68HC705SP7 datasheet(64/230 Pages) FREESCALE

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Resets

General Release Specification

MC68HC705JJ7/MC68HC705JP7 — Rev. 3.0

Resets

5.5.3 Low-Voltage Reset (LVR)

The LVR activates the RST reset signal to reset the device when the

voltage on the VDD pin falls below the LVR trip voltage. The LVR will

assert the pulldown device to pull the RESET pin low for three to four

cycles of the internal bus.

The LVR reset function can be enabled or disabled by programming the

LVREN bit in the MOR.

NOTE:

The LVR is intended for applications where the VDD supply voltage

normally operates above 4.5 volts.

5.5.4 Illegal Address Reset

An opcode fetch (execution of an instruction) at an address that is not in

the EPROM (locations $0700–$1FFF) or the RAM (locations

$0020–$00FF) generates an illegal address reset. The illegal address

reset will assert the pulldown device to pull the RESET pin low for three

to four cycles of the internal bus.

5.6 Reset States

The following paragraphs describe how the various resets initialize the

MCU.

5.6.1 CPU

A reset has the following effects on the CPU:

•

Loads the stack pointer with $FF

•

Sets the I bit in the condition code register, inhibiting interrupts

•

Loads the program counter with the user defined reset vector from

locations $1FFE and $1FFF

•

Clears the stop latch, enabling the CPU clock

•

Clears the wait latch, bringing the CPU out of the wait mode

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