Question:

Why would the CPU jump to an ILLEGAL_ISR during an EMI test.


Answer:

if you get an ILLEGAL_ISR, the normal course of events would be to try and shut down the system and then reset the device (you can do this by forcing a watchdog reset). ILLEGAL_ISR means the CPU has fetched an illegal op-code which means the CPU can no longer proceed in a normal state.

The most common cause of ILLEGAL_ISR type conditions we have seen under EMI testing, is either Power-Supply decoupling (not enough decoupling filters) or noise getting into the EMU0/1 + JTAG signals causing the device to go into test modes.

You want to make sure that the user has put strong pull-ups (~2K) on the EMU0 and EMU1 pins and a strong pull-down on the TRSTn (~2K) signal. Also, adding some small capacitance across the signals can help. All of this has to be placed close to the chip. Also make sure the JTAG header is not too far from the device.

Are the failures occurring while JTAG is connected? Another cause is ground loops through the JTAG Emulator. The solution to this sometimes involves using an "isolated" JTAG adaptor to avoid such issues.