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MAX803MEXR-T10

3-Pin Microprocessor Reset Circuits

Maxim Integrated 

3-Pin Microprocessor Reset Circuits

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 VCC

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 pull-up resistor to VCC is also recommended

for the MAX810 if RESET is required to remain valid for

VCC < 1V.

400

TA = +25°C

320

240

160

MAX8_ _J/L/M

80

MAX8_ _R/S/T/Z

0

1

10

100

1000

RESET COMPARATOR OVERDRIVE, VTH - VCC (mV)

Figure 1. Maximum Transient Duration Without Causing a

Reset Pulse vs. Reset Comparator Overdrive

VCC

VCC

MAX803

RESET

VCC

µP

MOTOROLA

RESET 68HCXX

INPUT

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

pull-up resistor allows either device to assert reset

(Figure 3).

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

the IC’s VCC 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 VCC decreases

VCC

MAX809

RESET

R1

GND

100k

Figure 2. RESET Valid to VCC = Ground Circuit

+3.3V

+5.0V

VCC

MAX803

RESET

RPULLUP

VCC

5V SYSTEM

RESET

INPUT

GND

GND

Figure 3. Interfacing to µPs with Bidirectional Reset I/O

GND

GND

Figure 4. MAX803 Open-Drain RESET Output Allows Use with

Multiple Supplies

6 _______________________________________________________________________________________

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