APB2 peripheral clock enable register (RCC_APB2ENR)
| AFIOEN | Alternate function I/O clock enable |
| IOPAEN | I/O port A clock enable |
| IOPBEN | I/O port B clock enable |
| IOPCEN | I/O port C clock enable |
| IOPDEN | I/O port D clock enable |
| ADC1EN | ADC 1 interface clock enable |
| SPI1EN | SPI 1 clock enable |
| USART1EN | USART1 clock enable |