Datasheet

MAX803/MAX809/MAX810
3-Pin Microprocessor Reset Circuits
6
Maxim Integrated
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
RESET
GND
RESET
INPUT
R
PULLUP
MAX803
5V SYSTEM
GND
V
CC
+3.3V
+5.0V
V
CC
Figure 4. MAX803 Open-Drain RESET Output Allows Use with
Multiple Supplies
RESET
GND
RESET
INPUT
MAX803
MOTOROLA
68HCXX
µP
GND
V
CC
V
CC
V
CC
Figure 3. Interfacing to µPs with Bidirectional Reset I/O
400
320
0
1 100 1000
160
80
240
RESET COMPARATOR OVERDRIVE, V
TH
- V
CC
(mV)
MAXIMUM TRANSIENT DURATION (µs)
10
T
A
= +25°C
MAX8_ _J/L/M
MAX8_ _R/S/T/Z
Figure 1. Maximum Transient Duration Without Causing a
Reset Pulse vs. Reset Comparator Overdrive
R1
100k
V
CC
GND
MAX809
RESET
Figure 2. RESET Valid to V
CC
= Ground Circuit