STM32U599ZJT Bootloader Stuck in Reset Loop - STM32 Bootloader

[Sz-Benjamin]

Hello Everyone!

I am facing an issue while trying to enter the STM32 bootloader via USART1 on an STM32U599ZJT.

Issue:

When applying 3.3V to BOOT0 and performing a single reset, the MCU enters a reset (boot?) loop, which persists until BOOT0 is deasserted.

Test Case:

1. BOOT0 → 3.3V
2. NRST pulled to GND once (manual reset)
3. MCU stuck in a reset loop (observed reset pulses on NRST, ~50-120 ms period)

However, when BOOT0 → GND, the reset loop stops, and the MCU behaves normally.

Additional Information:

• Custom PCB
• Following AN2606 guidelines
• Reference test performed successfully on an STM32G0

Any insights on what might be causing this behavior would be greatly appreciated!

Thank you in advance for your help!

[Sz-Benjamin · Accepted solution]

The USB_DM pin (PA12) must be pulled high with a strong pull-up resistor (maximum 2.7kΩ).