MAX803/MAX809/
MAX810
Pin Configuration
TOP VIEW
GND 1
3-Pin Microprocessor Reset Circuits
MAX803
MAX809
MAX810
RESET
(RESET) 2
3
V
CC
( ) ARE FOR MAX810.
MAX803 AVAILABLE IN SC70-3 ONLY.
SC70-3/SOT23-3
to V
CC
, starting 0.5V above the actual reset threshold
and ending below it by the magnitude indicated (reset
comparator overdrive). The graph indicates the maximum
pulse width a negative-going V
CC
transient can have with-
out causing a reset pulse. As the magnitude of the transient
increases (goes farther below the reset threshold), the
maximum allowable pulse width decreases. Typically, for
the MAX8__L and MAX8__M, a V
CC
transient that goes
100mV below the reset threshold and lasts 20μs or less will
not cause a reset pulse. A 0.1μF bypass capacitor mounted
as close as possible to the V
CC
pin provides additional
transient immunity.
Pin Description
PIN
1
NAME
GND
RESET
(MAX803/
MAX809)
2
RESET
(MAX810)
Ground
RESET
Output remains low while V
CC
is below the reset threshold, and for at
least 140ms after V
CC
rises above the
reset threshold.
RESET Output remains high while V
CC
is below the reset threshold, and for at
least 140ms after V
CC
rises above the
reset threshold.
Supply Voltage (+5V, +3.3V, +3.0V, or
+2.5V)
FUNCTION
When V
CC
falls below 1V, the MAX809
RESET
output no
longer sinks current—it becomes an open circuit.
Therefore, high-impedance CMOS logic inputs connect-
ed 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.
Ensuring a Valid Reset Output Down to
V
CC
= 0V
3
V
CC
A microprocessor’s (μP’s) reset input starts the μP in a
known state. The MAX803/MAX809/MAX810 assert reset
to prevent code-execution errors during power-up, power-
down, or brownout conditions. They assert a reset signal
whenever the V
CC
supply voltage declines below a preset
threshold, keeping it asserted for at least 140ms after V
CC
has risen above the reset threshold. The MAX803 uses
an open-drain output, and the MAX809/MAX810 have a
push-pull output stage. Connect a pullup resistor on the
MAX803’s
RESET
output to any supply between 0 and 6V.
Detailed Description
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
bidirectional reset pins, such as the Motorola 68HC11.
Connecting the μP supervisor’s
RESET
output directly to
the μC’s
RESET
pin with a single pullup resistor allows
either device to assert reset (Figure 3).
Applications Information
Negative-Going V
CC
Transients
In addition to issuing a reset to the μP during power-up,
power-down, and brownout conditions, the MAX803/
MAX809/MAX810 are relatively immune to short-duration
negative-going V
CC
transients (glitches).
Figure 1 shows typical transient duration vs. reset
comparator overdrive, for which the MAX803/MAX809/
MAX810 do
not
generate a reset pulse. The graph
was generated using a negative-going pulse applied
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 supply 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
pullup,
RESET
will be pulled high as V
CC
decays toward
0. The voltage where this occurs depends on the pullup
resistor value and the voltage to which it is connected.
MAX803 Open-Drain
RESET
Output Allows
Use with Multiple Supplies
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