[barbegal]

According to the documentation there is no calibration as such, the MSI clock simply runs in a phase locked loop (PLL) configuration with LSE (32.768 kHz). For example in 1MHz mode the MSI is setup to run at approximately 1Mhz, this clock then goes into a downscaler which downscales by a factor of 31 to approximately 32kHz and this is compared to the LSE clock to generate feedback for the MSI clock. When locked the MSI runs at 1015.8 kHz (32.768 * 31) so out by 1.58%.

It's also possible that the design hasn't been thoroughly tested and the PLL doesn't lock in certain conditions which could leave you with an unstable clock.

[readmodifywrite]

The lack of status bits on the auto-cal is really unfortunate.

Turning it off during a UART transaction definitely "fixes" it.

I'm somewhat tempted to do the manual calibration the HSI instead.

[barbegal]

Yeah a PLL without a status flag to indicate it is locked isn't good. I think there are also issues with stabilisation when using it with stop modes https://community.st.com/t5/stm32-mcus-products/msi-pll-mode...

If you really need the accuracy then regularly time the LSE clock using a timer clocked from MSI and apply the best trim values as described in this app note file:///home/tom/Downloads/an4736-how-to-calibrate-stm32l4-series-microcontrollers-internal-rc-oscillator-stmicroelectronics-1.pdf