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Central Processor Unit (CPU)

MC9S08GB/GT

118

MOTOROLA

clocks running in stop mode. This optionally allows an internal periodic signal to wake the target MCU

from stop mode.

When a host debug system is connected to the background debug pin (BKGD) and the ENBDM control

bit has been set by a serial command through the background interface (or because the MCU was reset into

active background mode), the oscillator is forced to remain active when the MCU enters stop mode. In this

case, if a serial BACKGROUND command is issued to the MCU through the background debug interface

while the CPU is in stop mode, CPU clocks will resume and the CPU will enter active background mode

where other serial background commands can be processed. This ensures that a host development system

can still gain access to a target MCU even if it is in stop mode.

Recovery from stop mode depends on the particular HCS08 and whether the oscillator was stopped in stop

mode. Refer to the Modes chart in Section 3 for more details.

7.5.5 BGND Instruction

The BGND instruction is new to the HCS08 compared to the M68HC08. BGND would not be used in

normal user programs because it forces the CPU to stop processing user instructions and enter the active

background mode. The only way to resume execution of the user program is through reset or by a host

debug system issuing a GO, TRACE1, or TAGGO serial command through the background debug

interface.

Software-based breakpoints can be set by replacing an opcode at the desired breakpoint address with the

BGND opcode. When the program reaches this breakpoint address, the CPU is forced to active

background mode rather than continuing the user program.

7.6 HCS08 Instruction Set Summary

Instruction Set Summary Nomenclature

The nomenclature listed here is used in the instruction descriptions in Table 7-1.

Operators

( )

Contents of register or memory location shown inside parentheses

← = Is loaded with (read: “gets”)

Boolean AND

Boolean OR

⊕ = Boolean exclusive-OR

× = Multiply

÷ = Divide

Concatenate

Add

–

Negate (two’s complement)

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