NXP Semiconductors MCF5253, Reference Manual, Page: 398 / 648

Background Debug Mode (BDM) Interface
MCF5253 Reference Manual, Rev. 1
20-28 Freescale Semiconductor
exception stack is created, the processor fetches a unique exception vector, 12, from the vector table (Refer
to the ColdFire Programmer’s Reference Manual).
Execution continues at the instruction address contained in this exception vector. All interrupts are ignored
while in emulator mode. Users can program the debug-interrupt handler to perform the necessary context
saves using the supervisor instruction set. As an example, this handler may save the state of all the
program-visible registers as well as the current context into a reserved memory area.
Once the required operations are completed, the return-from-exception (RTE) instruction is executed and
the processor exits emulator mode. Once the debug interrupt handler has completed its execution, the
external development system can then access the reserved memory locations using the BDM commands
to read memory.
Prior to the Rev. A implementation, if a hardware breakpoint (For example, a PC trigger) is left unmodified
by the debug interrupt service routine, another debug interrupt is generated after the RTE instruction
completes execution. In the Rev. A design, the hardware has been modified to inhibit the generation of
another debug interrupt during the first instruction after the RTE exits emulator mode. This behavior is
consistent with the existing logic involving trace mode, where the execution of the first instruction occurs
before another trace exception is generated. This Rev. A enhancement disables all hardware breakpoints
until the first instruction after the RTE has completed execution, regardless of the programmed trigger
response.
20.4.1.1 Emulator Mode
Emulator mode is used to facilitate non-intrusive emulator functionality. This mode can be entered in three
different ways:
• The EMU bit in the CSR may be programmed to force the ColdFire processor to begin execution
in emulator mode. This bit is only examined when RSTI is negated and the processor begins reset
exception processing. It may be set while the processor is halted before the reset exception
processing begins. Refer to
Section 20.3.1, “CPU Halt.”
• A debug interrupt always enters emulation mode when the debug interrupt exception processing
begins.
• The TCR bit in the CSR may be programmed to force the processor into emulation mode when
trace exception processing begins.
During emulation mode, the ColdFire processor exhibits the following properties:
• All interrupts are ignored, including level seven.
• If the MAP bit of the CSR is set, all memory accesses are forced into a specially mapped address
space signalled by TT = $2, TM = $5 or $6. This includes the stack frame writes and the vector
fetch for the exception which forced entry into this mode.
• If the MAP bit in the CSR is set, all caching of memory accesses is disabled. Additionally, the
SRAM module is disabled while in this mode.
The return-from-exception (RTE) instruction exits emulation mode. The processor status output port
provides a unique encoding for emulator mode entry ($D) and exit ($7).