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 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 microprocessor 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 monitor power supplies. This new family, which includes the STM809, STM810, STM811 and STM812, asserts a reset signal 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).

November 2003

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AN1772 - APPLICATION NOTE

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 internal registers and boot-code required for the microprocessor to function 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) input. This solution is inexpensive, but cannot be guaranteed 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 increased, which will slow the 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 device. 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).

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AN1772 - APPLICATION NOTE

Figure 1. Resistor-Capacitor Filter Circuit - Reset Delay

Different VCC Power-up slew rate, One RC reset delay.

0.9 VCC

Power-up VCC

Slow VCC slew rate gives

shorter Reset delay

µP Reset Input

VRC

VTH

VCC

µP Internal Reset

T2

(T2 < T1)

R

Microprocessor

T1

C

RST

One VCC Power-up slew rate, Different RC reset delay.

0.9 VCC

Power-up VCC

VRC

µP Reset Input

VTH

Slow reset slew rate

can cause problems

µP Internal Reset

T1

(T1 < T2)

T2

AI08625

Note: VRC = voltage on capacitor.

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