STM32 休眠模式下如何喂狗？

        在STM32开发中经常会用到独立看门狗（IWDG）和低功耗模式，看门狗是为了检测和解决由软件错误引起的故障，低功耗模式是为了在CPU不需要继续运行时进入到休眠模式用以节省电能。其中独立看门狗的时钟由独立的RC振荡器（STM32F10x一般为40kHz）提供，即使在主时钟出现故障时，也仍然有效，因此可以在停止和待机模式下工作。而且独立看门狗一旦启动，除了系统复位，它不能再被停止。但这样引发的一个问题是当MCU进入到低功耗模式后由于CPU停止运行无法喂狗，会导致系统频繁复位。那如何解决这个问题呢，难道独立看门狗和低功耗模式没法同时使用？

        一个很好的方式是在休眠模式下通过RTC定时唤醒来喂狗，喂完够在进入继续进入到休眠模式。比如看门狗复位的时间间隔为10s。那么在进入休眠模式前设置RTC闹钟中断时间为5s。这样每隔5s唤醒一次喂一次狗。便可以很好的解决这个问题。

while(1)
  {
    // 执行任务
	Task1();
	Task2();
	// ..

	// 喂狗
	dev_iwdg_feed();

	// 进入待机模式开关
	if(m_bEnterStandByMode)
	{	
	    // 使能外部中断，GPIOB3，用以MCU从待机模式唤醒
	    dev_exti_enable(TRUE);
ENTERSTOPMODE:	
		// 设置RTC闹钟，5秒钟产生一次RTC闹钟中断*/
		dev_rtc_setAlarm(5);
	
		// 进入停止模式（低功耗），直至外部中断触发时被唤醒
		PWR_EnterSTOPMode(PWR_Regulator_LowPower, PWR_STOPEntry_WFI);
	
		// 是否是RTC闹钟中断唤醒
		if(dev_rtc_isAlarm())
		{
			// 喂狗
			dev_iwdg_feed();
			// 喂完狗继续进入停止模式
			goto ENTERSTOPMODE;	
		}
		// 禁止外部中断 
		dev_exti_enable(FALSE);
		// 从停止模式唤醒后恢复系统时钟
		dev_clk_restore();
	}		      
  }

以下是完整的参考代码：

//**********************************************************************************************     
//  STM32F10x StopMode RTC Feed Dog 
//  compiler: Keil UV3     
//  2013-01-04 , By friehood     
//**********************************************************************************************  
#include "stm32f10x_lib.h"
#include "platform_config.h"
static Boolean g_bRTCAlarm = FALSE;

/*******************************************************************************
* Function Name  : RCC_Configuration
* Description    : Configures the different system clocks.
* Input          : None
* Output         : None
* Return         : None
*******************************************************************************/
void RCC_Configuration(void)
{
	/* RCC system reset(for debug purpose) */
	RCC_DeInit();

	/* Enable HSE */
	RCC_HSEConfig(RCC_HSE_ON);

	/* Wait till HSE is ready */
	if(RCC_WaitForHSEStartUp() == SUCCESS)
	{
		/* Enable Prefetch Buffer */
		FLASH_PrefetchBufferCmd(FLASH_PrefetchBuffer_Enable);

		//FLASH时序控制 
        //推荐值：SYSCLK = 0~24MHz   Latency=0 
        //        SYSCLK = 24~48MHz  Latency=1 
        //        SYSCLK = 48~72MHz  Latency=2
		//FLASH_SetLatency(FLASH_Latency_1);   		//警告:修改为1会对DMA值有影响（如ADC采集值会错位）
		FLASH_SetLatency(FLASH_Latency_2);

		/* HCLK = SYSCLK */
		RCC_HCLKConfig(RCC_SYSCLK_Div1); 

		/* PCLK2 = HCLK */
		RCC_PCLK2Config(RCC_HCLK_Div1); 

		/* PCLK1 = HCLK/2 */
		RCC_PCLK1Config(RCC_HCLK_Div2);

		/* PLLCLK = 12MHz * 3 = 36 MHz */
		RCC_PLLConfig(RCC_PLLSource_HSE_Div1, RCC_PLLMul_3);

		/* Enable PLL */ 
		RCC_PLLCmd(ENABLE);

		/* Wait till PLL is ready */
		while(RCC_GetFlagStatus(RCC_FLAG_PLLRDY) == RESET)
		{
		}

		/* Select PLL as system clock source */
		RCC_SYSCLKConfig(RCC_SYSCLKSource_PLLCLK);

		/* Wait till PLL is used as system clock source */
		while(RCC_GetSYSCLKSource() != 0x08)
		{
		}
	}
	/* Enable PWR and BKP clock */
	RCC_APB1PeriphClockCmd(RCC_APB1Periph_PWR | RCC_APB1Periph_BKP, ENABLE);

	/* Enable AFIO clock */
	RCC_APB2PeriphClockCmd(RCC_APB2Periph_AFIO, ENABLE);
}

/*******************************************************************************
* Function Name  : NVIC_Configuration
* Description    : Configures the nested vectored interrupt controller.
* Input          : None
* Output         : None
* Return         : None
*******************************************************************************/
void NVIC_Configuration(void)
{
  NVIC_InitTypeDef NVIC_InitStructure;

#ifdef  VECT_TAB_RAM
  /* Set the Vector Table base location at 0x20000000 */
  NVIC_SetVectorTable(NVIC_VectTab_RAM, 0x0);
#else  /* VECT_TAB_FLASH  */
  /* Set the Vector Table base location at 0x08000000 */
  NVIC_SetVectorTable(NVIC_VectTab_FLASH, 0x0);
#endif

  /* Configure one bit for preemption priority */
  NVIC_PriorityGroupConfig(NVIC_PriorityGroup_1);
}

/*******************************************************************************
* Function Name  : SysTick_Configuration
* Description    : Configures the SysTick to generate an interrupt each 1 millisecond.
* Input          : None
* Output         : None
* Return         : None
*******************************************************************************/
void SysTick_Configuration(void)
{
  /* Select AHB clock(HCLK) as SysTick clock source */
  SysTick_CLKSourceConfig(SysTick_CLKSource_HCLK);

  /* Set SysTick Priority to 3 */
  NVIC_SystemHandlerPriorityConfig(SystemHandler_SysTick, 3, 0);
   
  /* SysTick interrupt each 1ms with HCLK equal to 72MHz */
  SysTick_SetReload(72000);

  /* Enable the SysTick Interrupt */
  SysTick_ITConfig(ENABLE);
}

/*******************************************************************************
* Function Name  : Delay
* Description    : Inserts a delay time.
* Input          : nTime: specifies the delay time length, in milliseconds.
* Output         : None
* Return         : None
*******************************************************************************/
void Delay(u32 nTime)
{
  /* Enable the SysTick Counter */
  SysTick_CounterCmd(SysTick_Counter_Enable);
  
  TimingDelay = nTime;

  while(TimingDelay != 0);

  /* Disable the SysTick Counter */
  SysTick_CounterCmd(SysTick_Counter_Disable);
  /* Clear the SysTick Counter */
  SysTick_CounterCmd(SysTick_Counter_Clear);
}


/*******************************************************************************
* Function Name  : RTC_Configuration
* Description    : Configures RTC clock source and prescaler.
* Input          : None
* Output         : None
* Return         : None
*******************************************************************************/
void RTC_Configuration(void)
{
	EXTI_InitTypeDef EXTI_InitStructure;
	NVIC_InitTypeDef NVIC_InitStructure;
	
	/* RTC clock source configuration ------------------------------------------*/
	/* Allow access to BKP Domain */
	PWR_BackupAccessCmd(ENABLE);

	/* Reset Backup Domain */
	BKP_DeInit();

	/* Enable the LSI OSC */
	RCC_LSICmd(ENABLE);

	/* Wait till LSI is ready */
	while (RCC_GetFlagStatus(RCC_FLAG_LSIRDY) == RESET){}
	
	/* Select the RTC Clock Source */
	RCC_RTCCLKConfig(RCC_RTCCLKSource_LSI);

	/* Enable the RTC Clock */
	RCC_RTCCLKCmd(ENABLE);

	/* RTC configuration -------------------------------------------------------*/
	/* Wait for RTC APB registers synchronisation */
	RTC_WaitForSynchro();

	/* Set RTC prescaler: set RTC period to 1sec */
	RTC_SetPrescaler(40000);
	
	/* Wait until last write operation on RTC registers has finished */
	RTC_WaitForLastTask();

	/* Enable the RTC Alarm interrupt */
	RTC_ITConfig(RTC_IT_ALR, ENABLE);
	
	/* Wait until last write operation on RTC registers has finished */
	RTC_WaitForLastTask();

	/* Configure EXTI Line17(RTC Alarm) to generate an interrupt on rising edge */
    EXTI_ClearITPendingBit(EXTI_Line17);
    EXTI_InitStructure.EXTI_Line = EXTI_Line17;
	EXTI_InitStructure.EXTI_Mode = EXTI_Mode_Interrupt;
    EXTI_InitStructure.EXTI_Trigger = EXTI_Trigger_Rising;
	EXTI_InitStructure.EXTI_LineCmd = ENABLE;
    EXTI_Init(&EXTI_InitStructure);

	/* Enable the RTC Interrupt */
	NVIC_InitStructure.NVIC_IRQChannel = RTCAlarm_IRQChannel;
	NVIC_InitStructure.NVIC_IRQChannelPreemptionPriority = 0;
	NVIC_InitStructure.NVIC_IRQChannelSubPriority = 0;
	NVIC_InitStructure.NVIC_IRQChannelCmd = ENABLE;
	NVIC_Init(&NVIC_InitStructure);
}

/*******************************************************************************
* Function Name  : RTCAlarm_IRQHandler
* Description    : This function handles RTC Alarm interrupt request.
* Input          : None
* Output         : None
* Return         : None
*******************************************************************************/
void RTCAlarm_IRQHandler(void)
{
	if(RTC_GetITStatus(RTC_IT_ALR) != RESET)
	{
	    /* Set the RTC alarm flag */
	    g_bRTCAlarm = TRUE;

		/* Clear EXTI line17 pending bit */
		EXTI_ClearITPendingBit(EXTI_Line17);

		/* Check if the Wake-Up flag is set */
		if(PWR_GetFlagStatus(PWR_FLAG_WU) != RESET)
		{
			/* Clear Wake Up flag */
			PWR_ClearFlag(PWR_FLAG_WU);
		}																		
														 
		/* Wait until last write operation on RTC registers has finished */
		RTC_WaitForLastTask();   
		/* Clear RTC Alarm interrupt pending bit */
		RTC_ClearITPendingBit(RTC_IT_ALR);
		/* Wait until last write operation on RTC registers has finished */
		RTC_WaitForLastTask();
	}
}

/*******************************************************************************
* Function Name  : dev_rtc_setAlarm
* Description    : 设置RTC闹钟.
* Input          : 闹钟时间
* Output         : None
* Return         : None
*******************************************************************************/
void dev_rtc_setAlarm(u32 AlarmValue)
{
	/* Clear the RTC SEC flag */
	RTC_ClearFlag(RTC_FLAG_SEC);
	/* Wait clear RTC flag sccess */
    while(RTC_GetFlagStatus(RTC_FLAG_SEC) == RESET);
   	/* Wait until last write operation on RTC registers has finished */
	RTC_WaitForLastTask(); 

	/* Sets the RTC alarm value */
    RTC_SetAlarm(RTC_GetCounter() + AlarmValue);
	/* Wait until last write operation on RTC registers has finished */
	RTC_WaitForLastTask(); 
}

/*******************************************************************************
* Function Name  : dev_rtc_isAlarm
* Description    : RTC闹钟是否触发
* Input          : None
* Output         : None
* Return         : TRUE:已触发，FALSE，未触发
*******************************************************************************/
Boolean dev_rtc_isAlarm(void)
{
	if(g_bRTCAlarm)
	{
	    /* Clear the RTC alarm flag */
		g_bRTCAlarm = FALSE;
		return TRUE;
	}
	return FALSE;			
}

void dev_iwdg_init(void)
{
	/* Enable write access to IWDG_PR and IWDG_RLR registers */
	IWDG_WriteAccessCmd(IWDG_WriteAccess_Enable);
	/* IWDG counter clock: 40KHz(LSI) / 256 = 0.15625 KHz */
    IWDG_SetPrescaler(IWDG_Prescaler_256);
	/* Set counter reload value to 1562 */
	IWDG_SetReload(1562);	// 10s
	/* Reload IWDG counter */
	IWDG_ReloadCounter();
	/* Enable IWDG (the LSI oscillator will be enabled by hardware) */
	IWDG_Enable();
}

void dev_iwdg_feed(void)
{
	IWDG_ReloadCounter();
}

/*******************************************************************************
* Function Name  : dev_clk_restore
* Description    : Restore system clock after wake-up from STOP: enable HSE, PLL
*                  and select PLL as system clock source.
* Input          : None
* Output         : None
* Return         : None
*******************************************************************************/
void dev_clk_restore(void)
{
  /* Enable HSE */
  RCC_HSEConfig(RCC_HSE_ON);

  /* Wait till HSE is ready */
  HSEStartUpStatus = RCC_WaitForHSEStartUp();

  if(HSEStartUpStatus == SUCCESS)
  {
    /* Enable PLL */ 
    RCC_PLLCmd(ENABLE);

    /* Wait till PLL is ready */
    while(RCC_GetFlagStatus(RCC_FLAG_PLLRDY) == RESET)
    {
    }

    /* Select PLL as system clock source */
    RCC_SYSCLKConfig(RCC_SYSCLKSource_PLLCLK);

    /* Wait till PLL is used as system clock source */
    while(RCC_GetSYSCLKSource() != 0x08)
    {
    }
  }
}

/*******************************************************************************
* Function Name  : EXTI_Configuration
* Description    : Configures EXTI Line3.
* Input          : None
* Output         : None
* Return         : None
*******************************************************************************/
void EXIT_Configuration(void)
{
	EXTI_InitTypeDef EXTI_InitStructure;

	GPIO_EXTILineConfig(GPIO_PortSourceGPIOB,GPIO_PinSource3);
	EXTI_ClearITPendingBit(EXTI_Line3);
	EXTI_InitStructure.EXTI_Line = EXTI_Line3;
	EXTI_InitStructure.EXTI_Mode = EXTI_Mode_Interrupt;										  
	EXTI_InitStructure.EXTI_Trigger	= EXTI_Trigger_Falling;	
	EXTI_InitStructure.EXTI_LineCmd = ENABLE;
	EXTI_Init(&EXTI_InitStructure);
}

void dev_exti_enable(Boolean bEnable)
{
	NVIC_InitTypeDef NVIC_InitStructure;

	/* Clear the Key Button EXTI line pending bit */
	EXTI_ClearITPendingBit(EXTI_Line3);

	NVIC_ClearIRQChannelPendingBit(EXTI3_IRQChannel);
	NVIC_InitStructure.NVIC_IRQChannel = EXTI3_IRQChannel;
  	NVIC_InitStructure.NVIC_IRQChannelPreemptionPriority = 0;
	NVIC_InitStructure.NVIC_IRQChannelSubPriority = 0;
	NVIC_InitStructure.NVIC_IRQChannelCmd = bEnable ? ENABLE : DISABLE;
	NVIC_Init(&NVIC_InitStructure);	
}

/*******************************************************************************
* Function Name  : main
* Description    : Main program.
* Input          : None
* Output         : None
* Return         : None
*******************************************************************************/
int main(void)
{
  /* System Clocks Configuration */
  RCC_Configuration();

  /* NVIC configuration */
  NVIC_Configuration();

  /* Configure RTC clock source and prescaler */
  RTC_Configuration();

  /* Configure the SysTick to generate an interrupt each 1 millisecond */
  SysTick_Configuration();

  /* Configures EXTI Line3 */
  EXIT_Configuration();

  /* IWDG initialize*/
  dev_iwdg_init();

  while(1)
  {
    // 执行任务
	Task1();
	Task2();
	// ..

	// 喂狗
	dev_iwdg_feed();

	// 进入待机模式开关
	if(m_bEnterStandByMode)
	{	
	    // 使能外部中断，GPIOB3，用以MCU从待机模式唤醒
	    dev_exti_enable(TRUE);
ENTERSTOPMODE:	
		// 设置RTC闹钟，5秒钟产生一次RTC闹钟中断*/
		dev_rtc_setAlarm(5);
	
		// 进入停止模式（低功耗），直至外部中断触发时被唤醒
		PWR_EnterSTOPMode(PWR_Regulator_LowPower, PWR_STOPEntry_WFI);
	
		// 是否是RTC闹钟中断唤醒
		if(dev_rtc_isAlarm())
		{
			// 喂狗
			dev_iwdg_feed();
			// 喂完狗继续进入停止模式
			goto ENTERSTOPMODE;	
		}
		// 禁止外部中断 
		dev_exti_enable(FALSE);
		// 从停止模式唤醒后恢复系统时钟
		dev_clk_restore();
	}		      
  }
}