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G680HT1

Microprocessor reset IC

Global Mixed-mode Technology Inc 

Global Mixed-mode Technology Inc.

G680/G681

Pin Description

PIN

NAME

1

HTH

2

GND

3

LTH

4

RESET (RESET)

5

VCC

FUNCTION

High-Voltage Theshold (Input): Analog input to a comparator. When the level on this pin initially

rises above VTH, the delay generator cycles and the RESET remains low or RESET remains high

for a minimum of 140ms.

Ground

Low-Voltage Threshold (Input): Analog input to a comparator. This is the voltage monitor input

assigned to detect a low voltage condition. When the level on this pin falls below VTH, RESET or

RESET is asserted and the condition is latched until VHTH > VTH.

Reset (Output): Push-pull output. This output is asserted and latched when VLTH <VTH, indicating

a low voltage condition. This state remains latched until VHTH > VTH. The polarity of this signal

(active-high or low) is determined by the part number suffix. See ordering information.

Power Supply (Input): Independent supply input for internal circuitry.

Detailed Description

A microprocessor’s (µP’s) reset input starts the µP in a

known state. The G681L/G680L/G680H assert reset to

prevent code-execution errors during power-up,

power-down, or brownout conditions. They assert a

reset signal whenever the threshold voltage declines

below VTH, keeping it asserted for at least 140ms after

threshold voltage input has risen above the reset

threshold. The G681L uses an open-drain output, and

the G680L/G680H have a push-pull output stage.

Connect a pull-up resistor on the G681L’s RESET out-

put to any supply between 0 and 5.5V.

Battery voltage is monitored by a comparator via a

voltage divider network. The divided voltage is com-

pared to an internal reference voltage. When the volt-

age at the LTH input pin drops below the internal ref-

erence voltage, the output is asserted. At this point,

the voltage at HTH is assumed to be below the refer-

ence voltage.

350

300

250

200

150

100

50

G680

0

1

10

100

1000

Reset Comparator Overdrive, VTH- VLTH (mV)

Figure1. Maximum Transient Duration without

Causing a Reset Pulse vs. Reset Com-

parator Overdrive

Applications Information

Negative-Going VCC Transients

In addition to issuing a reset to the µP during

power-up, power-down, and brownout conditions, the

G681L/G680H/G680L are relatively immune to short-

duration negative-going VLTH transients (glitches).

Figure1. shows typical transient duration vs. reset

comparator overdrive, for which the G681L/G680H/

G680L do not generate a reset pulse. The graph was

generated using a negative-going pulse applied to

VLTH, starting 0.1V 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 VLTH 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, a VLTH transient that goes 100mV

below the reset threshold and lasts 7µs or less will not

cause a reset pulse. A 0.1µF bypass capacitor

mounted as close as possible to the VLTH pin provides

additional transient immunity.

Programming the Thresholds

The low-voltage threshold is calculated using:

VBAT (Io) = VREF

R1+R2+R3

R2+R3

The low-voltage threshold is calculated using:

VBAT (hi) = VREF

R1+R2+R3

R3

where, for both equations:

VBAT (hi) = 1.245V

In order to provide the additional criteria needed to

solve for the resistor values, the resistors can be se-

lected such that they have a given total value, that is,

R1 + R2 + R3 = RTOTAL.

Ver: 1.1

Jun 07, 2002

TEL: 886-3-5788833

http://www.gmt.com.tw

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