Guillermo Robles

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7 years, 9 months ago.

Powering a NUCLEO-L432KC with an external source

Hi,

I would like to connect a NUCLEO-L432KC to an external source through pin +5V. The idea is to disable ST-LINK to minimize the power consumption. The user manual (http://www.st.com/resource/en/user_manual/dm00231744.pdf) states clearly in Table 4 that it is possible to use pin 4: +5V, connected to an external source and the ST-LINK won't be powered. Unfortunately, no program runs when powering the NUCLEO-L432KC using this pin. I know that I have to open bridges SB14 and SB9 when using a 3.3 V external source, but nothing is said in the case of the input through +5V. However, there is an interesting discussion in https://developer.mbed.org/questions/68476/Nucleo-F303K8-fails-to-start-program-whe/ where the user J Roth defends that it is necessary to configure SB9 = OFF for a similar NUCLEO Board to input power through pin +5V.

There seems to be contradictory guidelines to power the NUCLEO with external sources; so, before unsoldering SB9 and SB14 my question is:

How can I power the NUCLEO with an external source connected to pin +5V?

Thank you

Question relating to:

Last updated: 16 Nov 2020
Affordable and flexible platform to ease prototyping using a STM32L432KCU6 microcontroller.

I have been trying a few things, looking for a solution to this same problem. I scoped the reset line and found that, with external power on the 5V Nano pin , it was bouncing around randomly. I gather this is because it is driven by the STLINK MCU, which is only powered from the VIN regulator's 5V, called E5V on the schematic. E5V is upstream from the 5V node (on the Nano pin). There is a diode between E5V and 5V, so that it possible to connect 5V to the Nano 5V pin without pushing voltage back on the regulator output. The problem is, with 5V on the Nano pin, there is no 5V on the STLINK. It would have been better to power the STLINK from the 5V node, as then STLINK would be brought up with either 5V Nano pin or voltage on VIN. This might also explain why one respondent had success by connecting 5V to both VIN and 5V pins. I think this is the best solution, as the STLINK should still be operational for programming and debugging. Removing the NRST solder bridge would presumably disable use of STLINK.

posted by Tom Dowad 12 Nov 2018

3 Answers

Jason Tully
7 years, 9 months ago.

I had to connect my projects psu 5Volt rail to both the VIN and +5V pins for it to be powered from an "external source" (not usb). This is working for me and no solder bridges were undone. As the datasheet allso mentions, make sure you power your external source first (your psu) before connecting USB, not the other way artound. This is also working, and I can debug and develop with the external source powered first, then usb plugged in.

Thank you Jason for your answer, my idea is to avoid the use of an USB connection so, once the program has been uploaded, I would only power the NUCLEO through pin +5V. If I also use pin VIN the consumption would be larger since the ST-LINK would also be powered.

posted by Guillermo Robles 26 Apr 2017

Thanks for your post, I just found it and would have never thought of it. So far it works great.

posted by Dexter Smith 22 Jun 2019
Hwee Choo Liaw
6 years, 3 months ago.

I would like to connect a NUCLEO-L432KC to an external source through pin +5V. The idea is to disable ST-LINK to minimize the power consumption. ...

My solution: The SB9 must be removed as the T_NRST line will ground the STM32L432KC's NRST pin if the ST_LINK is not powered. Further, the pin +5V input can be supplied by a 3.7V Li-Po battery input instead of a 5V input as the 3.3V regulator will stabilize the 3.3V to the microcontroller.

Bernd Glatz
7 years, 8 months ago.

Problem I have seen using external Power Supply on 5V pin is, that when you not powering ST-LINK, the reset line is not defined, probably (in my cases) held on GND -> The target MCU is in reset then.

My solution, I made a jumper circuit which applys 3V3 onto reset when in my "running mode". In debug mode, I remove just the jumper that the ST-LINK is able to reset the target mcu.

Many thanks Bernd. I haven't still tested your solution but it is worth a try! As soon as I can do some tests I would post the results.

posted by Guillermo Robles 26 Apr 2017

I have the same problem, but, apply 3V3 to Reset don't run the program, but stop LD2... perhaps, a short circuit... Anyone have a solution to use external 5V with this board ?

posted by Fabien Proriol 02 May 2018