Threshold Control register.
| RX_FIFO_LVL | RX FIFO Threshold Level.When the RX FIFO reaches this many bytes or higher, UARTn_INFTL.rx_fifo_level is set. |
| TX_FIFO_LVL | TX FIFO Threshold Level. When the TX FIFO reaches this many bytes or higher, UARTn_INTFL.tx_fifo_level is set. |
| RTS_FIFO_LVL | RTS threshold control. When the RX FIFO reaches this many bytes or higher, the RTS output signal is deasserted, informing the transmitting UART to stop sending data to this UART. |