APB1 peripheral clock enable register
| TIM2EN | TIM2 clock enable |
| TIM3EN | TIM3 clock enable |
| TIM4EN | TIM4 clock enable |
| TIM5EN | TIM5 clock enable |
| WWDGEN | Window watchdog clock enable |
| SPI2EN | SPI2 clock enable |
| SPI3EN | SPI3 clock enable |
| USART2EN | USART 2 clock enable |
| I2C1EN | I2C1 clock enable |
| I2C2EN | I2C2 clock enable |
| I2C3EN | I2C3 clock enable |
| PWREN | Power interface clock enable |