Serial Status Register
| TBI | Transmit bus idle flag |
| TDRE | Transmit data empty flag bit |
| RDRF | Received data full flag bit |
| ORE | Overrun error flag bit |
| FRE | Framing error flag bit |
| PE | Parity error flag bit (only functions in operation mode 0) |
| REC | Received error flag clear bit |