RTC stops at high temperature with LSE clock sourced from a crystal oscillator

Red_Mask · ‎2026-04-16

Hello!
I am using a 32.768 kHz TCXO (TG-3541CE) as the LSE to count the RTC.
The MCU is an STM32L452RE.
At room temperature, the RTC operates correctly, but at high temperatures (around 50 °C), the RTC stops.
Strangely, once it has stopped, touching the MCU by finger causes the RTC to start running again.
Suspecting an issue with the TCXO, I tried a different TCXO, but the same behavior occurred.

When I added a 200 Ω damping resistor between the TCXO and the MCU, the RTC no longer stopped even when the temperature was raised to 60 °C.
This seems to have solved the problem for now, but I am concerned about whether 200 Ω is an appropriate value and whether this is truly a fundamental solution.
If you have any ideas about possible root causes or considerations, I would really appreciate your advice.

mƎALLEm · ‎2026-04-17

Hello @Red_Mask and welcome to the ST community,

As you are using a crystal oscillator, try to isolate the crystal and supply it separately at 50°c, does it generate a normal output as for a room temperature? probe the output with an oscilloscope.

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LCE · ‎2026-04-17

The RCC is set to LSE BYPASS ?

Red_Mask · ‎2026-04-19

I have confirmed that the oscillator continues to output 32.768 Hz waveform even at temperatures above 50°C.

Red_Mask · ‎2026-04-19

Yes. I have set the RCC to LSE BYPASS.

waclawek.jan · ‎2026-04-20

Can you reproduce the problem at a "known good" board such as Nucleo?

JW

Red_Mask · ‎2026-04-20

OK! I will try it.

Red_Mask · ‎2026-04-26

I checked whether the problem occurs on a NUCLEO board.

As a result, we confirmed that the issue does not occur on a NUCLEO board.

After that, we found that even when setting the MCU (STM32L452RE) on our own board to the LSI and increasing the temperature, the RTC does not stop.
Given this, is it possible that an issue with the PCB layout or routing could cause the RTC driven by the LSE to stop?

waclawek.jan · ‎2026-04-27

How are *all* the supply and ground pins of the mcu connected? Is the power supply rock stable?

Is it STM32L452RExx or STM32L452RExxP?

JW