ST AN1772 Application note
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 prob lems 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 s upply voltage. Every microproces sor needs its pow er suppl y to be within a predef ined range, t o
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 h as developed a new family of l ow-power
supervisory devices, called Reset Circuits, specifically to monitor power supplies. This new family, which includes the
STM809, STM810, STM811 and STM81 2, asserts a reset signal whenever the power supply drops below a preset threshold
value, and keeps it asserted, until the vo ltage rises above that
threshold, for a minimum period of time. The STM811 and
STM812 also provide a push-button reset input signal (MR
).
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AN1772 - APPLICATION N OTE
POWER-UP/ RESET
Power-up is the first thing that must be considered in a new design, as every system requires a time interval for internal initialization after the power supply has stabilized.
During the power-up sequence, microprocessors and other complex digital devices, wait for the clock signal to stabilize and load the interna l registers and boot -code required for the microprocessor to funct ion
properly. For this reason, they always require a predefined time for power-up/reset. If the reset time is not
long enough, the system may not function properly.
Resistor-Capacitor Filter Circuit
One possible approach to delay the reset signal after power-up, is to use a Resistor-Capacitor (RC) lowpass filter at the microprocessor reset (RST
to work in all circumstances. It uses an exponential RC rise time for the reset delay after power-up (see
Figure 1). The delay can be changed easily by selecting the appropriate RC circuit.
Unfortunately, this approach has two drawbacks.
■ The reset delay depends on the power-up voltage slew rate - if the slew rate is too slow then the RC
curve tracks this rising voltage and the reset delay may be inadequate (See the dashed line in top
diagram in Figure 1).
■ The slew rate of reset signal at the microprocessor’s input depends on the reset time (typically 110-200
ms) - if longer pulses are required, the RC circuit values must be increas ed, whic h will s low th e slew
rate of the reset signal and may cause the microprocessor to malfunction, due to insufficient overdrive
at the Reset input.
Some manufacturers implement power-up/reset functions in the microprocessor to avoid these problems,
however in most cases the monitoring is not very precise.
Another solution is to use an external Reset Circuit dev ice. ST Reset Circuits have precise , predefined
voltage thresholds and reset delays and so do not encounter any of the drawbacks of the RC Filter Circuit
(see Integrated Reset Circuits section).
) input. This solution is inexpensive, but cannot be guaranteed
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Figure 1. Resistor-Capacitor Filter Circuit - Reset Delay
AN1772 - APPLICATION NOTE
Different V
Power-up V
µP Reset Input
V
CC
R
Microprocessor
µP Internal Reset
Power-up slew rate, One RC reset delay.
CC
0.9 V
CC
CC
V
RC
V
TH
Slow V
slew rate gives
CC
shorter Reset delay
T
2
(T2 < T1)
T
1
RST
C
One V
Power-up slew rate, Different RC reset delay.
CC
Power-up V
µP Reset Input
CC
0.9 V
CC
V
RC
V
TH
Slow reset slew rate
can cause problems
Note: VRC = voltage on capacitor.
µP Internal Reset
T
1
T
(T1 < T2)
2
AI08625
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