Realted: Here's an L4 snippet about calibrating the MSI: `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.`