#include <stdio.h>
#include <stm32l476xx.h>

int fputc(int ch, FILE *fp)
{
    if (fp == &__stdout)
    {
        if (ch == '\n')
        {
            while ((USART2->ISR & USART_ISR_TXE) == 0);
            USART2->TDR = '\r';
        }
        while ((USART2->ISR & USART_ISR_TXE) == 0);
        USART2->TDR = ch;
    }
    return ch;
}

int main(void)
{
    uint8_t flag = 0;
    RCC->APB1ENR1 |= RCC_APB1ENR1_PWREN;
    PWR->CR1 |= PWR_CR1_DBP; // 允许访问备用区域寄存器
    
    // 打开LSE
    if ((RCC->BDCR & RCC_BDCR_LSEON) == 0)
    {
        RCC->BDCR |= RCC_BDCR_LSEON;
        while ((RCC->BDCR & RCC_BDCR_LSERDY) == 0); // 等待LSE稳定
        flag = 1;
    }
    RCC->CR = (RCC->CR & ~RCC_CR_MSIRANGE) | RCC_CR_MSIRANGE_7 | RCC_CR_MSIRGSEL // MSI选8MHz时钟
                | RCC_CR_MSIPLLEN; // 且使用LSE校准MSI
    
    // 配置PLL: MSI/M*N/R=8MHz/1*18/2=72MHz
    RCC->PLLCFGR = RCC_PLLCFGR_PLLREN | RCC_PLLCFGR_PLLN_4 | RCC_PLLCFGR_PLLN_1 | RCC_PLLCFGR_PLLSRC_MSI;
    RCC->CR |= RCC_CR_PLLON;
    while ((RCC->CR & RCC_CR_PLLRDY) == 0); // 等待PLL稳定
    
    // 根据参考手册3.3.3 Read access latency, Table 11. Number of wait states according to CPU clock (HCLK) frequency
    // CPU时钟要达到64MHz以上, LATENCY必须为4WS
    // 由PWR_CR1_VOS可知当前V_CORE为Range 1 (最高时钟频率80MHz)
    FLASH->ACR |= FLASH_ACR_LATENCY_4WS; // 参阅: Increasing the CPU frequency
    while ((FLASH->ACR & FLASH_ACR_LATENCY) != FLASH_ACR_LATENCY_4WS);
    
    // 将PLL选作系统时钟
    RCC->CFGR |= RCC_CFGR_SW;
    while ((RCC->CFGR & RCC_CFGR_SWS) != RCC_CFGR_SWS);
    
    // LED灯配置
    RCC->AHB2ENR = RCC_AHB2ENR_GPIOAEN;
    GPIOA->MODER &= ~GPIO_MODER_MODE5_1;
    GPIOA->BSRR = GPIO_BSRR_BS5; // LED灯亮
    
    // 配置串口2
    RCC->APB1ENR1 |= RCC_APB1ENR1_USART2EN;
    GPIOA->MODER &= ~(GPIO_MODER_MODE2_0 | GPIO_MODER_MODE3_0 | GPIO_MODER_MODE9_0 | GPIO_MODER_MODE10_0);
    GPIOA->AFR[0] = GPIO_AFRL_AFSEL2_2 | GPIO_AFRL_AFSEL2_1 | GPIO_AFRL_AFSEL2_0;
    GPIOA->AFR[0] |= GPIO_AFRL_AFSEL3_2 | GPIO_AFRL_AFSEL3_1 | GPIO_AFRL_AFSEL3_0;
    USART2->BRR = 72000000 / 115200; // 被除数为时钟频率, 除数为波特率
    USART2->CR1 = USART_CR1_UE | USART_CR1_TE;
    
    // 需要延时一会儿才能使用USART2, 否则会乱码
    // 使用定时器2延时
    RCC->APB1ENR1 |= RCC_APB1ENR1_TIM2EN;
    //TIM2->ARR = 9999; // 10kHz/10000=1Hz -> 1s
    TIM2->ARR = 9; // 10kHz/10=1kHz -> 1ms
    TIM2->PSC = 7199; // 72MHz/7200=10kHz
    TIM2->CR1 = TIM_CR1_OPM | TIM_CR1_URS;
    TIM2->EGR = TIM_EGR_UG;
    TIM2->CR1 |= TIM_CR1_CEN;
    while ((TIM2->SR & TIM_SR_UIF) == 0);
    
    printf("It works!!!\n");
    if (flag)
        printf("LSE has been connected!\n"); // 只有上电时才会显示这句话
    printf("PWR->CR1=0x%08x\n", PWR->CR1);
    printf("RCC->BDCR=0x%08x\n", RCC->BDCR);
    printf("RCC->CFGR=0x%08x\n", RCC->CFGR);
    printf("RCC->CR=0x%08x\n", RCC->CR);
    printf("RCC->PLLCFGR=0x%08x\n", RCC->PLLCFGR);
    printf("USART2->BRR=%d\n", USART2->BRR);
    GPIOA->BRR = GPIO_BRR_BR5; // LED灯灭
    
    while (1);
}

【程序运行结果】
It works!!!
PWR->CR1=0x00000300
RCC->BDCR=0x00000003
RCC->CFGR=0x0000000f
RCC->CR=0x0300007f
RCC->PLLCFGR=0x01001201
USART2->BRR=625