INT4 configured but ISR doesn't work

Question: What care should be taken while configuring INT4?

 

Answer:

The bit 0 (AV4EN bit) of INTSETUP register must be set to '1'. This bit enables autovectoring on INT 4. To save the code and processing time required to sort out which FIFO interrupt occurred, the USB core provides a second level of interrupt vectoring, called Autovectoring. When the CPU takes a USB interrupt, it pushes the program counter onto its stack, and then executes a jump to address 53, where it expects to find a jump instruction to the INT4 service routine.