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| What is a Supervisor? |
| A Supervisor is a protection device. It monitors certain
system parameters and reacts to protect the system when
those parameters go out-of-tolerance. The most common supervisor
function is the Reset circuit or Voltage Monitor which monitors
the main system supply voltage and resets the sytem in case
the voltage is too high or too low. Other supervisor functions
include protection against runaway software, high temperature,
and intrusion, to name just a few. |
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| What is a Microprocessor (µP)
Supervisor? |
| The key Microprocessor Supervisory functions are the Reset
circuit and the Watchdog. These two functions are contained
in most Microprocessor Supervisors. |
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| Reset Circuit:
Also known as a Voltage Monitor, it protects the
microprocessor (and the system) by monitoring the system
power (Vcc) and generating a RESET signal to the µP
(micro) if the system power is out-of-tolerance. If the
voltage is too low, the micro can behave unpredictably,
and the Reset circuit prevents that.
In the figure to the right, when Vcc falls below Vref,
RESET is asserted. This is called, "Low-Voltage Detect
(LVD)." At power-up, the voltage monitor also asserts
RESET until Vcc has risen above Vref, then holds it for
a short time - typically 100ms - to ensure the system has
stabilized. This is called, "Power-on Reset (POR)."
Together, these two functions are called, "POR-LVD,"
providing yet another synonym for the basic Reset chip.
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Watchdog: The Watchdog is the
other most common Microprocessor Supervisory function. It
is a special timer which will reset the µP when it
times out. Software periodically resets the timer to prevent
that. If the software gets hung up or in a runaway condition,
it will not reset the timer, so the timer will then reset
the micro to restore the system to proper operation.
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| What is an NVRAM Supervisor? |
| NVRAM Supervisors turn standard, Low-Power
SRAMs into NVRAMs with control and battery back-up functions
at significant cost and space savings. The key NVRAM Supervisory
functions are the Battery Switchover circuit and the Write
Protect circuit.
Battery Switchover: The battery
switchover circuit monitors Vcc, and when it falls below
the switchover threshold, Vso, switches power to the battery,
thereby providing a continuous, uninterrupted supply to
the LPSRAM to preserve the data in it.
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Write Protect:
The Write Protect circuit monitors Vcc, and if it falls
below the Power-fail Detect threshold voltage, Vpfd, it
gates off access to the LPSRAM.
Since a microprocessor can behave erratically at low voltages,
the Write Protect circuit protects the LPSRAM contents from
possible corruption as power is failing.
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| Write Protect and Reset:
By bringing the internal Write Protect signal to an output
pin, the Write Protect circuit can be used to provide Power-on
Reset/Low Voltage Detect, as well as Write Protection.
Many of ST's NVRAM Supervisors and Serial RTCs include
a Reset or POR-LVD output.
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Push-Button Reset:
Many of ST's Reset circuits include reset inputs which can
trigger the same Reset output timing as provided by the
POR-LVD
function. These inputs are suitable for use with external
switches to provide a Push-button Reset function, often
necessary in systems.
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| Battery Monitor: Many of ST's
Reset, Supervisor, and Serial RTC devices include a battery
monitor to inform the system and the user when it's time
to replace the battery.
Once the battery falls to 2.5V, it needs to be replaced.
Some devices have a Battery Low pin while others have a
Battery Low Bit, accessible in a register in the RTC.
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| What is a Securitizor? |
A Securitizor is a Supervisor that includes ST's new Tamper
Detect/RAM Clear function. The M41ST87
is one such device. More are planned.
Tamper Detect and RAM Clear: Applications
requiring data protection are perfect candidates for this new
supervisory function. Whether Vcc is active, or the system is
in battery-backed mode, the Tamper Detect circuit is constantly
monitoring its two independent Tamper Detect inputs, looking for
any condition which would indicate someone was tampering with
the system. |
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When a Tamper Event occurs, the event time is automatically
stored, plus the user can configure up to three other responses:
• the circuit can interrupt the processor;
• the device's internal RAM can be cleared; and
• an external signal can be asserted to clear external RAM.
Tamper Detect options include:
• detect high-to-low or low-to-high transition, and
• detect the making or breaking of a connection.
The Tamper Detect/RAM Clear function is currently available on
the M41ST87.
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| Early Power-fail Warning |
| Also known as PFI-PFO (Power-fail In/Power-fail Out), this circuit
combines a comparator with a precision reference. It is used to
monitor the power supply upstream of the regulator and provide advanced
warning that the power is about to fail. By providing a few milliseconds
of advanced warning, the system can save any critical parameters
to NVRAM and gracefully shut down. |
| When the unregulated supply, Vunreg, first begins to
fall, Vcc will stay steady. Eventually, as Vunreg continues
to fall, Vcc will begin to fall. The time between these
two events can be used to gracefully shut down the system. |
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The resistors R1 and R2 determine the threshold voltage,
Vpfi-th. When Vunreg drops below Vpfi-th, PFO is asserted
to the microprocessor. After Vunreg has continued falling
for some time, Vcc will begin to fall. a short time later,
as Vcc falls out of tolerance - usually about 10% below
nominal - RESET will be asserted, and the system will shut
down abruptly.
"tsave," the time between the assertion
of PFO and RESET, can be used to save important status information
and parameters. The amount of time available is a function
of the system load current and the bulk store capacitance
in the regulator. This will vary widely, but can often be
10msecs or more.
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| Real-Time Clock (RTC): Since
Real-Time Clocks require Battery Switchover and Write Protect
circuits, it is a natural partnership to include RTCs on
Supervisor ICs. Several ST devices include this combination,
examples of which are the M41ST85
and the M48T212.
Both include Microprocessor Supervisory features, such as
Power-on Reset/Low-Voltage Detect, and Watchdog. NVRAM Supervisors
that incorporate the RTC are called TIMEKEEPER®
Supervisors.
Other ST Serial Real-Time Clocks also include Microprocessor
Supervisor functions, but without the NVRAM Switchover.
The M41ST84
and M41T94
are two such devices which offer POR-LVD
and Watchdog.
Alarm: The Alarm function works
just like an ordinary alarm clock. The user programs in
an alarm time, and when the current time is equal to the
alarm time, an interrupt is generated to the micro.
All TIMEKEEPER® Supervisors and many Serial
RTCs include the Alarm function. |
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| More Information |
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