Is it normal to have a long and irregular LSE startup time?

[JHuls.1]

I am using the STM32L151 processor.

I noticed that the LSE takes between 140ms and 712ms to get ready RCC_FLAG_LSERDY. I think Is a time of 700ms still a correct time? Fluctuations are normal?

code:

while ((uint16_t)RCC_GetFlagStatus(RCC_FLAG_LSERDY) == (uint16_t)RESET)

 {

    /* Waiting for LSE to be started */

}

[TDK · Accepted solution]

The datasheet says startup time is 1s typical and can vary significantly. Seems normal to me.

4. tSU(LSE) is the startup time measured from the moment it is enabled (by software) to a stabilized 32.768 kHz oscillation is reached. This value is measured for a standard crystal resonator and it can vary significantly with the crystal manufacturer.

[waclawek.jan]

LSE startup up to several seconds is absolutely normal, and within that, timing irregularities are normal, too.

In fact, paradoxically, the better the LSE design is, the longer it takes to start up.

JW

[JHuls.1]

Thanks for the answers.

In the In the Oscillator design guide (https://www.st.com/resource/en/application_note/cd00221665-oscillator-design-guide-for-stm8afals-stm32-mcus-and-mpus-stmicroelectronics.pdf)

I found also the following, in 3.6 Startup time: The startup time of a 32.768 kHz crystal ranges from 1 to 5 s.