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MPC555
/
MPC556
DEVELOPMENT SUPPORT
MOTOROLA
USER’S MANUAL
Rev. 15 October 2000
21-24
In debug mode the development port controls also the debug mode features of the
CPU. For more information
21.4.1 Debug Mode Support
The debug mode of the CPU provides the development system with the following basic
functions:
• Gives an ability to control the execution of the processor and maintain control on
it under all circumstances. The development port is able to force the CPU to enter
to the debug mode even when external interrupts are disabled.
• It is possible to enter debug mode immediately out of reset thus allowing the user
even to debug a ROM-less system.
• The user can selectively define, using an enable register, the events that will
cause the machine to enter into the debug mode.
• When in debug mode the user can detect the reason upon which the machine en-
tered debug mode by reading a cause register.
• Entering into the debug mode in all regular cases is restartable in the sense that
the user is able to continue to run his regular program from the location where it
entered the debug mode.
• When in debug mode all instructions are fetched from the development port but
load/store accesses are performed on the real system memory.
• Data Register of the development port is accessed using
mtspr
and
mfspr
in-
structions via special load/store cycles. (This feature together with the last one
enables easy memory dump & load).
• Upon entering debug mode, the processor gets into the privileged state (MSRPR
= 0). This allows execution of any instruction, and access to any storage location.
• An OR signal of all exception cause register (ECR) bits (ECR_OR) enables the
development port to detect pending events while already in debug mode. An ex-
ample is the ability of the development port to detect a debug mode access to a
non existing memory space.
The following figure illustrates the debug mode logic implemented in the CPU.
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Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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