3-Pin Microprocessor Reset Circuits
+5.0V
+3.3V
V
CC
V
V
CC
V
V
CC
CC
CC
R
PULLUP
µP
5V SYSTEM
MAX803
MAX803
MOTOROLA
68HCXX
RESET
INPUT
RESET
INPUT
RESET
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.
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.
A 100kΩ pull-up resistor to V
is also recommended
CC
for the MAX810 if RESET is required to remain valid for
V
CC
< 1V.
Interfacing to µPs with
Bidirectional Reset Pins
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.
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).
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.
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
___________________Chip Information
the IC’s V
pin. However, some systems may use the
CC
TRANSISTOR COUNT: 275 (SOT23)
380 (SC70)
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
decreases
CC
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
decays toward 0. The voltage where this
CC
occurs depends on the pull-up resistor value and the
voltage to which it is connected.
6
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