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STM32L451 shorts to ground while interfacing with ST-LINK/V2 ISOL


I have made a custom pcb for the STM32L451CCU6TR using the UFQFPN48 (7x7) package.


This circuit is placed on a 2 layered pcb. For debug purposes i have only soldered on the MCU, the decoupeling capacitors, 10Kohm to ground on the BOOT0 pin and a 20 pin header for interface with the ST-LINK/V2 ISOL.


I power the pcb externally with a bench power supply set to 3v and limit the current to ~200mA

While measuring the NRST pin with a DMM I found that its about 2.71V and the VDD and VDDA pins are 3.01V so i added an 10Kohm pullup resistor to the 3.3V powerplane and the NRST pin. without this i can't even read the chip with STMCubeProgrammer. With the pullup resistor i can read the chip with STMCubeProgrammer. However when browsing trough the tabs(not changing any parameters) the connection fails suddenly and the VDD shorts to ground. Even when increasing the bench supply to 500mA the voltage drops to 0.

After removing the power the mcu stays shorted

When I replace the MCU the circuit works again having no shorts.

The MCU is entirely fresh and uses consistently about ~75mA(measuring with a DMM is series with the external power supply) dropping to about 1mA after a short while staying there indefinitely.


These are the parameters I use to connect. The short happens after a random amount of time only when connected to the debugger in STMCubeProgrammer or STMCubeIDE. I use the standard 20pin ribbon cable that comes with the st-link

Read progress:
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Download verified successfully 

 ------ Switching context ----- 
COM frequency = 4000 kHz
Target connection mode: Default
Reading ROM table for AP 0 @0xe00fffd0
Hardware watchpoint supported by the target 
ST-LINK Firmware version : V2J43S7
Device ID: 0x462
PC: 0x1fff3f3a
ST-LINK detects target voltage = 3.33 V
ST-LINK device status: HALT_MODE
GDB session, device event: 6
GDB session, device event: 6
Stm32Device, pollAndNotify running...
GDB session, device event: 3
GDB session, device event: 3
GDB session, device event: 1
GDB session, device event: 1
GDB session terminated: Client connection lost
GdbSessionManager, session terminated: 2

Console output in STMCubeIDE. Lowering the frequency doesn't change anything.

I can't seem to pinpoint the issue. Are there any known issues regarding this problem or does anybody have an idea where to test next?