ISSUE: Attempting to read/program MCU mem has not been possible and I would like feedback on my setup. NOTE: Blink program flashed before removing still works.

Actual Error on STM32 ST LINK Utility:

Can not connect to target! Please select "Connect Under Reset" mode from Target->Settings menu and try again. If you're trying to connect to a low frequency application , please select a lower SWD Frequency mode from Target->Settings menu. No target connected

Due to chip shortage, I removed an STM32F303K8T6 from a Nucleo32 board. Removal was done with heat gone. No physical damaged sustained and was careful with ESD. I then used solder paste to solder MCU on a SMT breakout from Adafruit (This one), and installed pin headers. Finally checked for shorts and proper continuity on all pins.

The MCU now sits on a breadboard:

  1. Boot pin (#31) pulled low with 10k.
  2. NRST pin (#4) connected in between (10k to VDD) and (100nF cap to GND).
  3. Added 3 x 100nF Caps between VDD and GND (ST recommended 1 per VDD pin + VDDA)
  4. Pin #26 is connected to LED and through 510 resistor to ground (This works: More below)

STLink - V3SET Programmer (CN6 Described on Page 26) connections:

  1. VCC <> VDD (MCU pin # 1)

  2. GND <> GND (MCU pin # 32)

  3. NRST <> NRST (MCU pin # 4)

  4. DIO <> DIO (MCU pin # 23)

  5. CLK <> CLK (MCU pin # 24)

  6. Software: STM32 ST Link Utility

Upon Powering on, the breadboard setup draws about 25mA but the target isn't recognized by ST Link Utility. If I'm not mistaken, the MCU is running with its internal clock while on Nucleo32 by default.

All other pins were left floating and prior to removing MCU it had a blink program flashed. Program simply toggles pin #26.LED in breadboard confirms Program is still running.

NOTE: I was successful at using ST Link Utility and some jumpers to program MCU while on Nucleo board using essentially the same setup as above i.e. without USB connection. In this case the board draws between 30 and 35mA while programmer is connected to target.

Any help would be greatly appreciated. I'm happy to provide more info as needed. Thank you!

  • 1
    \$\begingroup\$ Looks ok, but it's probably got something to do with your setup. Make sure voltage levels on all Vcc are appropriate, you may have missed a ground pin or something. Also a breadboard is going to add inductance and resistance to every pin vs a PCB \$\endgroup\$
    – Voltage Spike
    Mar 24, 2022 at 18:09
  • 1
    \$\begingroup\$ Thank you @VoltageSpike Your comment prompted me to improve the connections and now it works. \$\endgroup\$
    – Mauricio
    Mar 24, 2022 at 21:20

1 Answer 1


So I got it to work removing long cables and re-wiring the whole thing with better quality/shorter jumper wires.

The connections didn't change from the initial post description.

The cables replaced were floating in the air and attached via grabbers (mini-hook test leads). The new connections used the breadboard connectivity exclusively via good quality jumpers i.e. All connections were made through the breadboard.

  • 1
    \$\begingroup\$ Yeah, those connections are among the first I route on a PCB because they make or break the whole deal with little workaround. \$\endgroup\$
    – DKNguyen
    Mar 25, 2022 at 0:19

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