3-Pin Microprocessor Reset Circuits
MAX803L/M/R/S/T/Z, MAX809J/L/M/R/S/T/Z, MAX810L/M/R/S/T/Z
+3.3V
+5.0V
V
CC
V
CC
V
CC
V
CC
R
PULLUP
V
CC
MAX803
RESET
RESET
INPUT
µP
MOTOROLA
68HCXX
5V SYSTEM
RESET
INPUT
MAX803
RESET
GND
GND
GND
GND
Figure 3. Interfacing to µPs with Bidirectional Reset I/O
Figure 4. MAX803 Open-Drain
RESET
Output Allows Use with
Multiple Supplies
ground, holding
RESET
low (Figure 2). 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 MAX810 if RESET is required to remain valid for
V
CC
< 1V.
Benefits of Highly Accurate
Reset Threshold
Most µP supervisor ICs have reset threshold voltages
between 5% and 10% below the value of nominal sup-
ply voltages. This ensures a reset will
not
occur within
5% of the nominal supply, but
will
occur when the sup-
ply is 10% below nominal.
When using ICs rated at only the nominal supply ±5%,
this leaves a zone of uncertainty where the supply is
between 5% and 10% low, and where the reset may or
may not be asserted.
The MAX8_ _L/T/Z use highly accurate circuitry to
ensure that reset is asserted close to the 5% limit, and
long before the supply has declined to 10% below
nominal.
Interfacing to µPs with
Bidirectional Reset Pins
Since the
RESET
output on the MAX803 is open drain,
this device interfaces easily with µPs that have bidirec-
tional reset pins, such as the Motorola 68HC11.
Connecting the µP supervisor’s
RESET
output directly
to the microcontroller’s (µC’s)
RESET
pin with a single
pull-up resistor allows either device to assert reset
(Figure 3).
MAX803 Open-Drain
RESET
Output
Allows Use with Multiple Supplies
Generally, the pull-up connected to the MAX803 will
connect to the supply voltage that is being monitored at
the IC’s V
CC
pin. However, some systems may use the
open-drain output to level-shift from the monitored sup-
ply to reset circuitry powered by some other supply
(Figure 4). Note that as the MAX803’s V
CC
decreases
below 1V, so does the IC’s ability to sink current at
RESET.
Also, with any pull-up,
RESET
will be pulled
high as V
CC
decays toward 0. The voltage where this
occurs depends on the pull-up resistor value and the
voltage to which it is connected.
___________________Chip Information
TRANSISTOR COUNT: 275 (SOT23)
380 (SC70)
6
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