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November 2003

AN1772

APPLICATION NOTE

How to Control Power-up/Reset and Monitor the Voltage

in Microprocessor Systems using ST Reset Circuits

CONTENTS

■

INTRODUCTION

■

POWER-UP/ RESET

– Resistor-Capacitor Filter

Circuit

■

MONITORING THE

SUPPLY VOLTAGE

DURING OPERATIONS

– Resistor-Capacitor-Di-

ode Filter Circuit

■

INTEGRATED RESET

CIRCUITS

– Reset Thresholds

– Manual Reset

■

CONCLUSION

■

REFERENCES

■

REVISION HISTORY

This Application Note describes the problems with monitoring

the power supply during Power-up/Reset in a microprocessor

system. It also explains how the family of ST family of Reset

Circuits can be used to solve them.

Further information on Reset Circuits can be found in the

STM809,

STM810,

STM811,

STM812

datasheet

on

www.st.com.

INTRODUCTION

Today, our microprocessor driven digital world, cannot exist

without the correct power supply voltage. Every microproces-

sor needs its power supply to be within a predefined range, to

be able to function properly. A small glitch in the power supply

can cause the system to crash. Incorrect power-up sequences,

slow ramp-up and glitches on the supply lines are common

problems that designers must take into account to minimize the

impact of faults resulting from the power supply.

There are several ways to solve these problems, some of

which are outlined in this document, but they all have certain

limitations and do not completely resolve the problem.

For this reason ST has developed a new family of low-power

supervisory devices, called Reset Circuits, specifically to mon-

itor power supplies. This new family, which includes the

STM809, STM810, STM811 and STM812, asserts a reset sig-

nal whenever the power supply drops below a preset threshold

value, and keeps it asserted, until the voltage rises above that

threshold, for a minimum period of time. The STM811 and

STM812 also provide a push-button reset input signal (MR).