AN1790 APPLICATION NOTE
How to Connect ST Reset Circuits to a Microprocessor
CONTENTS
INTROD UCTION RESET THRESHOLDS RESET OUTPUT MANU AL RESET INPUT CONN ECTING THE RESET CIRCUITS IN AN APPLICATION Connection in a Noisy Environment Connection to a Bi-directional Microprocessor Reset Connection With Reset Output Valid Down to VCC = 0V
This Application Note describes how to connect 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.
INTRODUC TION All digital systems require the power 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 STM812 also provide a push-button reset input signal (MR).
CONC LUSION REFERENCES REVISION HISTORY
December 2003
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RESET THRESHOLDS The STM8xx Reset Circuit devices have different reset threshold levels. Table 1, lists the thresholds, VRST, for each device and each temperature range. When considering the threshold level required for an application, it is also necessary to consider the maximum transient duration, not causing a reset pulse, with respect to voltage threshold overdrive. A large transient voltage overdrive needs less time to generate the Reset pulse, than a small transient voltage overdrive, which needs much longer. Refer to the characterization graph shown in Figure 1. Table 1. Reset Thresholds
Part Temperature Range ( C) Min 25 STM8xxL -40 to 80 25 STM8xxM -40 to 80 25 STM8xxT -40 to 80 25 STM8xxS -40 to 80 25 STM8xxR -40 to 80 2.55 2.70 2.85 2.58 2.63 3.00 2.68 3.00 2.88 2.93 3.15 2.98 4.25 3.03 3.08 4.50 3.14 4.50 4.30 4.38 4.75 4.46 4.54 VRST (V) Typ 4.63 Max 4.72
Figure 1. Max Transient Duration NOT Causing Reset Pulse vs. Reset Comparator Overdrive
700.00 600.00 Maximum Transient Duration (s) 500.00 400.00
R/S/T L/M
300.00 200.00 100.00 0.00
1
10
100
1000
AI07883
Reset Comparator Overdrive, VRST VCC (mV)
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RESET OUTPUT ST Reset Circuits feature a push-pull Reset output with the option of an active High (RST) or active Low (RST) output (see Figure 2 and Table 2). The push-pull output minimizes the external connections between 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
VCC
VCC
VRST
trec RST/RST RST trec RST Reset Circuit
VSS
AI08632
Table 2. Device Options
Part STM809 STM810 STM811 STM812 Reset Output active Low High Low High Manual Reset (MR) No No Yes Yes
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MANU AL RESET INPUT The STM811 and SMT812 devices feature a Manual Reset (MR) input, which activates the Reset output for a predefined time, trec. The simplest way to use this feature is to connect a push-button between the MR and VSS (ground), as shown in Figure 3. The Manual Reset has an internal 20 k pull-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 is driven Low, the Reset (RST or RST) output becomes active after a delay of tMLRL, and remains active during the time that MR is Low. When the switch is released, MR goes High (thanks to the internal pull-up resistor), but the Reset output remains active for a time of trec. The minimum pulse width of the MR signal (tMLMH(min) = 10s) must be taken into account in a design. If the MR pulse width is shorter than tMLMH(min), the RST/RST pulse generated is equal to the pulse width of MR, which can lead to the system not functioning correctly. Figure 3. Manual Reset Connection
VCC VCC VCC Microprocessor
MR Push-button Reset
RST VSS
RESET Input VSS
AI08637
Figure 4. Manual Reset Waveform
MR tMLRL RST
(1)
tMLMH
trec
AI08640
Note: 1. RST for STM810/812 2. Refer to STM8xx datasheet for the timing values.
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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
VCC STM810 VCC VCC Microprocessor VCC
STM809 Connection
VCC
RST VSS
RESET Input VSS
STM809
VCC
RST VSS
RESET Input VSS
STM812 Connection
VCC Microprocessor STM812 VCC VCC VCC
STM811 Connection
VCC
MR Push-button Reset
RST VSS
RESET Input VSS
STM811
VCC
MR Push-button Reset
RST VSS
RESET Input VSS
Microprocessor
AI08633
Microprocessor
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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 debounce 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 0.1F. Figure 6. Reset Device Connection in a Noisy Environment
VCC VCC STM812 VCC Microprocessor
MR Push-button Reset C = 0.1F
RST VSS
RESET Input VSS
AI08634
Connection to a Bi-directional Microprocessor Reset Sometimes designers need to connect a Reset Circuit device to a microprocessor's bi-directional Reset input. In this case a resistor should be connected between the RST output and the microprocessor's RESET input. The value of the resistor should be high enough to limit the current when the microprocessor pulls down the RESET input and low enough to respond when RST goes low. The recommended value of the resistor is 4.7k (see Figure 7). Figure 7. Reset Device Connection to a Bi-directional Microprocessor Reset
to other devices VCC VCC STM809 VCC Microprocessor
R = 4.7k RST VSS RESET Input VSS
AI08635
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Connection With Reset Output Valid Down to VCC = 0V When VCC falls below 1V, the RST (STM809/811) output no longer sinks current, but becomes an open circuit. In most applications this is not a problem, as most microprocessors do not operate below 1V. However, in applications where RST must be valid down to 0V, a pull-down resistor should be added to hold the RST output Low. The value of the resistor must be large enough not to load the RST output, and still be small enough to pull the output to ground. The recommended value of the resistor is 100k (see Figure 8). Figure 8. Reset Device Connection With RST Valid Down to VCC = 0V
VCC VCC STM809 VCC Microprocessor
RST
R=100k
RESET Input VSS
VSS
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 connected to the microprocessor, as no other external 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 A range and offer a wide range of voltage thresholds.
REFERENCES
STM 809, STM810, STM811, STM812
datasheet
REVISION HISTORY Table 3. Document Revision History
Date 04-Dec-2003 Version 1.0 First Issue. Revision Details
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