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Figure 3.

Interfacing to µPs with bidirectional

reset I/O

allows use with multiple supplies

Interfacing to µPs with Bidirectional Reset Pins

Since the RESET output on the SS8038L is open drain,

this device interfaces easily with µPs that have bidirec-

tional reset pins, such as the Motorola 68HC11. Con-

necting 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 (Fig-

ure 3).

Multiple Supplies

Generally, the pull-up connected to the SS8038L will

connect to the supply voltage that is being monitored at

open-drain output to level-shift from the monitored supply

to reset circuitry powered by some other supply (Figure

so does the IC’s ability to sink current at RESET . Also,

cays toward 0. The voltage where this occurs depends

on the pull-up resistor value and the voltage to which it is

connected.

Benefits of Highly Accurate Reset Threshold

Most µP supervisor ICs have reset threshold voltages

between 5% and 10% below the value of nominal supply

voltages. This ensures a reset will not occur within 5% of

the nominal supply, but will occur when the supply is 10%

below nominal.

When using ICs rated at only the nominal supply ±5%,

this leaves a zone of uncertainty where the supply is be-

tween 5% and 10% low, and where the reset may or

may not be asserted.

The SS803xx463/803xx308 use highly accurate circuitry

to ensure that reset is asserted close to the 5% limit, and

long before the supply has declined to 10% below nomi-

nal.

RESET

INPUT

GND

µP

V

CC

RESET

GND

V

CC

V

CC

MOTOROLA

68HCXX

RESET

INPUT

GND

µP

V

CC

RESET

GND

8038

V

CC

V

CC

MOTOROLA

68HCXX

RESET

INPUT

GND

5V SYSTEM

V

CC

RESET

GND

V

CC

+3.3V

+5.0V

RESET

INPUT

GND

5V SYSTEM

V

CC

RESET

GND

8038

V

CC

+3.3V

+5.0V

SS8037/8(G)

10/6/2004 Rev.2.20