STM32WLE5: LSCO pin behavior during Run ↔ Stop2 mode transitions

[Masud]

Hello,

I am working with STM32WLE5CC and using the LSCO output

(configured via HAL_RCCEx_EnableLSCO(RCC_LSCOSOURCE_LSI), source = LSI)

to observe the low‑speed clock externally.

In my application:

• LSI is used as the RTC clock source
• RTC is used as the wake‑up source from Stop2 mode
• RTC is configured in binary‑only mode
• AsynchPrediv = 30
• Wake‑up period = 5 seconds

According to RM0461:

• “The clock on LSCO is available in Run, Stop, Standby and Shutdown modes.”
• “In Stop 2 mode, all I/O pins keep the same state as in the Run mode.”

This observation is independent of RF RX/TX activity and refers only to the Run ↔ Stop2 transition.

However, on hardware I observe the following behavior:

• When transitioning from Run → Stop2, the LSCO pin stops toggling for a short time (~104 µs), then resumes.
• When transitioning from Stop2 → Run, the LSCO pin stops toggling for a longer time (~530 µs) before toggling again.

This occurs even though:

• LSCO is enabled
• LSI remains enabled
• RTC wake‑up from Stop2 works correctly
• GPIO configuration is unchanged

Once the device is fully in Run or Stop2 steady state, LSCO toggles correctly.

My questions

1. Is this temporary loss or static behavior of LSCO during Run ↔ Stop2 transitions expected by design on STM32WLE5?
2. Is LSCO pin driving not guaranteed during Stop2 entry/exit sequencing, even though LSCO is available in steady‑state Run and Stop2 modes?
3. Is there any official ST documentation, application note, or clarification that describes LSCO behavior during low‑power mode transitions (as opposed to steady states)?

Any clarification from ST on the intended electrical behavior of LSCO during Stop2 transitions would be very helpful.

Thank you.

[Imen.D]

Hello @Masud ,

There is no specific behavior about LSCO pin during low-power mode transitions.
You might find help in the section "2 Power modes" in the AN5568 Application note Ultra-low-power features of STM32WL5x/Ex microcontrollers.

In addition, this presentation STM32WL5-System-Power control (PWR) covers the power modes for STM32WL5.

[Masud]

Hi @Imen.D 

Thank you for your reply.

Actually I already went through these documents but I did not find anything during transition between stop2<>run
The document is for steady state If I am correct.

what do you mean by this?
There is no specific behavior about LSCO pin during low-power mode transitions.

Still I want answers for this
is the clock will be available always on LSCO pin?

I am attaching my observation result for the proof.

Please provide me the answers for this so that I can justify the behavior

or do i need some improvement in the source code or the hardware so that I can get clock always on the LSCO