Datasheet

MAX821/MAX822
Ensuring a Valid RESET Output
Down to V
CC
= 0V
When V
CC
falls below 1V, the MAX821 RESET output
no longer sinks current—it becomes an open circuit.
Therefore, high-impedance CMOS logic inputs con-
nected to the RESET output can drift to undetermined
voltages. This presents no problem in most applica-
tions, since most µP and other circuitry is inoperative
with V
CC
below 1V. However, in applications where the
RESET output must be valid down to 0V, adding a pull-
down resistor to the RESET pin will cause any stray
leakage currents to flow to ground, holding RESET low
(Figure 2a). R1’s value is not critical; 100kΩ is large
enough not to load RESET, and small enough to pull
RESET to ground.
A 100kΩ pull-up resistor to V
CC
is also recommended
for the MAX822 if RESET is required to remain valid for
V
CC
< 1V (Figure 2b).
Interfacing to µPs with
Bidirectional Reset Pins
µPs with bidirectional reset pins (such as the Motorola
68HC11 series) can contend with the MAX821 reset
output. For example, if the MAX821 RESET output is
asserted high and the µP wants to pull it low, indetermi-
nate logic levels may result. To correct such cases,
connect a 4.7kΩ resistor between the MAX821 RESET
output and the µP reset I/O (Figure 3). Buffer the reset
output to other system components.
___________________Chip Information
TRANSISTOR COUNT: 492
4-Pin µP Voltage Monitors with Pin-Selectable
Power-On Reset Timeout Delay
6 _______________________________________________________________________________________
4.7k
V
CC
GND
RESET
MAX821
RESET
V
CC
GND
µP
BUFFER
BUFFERED RESET
TO OTHER SYSTEM
COMPONENTS
Figure 3. Interfacing to µPs with Bidirectional Reset I/O
R1
V
CC
V
CC
GND
MAX821
RESET
R1
V
CC
V
CC
GND
MAX822
RESET
a)
b)
Figure 2. RESET/ RESET Valid to V
CC
= Ground Circuit