STM32F407 stuck. Latchup? How to find the weakness?

Lucast · ‎2026-01-21

Hi all,

our product, used in industrial traction, uses an STM32F407 MCU. In certain conditions, very hard to replicate, the MCU stucks and the watchdog does not reset the MCU. It seems completely blocked.

We're suspecting there are some EMC issue on certain brushed motors used the truck but it is not under our control.

After we remove the power supply of the board and give back the supply, the board starts to work again.

Someone suggest me some latchup phenomena and I think it could be a possible explanation. Now: how can I find what is the pin (or what are the pins) susceptible to the EMI? There could be some traces the phenomena leaves on the MCU?

Thanks!

Andrew Neil · ‎2026-01-21

Is it possibly for you to attach a debugger to a board in this "stuck" state?

See: Best Practices for Debugging Intermittent MCU Freezes on STM32.

@Lucast wrote:
We're suspecting there are some EMC issue on certain brushed motors used the truck but it is not under our control.

But the susceptibility of your system is under your control.

@Lucast wrote:
how can I find what is the pin (or what are the pins) susceptible to the EMI?

Has your product undergone EMC (particularly susceptibility) testing ?

In particular, testing relevant to "industrial traction" environments?

A complex system that works is invariably found to have evolved from a simple system that worked.
A complex system designed from scratch never works and cannot be patched up to make it work.

Lucast · ‎2026-01-21

Hi Andrew,

@Andrew Neil wrote:
Is it possibly for you to attach a debugger to a board in this "stuck" state?
See: Best Practices for Debugging Intermittent MCU Freezes on STM32.

Good point! I'm going to try during the next stuck.

But the susceptibility of your system is under your control.

@Lucast wrote:
how can I find what is the pin (or what are the pins) susceptible to the EMI?
Has your product undergone EMC (particularly susceptibility) testing ?
In particular, testing relevant to "industrial traction" environments?

Yep. It's full compliant. We replicate some reset on the MCU by using ESD gun at level beyond the level of the normative (i.e. 10-11kV instead of 8kV) but we're not sure it's the same problem because we were not able to replicate the freezing with ESD gun.

Andrew Neil · ‎2026-01-21

@Lucast wrote:
we were not able to replicate the freezing with ESD gun.

So it could be just a software issue.

Take a look at the linked thread - several suggestions of things to look for there...

In particular, do a review of your Watchdog management - are there places where the watchdog could be getting updated, even though the software is "stuck" ?

A complex system that works is invariably found to have evolved from a simple system that worked.
A complex system designed from scratch never works and cannot be patched up to make it work.