3-Pin Microprocessor Reset Circuits
MAX803L/M/R/S/T/Z, MAX809J/L/M/R/S/T/Z, MAX810L/M/R/S/T/Z
Therefore, high-impedance CMOS logic inputs con-
nected to
RESET
can drift to undetermined voltages.
This presents no problem in most applications since
most µP and other circuitry is inoperative with V
CC
below 1V. However, in applications where
RESET
must
be valid down to 0V, adding a pull-down resistor to
RESET
causes any stray leakage currents to flow to
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 pullup resistor to V
CC
is also recommended
for the MAX810 if RESET is required to remain valid for
V
CC
< 1V.
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
pullup resistor allows either device to assert reset
(Figure 3).
MAX803 Open-Drain
RESET
Output
Allows Use with Multiple Supplies
Generally, the pullup 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
400
MAXIMUM TRANSIENT DURATION (μs)
T
A
= +25°C
320
V
CC
240
MAX809
RESET
160
MAX8_ _J/L/M
80
MAX8_ _R/S/T/Z
0
1
10
100
1000
RESET COMPARATOR OVERDRIVE, V
TH
- V
CC
(mV)
GND
R1
100k
Figure 2.
RESET
Valid to V
CC
= Ground Circuit
Figure 1. Maximum Transient Duration Without Causing a
Reset Pulse vs. Reset Comparator Overdrive
+3.3V
V
CC
V
CC
V
CC
V
CC
R
PULLUP
V
CC
+5.0V
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
6
Figure 4. MAX803 Open-Drain
RESET
Output Allows Use with
Multiple Supplies
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