HP 68302 User Manual page 119

If you have problems
Listed below are common problems and their most common causes.
Problem: You get "ERROR: Stepping failed" when trying to step.
Cause: This is most likely caused by an invalid stack. In order to
step, the emulator will first modify the values on the stack,
and then execute an RTE. If the stack pointer points to an
invalid address, or the chip-select needed to access the
stack is not initialized, the stack reads caused by the RTE
will result in a double-bus fault.
Fix: Make sure the stack pointer is pointing to a legitimate
address and that any chip-selects for that address are
initialized.
Problem: You get "ERROR: Break failed" when trying to step.
Cause: You have configured the emulator for dedicated mode
interrupts but have not put the 68302 in dedicated mode
by modifying the GIMR and setting the most significant bit.
Or, the instruction that you single-stepped resulted in a
double-bus fault and the processor is now halted.
Fix: Make sure that the processor is set for the correct
interrupt mode before single-stepping. If the interrupt
mode is correct, then use the trace command before
stepping so that you see what went wrong.
Chapter 4: Plugging the Emulator into Target Systems
Configuring the Emulator for In-Circuit Operation
119