Datasheet

MC68HC908GR16 Data Sheet, Rev. 5.0
Freescale Semiconductor 135
Chapter 13
Resets and Interrupts
13.1 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.
13.2 Resets
A reset immediately returns the MCU to a known startup condition and begins program execution from a
user-defined memory location.
13.2.1 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 $FEFF in monitor mode
• Selects CGMXCLK divided by four as the bus clock
13.2.2 External Reset
A 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.
13.2.3 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.
13.2.3.1 Power-On Reset (POR)
A power-on reset (POR) is an internal reset caused by a positive transition on the V
DD
pin. V
DD
at the
POR must go below V
POR
to reset the MCU. This distinguishes between a reset and a POR. The POR is
not a brown-out detector, low-voltage detector, or glitch detector.