ST STM32L4x6 Reference Manual page 244

Reset and clock control (RCC)
Bits 23:22 SAI1SEL[1:0]: SAI1 clock source selection
Caution:
Bits 21:20 LPTIM2SEL[1:0]: Low power timer 2 clock source selection
Bits 19:18 LPTIM1SEL[1:0]: Low power timer 1 clock source selection
Bits 17:16 I2C3SEL[1:0]: I2C3 clock source selection
Bits 15:13 I2C2SEL[1:0]: I2C2 clock source selection
Bits 13:12 I2C1SEL[1:0]: I2C1 clock source selection
Bits 11:10 LPUART1SEL[1:0]: LPUART1 clock source selection
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These bits are set and cleared by software to select the SAI1 clock source.
00: PLLSAI1 "P" clock (PLLSAI1CLK) selected as SAI1 clock
01: PLLSAI2 "P" clock (PLLSAI2CLK) selected as SAI1 clock
10: PLL "P" clock (PLLSAI3CLK) selected as SAI1 clock
11: External input SAI1_EXTCLK selected as SAI1 clock
If the selected clock is the external clock, it is not possible to switch to another
clock if the external clock is not present.
These bits are set and cleared by software to select the LPTIM2 clock source.
00: PCLK selected as LPTIM2 clock
01: LSI clock selected as LPTIM2 clock
10: HSI16 clock selected as LPTIM2 clock
11: LSE clock selected as LPTIM2 clock
These bits are set and cleared by software to select the LPTIM1 clock source.
00: PCLK selected as LPTIM1 clock
01: LSI clock selected as LPTIM1 clock
10: HSI16 clock selected as LPTIM1 clock
11: LSE clock selected as LPTIM1 clock
These bits are set and cleared by software to select the I2C3 clock source.
00: PCLK selected as I2C3 clock
01: System clock (SYSCLK) selected as I2C3 clock
10: HSI16 clock selected as I2C3 clock
11: reserved
These bits are set and cleared by software to select the I2C2 clock source.
00: PCLK selected as I2C2 clock
01: System clock (SYSCLK) selected as I2C2 clock
10: HSI16 clock selected as I2C2 clock
11: reserved
These bits are set and cleared by software to select the I2C1 clock source.
00: PCLK selected as I2C1 clock
01: System clock (SYSCLK) selected as I2C1 clock
10: HSI16 clock selected as I2C1 clock
11: reserved
These bits are set and cleared by software to select the LPUART1 clock source.
00: PCLK selected as LPUART1 clock
01: System clock (SYSCLK) selected as LPUART1 clock
10: HSI16 clock selected as LPUART1 clock
11: LSE clock selected as LPUART1 clock
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