MC68HC908LB8_05 Datasheet, PDF (163/234 Pages) Freescale Semiconductor, Inc – M68HC08 Microcontrollers

Chapter 16
Resets and Interrupts
16.1 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.
16.2 Resets
A reset immediately returns the MCU to a known startup condition and begins program execution from a
user-defined memory location.
16.2.1 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
16.2.2 External Reset
A logic 0 applied to RST for a time, t_IRL, generates an external reset when pin PTA5/RST/KB5 is
configured as a reset pin. An external reset sets the PIN bit in the system integration module (SIM) reset
status register.
16.2.3 Internal Reset
Sources:
• Power-on reset (POR)
• Computer operating properly (COP)
• Low-power reset circuits
• Illegal opcode
• Illegal address
16.2.3.1 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 POR rearm voltage (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.
A power-on reset:
• Drives the RST pin low during the oscillator stabilization delay
MC68HC908LB8 Data Sheet, Rev. 1
Freescale Semiconductor
163