Frank,

"Software Flow Control" for this part is very well summarized in the scenario description on page 9 of the DataSheet.

Quad UART

The XON/XOFF example is for a typical case, file transfer. The receiver is programmed to issue XOFF when 60 of the 64 locations in the FIFO are in use (4 byte free-board).

The FIFO threshold interrupt is set to 52 to allow you to unload the Rx FIFO.

regards,
p