Datasheet

MC68HC908GZ16 • MC68HC908GZ8 Data Sheet, Rev. 4
Freescale Semiconductor 169
Chapter 14
Resets and Interrupts
14.1 Introduction
Resets and interrupts are responses to exceptional events during program execution. A reset re-initializes
the microcontroller (MCU) to its startup condition. An interrupt vectors the program counter to a service
routine.
14.2 Resets
A reset immediately returns the MCU to a known startup condition and begins program execution from a
user-defined memory location.
14.2.1 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
• Selects CGMXCLK divided by four as the bus clock
14.2.2 External Reset
A logic 0 applied to the RST pin for a time, t
RL
, generates an external reset. An external reset sets the
PIN bit in the system integration module (SIM) reset status register.
14.2.3 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.