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?

[msemegen]

We found workaround for that problem. It's probably bug in some MCUs we have or in STM32CubeProgrammer.

[Uwe Bonnes]

Please let us know the workaround...

[Piranha]

> we don't use RST pin

There is no point in researching other issues until the hardware is fixed. And it's NRST, not RST. The letter N has an important meaning. The datasheet, AN4467, all ST evaluation boards and even the popular but broken schematic of BluePill board all explain/show the necessary NRST capacitor.

[msemegen]

That strange behavior described above seems to appear only if flash is not empty. Workaround steps:

​1. Connect to mcu using Power down mode

2. Erase all flash concent.

3. Disconnect and connect again using Normal mode

​4. Deselect IWDG OB if it is selected and Applay changes.

5. Select IWDG OB and Applay changes

6. Flash firmware.

Now IWDG is inactive (I know it's illogical because in step 5 we activate IWDG, but after that MCU is not restarting every 400ms). It works for some of STM32L011F4P6.

[Piranha]

You can also check the OPTVERR bit in FLASH_SR, but anyway all of that can be a result of the missing NRST capacitor.

[msemegen]

So let's make things clear. That NRST capacitor is needed even if ​there is no reset button on the board?

[Piranha]

Yes.

[msemegen]

After applying NRST capacitor restarts still appears. So we have no idea what is going on...

[Piranha]

So read the FLASH_OPTR and FLASH_SR hex values in those cases where restarts happen.

[msemegen]

FLASH->OPTR = 0xA00AA (WDG_SW bit is off...)

FLASH->SR = 0xC