control register 2
| RXDMAEN | Rx buffer DMA enable |
| TXDMAEN | Tx buffer DMA enable |
| SSOE | SS output enable |
| NSSP | NSS pulse management |
| FRF | Frame format |
| ERRIE | Error interrupt enable |
| RXNEIE | RX buffer not empty interrupt enable |
| TXEIE | Tx buffer empty interrupt enable |
| DS | Data size |
| FRXTH | FIFO reception threshold |
| LDMA_RX | Last DMA transfer for reception |
| LDMA_TX | Last DMA transfer for transmission |