TN0941
Technical note
Reset and supervisor ICs: frequently asked questions (FAQ)
Introduction
•
When to and why use reset and supervisor ICs?
•
What is the difference between a voltage detector and a reset IC?
•
What are the main reset and supervisor IC categories?
•
Why should I use a standalone reset IC if one is already integrated in the microcontroller?
•
What are the following core features of reset and supervisor ICs used for?
– Power-on reset (POR)
– Low-voltage detect (LVD)
– Manual reset (MR)
– Reset pulse width (t
REC
)
– Push-pull and open drain reset outputs
•
What are the following additional options of reset and supervisor ICs used for?
– Backup battery switchover
– Battery freshness seal
– Watchdog
– Chip enable gating
– Early power fail
– Anti-tampering functions
– Reset input delay time (t
SRC
)
•
What is a Smart Reset™ and what is it used for?
•
With Smart Reset™ devices, is the reset asserted repeatedly if a push-button is pressed
for a long time?
August 2013
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Contents
TN0941
Contents
1
2
When to and why use reset and supervisor ICs? . . . . . . . . . . . . . . . . . 4
What is the difference between a voltage detector
and a reset IC? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
What are the main reset and supervisor IC
categories? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1
3.2
3.3
3.4
3.5
Reset and voltage detectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Microprocessor supervisors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Smart resets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Watchdog timers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Voltage protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3
4
Why should I use a standalone reset IC if one is
already integrated in the microcontroller? . . . . . . . . . . . . . . . . . . . . . . . 6
What are the following core features of reset and
supervisor ICs used for? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
5.1
5.2
5.3
5.4
5.5
Power-on reset (POR) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Low-voltage detect (LVD) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Manual reset (MR) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Reset pulse width (t
REC
) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Push-pull and open drain reset outputs . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
5
6
What are the following additional options of reset
and supervisor ICs used for? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
6.1
6.2
6.3
6.4
6.5
6.6
6.7
Backup battery switchover . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Battery freshness seal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Watchdog . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Chip enable gating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11
Early power failure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11
Anti-tampering functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Reset input delay time (t
SRC
) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
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Contents
7
8
What is a Smart Reset™ and what is it used for? . . . . . . . . . . . . . . . . 13
With Smart Reset™ devices, is the reset asserted
repeatedly if a push-button is pressed for a long
time? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Support material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
9
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When to and why use reset and supervisor ICs?
TN0941
1
When to and why use reset and supervisor ICs?
The reset and supervisor functions should be used in complex systems to ensure proper
wake-up during power-up, and/or to reboot the system when an application has failed or
frozen.
With the increased development and utilization of microprocessing units (example, 8-bit low
cost, 32-bit low power, 64-bit high performance, multi-core) in all kind of applications, there
is an ever-increasing need for reset and supervision functions.
Reset and supervisor ICs are like the lifeguard of a system: they protect against
software and power failure. Using them makes the final product more robust,
whatever its complexity, and increases the end-users perception of quality.
2
What is the difference between a voltage detector
and a reset IC?
The main difference is the reset pulse width (t
REC
) feature on the output pin (see
Section 5:
What are the following core features of reset and supervisor ICs used for?).
Voltage
detectors act as comparators and do not have any t
REC
whereas it is a major feature of a
reset IC (see
Figure 1).
Figure 1. Block diagram of (1) Voltage detector and (2) Reset IC
Voltage detectors also integrate accurate hysteresis functions (V
th+
/V
th-
) whereas only one
voltage threshold is usually specified for a reset IC (V
th
). In addition, the noise immunity of a
reset IC is native by design due to the t
REC
feature.
Suggested products
•
•
For further information on voltage detectors, please refer to
STM1061
For further information on reset ICs, please refer to
STM809
and
STM1001
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TN0941
What are the main reset and supervisor IC categories?
3
3.1
What are the main reset and supervisor IC
categories?
Reset and voltage detectors
These devices provide basic voltage monitoring and reset timing for the power-on reset
(POR) and low-voltage detect (LVD) function. Some devices feature an additional input
which can be used for push-button reset. Other devices are designed to monitor up to five
power supply voltages.
3.2
Microprocessor supervisors
These devices add additional features including a watchdog, early power fail warning, push-
button reset, and battery monitoring. Microprocessor supervisors with a switchover function
preserve SRAM data in the absence of system power by automatically switching to battery
backup when system power fails. A chip enable gate automatically write protects the SRAM
when V
CC
begins to fall.
3.3
Smart resets
Smart resets extend the functional capability of existing buttons so that users can simply
reset their frozen device with a long push of one or two buttons simultaneously. Smart
Reset™ devices provide a useful feature that ensures inadvertent short reset push-button
closures do not cause system resets. This is done by implementing extended Smart
Reset™ input delay time and combined push-button inputs, which together ensure a safe
reset and eliminate the need for a specific dedicated reset button.
3.4
Watchdog timers
Watchdog timers are mandatory products for applications requiring the highest security
level. They are a robust and reliable way of monitoring software code execution or hardware
failure and taking appropriate action (example, system reboot, high level interrupt
generation).
For higher efficiency, the watchdog function is usually not integrated into the
microprocessor, but implemented with standalone products. STMicroelectronics offers
various watchdog implementations, allowing a match with most application environments.
3.5
Voltage protection
These devices provide the necessary protection that prevent external overvoltage peaks
from damaging the internal circuitry. They are commonly implemented on the power
charging patch of portable devices, to prevent a USB or an AC charger power failure.
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