G692 datasheet(6/7 Pages) ETC

6 / 7 page

Ver: 1.0

Sep 23, 2002

TEL: 886-3-5788833

http://www.gmt.com.tw

G692/G693

Global Mixed-mode Technology Inc.

Applications Information

Negative-Going VCC Transients

In addition to issuing a reset to the µP during power-up,

power-down,

and

brownout

conditions,

the

G693L/G692H/G692L are relatively immune to short-

duration negative-going VCC transients (glitches).

Figure1 shows typical transient duration vs. reset

comparator overdrive, for which the G693L/G692H/

G692L do not generate a reset pulse. The graph was

generated using a negative-going pulse applied to VCC,

starting 0.5V above the actual reset threshold and

ending below it by the magnitude indicated (reset

comparator overdrive). The graph indicates the maxi-

mum pulse width a negative-going VCC transient can

have without causing a reset pulse. As the magnitude

of the transient increases (goes farther below the reset

threshold), the maximum allowable pulse width de-

creases. Typically, for the G69_ _463 and G69_ _438,

a VCC transient that goes 100mV below the reset

threshold and lasts 15µs or less will not cause a reset

pulse. A 0.1µF bypass capacitor mounted as close as

possible to the VCC pin provides additional transient

immunity.

Ensuring a Valid Reset Output Down to VCC = 0

When VCC falls below 1V, the G692 RESET output no

longer sinks current—it becomes an open circuit.

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 be-

low 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 pull-up resistor to VCC is also recommended

for the G693L if RESET is required to remain valid for

VCC < 1V.

Figure 3. Interfacing to µPs with Bidirectional Reset

I/O

Figure 4. G693L Open-Drain RESET Output Allows

Use with Multiple Supplies

Interfacing to µPs with Bidirectional Reset Pins

Since the RESET output on the G693L 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

(Figure 3).

G693L Open-Drain RESET Output Allows Use with

Multiple Supplies

Generally, the pull-up connected to the G693L 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 G693L’s VCC decreases

below 1V, so does the IC’s ability to sink current at

RESET . Also, with any pull-up, RESET will be pulled

high as VCC decays 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 sup-

ply voltages. This ensures a reset will not occur within

5% of the nominal supply, but will occur when the sup-

ply is 10% below nominal.

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

this leaves a zone of uncertainty where the supply is

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

may not be asserted.

The G69_ _463/G69_ _308 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

nominal.

RESET

INPUT

GND

5V SYSTEM

RESET

GND

G693

RPULL-UP

+3.3V

+5.0

RESET

INPUT

GND

5V SYSTEM

RPULL-UP

+3.3V

+5.0V

RESET

INPUT

GND

5V SYSTEM

RESET

GND

G693

RPULL-UP

+3.3V

+5.0

RESET

INPUT

GND

5V SYSTEM

RPULL-UP

+3.3V

+5.0V

RESET

INPUT

GND

µP

RESET

GND

G693

MOTOROLA

68HCXX

RPULL-UP

RESET

INPUT

GND

µP

RESET

GND

G693

MOTOROLA

68HCXX

RPULL-UP

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