SPI Transmit Control
| RESERVED | Reserved. Read value is undefined, only zero should be written. |
| TXSSEL0 | Transmit Slave Select 0. |
| TXSSEL1 | Transmit Slave Select 1. |
| TXSSEL2 | Transmit Slave Select 2. |
| TXSSEL3 | Transmit Slave Select 3. |
| EOT | End of Transfer. |
| EOF | End of Frame. |
| RXIGNORE | Receive Ignore. |
| RESERVED | Reserved. Read value is undefined, only zero should be written. |
| FLEN | Frame Length. |
| RESERVED | Reserved. Read value is undefined, only zero should be written. |