STM32U535RET6 – IWDG EWI and Window Mode Clarification

[Sharif]

Hello ST Community,

I am working with the STM32U535RET6 and need clarification on the interaction between the IWDG Window feature and Early Wake-up Interrupt (EWI).

I tested the following configurations:

1. Window enabled, EWI disabled

The Window feature works as expected:

• Early refresh → IWDG reset
• Late/no refresh → IWDG reset

2. EWI enabled, Window disabled

When only EWI is enabled, the EWI interrupt occurs when the counter reaches the configured EWIT value.

In the ISR, the HAL code acknowledges the EWI by setting EWIC.

According to RM0456, writing EWIC = 1 clears EWIF and also reloads the IWDG counter with RL[11:0].

However, in my observation, the EWI ISR is called only once, and I do not observe the expected counter reload after the EWI acknowledgement.

3. EWI and Window enabled together

When both features are enabled, I observe different behavior.

For example, with EWI configured for approximately 2 seconds, the EWI occurs repeatedly at approximately every 2 seconds. The IWDG counter appears to be reloaded after each EWI, so the counter never reaches the normal watchdog timeout.

Clarification required

Could you please clarify the following for the STM32U535RET6?

1. When only EWI is enabled, why do I observe only one EWI interrupt, although RM0456 states that EWIC = 1 reloads the IWDG counter?
2. When EWI and Window mode are enabled together, why does the EWI occur periodically, with the counter apparently reloaded after each EWI?
3. Is this the expected interaction between EWIC, IWDG counter reload, EWI, and Window mode?
4. What is the recommended configuration of RL, WIN, and EWIT to use both Window mode and EWI correctly, with EWI as an early warning while still allowing the IWDG to generate a reset if the application fails to refresh?
5. Is there a recommended HAL EWI ISR handling for this use case, considering that HAL_IWDG_IRQHandler() handles the EWI flag?

My intention is to use EWI only as an early warning, while retaining the normal IWDG Window watchdog reset protection.

Thank you in advance.