nucleo-L412KB powering from 3.3V

[ristoH]

Hi,

I'm powering nucleo with 3.3V directly to CN4/14 +3V3. SB14, SB9 are removed, JP1 is open. Works fine, BUT current is much higher than measured over JP1 when powered from USB?? 

USB is powering U5V which is powering ST- link 5V and 3V3 which is a 'parallel' power line to U3->VO->+3V3 and they should not have any sink in common??

So where is +3V3 'leaking'? to ST- link circuit??

In UM1956 there is in section 7.4.2 a paragraph:

+3V3 power supply
Using the +3V3 (CN4 pin 14) directly as a power input can be interesting, for instance, if
3.3 V is provided by a shield board. In this case, ST-LINK is not powered, thus programming
and debugging features are unavailable. When the board is powered by +3V3 (CN4 pin 14),
the solder bridge SB14 and SB9 (NRST) must be off.

So could someone please assist me what am I doing wrong or what is missing or....

BR, ristoH

[CYANG.1]

Hi,

Remove SB14 is to disconnect power from USB or E5V, remove SB9 is to avoid unpowered STLINK holding reset signal and keeping main MCU in reset status.

When the STlink is unpowered, it is possible to leaking 3.3V through the connection between main MCU and STLINK MCU on the board, through the signals of SWD (SWCLK-PA14, SWDIO-PA13) and VCP UART (PA2-TX, PA15-RX)

You can do two kinds of experiments to check if it is true. 

1. Remove SB2/SB3 to disconnect VCP UART. Set PA13/PA14 state to input/tristate, thus these two signals will not send out any powers.

2.Still power STLINK with USB 5V to guarantee STLINK MCU is powered. Because SB14 has already been removed, thus main MCU is powered by external 3.3V and STLINK MCU is powered by USB.

B.R.