October 2003
rev 1.0
5V
0V
T
RST
MR
Power Supply
MR
ASM811, ASM812
V
CC
V
TH
V
CC
V
CC
100kΩ
ASM811
RESET
GND
100kΩ
Power Supply
ASM812
MR
RESET
GND
5V
0V
5V
0V
5V
0V
T
MRST
RESET
ASM811
ASM812
RESET
Figures 2 & 3: RESET valid with V
CC
under 1.1V
Figure 1: Reset Timing and Manual Reset (MR)
Application Information
Reset Output Operation
In µP / µC systems it is important to have the processor and
the system begin operation from a known state. A reset
output to a processor is provided to prevent improper
operation during power supply sequencing or low voltage
brown-out conditions.
Bidirectional Reset Pin Interfacing
The ASM811/812 are designed to monitor the system power
supply voltages and issue a reset signal when the levels are
out of range. RESET outputs are guaranteed to be active for
V
CC
above 1.1V. When V
CC
exceeds the reset threshold, an
internal timer keeps RESET active for the reset timeout
period, after which RESET becomes inactive (HIGH for the
ASM811 and LOW for the ASM812). If V
CC
drops below the
reset threshold, RESET automatically becomes active.
Alternatively, external circuitry or an operator can initiate this
condition using the Manual Reset (MR) pin. MR can be left
open if it is not used. MR can be driven by TTL/CMOS logic
or even an external switch.
Valid Reset with V
CC
under 1.1V
To ensure logic inputs connected to the ASM811 RESET pin
are in a known state when V
CC
is under 1.1V, a 100kΩ pull-
down resistor at RESET is needed. The value is not critical.
A 100kΩ pull-up resistor to V
CC
is needed with the ASM812.
Bi-directional I/O Pin
Power supply
V
CC
Negative VCC Transients
Typically short duration transients of 100mV amplitude and
20µs duration do not cause a false RESET. A 0.1µF capacitor
at V
CC
increses transient immunity.
The ASM811/812 can interface with µP / µC bi-directional
reset pins by connecting a 4.7kΩ resistor in series with the
ASM811/812 reset output and the µP/µC bi-directional reset
input pin.
BUF
Buffered
RESET
ASM811
4.7kΩ
µC or µP
RESET
Input
GND
MR
RESET
GND
Figure 4: Bi-directional Reset Pin Interface
4 Pin µP Voltage Supervisor with Manual Reset
Notice: The information in this document is subject to change without notice
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ALSC [ ALLIANCE SEMICONDUCTOR CORPORATION ]
