M68HC08 datasheet(115/332 Pages) FREESCALE

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System Integration Module (SIM)

MC68HC908JB16 — Rev. 1.1

Technical Data

Freescale Semiconductor

System Integration Module (SIM)

115

Protecting flags in break mode ensures that set flags will not be cleared

while in break mode. This protection allows registers to be freely read

and written during break mode without losing status flag information.

Setting the BCFE bit enables the clearing mechanisms. Once cleared in

break mode, a flag remains cleared even when break mode is exited.

Status flags with a 2-step clearing mechanism — for example, a read of

one register followed by the read or write of another — are protected,

even when the first step is accomplished prior to entering break mode.

Upon leaving break mode, execution of the second step will clear the flag

as normal.

8.7 Low-Power Modes

Executing the WAIT or STOP instruction puts the MCU in a low-power-

consumption mode for standby situations. The SIM holds the CPU in a

non-clocked state. The operation of each of these modes is described

here. Both STOP and WAIT clear the interrupt mask (I) in the condition

code register, allowing interrupts to occur.

8.7.1 Wait Mode

In wait mode, the CPU clocks are inactive while the peripheral clocks

continue to run. Figure 8-14 shows the timing for wait mode entry.

A module that is active during wait mode can wake up the CPU with an

interrupt if the interrupt is enabled. Stacking for the interrupt begins one

cycle after the WAIT instruction during which the interrupt occurred. In

wait mode, the CPU clocks are inactive. Refer to the wait mode

subsection of each module to see if the module is active or inactive in

wait mode. Some modules can be programmed to be active in wait

mode.

Wait mode can also be exited by a reset or break. A break interrupt

during wait mode sets the SIM break stop/wait bit, SBSW, in the SIM

break status register (SBSR). If the COP disable bit, COPD, in the

configuration register (CONFIG) is logic 0, then the computer operating

properly module (COP) is enabled and remains active in wait mode.

M68HC08 Datasheet(PDF) 115 Page - Freescale Semiconductor, Inc