March 2007

5

Microprocessor Reset

The /RESET pin is asserted whenever V

reset threshold voltage. The /RESET pin remains asserted

for a period of 180ms after V

threshold voltage. The reset function ensures the micropro-

cessor is properly reset and powers up in a known condition

after a power failure. /RESET will remain valid with V

as 0.95V.

Transients on V

CC

The KB809 is inherently immune to very short “glitches” on

V

the under-voltage threshold without triggering a reset. As

shown in the graph of Figure 1, the narrower the transient, the

deeper the threshold overdrive that will be ignored. The line

on the graph represents the typical allowable transient dura-

tion for a given amount of threshold overdrive that will

not

generate a reset. The data from which Figure 1 is derived was

taken by adding negative-going square-wave pulses to a DC

input voltage set at 0.5V above the actual measured thresh-

old for the part being characterized.

0

5

10

15

20

0

200

400

600

RESET COMP. OVERDRIVE, V

Typical Transient

Response

V

TH

= 2.932

Figure 1. Comparator Overdrive

vs. Duration

Ensuring Proper Operation at Low V

CC

At levels of V

cannot always turn on sufficiently to produce a valid logic-low.

In this situation, other circuits driven by /RESET could be

allowed to float, causing undesired operation. (In most cases,

however, it is expected that the circuits driven by the KB809

will be similarly inoperative at V

If a given application requires that /RESET be valid below

V

resistor to the output. A value of 100k

Ω is recommended, as

this is usually an acceptable compromise of leakage current

and pull-down current. The resistor’s value is not critical,

however. See Figure 2.

/RESET

/RESET INPUT

Microcontroller

or

Microprocessor

GND

100k

GND

KB809

Figure 2. Ensuring a Valid

/RESET Signal at Low Voltage

Interfacing to Processors with Bidirectional Reset Pins

Some microprocessors have reset signal pins that are bidi-

rectional, rather than input only. The Motorola 68HC11 family

is one example. To prevent excessive current flow when the

processor asserts its reset output, a series resistor is em-

ployed between the KB809’s output and the reset pin. This

limits the current that will flow between the KB809 and the

reset pin to a theoretical maximum of V

this resistor also limits the drive capability of the supervisor’s

output, a buffer may be required to drive additional circuitry.

See Figure 3.

4.7k

BUFFER

RESET SIGNAL

to OTHER CIRCUITS

/RESET

GND

KB809

RESET I/O

Microcontroller

or

Microprocessor

GND

Figure 3. Interfacing to Processors

with Bidirectional Reset Pins

KB809

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Applications Information