STM32U385RGTQ Becomes Undetectable After First Programming

[sreejith_masensetech]

Hi everyone,

I am currently facing a persistent issue with the STM32U385RGT6Q where the MCU becomes undetectable after the first successful programming cycle.

Setup Overview

• MCU: STM32U385RGT6Q
• Programmer: STLINK-V3MINIE
• IDE: STM32CubeProgrammer
• OS used: Windows (initial attempts also on macOS)
• External protection: ESDALC6V1W5 on data lines

Schematics: MCU

Programming COnnector for ST Link

Observed Behavior

1. Initially, the STM32 was not detected by STM32CubeProgrammer
   → Root cause: faulty USB cable + macOS security restrictions
2. After switching to a proper data USB cable and Windows:
   → MCU detected and programmed successfully
   → Firmware runs (RGB LED turns green)
3. After reinstalling the ESD protection diode (ESDALC6V1W5):
   → MCU can still be programmed once successfully
4. After first programming cycle:
   • MCU becomes undetectable by ST-Link
   • RGB LED no longer functions
   • No signal observed on LED data line
   • Reprogramming is no longer possible
5. Replacing the MCU:
   → Same behavior repeats exactly
   → Works once, then becomes unresponsive

Error screenshot 

Key Suspicion

I suspect that ESDALC6V1W5 may be causing:

• Signal distortion on SWD lines (SWDIO / SWCLK)
• Back-powering or clamping issues
• Possible damage or latch-up after first execution

The issue consistently appears only after the diode is reinstalled.

 

Questions

1. Can the ESDALC6V1W5 interfere with SWD programming/debugging lines?
2. Is it possible that the diode is:
   • Injecting current into MCU pins?
   • Causing latch-up or permanent damage?
3. Are there recommended ESD protection strategies specifically for SWD lines on STM32U3 devices?
4. Could this behavior be related to option bytes or debug port being disabled after first flash?

 

Regards,
Sreejith

Engineer

Masensetech