AN1772 - STMicroelectronics

1/8November 2003

AN1772

APPLICATION NO TE

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

in Microp rocess or Systems usi ng ST Reset Circuits

CONTENTS

■INTRODUCTION

■POWER -U P/ R ESET

– Resistor-Capacitor Filter

Circuit

■MONITORING THE

SUPPLY VOLTAGE

DURING OPERATIONS

– Resistor-Capacitor-Di-

ode Fil ter Circuit

■INTEGRATED RESET

CIRCUITS

– Reset Threshol ds

– 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 vol tage. Every m icroproces-

sor nee ds its power supply to be within a predef ined range, t o

be able to function properly. A sm all glitch in the p ower supply

can cause the sy stem t o crash. I ncorrect 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 resulti ng 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, specif i cally to mon-

itor power supplies. This new family, which includes the

STM809, STM8 10, STM811 and STM81 2, asserts a reset sig-

nal whenever the power supply drops below a preset t hr eshold

value, and keeps it ass erted, until the vo ltage rises above that

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

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

AN1772 - APPLICATION NOTE

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POWER-UP/ RESET

Power-up is the first thing that must be considered in a new design, as every system requires a time in-

terval for internal initialization after the power su pply has stabilized.

During the power-up sequence, microprocessors and ot her complex digital dev ices, wait for the clock sig-

nal to stabilize and l oad the interna l registers and boot -code required for the micropro cessor to funct ion

properly. For thi s reas on, t hey always require a predefined time for power-up/reset . If the reset ti me is not

long enough, the system may not function properly.

Resistor-Capacitor Filter Circu it

One possible approac h to delay the reset signal after power-up, is to use a Resistor-Capacitor (RC) low-

pass filter at the microprocessor reset (RST) input. This solution is inexpensiv e, but cannot be guaranteed

to work in all circumstances. It uses a n exponential RC rise time for the reset delay a fter power-up (see

Figure 1). The delay can be cha nged 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 ra te is too slow then the RC

curve tracks this rising voltage and the reset delay may be inadequat e (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 (typic ally 110-200

ms) - if longer pulses are required, the RC circuit values must be increas ed, w hic 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 t he microprocessor to avoi d these problems,

however i n 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 drawback s of the RC Filter Circuit

(see Integrated Res et Circuits section).

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AN1772 - APPLICATION NOTE

Figure 1. Resistor -Capacitor Filter Circuit - Reset Delay

Note: VRC = vol ta ge on capac i tor.

AI08625

RMicroprocessor

VCC

RST

Power-up VCC

µP Internal Reset

µP Reset Input

VTH

VRC

0.9 VCC

T2(T1 < T2)

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

Power-up VCC

µP Internal Reset

µP Reset Input

VTH

VRC

0.9 VCC

T1(T2 < T1)

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

Slow VCC slew rate gives

shorter Reset delay

Slow reset slew rate

can cause problems

AN1772 - APPLICATION NOTE

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MONITORING THE SUPPLY VOLTAGE DURING OPERATIONS

For th e microproce sso r and o the r d e vi ce s in th e system to function co rrectly, it is also nec es sa ry to mon-

itor the su pply voltage during operat ions.

Brownout condi tions, or gl itches on th e power supply li nes, c an cause unw anted chang es in the i nternal

registers, which can lead to instruc tions being incorrectly executed, incorrect output signals and errors in

th e operations results.

The RC filter solution proposed for monitoring the voltage during power-up and reset is not sufficient to

monitor the voltage correctly during o perations . It can filter out fast undershoots, however, as the micro-

processor is unaware that the voltage dropped bel ow the threshold, due to the time constant of the RC

delay, it continues to operate, which can lead to instructions being incorrectly execu ted.

Resistor-Capacitor-Diode Filter Circuit

A Resistor-Capacitor-Diode (RCD) filter circuit (the same as the RC circuit with a diode added parallel to

the resistor), can i mprove the response time to f ast undershoots, but the drop in the voltage level must be

greater than the forward turn-on voltage of the diode, before the diod e can start to pull RST low.

Anot her limitation of this solution is that, the RCD filter circuit can only m oni tor drops in the voltage supply ,

that are greate r than the microprocess or’s tolerance voltage plus the diode drop.

As for the Power-up/Reset phase, a better solution is to use an external Reset device like the ST Reset

Circuits, which are also ideal for monitoring the voltage level during operations (see Int egrated Reset Cir-

cuits section).

Figure 2. Resistor-Capacitor-Diod e Filter Circuit

Note: VD = Voltage across diode, V RC = voltage on capacitor.

AI08626

RMicroprocessor

VCC

RST

Power-up VCC

µP Internal Reset

µP Reset Input

VTH

RC constant, without parallel diode,

filters out fast voltage undershoots

VRC

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AN1772 - APPLICATION NOTE

INTEGRATED RESET CIRCUITS

Integrated Reset Circui ts, like the S TM809-8 12, are devices that are dedi cated for reset and monitoring

purposes. They monitor the input voltage supply and send a reset signal to the microprocess or f or a preset

time interval, if the voltage supply goes outside the preset limit.

A typi cal reset dev ice cons ists of a voltage reference, a voltage comparator, a supply volt age resistor-di-

vider network, a fixed delay time circui t and an output dri ver ( see Figure 3).

The internal voltage comparat or monitors the input voltage supply and compares it with a predefined volt -

age reference. If the suppl y voltage is under the threshold, a reset signal RST, is as serted . The signal re-

mains asserted unt il the supply vol tage reaches the threshold and st ays above it for a preset t ime interval.

The output of the Reset Circuit is independent of the Power-up slew rate, glitches and other brownout con-

ditions.

Figure 3. Integrated Reset Circuit

Note: 1. STM811/812 only.

AI08627

COMPARE

RST

DEBOUNCE

Delay

(1)

RST

Output

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Reset Thresholds

The Reset Circuits are available with multiple threshold voltages (refer to Table 1), so the designer can

choose the reset threshol d level best suited to the system’ s power supply.

The sensitivity of the Reset Circuit depends on:

■the durat ion of the vol tage unders hoot,

■the magnitude of t he undershoot below the minimum threshold.

The generat ion of the reset si gnal d epends o n, the ma ximum vol tage tran sient durat ion versus the reset

comparator overdrive, as is shown in Figure 4. Conditions fall ing int o the area below the curve ar e fil tered

and therefore do not generate a reset signal.

Figure 4. Reset Thresh olds

Table 1. Reset Threshold Levels

Manual Reset

A manual reset is useful in many applications, from debugging new systems to waking-up from an idle or

sleep mode.

Usually an external reset is done with a switch, however switches are not ideal because of switching

bounce (voltage spikes that can occur during the switching phase). The problem can be overc ome by de-

signing a specific circuit, but this requires additional discrete logic gates. The STM811 and STM812 Reset

Circuits solve the problem with a push-butt on reset input signal (MR). These devices have an i nternal de-

bounce circuit (see Figure 3), usually with an internal pull-up resistor, so there are no requirement s for any

other discrete circui ts connecte d to the device.

Device Reset Thres hold Voltag e

Min Max Unit

STM8xxL 4.50 4.75 V

STM8xxM 4.25 4.50 V

STM8xxT 3.00 3.15 V

STM8xxS 2.85 3.00 V

STM8xxR 2.55 2.70 V

AI08628

µS

Max. voltage transient duration

(not producing reset pulse)

Reset Comparator Overdrive, mV

STM8xxL/ M

STM8xxR/S/T

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AN1772 - APPLICATION NOTE

CONCLUSION

ST Reset Circuits are small devices, that take little board space, but monitor th e most impo rtant thing in

the system - the power supply. They are b ecoming i ncre asingly popular with designers, thanks to their

dedicated features and low cost. They are av ailable in small SOT23 and SOT143 packages; they require

only a low supply current, typically in the 5-10 µA range and offer a wide range of voltage thresholds.

They can b e used in man y different appl ications f rom b a ttery dev ices with low p ower requi remen ts to in-

dust rial machines with reliability constraints.

REFERENCES

■STM809, STM810, STM811, STM812 datasheet

RE VISION H ISTORY

Table 2. Document Revi sion History

Date Version Revision Details

27-Nov-2003 1.0 First Issue.

AN1772 - APPLICATION NOTE

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