Datasheet

Detailed Description
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.
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
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.
Ensuring a Valid Reset Output Down to
V
CC
= 0V
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.
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).
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 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.
PIN NAME FUNCTION
1 GND Ground
2
RESET
(MAX803/
MAX809)
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
(MAX810)
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.
3 V
CC
Supply Voltage (+5V, +3.3V, +3.0V, or
+2.5V)
TOP VIEW
RESET (RESET)
1
GND
V
CC
MAX803
MAX809
MAX810
SC70-3/SOT23-3
2
3
( ) ARE FOR MAX810.
MAX803 AVAILABLE IN SC70-3 ONLY.
MAX803/MAX809/
MAX810
3-Pin Microprocessor Reset Circuits
www.maximintegrated.com
Maxim Integrated │ 5
Pin Description
Pin Conguration