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Is there a way to make the internal MSI (RC) clock recalibrate itself (automatically in hardware) onto the LSE 32.786kHz clock input, effectively reaching the same precision as a sealed-external oscillator? the slow speed clock (LSE) cannot be fed into the PLL circuitry which provides e.g the 80Mhz core clock.

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The Reference Manual for your part states in section 6.2.3

Hardware auto calibration with LSE (PLL-mode) When a 32.768 kHz external oscillator is present in the application, it is possible to configure the MSI in a PLL-mode by setting the MSIPLLEN bit in the Clock control register (RCC_CR). When configured in PLL-mode, the MSI automatically calibrates itself thanks to the LSE. This mode is available for all MSI frequency ranges. At 48 MHz, the MSI in PLL-mode can be used for the USB FS device, saving the need of an external high-speed crystal.

So yes, it does look possible.

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According to ref. manual, chapter "6.2.3 - MSI clock", if you use MSI + PLL, it automatically autocalibrate by hardware when an LSE is present.

I've not tested it, but according to ref. manual you have to do nothing to achieve what you want.

However, this is obvious just reading the ref. manual. If you hadn't search this in the manual before asking, you should do it in the future, you are expected to search for the solution by yourself before asking. If what happens is that this is not what you need, maybe you should clarify your question.

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