NUCLEO-U083RC - LPUART2 wake-up from Stop2 fails after power cycling

[Errorr__]

Hello all,

We are testing LPUART wake-up from Stop 2 mode using the STM32CubeU0 LL example:

STM32Cube_FW_U0_V1.2.0/Projects/NUCLEOU083RC/Examples_LL/LPUART/LPUART_WakeUpFromStop2_Init

We made a minimal modification to the example:

• Replaced LPUART1 with LPUART2
•  Used the corresponding GPIO pins for LPUART2 (PB10,PB11)
• Configuration otherwise unchanged (same clock source, wake-up on RX, interrupt-based RX)

Observed behavior on NUCLEO-U083RC

1. After programming the board using STM32CubeProgrammer in hardware reset mode, the example works correctly:
   1. MCU enters Stop 2
   2. RX activity on LPUART2 wakes the system as expected
2. However, after disconnecting and reconnecting the power supply LPUART2 no longer wakes the MCU from Stop 2
3. RX data arrives on the pin, but no wake-up occurs
4. If we again connect and disconnect the programmer (in hardware reset mode) the system starts working normally again, wake-up from Stop 2 via LPUART2 is restored

Additional observations

• Using LPUART1 instead of LPUART2, with the same configuration approach, works reliably
• The issue appears specific to LPUART2 wake-up after a cold power cycle
• The same behavior was also observed on a custom board using STM32U073CBT6

Has this behavior been observed before?
Is there any known requirement or additional reset/initialization step specific to LPUART2 for wake-up from Stop 2 after a power-on reset? I have attached the (modified) example code for your reference.

Thanks in advance for your support.

[Gyessine · Accepted solution]

Hello @Errorr__ 
Based on the reference manual table 22
LPUART 2 and 3 does not support wakeup capability from stop 2 mode.

Only LPUART1 supports this feature. This limitation is intentional to make stop 2 mode more power efficient.
I recommend using LPUART1, or use stop 0 or stop 1 mode if you need to use LPUART2 or LPUART3.

BR
Gyessine

[Errorr__]

Hi @Gyessine ,

Thank you for the response. When we checked the reference manual, Table 22 indicates that LPUART (x = 1, 2, 3) supports wake-up from Stop 2 mode (I’ve attached a screenshot for reference). However, it appears that this information differs between document versions. The document you referred to is RM0503 Rev. 4, whereas we were referring to RM0503 Rev. 2.

I also reverified the ST example using LPUART2 with Stop 1 mode instead of Stop 2, and it works correctly.

[Gyessine]

Hello @Errorr__ 
the fact that LPUART2 and LPUART3 supports wake-up capability in stop 2 mode in revision 2 is a documentation error that was corrected in newer versions
I am glad you solved your issue. Please mark it as solved so that it can serve as a reference for other community users.
BR

Gyessine

[Andrew Neil]

---

@Errorr__ wrote:

The document you referred to is RM0503 Rev. 4, whereas we were referring to RM0503 Rev. 2.

---

Always good to make sure you have the most recent revision!

If you look at the revision history in Rev 4, you'll see that a lot was changed since Rev 2.

[Errorr__]

Yes, this was actually good lesson!