July 2005

5

M9999-071305

MIC809-5

Micrel

Applications Information

Microprocessor Reset

reset threshold voltage. The /RESET pin remains asserted

threshold voltage. The reset function ensures the micropro-

cessor is properly reset and powers up in a known condition

low as 1V.

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.

0

5

10

15

20

25

0

200 400

600 800 1000

Typical Transient

Response

V

Figure 1. Comparator Overdrive

vs. Duration

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

If a given application requires that /RESET be valid below

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.

/RESET

/RESET INPUT

Microcontroller

or

Microprocessor

GND

100k

GND

MIC809

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

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.

4.7k

BUFFER

RESET SIGNAL

to OTHER CIRCUITS

/RESET

GND

MIC809

RESET I/O

Microcontroller

or

Microprocessor

GND

Figure 3. Interfacing to Processors

with Bidirectional Reset Pins