Indeterminate state of MCU

[PChan.17]

We use STM32412 for our battery operated system.

A strange case is observed where MCU stopped responding.

Neither it got powered up, also seems the code was not being executed.

Our NRST pin is powered by the same source by which MCU is powered, so NRST did went low, as soon as the MCU got stuck. When tried to RESET MCU by powering up the system using external power the MCU did not respond to RESET. Throughout the process we did not Remove the battery from the system

Also the backup domain of the MCU was active. as the battery was not removed from the system.

Now when we removed the battery and connected again to system it started working normal. MCU responded well to all commands.

Attached is the schematic for better understanding.

Circuit Explanation:

The switch S1 when pressed momentarily gets the LDO enable signal high, which turns ON the 3v power supply which powers the MCU, after this the switch is released.

MCU on power-up will take control of LDO enable signal keeping it high and keeping the power ON. This same power is sourcing the NRST pin.

What we wish to know is that what possibly could go wrong with MCU?

How does back-up domain affect turning on of the MCU?

When will MCU not even respond to NRST reset?

[Tesla DeLorean]

>>A strange case is observed where MCU stopped responding. Neither it got powered up, also seems the code was not being executed.

If BOOT0 is High it is going to execute ROM code and not yours. You could check if the System Loader is communicative via USART1 PA9/PA10 in this scenario.

The other issue could be some kind of latch-up issue, perhaps ESD related.

[PChan.17]

Clive, Thanks a lot 😊, for prompt and apt answer.

There is a little possibility of BOOT0 going high, but ESD seems an interesting point to ponder.

It would be nice of you if you can throw some light on how ESD can affect MCU working, It will motivate me to dig deeper, also of you are running out of time, please suggest where I should look in more for this.

[Tesla DeLorean]

https://en.wikipedia.org/wiki/Latch-up

[PChan.17]

Hi! good morning Clive,

I have gone through the wiki article, regarding the latch-up condition, it suggests to have a power cycle to correct the latch-up condition.

In the schematic attached as you can see, the MCU is powered up form a LDO which upon reset will ultimately stop powering MCU and will turn on MCU again when Switch S1 is pressed.

This is how the power cycle is sequenced, during this whole process, only back-up domain of MCU i.e. back-up registers and RTC is powered, because they are directly connected to battery.

Does this back-up domain or RTC play any role in hanging the MCU?

By power cycle do we need to remove power from back-up domain too?

[PChan.17]

Hi! good morning Clive,

I have gone through the wiki article, regarding the latch-up condition, it suggests to have a power cycle to correct the latch-up condition.

In the schematic attached as you can see, the MCU is powered up form a LDO which upon reset will ultimately stop powering MCU and will turn on MCU again when Switch S1 is pressed.

[PChan.17]

Happy New Year Clive!

Any other suggestions you would like me to look into for probable causes, of such condition?

Regards,

Puneet