MC68HC908GR4MP datasheet(43/286 Pages) FREESCALE

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MC68HC908GR8 • MC68HC908GR4 Data Sheet, Rev. 7

Freescale Semiconductor

Chapter 4

Resets and Interrupts

4.1 Introduction

Resets and interrupts are responses to exceptional events during program execution. A reset re-initializes

the MCU to its startup condition. An interrupt vectors the program counter to a service routine.

4.2 Resets

A reset immediately returns the MCU to a known startup condition and begins program execution from a

user-defined memory location.

4.2.1 Effects

A reset:

•

Immediately stops the operation of the instruction being executed

•

Initializes certain control and status bits

•

Loads the program counter with a user-defined reset vector address from locations $FFFE and

$FFFF, $FEFE and $FFFF in monitor mode

•

Selects CGMXCLK divided by four as the bus clock

4.2.2 External Reset

A logic 0 applied to the RST pin for a time, tIRL, generates an external reset. An external reset sets the

PIN bit in the SIM reset status register.

4.2.3 Internal Reset

Sources:

•

Power-on reset (POR)

•

Computer operating properly (COP)

•

Low-power reset circuits

•

Illegal opcode

•

Illegal address

All internal reset sources pull the RST pin low for 32 CGMXCLK cycles to allow resetting of external

devices. The MCU is held in reset for an additional 32 CGMXCLK cycles after releasing the RST pin.

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