APB1 peripheral clock enable register (RCC_APB1ENR)
| TIM3EN | Timer 3 clock enable |
| TIM6EN | Timer 6 clock enable |
| TIM7EN | TIM7 timer clock enable |
| TIM14EN | Timer 14 clock enable |
| WWDGEN | Window watchdog clock enable |
| SPI2EN | SPI 2 clock enable |
| USART2EN | USART 2 clock enable |
| USART3EN | USART3 clock enable |
| USART4EN | USART4 clock enable |
| USART5EN | USART5 clock enable |
| I2C1EN | I2C 1 clock enable |
| I2C2EN | I2C 2 clock enable |
| USBRST | USB interface clock enable |
| PWREN | Power interface clock enable |