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PDF EB413 Data sheet ( Hoja de datos )
| Número de pieza | EB413 | |
| Descripción | Resetting MCUs | |
| Fabricantes | Motorola Inc | |
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Freescale Semiconductor, Inc.
Order this document
by EB413/D
Motorola Semiconductor Engineering Bulletin
EB413
Resetting MCUs
By Ross Mitchell
MCU Applications
Motorola Ltd.
East Kilbride, Scotland
Introduction
A simple function such as reset can cause many problems since
different applications impose very different conditions on the start up and
power down of the microcontroller unit (MCU). This document covers the
main issues relating to reset and aims to lead the user of the M68HC05
and M68HC11 devices to a safe a reliable approach for the application.
Reset in its most basic function ensures that the MCU starts executing
code in a controlled manner once power is applied. It may also be used
to prevent the device running out of specification (often in conjunction
with external devices) and can cause a system reset at the board level
whenever the MCU has not executed the code in the expected way
(watchdog).
© Motorola, Inc., 1993, 2000
EB413
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1 page  
Freescale Semiconductor, Inc.
Engineering Bulletin
Power-On Reset (POR) Detection
EEPROM On Chip discusses protection for on-chip EEPROM where the
use of an external LVR device is highly recommended.
Power-On Reset (POR) Detection
While the LVR circuit can protect against a partial power loss, the POR
function can only indicate a total loss of power followed by power up. The
VDD supply must drop to less than 0.2 V to ensure that the POR register
is reset correctly. On power up, a bit in one of the HC05 registers (often
the miscellaneous register if it exists) will be set to a logic high state. This
bit once cleared (by reading the bit) will remain in the logic low state even
if an external reset occurs and only be reset to a 1 when the power is
totally lost and returned.
System Power-Up Sequence
When power is applied to the system, it will normally take a finite period
to achieve the designed operating voltage. During this time, the MCU
may be required to maintain a specific state on its output pins and hold
off any execution of code until it is safe to commence running the
application.
Once power is applied to an HC05 device, the oscillator will start running
whether reset is held in the active state or not. Of course the oscillator
takes a short time to start up (4-MHz crystals take about 5 ms to start
and 32-kHz crystals take much longer, often as much as 500 ms), but
once the first couple of clocks have occurred, the active state (logic 0) of
the RESET pin normally configures the port lines to be inputs or to a
known state if they are only able to operate in output mode. In this way,
the external circuitry can assume a known state on the MCU pins just a
short time after power on but before the MCU reset is released.
Please remember that the port data registers are not usually initialized
by power on or reset and so a reset will normally leave the port data
unchanged and simply place the port in input mode.
EB413
MOTOROLA
For More Information On This Product,
Go to: www.freescale.com
5
5 Page 
Freescale Semiconductor, Inc.
Power Applied Reset
Engineering Bulletin
Power Applied Reset
It is often a requirement of the application that an external circuit controls
the reset of the MCU. In these conditions, reset may be applied and
released without power down of the application. The 4064-cycle delay is
only applicable to a power-on condition or after the MCU comes out of
stop mode. In each case the oscillator must be restarted and so a short
delay for the oscillator to stabilize is required. When a reset pulse is
subsequently received, the oscillator continues to run and there is no
start-up delay. See Power-On Reset and External Reset Timing for
the MC68HC05J1 for timing diagrams showing power-on reset and the
external reset. The minimum low time for the reset pulse is 1.5 times the
bus cycle time to ensure that a full reset occurs. It is up to the user to
ensure that the minimum low time is met. A shorter low time may cause
a partial reset of the MCU.
Power-On Reset and External Reset Timing for the MC68HC05J1
VDD
OSC1 PIN
tVDDR
(NOTE 1)
4064 tcyc
INTERNAL
CLOCK
INTERNAL
ADDRESS BUS
07FE 07FE 07FE 07FE 07FE
INTERNAL
DATA BUS
Notes:
1. Power-on reset threshold is typically between 1 V and 2 V.
2. Internal clock, internal address bus, and internal data bus are not available externally.
Figure 3. Power-On Reset Timing
EB413
MOTOROLA
For More Information On This Product,
Go to: www.freescale.com
07FE 07FF
NEW NEW
PCH PCL
11
11 Page | ||
| Páginas | Total 12 Páginas | |
| PDF Descargar | [ Datasheet EB413.PDF ] | |
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