MC908QT4MPE datasheet(105/184 Pages) FREESCALE

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Reset and System Initialization

MC68HC908QY/QT Family Data Sheet, Rev. 5

Freescale Semiconductor

105

Figure 13-2. SIM Clock Signals

13.3.1 Bus Timing

In user mode, the internal bus frequency is the oscillator frequency (BUSCLKX4) divided by four.

13.3.2 Clock Start-Up from POR

When the power-on reset module generates a reset, the clocks to the CPU and peripherals are inactive

and held in an inactive phase until after the 4096 BUSCLKX4 cycle POR time out has completed. The

IBUS clocks start upon completion of the time out.

13.3.3 Clocks in Stop Mode and Wait Mode

Upon exit from stop mode by an interrupt or reset, the SIM allows BUSCLKX4 to clock the SIM counter.

The CPU and peripheral clocks do not become active until after the stop delay time out. This time out is

selectable as 4096 or 32 BUSCLKX4 cycles. See 13.7.2 Stop Mode.

In wait mode, the CPU clocks are inactive. The SIM also produces two sets of clocks for other modules.

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.

13.4 Reset and System Initialization

The MCU has these reset sources:

•

Power-on reset module (POR)

•

External reset pin (RST)

•

Computer operating properly module (COP)

•

Low-voltage inhibit module (LVI)

•

Illegal opcode

•

Illegal address

All of these resets produce the vector $FFFE–FFFF ($FEFE–FEFF in monitor mode) and assert the

internal reset signal (IRST). IRST causes all registers to be returned to their default values and all

modules to be returned to their reset states.

An internal reset clears the SIM counter (see 13.5 SIM Counter), but an external reset does not. Each of

the resets sets a corresponding bit in the SIM reset status register (SRSR). See 13.8 SIM Registers.

÷ 2

BUS CLOCK

GENERATORS

SIM

SIM COUNTER

FROM

OSCILLATOR

FROM

OSCILLATOR

BUSCLKX2

BUSCLKX4

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