    // GPIOB使能
    RCC->MP_AHB4ENSETR |= (0x1 << 1);
    // GPIOG使能
    RCC->MP_AHB4ENSETR |= (0x1 << 6);
    // UART4使能
    RCC->APB1RSTCLRR |= (0x1 << 16);

    // GPIOG设置为复用
    GPIOG->MODER &= (~(0x3 << 22));
    GPIOG->MODER |= (0x2 << 22);
    // GPIOB设置为复用
    GPIOB->MODER &= (~(0x3 << 4));
    GPIOB->MODER |= (0x2 << 4);

    // GPIOB的复用功能设置
    GPIOB->AFRL &= (~(0xF << 8));
    GPIOB->AFRL |= (0x8 << 8);
    // GPIOG的复用功能设置
    GPIOG->AFRL &= (~(0xF << 12));
    GPIOG->AFRL |= (0x6 << 12);

    // UART初始化
    USART4->CR1 &= (~(0x1 << 12));
    USART4->CR1 &= (~(0x1 << 28));
    USART4->CR1 &= (~(0x1 << 10));
    USART4->CR1 &= (~(0x1 << 15));
    USART4->CR1 |= (0x1 << 2);
    USART4->CR1 |= (0x1 << 3);
    USART4->CR1 |= (0x1);

    // UART设置停止位
    USART4->CR2 &= (~(0X3 << 12));

    // UART设置波特率
    USART4->BRR &= (~(0XFFF));
    USART4->BRR |= (0X22B);
}

// 发送一个字符
void hal_put_char(const char str)
{
    while (!(USART4->ISR & 1 << 6))
        ; // 如果不为1则一直循环等待上一位发送完毕
    USART4->TDR = str;
}

// 发送一个字符串
void hal_put_string(const char *string)
{
    while (*string != '\0')
    {
        hal_put_char(*string++);
    }
}

// 接收一个字符
char hal_get_char()
{
    while (!(USART4->ISR & 1 << 5))
        ; // 等待一直到接收到数据为止
    return USART4->RDR;
}

// 接受一个字符串
char *hal_get_string()
{
    char buff[128];
    int i = 0;
    while (hal_get_char() != '\n' || hal_get_char() != '\r')
    {
        buff[i++] = hal_get_char();
    }
    buff[i] = '\0';
    return buff;
}