STM32F4 Independent Watchdog (IWDG) not triggering as expected

Question

Im trying to understand how Independent watchdog really works and i wrote a piece of code that runs on STM32F411VE Eval board. I believe i have configured the Watchdog Timer correctly according to Reference manual.

The Independent WDOG Timer is configured with Internal LSI with a frequency of 32KHz and a pre-scaler of 64. The counter is set to 256. So if the Reload counter is not loaded within 0.5 seconds, the MCU should be reset. Is my understanding wrong?

What i dont understand is, the watchdog reset comes way too late than expected (5-6 seconds) if the counter value is not reloaded. I fail to understand why. Here's my Code.

//Initialise Independent Watchdog
void init_independent_wdog(void){

 IWDG_WriteAccessCmd(IWDG_WriteAccess_Enable);
 if(!IS_BIT_SET(IWDG->SR,0)){
      IWDG_SetPrescaler(IWDG_Prescaler_64);
 }
 //Set counter value to 256
 if(!IS_BIT_SET(IWDG->SR,1)){
       IWDG_SetReload(0x0100);
 }
 IWDG_Enable();
}

int main(void)
{
   SystemInit();
   SysTick_Init();

   //Initialize On-board LED GPIOs
   init_led_gpios();

   //Initialize On-board USART
   init_usart6_gpio();
   init_usart6_comm_module();

  //Initialize External Interrupt Button
  init_exti_struct();
  enable_exti_irq();

  //Initialize Independent WDOG
  init_independent_wdog();
  USART_TX_string("Hello1\n\r");

  while (1);      
 }


 //Interrupt triggered when User Button at PA0 is pressed
 void EXTI0_IRQHandler(void)
 {
    if (EXTI_GetITStatus(EXTI_Line0) != RESET)
    {
        GPIO_ToggleBits(GPIOD, GPIO_Pin_15);
        IWDG_ReloadCounter();
        EXTI_ClearITPendingBit(EXTI_Line0);
    }
  }

Answer 1

Your init-function is probably not doing what you want.

Currently you only call the set functions if the bits are in a certain state. What you want to do is wait until the bit is in the correct state and then execute the set function.

---

Wrong guess:

So my guess is that the second statement is never executed and the reload register stays at the standard value of 4095 which would result in a ~8 s timeout (RC oscillator can vary quite a bit, so 6 s is in the range of what's possible).

---

Correct guess:

The reference manual states this:

When the independent watchdog is started by writing the value 0xCCCC in the Key register (IWDG_KR), the counter starts counting down from the reset value of 0xFFF. When it reaches the end of count value (0x000) a reset signal is generated (IWDG reset).

Whenever the key value 0xAAAA is written in the IWDG_KR register, the IWDG_RLR value is reloaded in the counter and the watchdog reset is prevented.

So the first time the watchdog will start counting down from 0xFFF which results in a time of around 8 seconds with a divider of 64 in place. To prevent that, we have to "reload" the value right after enabling the watchdog.

So try this (also corrected the waiting for the bits):

//Initialize Independent Watchdog
void init_independent_wdog(void){

    IWDG_WriteAccessCmd(IWDG_WriteAccess_Enable);
    while(IS_BIT_SET(IWDG->SR,0))
    {
        // wait for PVU bit reset
    }
    IWDG_SetPrescaler(IWDG_Prescaler_64);

    //Set counter value
    while(IS_BIT_SET(IWDG->SR,1))
    {
        // wait for RVU bit reset
    }

    IWDG_SetReload(256);

    IWDG_Enable();
    IWDG_ReloadCounter(); // load the wanted value
}