NXP Semiconductors K22F series, Reference Manual, Page: 1372 / 1407

The transmit warning flag is set when the number of entries in any of the enabled
transmit FIFOs is empty and is cleared when the number of entries in each enabled
transmit FIFO is not empty.
The receive warning flag is set when the number of entries in any of the enabled receive
FIFOs is full and is cleared when the number of entries in each enabled receive FIFO is
not full.
The FIFO warning flag can generate an Interrupt or a DMA request.
49.4.7.3 FIFO error flag
The transmit FIFO error flag is set when the any of the enabled transmit FIFOs
underflow. After it is set, all enabled transmit channels will transmit zero until
TCSR[FEF] is cleared and the next transmit frame starts. All enabled transmit FIFOs
must be reset and initialized with new data before TCSR[FEF] is cleared.
When TCR4[FCONT] is set, the FIFO will continue transmitting data following an
underflow without software intervention. To ensure that data is transmitted in the correct
order, the transmitter will continue from the same word number in the frame that caused
the FIFO to underflow, but only after new data has been written to the transmit FIFO.
Software should still clear the TCSR[FEF] flag, but without reinitializing the transmit
FIFOs.
RCSR[FEF] is set when the any of the enabled receive FIFOs overflow. After it is set, all
enabled receive channels discard received data until RCSR[FEF] is cleared and the next
next receive frame starts. All enabled receive FIFOs should be emptied before
RCSR[FEF] is cleared.
When RCR4[FCONT] is set, the FIFO will continue receiving data following an
overflow without software intervention. To ensure that data is received in the correct
order, the receiver will continue from the same word number in the frame that caused the
FIFO to overflow, but only after data has been read from the receive FIFO. Software
should still clear the RCSR[FEF] flag, but without emptying the receive FIFOs.
The FIFO error flag can generate only an interrupt.
Functional description
K22F Sub-Family Reference Manual, Rev. 4, 08/2016
1372 NXP Semiconductors