MC68HC908GT16_07 Datasheet, PDF (137/292 Pages) Freescale Semiconductor, Inc – Microcontrollers

Chapter 13
Resets and Interrupts
13.1 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.
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
• Selects CGMXCLK divided by four as the bus clock
13.2.2 External Reset
A logic 0 applied to the RST pin for a time, t_IRL, generates an external reset. An external reset sets the
PIN bit in the 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. See
Figure 13-1.
MC68HC908GT16 • MC68HC908GT8 • MC68HC08GT16 Data Sheet, Rev. 5.0
Freescale Semiconductor
137