USART_INT Interrupt Enable Set
| DRE | Data Register Empty Interrupt Enable |
| TXC | Transmit Complete Interrupt Enable |
| RXC | Receive Complete Interrupt Enable |
| RXS | Receive Start Interrupt Enable |
| CTSIC | Clear To Send Input Change Interrupt Enable |
| RXBRK | Break Received Interrupt Enable |
| ERROR | Combined Error Interrupt Enable |