How to run NUCLEO-F031K6 on 3.3v supply

[srobi.1]

I have a working project* based on the STM32f031k6 (NUCLEO-F031K6). I have been running it successfully from a 4.8V Ni-Mh battery connected to the VIN pin for some time.

Now, I want to modify the design to run from 3.3V. I have disconnected the VIN connection, removed SB9 and SB14 as instructed in the Nucleo-32 documentation (UM1956) and connected 3.3V to CN4.14. But, now my application does not run.

If I connect the Nucleo to my PC via the USB cable**, the program runs OK. Now the strange part, if I remove the USB cable, so that the Nucleo is not connected to the PC, the program continues to run correctly from the 3.3V supply.

I did think this might be something to do with the processor being held in reset, so I put a 10k pullup on NRST but that did not help.

Can anyone tell me what else I must do to get the STM32f031k6 running on 3.3V?

*(GPS module data arriving via the UART and the data displayed on a TFT display via SPI)

** (micro to USB-A)

[srobi.1]

I have found that by connecting the 3.3V to BOTH the 3.3V pin and the 5V pin of the Nucleo, my project starts OK. I am unsure why this happens or whether it is safe. Help please?

[Peter BENSCH]

It might happen the SWD pins, i.e. SWDIO and SWCLK, catch some noise during Power-Up Reset and interprete it as the start of some debugging activity. Even you disable them in your program, the SW-DP are assigned as dedicated pins which are immediately usable by the debugger host after reset (see RM0091, section 32.3).

It shouldn't be a problem if you connect 3.3V to both 3.3V and 5V, as the NUCLEO boards are only intended for evaluation anyway and you will certainly start your own layout, won't you?

Good luck!

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/Peter