AVR180
8
Alternative 3: Integrated Circuit Solutions
Characteristics
•
Easy to Mount
•
Very Accurate Threshold Voltage
•
Low Power Consumption
•
Small Footprint
•
Low Component Count
•
Wide Variety in Additional Functionality
Introduction
A selection of integrated circuits is available from various
semiconductor suppliers. They vary from simple 3-pin fixed
voltage detectors to advanced circuitry containing watch-
dog timers and power on reset (POR) time-outs. Because
all AVR MCUs have built-in watchdog and POR circuitry,
these functions do not require handling by the external IC.
The threshold accuracy is better than ±1% for most circuits.
Current consumption is in the µA range. Make sure to
choose a device with an active low output. A wide variety of
package types are available, ranging from miniature three-
pin SOT-23 to large packages with high pin count.
Figure 5a. Detector with Push-Pull Output
Figure 5b. Detector with Open-Drain Output
Figure 5c. Alternative Location of Manual Reset Switch
Output Driver
The IC reset output can be push-pull or open drain (open
collector), either CMOS or TTL output levels. Open drain
solutions allow easy connection of a manual reset button
and/or in-system programmers. This feature can also be
implemented with push-pull outputs, with the addition of a
resistor in series with the output. The ISP and/or manual
button is connected between the resistor and the AVR
RESET input (see Figure 5a and Figure 5b).
Figure 6. Reset Pulse Stretching
Reset Pulse Stretching
An additional feature in some of these circuits is stretching
of the reset pulse. The reset is held active for a defined
amount of time after the condition (power-on reset, brown-
out reset etc.) that caused the reset has returned to normal
(see Figure 6). Some of these devices also provide this
feature for the manual reset. The device senses the output
level, detecting the closing and opening of a reset button.
When the button is released, the device keeps the reset
line active for an additional amount of time. WARNING!
This feature will interfere with the operation of an in-system
programmer, which toggles the RESET line actively.
Power Regulator
Several integrated power regulators includes the low-volt-
age detector, combining both functionalities in one device.
This reduces part count, and often adds the functionality at
no extra cost.
Battery Backup Solutions
Some systems contain a battery to supply power when the
main power drops. The power regulator in such systems
often provides a status signal to the MCU telling which
source currently supplies power to the circuit. Connecting
this signal to RESET will shut the AVR down when battery
power is used, preserving RAM contents but halting execu-
tion. Alternatively, connecting this signal to an input pin, the
AVR can detect the event and execute a safe power down
sequence, switching off power hungry peripheral equip-
ment (motor, display etc.) before entering power-down
mode. (The power consumption in RESET is the same as
in normal active running mode, whereas the consumption
in power down mode is in the µA range.) When main power
supply voltage returns to an acceptable level, the AVR
V
CC
R1
1 - 4K
OPTIONAL
RESET
SWITCH
ISP
RESET
DETECTOR
LOGIC
V
CC
OPTIONAL
RESET
SWITCH
ISP
RESET
DETECTOR
LOGIC
V
CC
GND
RESET
V
CC
RESET WITHOUT
PULSE STRETCHING
V
CC
V
T
RESET WITH
PULSE STRETCHING
t
BROWN-OUT
t
BROWN-OUT
t
RST
Summary of Contents for AVR180
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