I'm not sure what this should look like (I'm still learning the concepts behind embedded-hal), but I'd like to have the ability to configure the MSI clock (MSIRANGE in RCC_CR), and use the LSE to calibrate MSI (MSIPLLEN in RCC_CR), as in 6.2.3 "Hardware auto calibration with LSE".

Right now, it looks like the default is to use either the PLL or HSI16 depending on the clock speed requested, right?

Need to finish following parts for release:

• UART #18
• I2C
• DMA
• Wait svd 0.14.0? brandonedens/stm32l4x6#5
• Merge with https://github.com/MabezDev/stm32l4xx-hal

@thenewwazoo Did you want to make PR for UART?

When configuring clocks without sysclock setting, HAL currently uses MSI with 4MHz frequency as default.
Which is per Ch 6.2:

The MSI is used as system clock source after startup from Reset, configured at 4 MHz.

But actually, there is significant difference in timing between using this default clock and not configuring clock at all(i.e. leaving it as it is on start).
As consequence the same configration of SysTick exception leads to different timings of exceptions depending on whether user would configure default sysclock through HAL or not.

Should be investigated