ST AN1790 Application note

AN1790
APPLICATION NOTE
How to Connect ST Reset Circuits to a Microprocessor

CONTENTS

INTRODUCTION
RESET THRESHOLDS
RESET OUTPUT
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.
V
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
(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

V
CC
V
CC
V
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
V
SS
RST
trec
RST
AI08632
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AN1790 - APPLICATION NOTE

MANUAL RESET INPUT

The STM811 and SMT812 devices fea ture a Manual Reset (MR for a predefined time, t
and VSS (ground), as shown in Figure 3.
MR
. The simplest way to use this feature is to connect a push-button between the
rec
The Manual Reset has an internal 20 k pul l-up resistor and a debounce circuit, which solves the problem of any noise generated by switching the push-button.
The timing diagram represented in Figure 4 shows how the Manual Reset input functions. When MR driven Low, the Reset (RST or RST ing the time that MR
is Low. When the switch is released, MR goes High (thanks to the internal pull-up
) output becomes active after a delay of t
resistor), but the Reset output remains active for a time of t The minimum pulse width of the M R
the M R of MR
pulse width is shorter than t
, which can lead to the system not functioning correctly.
signal (t
MLMH(min)
MLMH(min)
, the RST/RST pulse generated is equal to the pulse width
= 10µs) must be taken into account in a design. If

Figure 3. Manual Reset Connection

V
CC
) input, which activates the Reset output
, and remains active dur-
MLRL
.
rec
is
Push-button
Reset

Figure 4. Manual Reset Waveform

MR
tMLRL
(1)
RST
tMLMH
Note: 1. RST for STM 810/812
2. Refer to STM 8xx datasheet for the tim i ng values .
MR
V
CC
RST
V
SS
trec
V
RESET Input
V
CC
Microprocessor
SS
AI08637
AI08640
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AN1790 - APPLICATION NOTE

CONNECTING THE RESET CIRCUITS IN AN APPLICATION

The standard connection of the Reset devices is very simple, as normally no other circuits are required. The standard connections for the STM809, STM810, STM811 and STM812 are shown in Figure 5.

Figure 5. Standard Reset Device Connections

STM810 Connection STM809 Connection
V
CC
V
CC
V
CC
V
CC
V
CC
V
CC
V
CC
Push-button
Reset
MR
STM810
RST
V
SS
RESET Input
V
Microprocessor
SS
STM809
RST
V
SS
RESET Input
V
Microprocessor
SS
STM811 ConnectionSTM812 Connection
V
CC
V
CC
V
CC
STM812
RST
V
SS
RESET Input
V
Microprocessor
SS
Push-button
Reset
MR
V
CC
STM811
RST
V
SS
V
RESET Input
V
CC
Microprocessor
SS
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Connection in a Noisy Environment

The internal debounce circuit in the ST Reset devices is especially designed to remove any noise gener­ated by switching the push-button at the Manual Reset input. However the internal debounc e circuit may not be sufficient to filter external noise in extremely noisy environments, for example, long wires in a noisy environment or high speed buses near the Manual Reset input. In such cases, an external capacitor can be added to solve the problem (see Figure 6). The recommended value of the capacitor is 0.1µF.

Figure 6. Reset Device Connection in a Noisy Environment

V
CC
V
RESET Input
V
CC
Microprocessor
SS
AI08634
Push-button
Reset
C = 0.1µF
MR
V
CC
STM812
RST
V
SS

Connection to a Bi-directional Microprocessor Reset

Sometimes designers need to connect a Reset Circuit device to a m icroprocessor’s bi -directional Reset input. In this case a resistor should be connected between the RST
output and the microprocessor’s RE­SET inp ut. T he va lue o f the resisto r s hould be high enough to lim it the c urrent when the microprocess or pulls down the RESET
input and low enough to respond when RST goes low. The recomm ended value
of the resistor is 4.7kΩ (see Figure 7).

Figure 7. Reset Device Connection to a Bi-directional Microprocessor Reset

to other devices
V
CC
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V
CC
STM809
RST
V
SS
R = 4.7k
V
RESET Input
V
CC
Microprocessor
SS
AI08635
AN1790 - APPLICATION NOTE

Connection With Reset Output V alid Dow n t o VCC = 0V

When V circuit. In most applications this is not a problem, as most microprocessors do not operate below 1V. How­ever, in applications where RST the R ST be small enough to pull the output to ground. The recommended value of the resistor is 100kΩ (see Figure
8).
falls bel ow 1V , th e RS T (S TM809/811) output no longer sinks curren t, but becomes a n open
CC
must be valid d own to 0V, a pull-down resi stor shou ld b e added to hold
output Low. The value of the resistor must be large enough not to load the RST outpu t, and still
Figure 8. Reset Device Connection With RST
V
CC
V
CC
RST
V
SS
Valid Down to VCC = 0V
STM809
R=100k
V
RESET Input
V
CC
Microprocessor
SS
AI08636

CONCLUSION

ST Reset Circuits are low-power supervisory devices, that have been specifically designed to monitor power supplies in microprocessor systems.
In standard applications, they can be directly connec ted to the mi croprocessor, as no other ext ernal cir­cuits are required. However, they can also be used in specific application environments, with the simple addition of a resistor or capacitor.
They are available 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.

REFERENCES

STM809, STM810, STM811, STM812 datasheet

REVISION HIST ORY

Table 3. Document Revision History

Date Version Revision Details
04-Dec-2003 1.0 First Issue.
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s
d
t t
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