Hi John,What type of test environment you are using?- microcontroller / silicon?- FPGA?- Verilog simulation?A few things to check:- value of PRIMASK (if set, it disable all interrupts)- value of IPSR (if it is already running another interrupt, then the current priority level might be higher or same as the interrupt).- what happen with the program? Does it still running, or does it hangs/crashed when the interrupt take place? Could you check if the vector table is set correctly in the compiled program image?regards,Joseph