The threshold value allows you to provide early indication to the processor that the receive FIFO is nearly full. A receive
FIFO overrun interrupt (ssi_rxo_intr) is generated when the receive shift logic attempts to load data into a completely full
receive FIFO. However, this newly received data are lost. A receive FIFO underflow interrupt (ssi_rxu_intr) is generated if
you attempt to read from an empty receive FIFO. This alerts the processor that the read data are invalid.