ST AN1790 Application note

AN1790

APPLICATION NOTE

How to Connect ST Reset Circuits to a Microprocessor

■ INTRODUCTION

■ RESET THRESHOLDS

■ RESET OUTPUT

■ MANUAL RESET INPUT

■ CONNECTING THE

RESET CIRCUITS IN AN APPLICATION

– Connection in a Noisy

Environment

– Connection to a Bi-direc-

tional Microprocessor Reset

– Connection With Reset

Output Valid Down to VCC = 0V

■ CONCLUSION

■ REFERENCES

■ REVISION HISTORY

This Application Note describes how to con nect the STM809, STM810, STM811 and STM812 Reset Circuits to Microprocessor Systems.

Further information on Reset Circuits can be found in the STM809, STM810, STM811, STM812 datasheet on www.st.com.

INTRODUCTION

All digital systems require the p ower supply to be controlled during power-up and normal operations. Noise resulting from ground loops or switching many signals at once, can create glitches in the power supply which can cause the system to malfunction.

For this reason ST has developed a new type of low-power supervisory device, called Reset Circuits, specifically to monitor power supplies. The Reset Circuits, STM809, STM810, STM811, STM812, assert a reset signal whenever the power supply drops below a preset threshold value, and keep it asserted until the voltage rises above that threshold for a minimum period of time. The STM811 and S TM 812 al so pro vide a push-button reset input signal (MR

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AN1790 - APPLICATION NOTE

RESET THRESHOLDS

The STM8xx Reset Circuit devices have different reset threshold levels. Table 1, lists the thresholds,

, for each device and each temperature range.

RST

When considering the threshold level required for an application, it is also necessary to consider the maximum transient duration, not c ausing a reset pulse, with respect to voltage thresh old overdrive. A large transient voltage overdrive need s less time to g enerate the Reset pulse, than a s mall transient voltage overdrive, which needs much longer. Refer to the characterization graph shown in Figure 1.

Table 1. Reset Thresholds

(V)

Part Temperature Range (°C)

Min Typ Max

25 4.54 4.63 4.72

STM8xxL

-40 to 80 4.50 4.75 25 4.30 4.38 4.46

STM8xxM

-40 to 80 4.25 4.50 25 3.03 3.08 3.14

STM8xxT

-40 to 80 3.00 3.15

RST

STM8xxS

25 2.88 2.93 2.98

-40 to 80 2.85 3.00 25 2.58 2.63 2.68

STM8xxR

-40 to 80 2.55 2.70

Figure 1. Max Transient Duration NOT Causing Reset Pulse vs. Reset Comparator Overdrive

700.00

600.00

500.00

400.00

300.00

200.00

100.00

Maximum Transient Duration (µs)

0.00 1 10 100 1000

Reset Comparator Overdrive, V

– VCC (mV)

RST

AI07883

R/S/T L/M

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AN1790 - APPLICATION NOTE

RESET OUTPUT

ST Reset Circuits feature a push-pull Reset out put with t he opt ion of an ac tive High (RST ) or ac tive Low

) output (see F igure 2 and Table 2). The push-pull output minimi zes the external c onnections be-

(RST tween the Reset Circuit and the microprocessor or other logic devices. It offers high speed, almost rail-torail response, and the capability to source or sink current.

Figure 2. Reset Output

RST

trec

Reset Circuit

Table 2. Device Options

Part Reset Output active Manual Reset (MR)

STM809 Low No STM810 High No STM811 Low Yes STM812 High Yes

RST/RST

RST