Motorola ColdFire MCF5281 - Manual (Page 656)

Debug Support
30-38 Freescale Semiconductor
enabled when interrupt sampling occurs. For address and data breakpoints, reporting is considered
imprecise because several instructions may execute after the triggering address or data is detected.
As soon as the debug interrupt is recognized, the processor aborts execution and initiates exception
processing. This event is signaled externally by the assertion of a unique PST value (PST = 0xD) for
multiple cycles. The core enters emulator mode when exception processing begins. After the standard
8-byte exception stack is created, the processor fetches a unique exception vector, 12, from the vector
table.
Execution continues at the instruction address in the vector corresponding to the breakpoint triggered. All
interrupts are ignored while the processor is in emulator mode. The debug interrupt handler can use
supervisor instructions to save the necessary context such as the state of all program-visible registers into
a reserved memory area.
When debug interrupt operations complete, the RTE instruction executes and the processor exits emulator
mode. After the debug interrupt handler completes execution, the external development system can use
BDM commands to read the reserved memory locations.
If a hardware breakpoint such as a PC trigger is left unmodified by the debug interrupt service routine,
another debug interrupt is generated after the completion of the RTE instruction.
30.6.1.1 Emulator Mode
Emulator mode is used to facilitate non-intrusive emulator functionality. This mode can be entered in three
different ways:
• Setting CSR[EMU] forces the processor into emulator mode. EMU is examined only if RSTI is
negated and the processor begins reset exception processing. It can be set while the processor is
halted before reset exception processing begins. See
Section 30.5.1, “CPU Halt .”
• A debug interrupt always puts the processor in emulation mode when debug interrupt exception
processing begins.
• Setting CSR[TRC] forces the processor into emulation mode when trace exception processing
begins.
While operating in emulation mode, the processor exhibits the following properties:
• All interrupts are ignored, including level-7 interrupts.
• If CSR[MAP] = 1, all caching of memory and the SRAM module are disabled. All memory
accesses are forced into a specially mapped address space signaled by TT = 0x2, TM = 0x5 or 0x6.
This includes stack frame writes and the vector fetch for the exception that forced entry into this
mode.
The RTE instruction exits emulation mode. The processor status output port provides a unique encoding
for emulator mode entry (0xD) and exit (0x7).
30.6.2 Concurrent BDM and Processor Operation
The debug module supports concurrent operation of both the processor and most BDM commands. BDM
commands may be executed while the processor is running, except those following operations that access
processor/memory registers:
• Read/write address and data registers
• Read/write control registers
For BDM commands that access memory, the debug module requests the processor’s local bus. The
processor responds by stalling the instruction fetch pipeline and waiting for current bus activity to
MCF5282 and MCF5216 ColdFire Microcontroller User’s Manual, Rev. 3