how to use Nios Jtag uart fifo interrupt

http://forum.niosforum.com/work2/style_emoticons/<#EMO_DIR#>/sad.gif when a core of JTAG_UART is inserted to a system, the value of Write FIFO IRQ threshold and Read FIFO IRQ threshold can be set, but if I can not use a system clock and I have to use this FIFO, how can I make it to raise an intterrupt when i only ONE byte. http://forum.niosforum.com/work2/style_emoticons/<#EMO_DIR#>/blink.gif