上次遗漏了HSI时钟设置，可以跑到32MHz。
修改system_stm32l1xx.c文件
  * @brief  Configures the System clock frequency, AHB/APBx prescalers and Flash
  *         settings.
  * @note   This function should be called only once the RCC clock configuration  
  *         is reset to the default reset state (done in SystemInit() function).            
  * @param  None
  * @retval None
  */
static void SetSysClock(void)
{
  __IO uint32_t StartUpCounter = 0, HSIStatus = 0;
  
  /* SYSCLK, HCLK, PCLK2 and PCLK1 configuration ---------------------------*/
  /* Enable HSI 内部时钟使能*/
  RCC->CR |= ((uint32_t)RCC_CR_HSION);                //RCC_CR_HSEON替换成RCC_CR_HSION

  /* Wait till HSI is ready and if Time out is reached exit */
  /* 等待外部时钟就绪，超时推出 */
  do
  {
    HSIStatus = RCC->CR & RCC_CR_HSIRDY;        //RCC_CR_HSEON替换成RCC_CR_HSION，HSIStatus替换成HSEStatus
    StartUpCounter++;
  } while((HSIStatus == 0) && (StartUpCounter != HSI_STARTUP_TIMEOUT));//HSIStatus替换成HSEStatus

  if ((RCC->CR & RCC_CR_HSIRDY) != RESET)        //RCC_CR_HSEON替换成RCC_CR_HSION
  {
    HSIStatus = (uint32_t)0x01;        //超时置1        //HSEStatus替换成HSIStatus
  }
  else
  {
    HSIStatus = (uint32_t)0x00;                        //HSEStatus替换成HSIStatus
  }
  
  if (HSIStatus == (uint32_t)0x01)                //HSEStatus替换成HSIStatus
  {
    /* Enable 64-bit access */
    FLASH->ACR |= FLASH_ACR_ACC64;
   
    /* Enable Prefetch Buffer */
    FLASH->ACR |= FLASH_ACR_PRFTEN;

    /* Flash 1 wait state */
    FLASH->ACR |= FLASH_ACR_LATENCY;
   
    /* Power enable */
    RCC->APB1ENR |= RCC_APB1ENR_PWREN;
  
    /* Select the Voltage Range 1 (1.8 V) */
    PWR->CR = PWR_CR_VOS_0;
  
    /* Wait Until the Voltage Regulator is ready */
    while((PWR->CSR & PWR_CSR_VOSF) != RESET)
    {
    }
        
    /* HCLK = SYSCLK /1*/
    RCC->CFGR |= (uint32_t)RCC_CFGR_HPRE_DIV1;
  
    /* PCLK2 = HCLK /1*/
    RCC->CFGR |= (uint32_t)RCC_CFGR_PPRE2_DIV1;
   
    /* PCLK1 = HCLK /1*/
    RCC->CFGR |= (uint32_t)RCC_CFGR_PPRE1_DIV1;
   
    /*  PLL configuration */
    RCC->CFGR &= (uint32_t)((uint32_t)~(RCC_CFGR_PLLSRC | RCC_CFGR_PLLMUL |  RCC_CFGR_PLLDIV));
    RCC->CFGR |= (uint32_t)(RCC_CFGR_PLLSRC_HSI | RCC_CFGR_PLLMUL4 | RCC_CFGR_PLLDIV2);                //RCC_CFGR_PLLSRC_HSE替换成RCC_CFGR_PLLSRC_HSI

    /* Enable PLL */
    RCC->CR |= RCC_CR_PLLON;

    /* Wait till PLL is ready */
    while((RCC->CR & RCC_CR_PLLRDY) == 0)
    {
    }
        
    /* Select PLL as system clock source */
    RCC->CFGR &= (uint32_t)((uint32_t)~(RCC_CFGR_SW));
    RCC->CFGR |= (uint32_t)RCC_CFGR_SW_PLL;

    /* Wait till PLL is used as system clock source */
    while ((RCC->CFGR & (uint32_t)RCC_CFGR_SWS) != (uint32_t)RCC_CFGR_SWS_PLL)
    {
    }
  }
  else
  {
    /* If HSI fails to start-up, the application will have wrong clock
       configuration. User can add here some code to deal with this error */
  }


然后

int main(void)
{
        RCC_GetClocksFreq(&RCC_Clocks);
        //GPIO端口配置
        GPIO_InitTypeDef GPIO_InitStructure;
        USART_InitTypeDef USART_InitStructure;
        NVIC_InitTypeDef NVIC_InitStructure;
        /* Enable GPIO clock */
        RCC_AHBPeriphClockCmd(RCC_AHBPeriph_GPIOA, ENABLE);
        
        /* Enable USART1 clock */
        RCC_APB2PeriphClockCmd(RCC_APB2Periph_USART1, ENABLE);
        
        /* Configure USART1 EN as alternate function push-pull */
        GPIO_InitStructure.GPIO_Pin = GPIO_Pin_11;
        GPIO_InitStructure.GPIO_Mode = GPIO_Mode_OUT;        //输出模式
        GPIO_InitStructure.GPIO_Speed = GPIO_Speed_40MHz;
        GPIO_InitStructure.GPIO_OType = GPIO_OType_PP;        //推挽
        GPIO_InitStructure.GPIO_PuPd = GPIO_PuPd_UP;
        GPIO_Init(GPIOA, &GPIO_InitStructure);
        
        /* Configure USART Tx as alternate function push-pull */
        GPIO_InitStructure.GPIO_Pin = GPIO_Pin_9 | GPIO_Pin_10;
        GPIO_InitStructure.GPIO_Mode = GPIO_Mode_AF;
        GPIO_InitStructure.GPIO_Speed = GPIO_Speed_40MHz;
        GPIO_InitStructure.GPIO_OType = GPIO_OType_PP;
        GPIO_InitStructure.GPIO_PuPd = GPIO_PuPd_UP;
        GPIO_Init(GPIOA, &GPIO_InitStructure);

        /* Connect PXx to USARTx_Tx */
        GPIO_PinAFConfig(GPIOA, GPIO_PinSource9,GPIO_AF_USART1);

        /* Connect PXx to USARTx_Rx */
        GPIO_PinAFConfig(GPIOA,GPIO_PinSource10,GPIO_AF_USART1);

        /* USART configuration */
        USART_InitStructure.USART_BaudRate = 9600;
        USART_InitStructure.USART_WordLength = USART_WordLength_8b;
        USART_InitStructure.USART_StopBits = USART_StopBits_1;
        USART_InitStructure.USART_Parity = USART_Parity_No;
        USART_InitStructure.USART_HardwareFlowControl = USART_HardwareFlowControl_None;
        USART_InitStructure.USART_Mode = USART_Mode_Rx | USART_Mode_Tx;
        USART_Init(USART1,&USART_InitStructure);
        
        /* Enable the USARTx Interrupt */
        NVIC_InitStructure.NVIC_IRQChannel = USART1_IRQn;
        NVIC_InitStructure.NVIC_IRQChannelPreemptionPriority = 0;
        NVIC_InitStructure.NVIC_IRQChannelSubPriority = 6;
        NVIC_InitStructure.NVIC_IRQChannelCmd = ENABLE;
        NVIC_Init(&NVIC_InitStructure);
        
        USART_ITConfig(USART1, USART_IT_RXNE,ENABLE);
        while(1)
        {
                //...
        ｝
}

单步没问题而全速不行的，我调试stm32f091的串口时碰到过一次，后来发现是因为全速时有个标志没有判断，导致运行到下一句时上一个字节还没有发送完成，而单步调试时因为每一步时间都足够所以没有问题，供参考。