STM32 STM32F4 寄存器怎么配置不上， 无法往寄存器写入数据

当出现这个问题时，往往是因为你没有在RCC寄存器中把相关的时钟使能打开。

配置寄存器之前记得调用"RCC_AxxxPeriphClockCmd"先打开需要配置的时钟源，别调用了“RCC_AxxxPeriphResetCmd"。

相关函数定义源代码如下：

/**
  * @brief  Enables or disables the AHB peripheral clock.
  * @param  RCC_AHBPeriph: specifies the AHB peripheral to gates its clock.
  *   
  *   For @b STM32_Connectivity_line_devices, this parameter can be any combination
  *   of the following values:        
  *     @arg RCC_AHBPeriph_DMA1
  *     @arg RCC_AHBPeriph_DMA2
  *     @arg RCC_AHBPeriph_SRAM
  *     @arg RCC_AHBPeriph_FLITF
  *     @arg RCC_AHBPeriph_CRC
  *     @arg RCC_AHBPeriph_OTG_FS    
  *     @arg RCC_AHBPeriph_ETH_MAC   
  *     @arg RCC_AHBPeriph_ETH_MAC_Tx
  *     @arg RCC_AHBPeriph_ETH_MAC_Rx
  * 
  *   For @b other_STM32_devices, this parameter can be any combination of the 
  *   following values:        
  *     @arg RCC_AHBPeriph_DMA1
  *     @arg RCC_AHBPeriph_DMA2
  *     @arg RCC_AHBPeriph_SRAM
  *     @arg RCC_AHBPeriph_FLITF
  *     @arg RCC_AHBPeriph_CRC
  *     @arg RCC_AHBPeriph_FSMC
  *     @arg RCC_AHBPeriph_SDIO
  *   
  * @note SRAM and FLITF clock can be disabled only during sleep mode.
  * @param  NewState: new state of the specified peripheral clock.
  *   This parameter can be: ENABLE or DISABLE.
  * @retval None
  */
void RCC_AHBPeriphClockCmd(uint32_t RCC_AHBPeriph, FunctionalState NewState)
{
  /* Check the parameters */
  assert_param(IS_RCC_AHB_PERIPH(RCC_AHBPeriph));
  assert_param(IS_FUNCTIONAL_STATE(NewState));

  if (NewState != DISABLE)
  {
    RCC->AHBENR |= RCC_AHBPeriph;
  }
  else
  {
    RCC->AHBENR &= ~RCC_AHBPeriph;
  }
}

/**
  * @brief  Enables or disables the High Speed APB (APB2) peripheral clock.
  * @param  RCC_APB2Periph: specifies the APB2 peripheral to gates its clock.
  *   This parameter can be any combination of the following values:
  *     @arg RCC_APB2Periph_AFIO, RCC_APB2Periph_GPIOA, RCC_APB2Periph_GPIOB,
  *          RCC_APB2Periph_GPIOC, RCC_APB2Periph_GPIOD, RCC_APB2Periph_GPIOE,
  *          RCC_APB2Periph_GPIOF, RCC_APB2Periph_GPIOG, RCC_APB2Periph_ADC1,
  *          RCC_APB2Periph_ADC2, RCC_APB2Periph_TIM1, RCC_APB2Periph_SPI1,
  *          RCC_APB2Periph_TIM8, RCC_APB2Periph_USART1, RCC_APB2Periph_ADC3,
  *          RCC_APB2Periph_TIM15, RCC_APB2Periph_TIM16, RCC_APB2Periph_TIM17,
  *          RCC_APB2Periph_TIM9, RCC_APB2Periph_TIM10, RCC_APB2Periph_TIM11     
  * @param  NewState: new state of the specified peripheral clock.
  *   This parameter can be: ENABLE or DISABLE.
  * @retval None
  */
void RCC_APB2PeriphClockCmd(uint32_t RCC_APB2Periph, FunctionalState NewState)
{
  /* Check the parameters */
  assert_param(IS_RCC_APB2_PERIPH(RCC_APB2Periph));
  assert_param(IS_FUNCTIONAL_STATE(NewState));
  if (NewState != DISABLE)
  {
    RCC->APB2ENR |= RCC_APB2Periph;
  }
  else
  {
    RCC->APB2ENR &= ~RCC_APB2Periph;
  }
}

/**
  * @brief  Enables or disables the Low Speed APB (APB1) peripheral clock.
  * @param  RCC_APB1Periph: specifies the APB1 peripheral to gates its clock.
  *   This parameter can be any combination of the following values:
  *     @arg RCC_APB1Periph_TIM2, RCC_APB1Periph_TIM3, RCC_APB1Periph_TIM4,
  *          RCC_APB1Periph_TIM5, RCC_APB1Periph_TIM6, RCC_APB1Periph_TIM7,
  *          RCC_APB1Periph_WWDG, RCC_APB1Periph_SPI2, RCC_APB1Periph_SPI3,
  *          RCC_APB1Periph_USART2, RCC_APB1Periph_USART3, RCC_APB1Periph_USART4, 
  *          RCC_APB1Periph_USART5, RCC_APB1Periph_I2C1, RCC_APB1Periph_I2C2,
  *          RCC_APB1Periph_USB, RCC_APB1Periph_CAN1, RCC_APB1Periph_BKP,
  *          RCC_APB1Periph_PWR, RCC_APB1Periph_DAC, RCC_APB1Periph_CEC,
  *          RCC_APB1Periph_TIM12, RCC_APB1Periph_TIM13, RCC_APB1Periph_TIM14
  * @param  NewState: new state of the specified peripheral clock.
  *   This parameter can be: ENABLE or DISABLE.
  * @retval None
  */
void RCC_APB1PeriphClockCmd(uint32_t RCC_APB1Periph, FunctionalState NewState)
{
  /* Check the parameters */
  assert_param(IS_RCC_APB1_PERIPH(RCC_APB1Periph));
  assert_param(IS_FUNCTIONAL_STATE(NewState));
  if (NewState != DISABLE)
  {
    RCC->APB1ENR |= RCC_APB1Periph;
  }
  else
  {
    RCC->APB1ENR &= ~RCC_APB1Periph;
  }
}