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I have a device that works with two voltage levels (5V and 3V3). The first one (5V) is used for addressable LEDs (and other peripherals), and the second one (3V3) for an MCU (STM32F3). The device itself uses LDO to turn 5V into 3V3. Once the power source is connected to 5V, there should not be any problems with this.

The MCU is programmed via the ST-LINK present on the NUCLEO board, which at the same time I use as the source of 5V (USB connected to computer). ST-LINK pinout.

Now I have a problem that I am connecting two 3V3 voltages in parallel. I drew a block schematic for easier interpretation. Block schematic during programming.

Now I see three options for connecting my device during programming.

  1. Connect both 5V and 3V3 (VDD_TARGET pin) from NUCLEO to my device, which will result in two parallel 3V3 voltage sources.
  2. Connect only 5V from NUCLEO, which will be converted by LDO on my board to 3V3 and powering the MCU.
  3. Connect only 3V3 from NUCLEO, which will power the MCU, but not the peripherals utilizing 5V.

Which option should I choose?

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VDD_TARGET CN4 Pin 1 is described as VDD from application. I read that as an input to the ST-LINK to sense the supply voltage of the connected STM32 target. Whereas the question seems to assume that VDD_TARGET is an output from the ST-LINK to supply power to the connected STM32.

I have seen other documents such a UM1075 ST-LINK/V2 in-circuit debugger/programmer for STM8 and STM32 have the following note for the Target VCC pin:

The power supply from the application board is connected to the ST-LINK/V2 debugging and programming board to ensure signal compatibility between both boards.

On searching found MB1136 schematic of which sheet 4 is for the STLINK/V2-1 and the reference designators match those shown in the question. The following part of the schematic shows CN4 pin 1 is an input.

MB1136 CN4 section

AIN_1 is connected to pin PA0 of the U2 STM32F103CBT6 used on the ST-LINK, which presumably is used by the ST-LINK software to sense the power supply of the connected STM32 target.

From the options in the question:

  1. Connect both 5V and 3V3 (VDD_TARGET pin) from NUCLEO to my device, which will result in two parallel 3V3 voltage sources.

My understanding is that is OK, since VDD_TARGET is an input to the ST-LINK, and therefore won't result in two parallel 3.3 V voltage sources to the STM32 target device.

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  • \$\begingroup\$ I will try it tomorrow and report back. \$\endgroup\$ Commented Jul 5 at 18:07
  • \$\begingroup\$ Testing connectivity resulted as you said. Programming was also successful. \$\endgroup\$ Commented Jul 7 at 18:02

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