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1/8November 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
).
