ST STM32L4x6 Reference Manual page 245

RM0351
Bits 9:8 UART5SEL[1:0]: UART5 clock source selection
Bits 7:6 UART4SEL[1:0]: UART4 clock source selection
Bits 5:4 USART3SEL[1:0]: USART3 clock source selection
Bits 3:2 USART2SEL[1:0]: USART2 clock source selection
Bits 1:0 USART1SEL[1:0]: USART1 clock source selection
These bits are set and cleared by software to select the UART5 clock source.
00: PCLK selected as UART5 clock
01: System clock (SYSCLK) selected as UART5 clock
10: HSI16 clock selected as UART5 clock
11: LSE clock selected as UART5 clock
This bit is set and cleared by software to select the UART4 clock source.
00: PCLK selected as UART4 clock
01: System clock (SYSCLK) selected as UART4 clock
10: HSI16 clock selected as UART4 clock
11: LSE clock selected as UART4 clock
This bit is set and cleared by software to select the USART3 clock source.
00: PCLK selected as USART3 clock
01: System clock (SYSCLK) selected as USART3 clock
10: HSI16 clock selected as USART3 clock
11: LSE clock selected as USART3 clock
This bit is set and cleared by software to select the USART2 clock source.
00: PCLK selected as USART2 clock
01: System clock (SYSCLK) selected as USART2 clock
10: HSI16 clock selected as USART2 clock
11: LSE clock selected as USART2 clock
This bit is set and cleared by software to select the USART1 clock source.
00: PCLK selected as USART1 clock
01: System clock (SYSCLK) selected as USART1 clock
10: HSI16 clock selected as USART1 clock
11: LSE clock selected as USART1 clock
DocID024597 Rev 3
Reset and clock control (RCC)
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