Implementing separate power supply for IWDG (watchdog)

[Itchy]

Hi

In the STM32H7 dual-core safety manual (UM2840) Section 4.2.1 Power supply, ST states that it is recommended to use a separate power supply for IWDG. In the reference manual (RM0399) in figure 576 it is displayed that IWDG is supplied from Vdd domain. Does this mean that Vdd domain and Core domain shall have different power supplies? If not, which rails are meant?

Thanks

[Petr Sladecek]

Hello,

please study reference safety architectures at safety manual and sense of WMONe, WDTe and PEv especially. The target is to use system components capable to operate even if the micro is depowered. So, the task is not to find some internal power rail but perform safety function fully independent on the micro. No embedded IWDG or power monitor (dependent on micro power) can fully satisfy this requirement. Role of these embedded parts is rather at level of efficient additional protective mechanisms. See detailed description of CPU_SM_5 vs CPU_SM_6, too.

Best regards,

Petr

[Itchy]

Thank you for the answer. But why is the safety manual referring to a separate power supply?

[Petr Sladecek]

Please distinguish between two quite different independent watchdog systems - external (WDTe required to be powered separately) and internal (STM32 embedded IWDG).

[Itchy]

In the safety manual is written that a separate power supply for IWDG is needed. Please help me to understand this.

Safety manual (UM2840) 4.2.1

"the external watchdog is different from the digital core of the MCU, and this diversity helps to

mitigate dependent failures related to the main supply alterations. As reported in VSUP_SM_2 description,

separate power supply for IWDG or/and the adoption of an external watchdog (CPU_SM_5) increase such

diversity."

[Petr Sladecek]

Please accept that both VSUP_SM_2 and CPU_SM_5 refers to an external watchdog and address efficiency of independent both clocking and power supply. I believe the word IWDG applied at the text there is used at common sense and doesn't mean the STM32 embedded one primarily (despite dual H7 features separated VCAP and VDD what could be beneficial, too).