STM32CubeIDE F411RE Can't Set LSE Source in UI

[JBell.2]

I have a Nucleo F411RE board and I noticed in STM32CubeIDE Version 1.3.0, that I can't change the LSE clock source to external. However, I can change the HSE clock source to external (see attached).

On the Nucleo board it looks like only the LSE clock is populated (X2) and the HSE clock is unpopulated (X3).

Is this a bug in STM32CubeIDE or did I miss something? FWIW I'm not using any external clock for my particular design, just playing around when I noticed this.

[TDK · Accepted solution]

You need to enable the RTC before you can change the "RTC clock mux".

Even though the board doesn't have an HSE crystal, it does have an 8MHz HSE bypass clock from the ST-link. Look at the RCC section. This is why you can enable the HSE clock.

[waclawek.jan]

CubeMX does not necessarily reflect how the chip exactly works, as it presents only a limited number of options to the users, those, which are "normally used".

In reality, of course, you can change the RTC clock source regardless of whether RTC is enabled or not. There's an interesting detail, though - once you chose RTC clock source, you can't change again it until you perform a backup domain reset. Read description of RCC_BDCR in RM.

JW

[JBell.2]

I missed both of these things:

I initially thought that mux was a "LS Clock mux", not an "RTC mux". It makes sense now why it wasn't able to change without enabling the RTC.

Also I missed that in the schematics for the board, the clock signal is coming from the ST-Link MCO line, and the resistors to the not-populated X3 pads are N/A.

Thanks for the quick and expert response.