HCS08 datasheet(34/290 Pages) MOTOROLA

34 / 290 page

Modes of Operation

MC9S08GB/GT

MOTOROLA

Once in 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 when 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 only be executed 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 MC9S08GB/GT is

shipped from the Motorola 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 the Development Support section.

3.5 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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