Kingbor Technology Co.,Ltd
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KB809
Applications Information
VCC
Microprocessor Reset
VCC
The /RESET pin is asserted whenever V falls below the
reset threshold voltage. The /RESET pin remains asserted
for a period of 180ms after V has risen above the reset
CC
VIN
KB809
/RESET
CC
/RESET INPUT
threshold voltage. The reset function ensures the micropro-
cessor is properly reset and powers up in a known condition
afterapowerfailure./RESETwillremainvalidwithV aslow
Microcontroller
or
Microprocessor
GND
RPULL-DOWN
100k
CC
as 0.95V.
GND
Transients on V
CC
TheKB809isinherentlyimmunetoveryshort“glitches”on
. In the case of very brief transients, V may drop below
V
CC
CC
the under-voltage threshold without triggering a reset. As
showninthegraphofFigure1, thenarrowerthetransient, 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
generateareset. ThedatafromwhichFigure1isderivedwas
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.
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, ratherthaninputonly. TheMotorola68HC11family
is one example. To prevent excessive current flow when the
processor asserts its reset output, a series resistor is em-
ployedbetweentheKB809’soutputandtheresetpin.This
limits the current that will flow between the KB809 and the
Typical Transient
reset pin to a theoretical maximum of V /R
. Because
Response
CC SERIES
20
this resistor also limits the drive capability of the supervisor’s
output, a buffer may be required to drive additional circuitry.
See Figure 3.
V
= 2.932
TH
15
10
5
BUFFER
RESET SIGNAL
to OTHER CIRCUITS
VCC
0
VCC
0
200
400
RESET COMP. OVERDRIVE, V –V (mV)
600
VIN
TH
CC
KB809
4.7k
Figure 1. Comparator Overdrive
vs. Duration
/RESET
GND
RESET I/O
Microcontroller
or
Microprocessor
Ensuring Proper Operation at Low V
CC
At levels of V
below 1.4V, the KB809’s output driver
CC
cannotalwaysturnonsufficientlytoproduceavalidlogic-low.
In this situation, other circuits driven by /RESET could be
allowedtofloat,causingundesiredoperation.(Inmostcases,
however, itisexpectedthatthecircuitsdrivenbytheKB809
GND
Figure 3. Interfacing to Processors
with Bidirectional Reset Pins
will be similarly inoperative at V =0.95V.)
CC
If a given application requires that /RESET be valid below
=1.4V, this can be accomplished by adding a pull-down
V
CC
resistor to the output. A value of 100kΩ 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.
March 2007
5
KINGBOR [ KINGBOR TECHNOLOGY CO ]
