I've set up a the internal uart interrupt to fire when fifo is not empty. It works when the buffer size is default to 8 but when I increase the buffer size, the interrupt no longer fires. I see that for buffers greater than 8 bytes, the box is checked and greyed out so I am assuming that it is always active for fifos larger than 8. Am I wrong?
Just a bit wrong. When the buffer size is larger than the FIFO size an internal interrupt gets activated which shuffels data from FIFO to buffer without any user action. Poll using GetRxBufferSize() which returns the number of bytes in the buffer (not the size of buffer) and read them with GetByte(). Do not interfere with internal interrupt by reading status of UART.