ST STM32L4x6 Reference Manual page 202

Reset and clock control (RCC)
Bits 1:0 SW[1:0]: System clock switch
202/1693
Set and cleared by software to select system clock source (SYSCLK).
Configured by HW to force MSI oscillator selection when exiting Standby or Shutdown mode.
Configured by HW to force MSI or HSI16 oscillator selection when exiting Stop mode or in
case of failure of the HSE oscillator, depending on STOPWUCK value.
00: MSI selected as system clock
01: HSI16 selected as system clock
10: HSE selected as system clock
11: PLL selected as system clock
DocID024597 Rev 3
RM0351