HAL_IWDG_Refresh causes watchdog to immediately trip on STM32L432KC

Question

I'm experiencing an odd issue with a Nucleo L432KC devboard from ST. When I start the IWDG using the following code:

IWDG_HandleTypeDef IwdgHandle;

IwdgHandle.Instance = IWDG;
IwdgHandle.Init.Prescaler = IWDG_PRESCALER_256;
IwdgHandle.Init.Reload    = 30 * 125; 
// 30 sec * 125 watchdog pulses per sec using the LSI frequency of ~32kHz and prescaler 256

and then try to "pet" the watchdog using the following:

  HAL_IWDG_Refresh(&IwdgHandle); // resets the IWDG counter back to value of reload

The device immediately resets and the RCC_FLAG_IWDGRST is set, indicating the reset was due to the watchdog timeout.

This issue using the same code worked perfectly on a Nucleo L152RE (the only difference is the dependencies now are for the L4xx series instead of L1xx)

Any ideas on why this reset is occuring?

Answer 1

Independent watchdog in the Stm32L4 series have window option. When enabled, it will reset the mcu if watchdog is reloaded too early - when watchdog counter value is above the window value.

IwdgHandle.Init.Window should be set at 0xFFF (max value) if option is not required. If you have IwdgHandle declared in bss - Window field will be zero, resulting in a reset on watchdog refresh if counter is above zero.

typedef struct
{
  uint32_t Prescaler;  /*!< Select the prescaler of the IWDG.
                        This parameter can be a value of @ref IWDG_Prescaler */

  uint32_t Reload;     /*!< Specifies the IWDG down-counter reload value.
                        This parameter must be a number between Min_Data = 0 and Max_Data = 0x0FFF */

  uint32_t Window;     /*!< Specifies the window value to be compared to the down-counter.
                        This parameter must be a number between Min_Data = 0 and Max_Data = 0x0FFF */
} IWDG_InitTypeDef;

PS: Watchdog's timer is a downcounter. HAL_IWDG_Refresh reloads counter with Reload value, not resets to zero