M68HC05 Datasheet, PDF (50/128 Pages) Motorola, Inc – M68HC05 MICROCONTROLLERS

Resets
Freescale Semiconductor, Inc.
5.3 Reset Types
A reset immediately stops the operation of the instruction being
executed, initializes certain control bits, and loads the program counter
with a user-defined reset vector address. These conditions produce a
reset:
• Initial power-up (power-on reset)
• A logic 0 applied to the RESET pin (external reset)
• Timeout of the mask-optional computer operating properly (COP)
watchdog (COP reset)
• An opcode fetch from an address not in the memory map (illegal
address reset)
Figure 5-1 is a block diagram of the reset sources.
COP WATCHDOG
(MASK OPTION)
V_DD
POWER-ON RESET
ILLEGAL ADDRESS RESET
INTERNAL ADDRESS BUS
RESET
INTERNAL CLOCK
R
RST
RESET
LATCH
TO CPU
AND
SUBSYSTEMS
Figure 5-1. Reset Sources
5.3.1 Power-On Reset
A positive transition on the V_DD pin generates a power-on reset. The
power-on reset is strictly for power-up conditions and cannot be used to
detect drops in power supply voltage.
Technical Data
Resets
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MC68HC05J1A — Rev. 3.0