STM32U585QII - RCC LSE Issue over temperature using TCXO MEMS-type oscillator

[Alexis-G]

Hello,

We are currently experiencing an issue with the STM32U585QII3 microcontroller.

We use the LSE pin of the microcontroller to inject a 32.768 kHz signal generated by an external TCXO MEMS-type oscillator, with the input configured in RCC_LSE_BYPASS mode. This clock supplies both a timer and the RTC function.

We have observed a loss of clock functionality at low temperatures, around -10°C to -20°C.

To further investigate this phenomenon, we routed the LSE clock to the MCO pin (since the LSCO pin is not available on our hardware).

On the functional boards, the duty cycle observed on the MCO pin is approximately 60% ON / 40% OFF, with a slight influence from temperature. (The TCXO duty-cycle is 50/50)

However, on systems exhibiting the fault, we observe at room temperature a duty cycle of around 80% ON / 20% OFF, which shifts to 98% ON / 2% OFF when cooled, until the clock on the MCO pin is lost entirely.
This also causes the RTC function and the timer to stop working. Once the board returns to room temperature, normal operation resumes.

We have also noticed that the revisions of our microcontrollers differ: systems operating normally are revision W, while those exhibiting the issue at low temperature are revision X.

Have you encountered a similar issue before? Are there any specific parameters or recommendations we should consider in this context?

Additionally, I noticed in the errata sheet a limitation related to the LSE pin between revisions X and W. Could this be linked to our problem?

Thank you very much for your feedback and recommendations.

Alexis-G

[Uwe Bonnes]

Does the input LSE clock have around 50 % duty cycle? Did you enable  LSESYSEN to use for more sinks than RTC and Tamper?

[mƎALLEm]

Hello @Alexis-G and welcome to the ST community,

As it's "TCXO MEMS-type oscillator" I don't think it's a STM32 issue but it's linked to the oscillator itself.

Did you try to isolate that oscillator (disconnect it from STM32) and see what happens on its output in that range of temperature: -10°C to -20°C?

Did you check the crystal datasheet PPM vs temp?

[Alexis-G]

Thank you for your reply.

To answer your questions: yes, the input clock at the LSE pin has a duty cycle of approximately 50%. Also, LSESYSEN is enabled.

[Alexis-G]

Thank you very much for your welcome and your reply.

Indeed, I have tested the oscillator both connected and disconnected from the microcontroller, using a shunt resistor.
The oscillator works perfectly even at -40°C and maintains its performance across the whole temperature range.