Reset pin goes down every 400ms for 50us

[msemegen]

Hi,

we have problem with our custom designed board. We are using STM32L011F4P6 MCUs and some of them behaves very strange. After reset, every 400ms for 50us, reset pin (which is internally pulled up), goes down. We don't known what causes that behavior. For sure it's not software - even txt uploaded on flash file causes the same behavior.

Did anyone encountered that before?

[waclawek.jan]

Watchdog reset, due to "hardware watchdog" enabled in Option bytes?

JW

[msemegen]

Nope,

no watchdog is activated and power supply is stable

[Piranha]

And yet that period is the default period for IWDG, which means that you didn't read everything that Jan said...

[msemegen]

IWDG is not activated in OB​... If period looks like default IWDG period, this is really strange...

[Tesla DeLorean]

Attached to anything else in-circuit that might also do this via the NRST connection?

[msemegen]

Unfortunately not...

[Piranha]

Show us the FLASH_OPTR value as a hex number.

[Uwe Bonnes]

A 50 us low pulse on NRST als shows that the recommend 100 nF to ground are not connected to NRST. That makes NRST suscesible to noise.

[msemegen]

We don't have any reset button or something like that, so we don't use RST pin.