Nucleo-L4R5 not working when powered externally

[b.a.]

I have a Nucleo-L4R5 board that I want to power externally and for power consumption reasons, deactivate the ST-Link.

For that reason I removed the jumper from JP6 and applied 5V power to CN8 pin 9 (5V)and 13 (Gnd).

USB is plugged to the ST-Link and I program the board with the timedWakeUp-example from the STM32Duino-LowPower library.

The builtin LED starts blinking as expected, but as soon as I pull the USB from the ST-Link part of the board, the blinking stops.

Board version is MB1312-A01 to which I cannot find the documentation/schematic. Only newer A03 Version is available.

When I look into the A03 schematic, all should be fine and the MCU running when JP6 jumper is pulled. The 3V3 regulator U6 is powered and I can measure that the 3V3 voltage is present.

Any idea what might be wrong?

[Andrew Neil]

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@b.a. wrote:

I have a Nucleo-L4R5 

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You mean the Nucleo-L4R5ZI ?

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@b.a. wrote:

 for power consumption reasons, deactivate the ST-Link.

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That board has the break-off ST-Link section (highlighted above). The surest way to deactivate the ST-Link is to break it off!

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[b.a.]

Yes, that's the board I use.

Of course I am aware that the ST-Link can be broken off.

So if I am asking here, maybe that is not the solution I would prefer...

Furthermore, I really would like to understand what is going on.

I am quite sure that breaking of the ST-Link will not change the current behaviour of the board not running the program with deactivated ST-Link.

So I'll end up with a broken board AND no solution ...

[Andrew Neil · Accepted solution]

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@b.a. wrote:

Of course I am aware that the ST-Link can be broken off.

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That's not a given - many don't realise that !

My guess would be that the "deactivated" (but still connected) ST-Link is holding the NRST line down.

It looks like there is a link to disconnect NRST - so you could try that.

You said this was specifically about power consumption. In that case, you really do need to be sure that all connections to the ST-Link are properly disconnected - otherwise there will be leakage ...

[b.a.]

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ST-Link is holding the NRST line down.

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nailed it :)

Thank you very much, as well for the hint to cutting all the connections.

[b.a.]

Oh! Just realized, that when I power the ST-Link from a different source, and connect JP6 to E5V, it draws no power from the E5V!
So that is a quick way to roughly measure power consumption of the MCU part :)

[Andrew Neil]

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@b.a. wrote:

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ST-Link is holding the NRST line down.

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nailed it :)

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Glad that helped!

The User Manual mentions it for external 3.3V operation, but not 5V:

https://www.st.com/resource/en/user_manual/um2179-stm32-nucleo144-boards-mb1312-stmicroelectronics.pdf#page=23

@STTwo-32 - is this a documentation error/omission?