Clock configuration register 3
| USART1SW | USART1 clock source selection |
| I2C1SW | I2C1 clock source selection |
| I2C2SW | I2C2 clock source selection |
| I2C3SW | I2C3 clock source selection |
| TIM1SW | Timer1 clock source selection |
| TIM8SW | Timer8 clock source selection |
| USART2SW | USART2 clock source selection |
| USART3SW | USART3 clock source selection |
| UART4SW | UART4 clock source selection |
| UART5SW | UART5 clock source selection |