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MIC809-5 Datasheet PDF : 6 Pages
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MIC809-5
Applications Information
Microprocessor Reset
The /RESET pin is asserted whenever VCC falls below the
reset threshold voltage. The /RESET pin remains asserted
for a period of 30ms after VCC has risen above the reset
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 VCC as
low as 1V.
Transients on VCC
The MIC809-5 is inherently immune to very short “glitches”
on VCC. In the case of very brief transients, VCC may drop
below the reset-voltage threshold without triggering a reset.
As shown in the graph, the MIC809-5 is relatively immune
to transients with typical duration of 20µs. The lines on the
graph of Figure 1 represent the typical allowable transient
duration for a given amount of threshold overdrive that will
not generate a reset. The data was taken by adding nega-
tive-going square-wave pulse to a DC input voltage set at
0.5V above actual measured threshold for the part being
characterized.
Typical Transient
Response
25
VTH = 2.932
20
15
10
5
0
0 200 400 600 800 1000
RESET COMP. OVERDRIVE, VTH –VCC (mV)
Figure 1. Comparator Overdrive
vs. Duration
Ensuring Proper Operation at Low VCC
At levels of VCC below 1V, the MIC809-5’s output driver 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 MIC809-5 will be
similarly inoperative at VCC =1V.)
If a given application requires that /RESET be valid below
VCC = 1V, this can be accomplished by adding a pull-down
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.
Micrel
VCC
VIN
MIC809
/RESET
GND
RPULL-DOWN
100k
VCC
/RESET INPUT
Microcontroller
or
Microprocessor
GND
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 fam-
ily is one example. To prevent excessive current flow when
the processor asserts its reset output, a series resistor is
employed between the MIC809-5’s output and the reset pin.
This limits the current that will flow between the MIC809-5
and the reset pin to a theoretical maximum of VCC/RSERIES.
Because this resistor also limits the drive capability of the
supervisor’s output, a buffer may be required to drive ad-
ditional circuitry. See Figure 3.
BUFFER
RESET SIGNAL
to OTHER CIRCUITS
VCC
VIN
MIC809
/RESET 4.7k
GND
VCC
RESET I/O
Microcontroller
or
Microprocessor
GND
Figure 3. Interfacing to Processors
with Bidirectional Reset Pins
July 2005
5
M9999-071305
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