MCU Modes of Operation
MC1321x Reference Manual, Rev. 1.6
10-2
Freescale Semiconductor
•
When encountering a DBG breakpoint
After entering Active Background Mode, the CPU is held in a suspended state waiting for serial
background commands rather than executing instructions from the user’s application program.
Background commands are of two types:
•
Non-intrusive commands, defined as commands that can be issued while the user program is
running. Non-intrusive commands can be issued through the BKGD pin while the MCU is in run
mode; non-intrusive commands can also be executed while the MCU is in the Active Background
Mode. Non-intrusive commands include:
— Memory access commands
— Memory-access-with-status commands
— BDC register access commands
— The BACKGROUND command
•
Active background commands, which can be executed only while the MCU is in Active
Background Mode. Active background commands include commands to:
— Read or write CPU registers
— Trace one user program instruction at a time
— Leave Active Background Mode to return to the user’s application program (GO)
The Active Background Mode is used to program a bootloader or user application program into the
FLASH program memory before the MCU is operated in run mode for the first time. When the HCS08 is
shipped from the Freescale Semiconductor factory, the FLASH program memory is erased by default
unless specifically noted so there is no program that could be executed in run mode until the FLASH
memory is initially programmed. The Active Background Mode can also be used to erase and reprogram
the FLASH memory after it has been previously programmed.
For additional information about the Active Background Mode, refer to
10.5
Wait Mode
Wait Mode is entered by executing a WAIT instruction. Upon execution of the WAIT instruction, the CPU
enters a low-power state in which it is not clocked. The I bit in CCR is cleared when the CPU enters the
Wait Mode, enabling interrupts. When an interrupt request occurs, the CPU exits the Wait Mode and
resumes processing, beginning with the stacking operations leading to the interrupt service routine.
While the MCU is in Wait Mode, there are some restrictions on which background debug commands can
be used. Only the BACKGROUND command and memory-access-with-status commands are available
when the MCU is in Wait Mode. The memory-access-with-status commands do not allow memory access,
but they report an error indicating that the MCU is in either stop or Wait Mode. The BACKGROUND
command can be used to wake the MCU from Wait Mode and enter Active Background Mode.
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