# STM32F401RE - NRST held low

**URL:** <https://forum.digikey.com/t/stm32f401re-nrst-held-low/19566>\
**Category:** Integrated Circuits (ICs)\
**Tags:** stmicroelectronics, embedded\
**Created:** [January 24, 2022, 11:27am UTC](https://forum.digikey.com/t/stm32f401re-nrst-held-low/19566 "2022-01-24T11:27:23Z")\
**Posts on this page:** 3\
**Page:** 1

<div class="post-metadata">

**Author:** ![vale.niskanen](https://avatars.discourse-cdn.com/v4/letter/v/b782af/32.png) [@vale.niskanen](https://forum.digikey.com/u/vale.niskanen)\
**Post date:** [January 24, 2022, 11:27am UTC](https://forum.digikey.com/t/stm32f401re-nrst-held-low/19566/1 "2022-01-24T11:27:23Z")

</div>

I started tinkering with Nucleo STM32F401RE. I was just blinking led succesfully at PA0 pin and suddenly the problem occured.

The CubeIDE gives me an error code when I’m trying to program it (Target Held under Reset).

I measured the Voltage on NRST pin (68mV). So I think something is holding the NRST pin LOW all the time.

I have tried different boot methods with Cube programmer but none of them have worked. I suspect there is something wrong with the board (maybe something shortcuts the NRST to GND)?

Does anyone have any suggestions?

---

<div class="post-metadata">

**Author:** ![Kristof\_2649](https://sea2.discourse-cdn.com/digikey/user_avatar/forum.digikey.com/kristof_2649/32/2325_2.png) [@Kristof\_2649](https://forum.digikey.com/u/Kristof_2649)\
**Post date:** [January 24, 2022, 2:16pm UTC](https://forum.digikey.com/t/stm32f401re-nrst-held-low/19566/2 "2022-01-24T14:16:55Z")

</div>

> [@vale.niskanen](#):
>
> STM32F401RE

Hello Vale Niskanen,

Thank you for your inquiry.

Per Page 22 of the datasheet for STM32F401RE

> In regulator OFF mode, the following features are no more supported:  
> • PA0 cannot be used as a GPIO pin since it allows to reset a part of the V12 logic power  
> domain which is not reset by the NRST pin.  
> • As long as PA0 is kept low, the debug mode cannot be used under power-on reset. As  
> a consequence, PA0 and NRST pins must be managed separately if the debug  
> connection under reset or pre-reset is required

---

<div class="post-metadata">

**Author:** ![vale.niskanen](https://avatars.discourse-cdn.com/v4/letter/v/b782af/32.png) [@vale.niskanen](https://forum.digikey.com/u/vale.niskanen)\
**Post date:** [January 24, 2022, 6:51pm UTC](https://forum.digikey.com/t/stm32f401re-nrst-held-low/19566/3 "2022-01-24T18:51:41Z")

</div>

Thanks Kristofer for your answer.

As far as I understand the nucleo board (STM32F401RE) has LQFP64 package. So does this chip even have the Regulator OFF option(page 22)?

Also I’m quite beginner with these boards so I would appreciate little more help/instructions with the issue 🙂