STM32G0 Nucleo MB1455C board does not run on 5V supply

[KPN1]

Hi There,

I am using a STM32G0 Nucleo MB1455C board with STM32G031K8 processor.

Firmware loads ok through USB and runs as intended.

For my target application I want to power the board from a 5V supply. According to the user manual that should be possible without any changes to the solder bridges on the board.

However when I connect 5V to CN4 pin 4 the red power LED LD2 lights up, 3V3 is present but the processor does not start. It runs when I put 8V onto the Vin pin (CN4 pin 1).

This feels like a configuration issue, but the manual seems to indicate this is not needed. A schematic would go a long way, but I could not find one for this board.

Any idea what needs to change for the 5V power input to work?

Appreciate your assistance.

Regards, Peter

[mfgkw]

Hi Peter,

the schematics goes here: https://www.st.com/resource/en/schematic_pack/mb1455-g031k8-c01_schematic.pdf

Looking on the schematics above I agree with you - the board SHOULD run with +5V on CN4-4.

(Only SB11 must be connected, But it is per default, and on my board I can see a 0 Ohm there. Without SB11 powering with VIN would not succeed as well.)

But I have some unused NUCLEO-STM32G031K8. I took one of them and see the same behaviour:

- only USB connected: the board runs

- only VIN connected  on CN4-1(~5.3V or above): the board runs

- only +5V connected on CIN4-4: red LD2 lights, but the board does NOT run.

So I can not explain what's going on. But you are not alone...

Maybe ST changed something without note, you are the first who ever tried this.

[KPN1]

Thanks for the schematic link.

Looking at this, the 5V on CN4 pin 4 does not power the regulator for the ST-Link processor. The diodes prevent that.

Not that I need that in normal operation, but the ST-Link connection is likely what is holding the reset down and stopping the micro from running.

I could bridge the diode D5 to feed the 5V back to the E5V rail that powers the U1 regulator for the ST Link section. Any better ideas?

Regards, Peter

[mfgkw]

Probably you are right and G0 is kept on Reset by ST-Link. But I can not read this from UM2591, according to that it should work with +5V out of the box without ST-Link powered from USB.

[KPN1]

Thanks for looking into this for me.

Bridging D5 does work by powering the ST Link subsection and subsequent release of the reset line. This also feeds the linear regulator U8 backwards, but I think it can take that.

Maybe not elegant, but sometimes a pragmatic approach works.

Next time I know to feed it from the Vin instead, but I already had a PCB made for this test application.

Thanks for your help!

Peter

[Andrew Neil]

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@KPN1 wrote:

the ST-Link connection is likely what is holding the reset down and stopping the micro from running.

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Yes, that would be the case.

https://community.st.com/t5/stm32-mcus-boards-and-hardware/external-powering-of-nucleo-l031k6-hsi-able-to-run/m-p/663084/highlight/true#M18423

You should break SB2 to disconnect the Reset connection to the ST-Link;

Possibly also SB3 - to disconnect the VCP:

[KPN1]

Thanks Andrew,

I did see that comment, although it only seems to apply when powering from the 3V3 pin. Even then it does not make sense looking at the schematic. SB2 is the reset of the STLink MCU and SB3 one of the comms lines to the USB. The design and the manual have drifted apart somehow.

From the MB1455C schematic I can see that the T_NRST (reset from U2 / STM32G031K8 / target MCU) goes directly to the PB0 of the STLink MCU. So the port protection diodes in U7 will hold the line down (to one diode drop) unless the STLink section is powered. That is exactly what I am seeing.

The least invasive solution is still to power the STLink part, since I can't easily disconnect it from the target.

Regards, Peter

[Andrew Neil]

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@KPN1 wrote:

although it only seems to apply when powering from the 3V3 pin.

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Indeed, the manual does seem misleading/unhelpful in that respect.

But it will apply in any situation where the ST-Link is not powered.

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@KPN1 wrote:

The design and the manual have drifted apart somehow.

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Hmmm ... so they do!

Perhaps @STOne-32 could look into these documentation issues ...

[KPN1 · Accepted solution]

I think this is more of a generic question:

Are these eval boards meant to be used for anything other than USB connected debugging?

If so, it would be nice to implement a way to disconnect the STLink connection, so you can only power the target MCU. Short of that, the manual needs to be clear that the unit can only work if everything is powered.

In some older boards you could break the STLink section off, but that means any firmware upgrade later requires connecting a separate programmer. Also not practical with the small form factor PCBs that minic Arduinos.

For now, I will bridge D5 to keep the STLink powered from the 5V pin so I can use it. 

regards, Peter

[Andrew Neil]

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@KPN1 wrote:

Are these eval boards meant to be used for anything other than USB connected debugging?

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Yes, that is their key use case - a quick and simple "get you started" platform.

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@KPN1 wrote:

it would be nice to implement a way to disconnect the STLink connection, so you can only power the target MCU.

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Indeed - see also:

Excess Current Consumption Running B-L072Z-LRWAN1 From Battery??

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@KPN1 wrote:

In some older boards you could break the STLink section off,

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Indeed - that's what I always recommend as the surest way to have the ST-Link disconnected.

On this board, you should have the SB2 & SB3 links to achieve that - but it seems to have got messed-up somehow.