MAX809S datasheet(6/8 Pages) MAXIM

6 / 8 page

MAX803/MAX809/MAX810

3-Pin Microprocessor Reset Circuits

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 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; 100k

Ω is large enough not to load RESET and

small enough to pull RESET to ground.

A 100k

Ω pullup resistor to VCC is also recommended

for the MAX810 if RESET is required to remain valid for

VCC < 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 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

RESET

GND

RESET

INPUT

RPULLUP

MAX803

5V SYSTEM

GND

VCC

+3.3V

+5.0V

VCC

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

Multiple Supplies

RESET

GND

RESET

INPUT

MAX803

MOTOROLA

68HCXX

µP

GND

VCC

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

400

320

100

1000

160

240

RESET COMPARATOR OVERDRIVE, VTH - VCC (mV)

TA = +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

100k

VCC

GND

MAX809

RESET

Figure 2. RESET Valid to VCC = Ground Circuit

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