RTC power supply and voltage amplitude

[PWang.5]

For STM32F4 series, when PC14/PC is used for external crystal(32.768k), which power supply is output for crystal? What is the voltage amplitude？

[waclawek.jan]

The RTC module including the LSE oscillator is in the backup voltage domain, i.e. it is supplied from VDD when it is present, or from VBAT when VDD is low.

The voltage you see on PC14/PC15 may be unrelated to the supply voltage. The RC oscillator is an analog circuit which generates bias itself appropriate to its operation. Its value is not specified.

The oscillation amplitude is not specified either. As the LSE oscillator is a very low power circuit, by probing it using common oscilloscope probe you can change this amplitude or even stop oscillations altogether.

JW

[TDK]

The crystal is a current-driven thing. The voltage will change based on capacitors, external resistors, and crystal selection, and drive level.

Why? Nothing else should be connected to these pins.

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