[STM32U575] Intermittent Wake-Up Freeze with PM Enabled + Flashing Requires Power Cycle

[zafersn]

Hey folks, I’m running into some issues related to power management and STOP modes on the STM32U575, and I'm hoping to get some help or pointers.

I've had Zephyr’s power management enabled on this board since v3.6, and it’s generally been working, but I didn’t really investigate deeper until now. I'm calling this a PM-related issue because none of these problems show up when PM is disabled.

Issue 1: Flashing Requires Power Cycle

Whenever I flash a new .hex file to the board, it freezes and becomes completely unresponsive until I power cycle it. This is 100% reproducible. I saw a similar discussion here: #91112 and added a comment describing my experience.

Issue 2: Freeze During Wake-Up After Long Uptime

The second and more serious issue is that the board runs fine for a while, sometimes even a couple of days and then freezes during the wake-up process. When this happens, the watchdog eventually kicks in and resets the board.

My application is super basic: it wakes up once a day, performs a quick SPI communication, then goes back to sleep. That’s it. The delays in the SPI sequence are done with k_msleep(). Based on logs and behavior, I suspect the freeze might be happening around those sleep/delay calls, though I’m not 100% sure. There's very little else happening in the app, so it's hard to blame anything else at the moment.

Other Details:

• I assume the board is entering STOP2, since the devicetree (stm32u5.dtsi) defines only STOP0, STOP1, and STOP2:

    		cpu-power-states = < &stop0 &stop1 &stop2 >; /* in zephyr\dts\arm\st\u5\stm32u5.dtsi:41 */
  

• I’m not using CONFIG_PM_S2RAM, so that's disabled.

• Debugging is tough because the issue never occurs while using ST-Link in debug mode. So I'm relying on log outputs for now.

• I'm using the latest nucleo_u575zi_q DTS, and all low-power timer (lp_time) definitions are in place.

• My board setup (CMake and config) is the same as nucleo_u575zi_q.

• I tried upgrading from Zephyr 3.6.0 to 4.2.99, but unfortunately that didn’t change anything.

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What I’m Looking For:

• Suggestions for how to debug PM/wake-up issues more effectively without a debugger.
• Ideas on what might be freezing the system during or after STOP mode.
• Anyone who has seen similar behavior on U5 or other STM32 PM setups.
• Tips on ensuring k_msleep() and timer-based delays are safe in STOP modes (especially STOP2).
• Do I need to enable CONFIG_STM32_LPTIM_STDBY_TIMER to make k_msleep() reliable during STOP0-2?

Any help is appreciated, thanks in advance!