tfe=not_empty, tfnf=full, busy=idle, rfne=empty, rff=not_full
UART Status Register
| busy | UART Busy Bit 0 (idle): undefined 1 (busy): undefined |
| tfnf | TX FIFO Not Full 0 (full): undefined 1 (not_full): undefined |
| tfe | TX FIFO Empty 0 (not_empty): undefined 1 (empty): undefined |
| rfne | RX FIFO Not Empty 0 (empty): undefined 1 (not_empty): undefined |
| rff | RX FIFO Full 0 (not_full): undefined 1 (full): undefined |